Boilers / Exhaust gas Economizers Flashcards

Question

How often is boiler surveying carried out?

Answer 1

All boilers must have a minimum of two internal examinations every 5 years during dry dock. A general external examination is to be carried out at the time of the Annual Survey. An external survey of boilers including tests of safety and protective devices, and tests of safety valves using their relieving gear, is to be carried out annually within the range dates of the Annual Survey of the ship. The safety valves are to be tested by the Chief Engineer at sea within the dates of the Annual Survey. This test is to be recorded in the log book and reviewed by the attending Surveyor prior to completion

Answer 2

The survey of auxiliary boilers includes ancillary equipment which, together with the controls, should be examined under working conditions and the correct adjustment of any safety devices to be verified. A thorough external examination of boilers is to be undertaken. It is to be verified that boiler supports, chocks, rolling stays, uptakes and the funnel base are in satisfactory condition. Thorough examination of each boiler, together with super heater, superheat control, air heater and economiser, if fitted. If a boiler has not been sufficiently cleaned to allow a proper examination of pressure parts, the survey cannot be regarded as complete until this has been done. Where the construction of a boiler does not allow direct visual internal examination of the shell, drums or headers, the attending Surveyor(s) are to be satisfied that the boiler is in a safe working condition by using remote viewing instruments, Ultrasonic Testing (UT), or hydraulic testing to determine a safe working pressure. Longitudinal stays attached to end plates by external and internal welds should be examined at the inner welds for possible cracking. All such stays should be drilled at the ends with holes 5 mm diameter and of sufficient depth to penetrate the centre of the stay beyond the inner weld. Every effort is to be made to complete the survey prior to departure. If, for any reason, the survey is not completed, the definite intentions of the Owner/Operator regarding completion are to be ascertained. If a later date, more than two months after the date on which the internal examination was held, is required in cases where a ship remains in port without the appliance being placed under steam, a further internal survey is to be carried out Rolling stays, expansion/sliding feet arrangements and freedom of pins and holding down bolts are to be specially examined in way of attachments to the shell or casings and freedom of pins is to be verified The joints of manhole, hand hole doors are to be examined and to ensure that the jointing faces are in good condition and that the clearances do not exceed 1.5 mm

Answer 3

Verify the calibration of the boiler pressure gauges. Where the calibration cannot be readily verified, a written statement is to be obtained from the Master/Chief Engineer confirming that the gauges have been calibrated. Alternatively, consideration may be given to a second gauge for verification of the primary gauge. Establishing that water level indicators are operating correctly by utilising a methodical blow down sequence. Comparison of bench tested valves previously set under workshop conditions to respective pressures as set at sea. Establishing that water level indicators are operating correctly by utilising a methodical blow down sequence. Testing all safety and alarm equipment. Observation of boiler starting sequences and firing. Verification of the calibration of the boiler pressure gauges. Where the calibration cannot be readily verified, a written statement is to be obtained from the Master/Chief Engineer confirming that the gauges have been calibrated. Alternatively, consideration may be given to a second gauge for verification of the primary gauge. Testing all safety and alarm equipment & observation of boiler starting sequences and firing. A general examination of the associated oil fuel system including fuel tank valves, pipes, deck control gear are also to be carried out.

Answer 4

Survey of alarms, controls and safeguard lights during periodical surveys is a requirement of the appropriate Rules and Regulations. The purpose is to ensure that the alarms, controls or safeguards remains functional in service. Records of changes to the hardware and software used for control and monitoring systems for propelling and essential auxiliary machinery since the original issue (and their identification) are to be reviewed. Satisfactory operation of the alarms, controls and safeguards is to be verified.

Answer 5

Safety valves are to be adjusted under steam to not more than 103 per cent of the approved design pressure shown in the Register Book and the Survey Preparation Package (SPP). In all cases, a record of the pressure to which the safety valves have been set should be left on board. Where more than one safety valve used, the other(s) are to be gagged (prevented from opening) whilst the safety valve is tested. On completion this role is to be reversed so that in turn, all valves have been tested. Verification of the calibration of the boiler pressure gauges When surveying Main Boilers, Auxiliary Boilers, Exhaust Gas Boilers, Economisers or any steam plant, examination of the drain lines fitted to the safety valves to ensure they are independent, clear, unblocked and in accordance with the appropriate Rules and Regulations.

Answer 6

hydrostatic test is a stress procedure used to determine the integrity of all aspects of a given boiler unit. The test involves filling the boiler with water and pressurizing to at least one and half times the maximum specified operating pressures for a short duration.

Answer 7

After boiler repairs a hydraulic test is done. In preparation of the test all equipment and foreign matter is removed from the water space of the boiler and everything is examined. Any permanent welding should be done by a professional welder and welds are subjected to non destructive testing techniques. Test is preformed to hold the safety valves and boiler mountings closed while equipment is subjected to a hydrostatic test. With the exception of feed check valve and air cock. To avoid damage to the spindle and/or seat, care is required so the gag screw is not tightened. Boiler is then filled with clean fresh water and purged of air. Using a hydraulic pump unit connected by a small bore pipe to boiler directly or to feed line and then pressure is gradually applied. Testing pressure used is 1.5 times the working pressure applied for at least 30mins. When boiler is under pressure its examined for leakages and faults. Weld repairs should be checked repeatedly with blows by a hammer.

Answer 8

If an examination of boiler has to be done boiler has to be emptied of water.its always better if time is available to allow boiler to cool down for a period of time after shut down and then you pump the water out. Avoid sudden shock of cooling due to complete blow down. Allow as much time for blow down as possible after shut down and before starting if blowing down to the sea. Ship side blow down cock must be opened first and then blow down valve for boiler is gradually opened. This way engineer has better control of operation. For example if external blow down pipe between boiler and ship side is in a corroded condition and engineer opens boiler blow down valve first this leads to rupturing of blow down pipe. As a safety measure when handle is removed valve must be in closed position this way the valve cannot be accidentally left open. When blow down process starts noise level falls and pressure is low. Ensure no cold sea water can enter boiler. When boiler is empty of water it still contains steam, which can condense and cause a vacuum condition. This increases risk of entry of cold sea water but to prevent entry boiler blow down valve is non return type with some water tube boilers having a double shut off. Even with these arrangements its recommended to start closing boiler blow down valve when pressure is low enough and down to the desired value, the blow down valve is then closed tightly as well as ship side cock. After this allow time for boiler to cool down and lose all its pressure. When pressure is atmospheric, open air cock and gauge glass drain to ensure pressure inside boiler is atmospheric. Then boiler door is unbolted and manholes knocked in top or bottom but not both. If top door secure rope to eye bolt then secure other end of rope. Loosen the retaining nuts but don’t take them off yet. Then take a long plank of wood stand back and knock door down. When door open remove retaining nuts. Do not then immediately open bottom manhole because if boiler is hot this will lead cool air passing through as well as thermal shock.

Answer 9

see EOOW ORAL Sketch Pack for drawing A three element boiler water level control system is shown above. The measured variables or elements are ‘steam flow’, ‘drum flow’ and ‘feed water flow’. Since in a balanced situation steam flow must equal feed flow, these two signals are compared in a differential relay. The relay output is fed to a two term controller and comparator into which the measured drum level signal is also fed. Any deviation between the desired and actual drum level and any deviation between feed and steam flow will result in controller action to adjust the feed water control valve. The drum level will then be returned to its correct position. A sudden increase in steam demand would result in a deviation signal from the differential relay and an output signal to open the feed water control valve. The swell effect would therefore not influence the correct operation of the control system. For a reduction in steam demand, an output signal to close the feed water control valve would result, thus avoiding shrinkage effects. Any change in feed water pressure would result in feed water control valve movement to correct the change before the drum level was affected.

Answer 10

see EOOW ORAL Sketch Pack for drawing the above drawing shows an ogee ring for a boiler Tell-tale holes are drilled at equal circumferential intervals in the boiler shell this enable leakage between the Ogee ring and the boiler shell. Connecting the bottom of the furnace to the boiler shell plating is a seamless Ogee ring. This ring is pressed out of thicker plating than the furnace; the greater thickness is necessary since circulation in its vicinity is not as good as elsewhere in the boiler and deposits can accumulate between it and the boiler shell plating. Hand hole cleaning doors are provided around the circumference of the boiler in the region of the ‘Ogee’ ring.

Answer 11

The reactions of steam and water in the drum are complicated and require a control system based on a number of measured elements. When a boiler is operating the water level in the gauge glass reads higher than when the boiler is shut down. This is because of the presence of steam bubbles in the water, a situation which is accepted in normal practice. If however there occurs a higher increase in steam demand from the boiler the pressure in the drum will fall. Some of the water present in the drum at the higher pressure will now ‘flash off’ and become steam. These bubbles of steam will cause the drum level to rise. The reduced mass of water in the drum will also results in more steam being produced, which will further raise the water level. This effect is known as ‘swell’. When the boiler load returns to normal the drum pressure will rise and steam bubble formation will reduce, causing a fall in water level. Incoming colder feed water will further reduce steam bubble formation and what is known as ‘shrinkage’ of the drum level will occur. The problems associated with swell and shrinkage are removed by the use of a second measuring element, ‘steam flow’ . A third element ‘feed water flow’ is added to avoid problems that would occur if the feed water pressure were to vary.

Answer 12

Simple Pressure Jet Atomizer Spill type of pressure jet atomizer Constant differential type atomizer Plunger type of pressure jet atomizer Rotary or spinning cup jet boiler fuel oil atomizers

Answer 13

see EOOW ORAL Sketch Pack for drawing Sketch above shows diagrammatically the principle of the simple pressure jet boiler fuel oil atomizers. Oil fuel at a pressure of not less than 8 bar is supplied to the tangentially arranged ports, wherein it falls in pressure, resulting in the oil swirling around at high velocity inside the chamber. The greater the pressure drop the greater would be the velocity, but generally supply oil pressure would not be greater than 25 bar, high pressures create pumping and sealing problems. At outlet from the short sharp edged orifice (care must be taken not to damage the orifice during cleaning) a hollow expanding cone of fuel droplets is produced whose initial film thickness reduces as oil supply pressure increases.

Answer 14

1.Simple, inexpensive and robust. 2.No moving parts, hence no possibility of seizure. 3.Large range of droplet size for one pressure. 4.Turn down ratio is low, about 2.5 to 1. Turn-down ratio is the ratio of the burner throughput maximum to minimum. If say the maximum throughput is 2500 kg/hr, then with a turn down ratio of 2.5 to 1 the minimum throughput will be 1000 kg/hr. Maximum throughput is about 3200 kg/hr 5.No alteration in air register arrangement for all outputs, this is a consequence of the low turndown ratio of this type of burner. 6.To vary throughput the pressure must be altered, this results in variation in droplet size, hence atomization and combustion will be affected.

Answer 15

With the differential pressure between delivery and spill kept constant the pressure drop across the tangential ports is kept constant and the rotational velocity of the fuel in the swirl chamber will not alter. This will keep fuel cone angle and atomization constant. The main disadvantage with these types of boiler fuel oil atomizers is the large quantity of hot oil being returned, it may prove difficult to keep control over oil temperature.

Answer 16

see EOOW ORAL Sketch Pack for drawing drawing above shows the principle of operation of this type of burner, as the oil supply pressure is increased the spring loaded plunger moves to uncover extra tangential oil entry holes. The pressure drop and hence the rotational velocity of the oil remains nearly constant.

Answer 17

see EOOW ORAL Sketch Pack for drawing As seen in the drawing above the atomizer consists of a motor driven fan, metering pump and fuel cup. The fuel cup which rotates at 70 to 100 rev/second is supplied with oil at low pressure (1.7 to 4.5 bar) from the metering pump which is used to control the throughput. Due to centrifugal force and opposite swirl of air to fuel the oil film leaving the cup is rapidly broken down into relatively even sized droplets.

Answer 18

1.High output possible, up to 3600 kg/h. 2.Low oil supply pressure. 3.At low throughputs atomization due to reduced oil film thickness is improved. 4.Wide turndown ratio, up to 20:1 possible. 5.Oil viscosity need only be reduced to 400 seconds Redwood number 1 for satisfactory operation.

Answer 19

internal desuperheater spray type external desuperheater external desuperheater

Answer 20

Desuperheaters remove the superheat in the steam that is to be used for auxiliary purposes, the reason is that if high temperature steam was used for auxiliaries then the materials used would have to be capable of withstanding high temperatures, this leads to increased initial cost.

Answer 21

see EOOW ORAL Sketch Pack for drawing Desuperheaters are generally coils situated in the water spaces that are supplied with steam from the superheated section outlet; this type of Desuperheaters is generally called as internal Desuperheaters. If this type of Desuperheaters develops a leakage then water loss from the boiler occurs since boiler pressure is greater than steam pressure in the Desuperheaters coil. This water loss could result in water hammer and subsequent damage in the auxiliary system.

Answer 22

see EOOW ORAL Sketch Pack for drawing Spray type external Desuperheaters is shown in the diagram above, in which feed water of low solids content at 3.5 bar above steam pressure is supplied at a controlled rate into a vertically arranged vessel to mix with superheated steam. The excess water is kept to a minimum, collects at the bottom of the vessel and passes out through the drain and then the superheated steam moves up the annular space formed by the mixing compartment and the outer container to the auxiliary steam supply line.

Answer 23

see EOOW ORAL Sketch Pack for drawing Another type of external Desuperheaters is shown in the figure below. This is really part of the boiler externally arranged. It has a steam and water connection to the boiler drum so that the circulation through the Desuperheaters takes place, taking heat from the superheated steam in the solid drawn steel ‘U’ shaped tubes. A blow down valve is fitted in order to remove any sludge that may accumulate in the lower portion of the vessel. This type of Desuperheaters is for large steam demands since the 3 pass U tube arrangement would be too large to accommodate in the water space of the steam drum. Good combustion is essential for the efficient running of the boiler. It gives the best possible heat release and minimum amount of deposits on the heating surfaces.

Answer 24

Boiler is a pressure vessel (unit) used on ships in which the water is heated to evaporate and generate the steam and the unit is arranged so that the generated steam is accumulated in it. The two main types are water tube and fire tube. steam is then used for heating of fuel and water as diesel oil is in limited supply and expensive

Answer 25

Steam can be classified into two main types saturated and superheated. Saturated steam Saturated steam is steam that is in equilibrium with water at the same temperature and pressure. This means that saturated steam contains both water vapour and liquid water droplets. The temperature and pressure of saturated steam depend on each other and are determined by the boiling point of water at that pressure. For example, at atmospheric pressure (101.3 kPa), the boiling point of water is 100°C, and the saturated steam has the same temperature. However, if the pressure is increased to 10 bar (1000 kPa), the boiling point of water rises to 180°C, and the saturated steam also has a higher temperature. Superheated steam Superheated steam is steam that has been heated to a temperature higher than its saturation temperature at the same pressure. This means that superheated steam does not contain any liquid water droplets and is completely dry, high-temperature steam with significantly more energy content. Superheated steam can be produced by passing saturated steam through a separate heating device called a super heater , which transfers additional heat to the steam by contact or by radiation.

Answer 26

1.High efficiency (generally greater than 85 %) hence reduced fuel consumption. 2.Flexibility of design – important space consideration. 3.Capable of high output (i.e. high evaporation rate). 4.High pressures and temperatures improve turbine plant efficiency. 5.Flexible in operation to meet fluctuating demands of the plant – superheat control rapidly responsive to changing demands. 6.Generally all surfaces are circular hence no supporting stays are required. 7.Steam can be raised rapidly from cold if the occasion demands (3 to 4 hours compared to 24 hours for smoke tube or fire tube boilers) because of the positive circulation. 8.Consider a fire tube or smoke tube boiler and a water tube boiler with similar evaporation rates the water tube boiler would be compact and relatively light by comparison and its water content would be about 7 tonnes or less compared with the smoke tube or fire tube boiler’s 30 tonnes. 9.With double casing radiation loss can be cut to 1 % or less.

Answer 27

when starting-up boiler after any repair work of the refractory, it is very important to reduce boiler load. This is because the new refractory still contains a small amount of water. When heated the water vaporises and expands which might cause fissures and cracks in the refractory. The burner must therefore only be operated at minimum load and in intervals of 1-2 minutes for the first hours. Between each operation interval the burner should remain stopped for approximately 8-10 minutes. Boiler refractory is heat-resistant materials that are used to line the boiler to help it retain heat. This prevents thermal loss during boiler operation. It also helps the boiler retain heat even when it is not firing.

Answer 28

Before start-up of the boiler plant, following general work and check procedures must be followed 1.Check that the main steam valve, by-pass valve and circulation valves if provided, scum valve, and blow-down valves are closed. 2.Open the feed water valves and the air valve. Fill the boiler with feed water to approximately 50 mm below normal water level. The water level rises due to expansion when the boiler is heated. If the temperature difference between the boiler and feed water exceeds approximately 50ºC, the boiler must be filled very slowly. 3.When filling a pressure less boiler, the shut-off valve after the feed water pump must be throttled. Otherwise the pump motor will be overloaded. 4.Check the water level in the water level gauges. Check frequently during the complete start-up. The water level gauges should be blown down several times to ensure a correct indication. 5.Check that the water level control system is connected and operational. 6.Check the oil system and start the fuel oil supply pump. Pre-heat the fuel oil if the burner should operate on heavy fuel oil. 7.Check the burner and the safety functions according to the manufacturer instruction.

Answer 29

The following work procedures must be followed during start-up of the boiler. 1.Check that the gauge board valve and pressure gauge valves are opened. 2.Check that the air valve is open if the boiler pressure is below 1.0 bar. 3.Start the burner on manual control and on low load. Check that the water level does not rise too high during the pressure rising period. 4.Drain via the blow down valves if the water level is too high. 5.If the air valve was opened close it when only steam blows out. A pressure reading should be indicated on the boiler pressure gauge before the air valve is closed. 6.Tighten all covers such as manholes, hand holes, inspection doors, etc. during the pressure rising period. If required, check all flange joints on the plant. 7.Change to automatic control of the burner when the boiler pressure is 0.5 bar lower than the working pressure of the boiler. 8.Open the by-pass valve slowly to heat-up and pressurise the steam system. If the boiler is not provided with a by-pass valve, the main steam valve should be used to heat-up and pressurise the steam system. 9.Open the main steam valve and close the by-pass valve. 10.Open the valves to the steam consumers carefully in order to avoid water chocks. 11.When the boiler is in normal operation, check that the water level control system and the gauge board functions are fully operational.

Answer 30

Change over boiler fuel oil to diesel oil. Make sure all diesel engines are stopped or changed over to diesel oil. Electric fuel oil heaters can be put in use if fitted on board. Shut main steam stop valve and put off feed water pump. Carry out scum blow down to remove floating oil, sludge, etc. Carry out bottom blow down after scum blow down, otherwise oil carry over may occur. Boiler blow down to be carried out in steps to avoid thermal stresses. Excessive stresses may lead to cracking. Sudden cooling down and drop in pressure may cause stresses in boiler. So reduction of pressure must be gradual. Make sure nobody is standing near ship side blow down valve. Gradually reduce boiler pressure in steps, 5bar—–>4.5bar——>4bar and so on. This helps in gradual reduction in boiler temperature also. Safety gear to be used by the personnel operating blow down valves. When boiler pressure reaches 1bar, open up the vent. Now if the ship is loaded and having high draft, it may not be possible to drain boiler water completely. In such case arrangements to be made for transferring remaining boiler water to bilges. Once the boiler is fully de pressurized, slack top manhole door first, make sure no pressure is inside, support the door with chain block, and slowly remove it and secure enter the boiler when it is sufficiently cold. Check the welding in the boiler. A careful examination should be carried out with respect to any possible corrosion or crack formation. Special care should be taken to the water line area in the pressure vessel where oxygen pitting may occur. If deposits are found to be forming in the boiler tubes, the boiler should be chemically cleaned. It is advisable to consult a company of cleaning specialists who will examine the boiler deposits and treat the boiler accordingly. After chemical treatment the boiler should be blown-down at least twice a day for approximately one week. This will ensure that excessive sludge deposits due to chemical treatment do not collect in the bottom of pressure vessel.

Answer 31

If the boiler is contaminated with foreign substances like oil, chemicals, corrosion products etc., it is very important to act immediately to avoid damages to the boiler. Layers of thin oil films, mud, etc. exposed to the heating surfaces causes a bad heat transfer in the boiler, leading to overheating followed by burned out pressure parts. In order to remove such contamination, a boiling out or acid cleaning have to be performed immediately. Corrosion products from the pipe system or insufficient boiler water treatment may result in corrosion in the boiler itself. It is therefore important to observe that such circumstances do not occur in the system.

Answer 32

The furnace should be inspected at least twice a year. During this inspection the following issues should be taken into consideration. Check for cracks at the refractory lining and that the furnace walls are free from excessive soot deposits. Examine carefully the area opposite the burner. Too much soot deposits indicate that the burner should be adjusted. Check that the pin-tube elements are intact and that soot deposits are within normal limits.

Answer 33

1.Check the boiler steam pressure and the water level. 2.Check that the feed water control system is operational. 3.Check the boiler water condition and make necessary countermeasures with regard to the feed and boiler water treatment. If necessary blow-down the boiler. 4.Check the function of the oil burner at different capacities through the inspection holes on the boiler. 5.Check the flue gas (a mixture of gases produced by the burning of fuel) temperature after and/or the draft loss across the boiler. If either the temperature or the draft loss is too high, the pin-tube section must be cleaned.

Answer 34

Regularly inspect boiler flame to ensure correct air fuel ratio is maintained. This makes complete combustion with clean flue gas. Carry out boiler burner and swirler routines as per PMS. Regularly soot blow economisers and water wash periodically to remove soot deposits. Use proper grade of fuel oil for boiler. Fuel oil used for boiler to be treated well. Avoid low load operation of boiler for long period. Running boiler near full load periodically, helps in expelling out carbon deposits adhered in uptake passages and reduces chances of boiler uptake fire considerably.

Answer 35

Auxiliary boiler uptake or flue gases normally pass through economisers or air pre-heaters or feed water heaters. All these types of heat exchangers are fin type. Hence large amount of carbon or soot deposits on the fins. Along with the flue gases some un-burnt carbon particles also get deposited on the economisers. Normally dry soot deposits have a very high ignition temperature. But when the soot gets wet with hydrocarbon vapours, their ignition temperature comes down to around 150 degree Celsius. This may result in boiler soot fire or boiler uptake fire. When an uptake fire is suspected, never soot blow the economiser or boiler. Soot blow with steam causes steam to convert as hydrogen gas and result in metal fire.

Answer 36

It is inter crystalline fracture. It is cause by high concentration of caustic soda and the material under stress. The stress corrosion cracks follow the grain or crystal boundaries of the material and failure. Sodium sulphate or sodium nitrate is used for the prevention of caustic embrittlement. It can be found in highly stress area in boiler. Such as tube and tube plate connection, riveted head, seam and boiler mountings.

Answer 37

If water level has not yet dropped completely out the sight glass, water may be put into the boiler. If water drops completely out of sight glass, check another sight glass, if both disappear water; do not add water until the boiler is cool enough to prevent any possible damage due to rapid cooling of over heated plate. The fire must be immediately stopped by extinguishing it with appropriate means of fire extinguishing media. The main steam stop valve must be closed. Blow down and cool down the boiler. Check leakage, drum outside, locate the cause of trouble and make necessary repair. Enter the boiler after it has cooled and examined any possible damage. After repair, water fill up slowly and restore to normal operation. If no damage occurred, inject the water slowly into the boiler and restore it to operation.

Answer 38

Normal boiler shut down Boiler can be stopped by turning selector switch at control panel to stop. When the boiler is stopped, sudden temperature and pressure drops should be avoided as they might expose mountings, pipe lines, and the boiler plant to inadmissible temperature gradients. When minimum load is obtained, stop the burner. Keep the water level at normal level until the boiler stops producing steam. Stop the feed water pump and close the feed water valves. Close the main steam valve.

Answer 39

The boiler must be taken out of service immediately if parts of the heating surface have been glowing or the boiler shows recognisable deformations. The supervising authorities must be informed, and the boiler must not be used until approval from these authorities is available a substantial loss of water is noted the feed water system is unable to provide the necessary amount of feed water, e.g. due to failure of parts the safety valve cannot function sudden cracks or damage are noted in the refractory, and if steam or moisture is coming out of the refractory oil in the feed water is detected too high salinity level is detected. If an emergency shut down must be carried out, the fuel supply should be stopped. The main steam valve should be closed gradually, and the boiler must be cooled. The safety valves must not be operated. Parallel working boilers should be disconnected at once.

Answer 40

1.Clean the boiler from soot with water. 2.Operate the burner for at least 15 minutes after the soot removal to dry out the remaining water. 3.Stop the boiler as mentioned previously. 4.Check the furnace and the pin tubes with regard to cleanliness. 5.Empty the boiler from water and clean it. Check if lime stone appears. 6.Check and clean the outer fittings. Change gaskets where required. 7.Clean the feed water tank and feed water pipes. 8.Clean and grease the bearings of motor, pump, and fan. 9.Check and align the burner, if necessary. 10.If the boiler is shut down for a long period of time, the pin tubes must be thoroughly cleaned. 11.Check that the necessary spare parts are available. Order complementary parts in time. It is of extreme importance that the boiler is NOT operated without water when the oil burner is in operation, e.g. due to disconnection of the water level safety devices. This will immediately cause complete break down of the boiler.

Answer 41

1.Low pressure boiler – up to 10Kg / Cm^2 2.Medium pressure boiler – 10-25 Kg /Cm^2 3.High pressure boiler – Over 25 Kg / Cm^2 kg/cm^2 is not the standard unit in the International System of Units (SI). In the SI system, the unit of pressure is the pascal (Pa), which is defined as one newton per square meter (N/m2). 1 kg/cm² = 98066.5 pascals 1 Pa = 1 N / m2

Answer 42

It is the impact of moving water in steam line when the steam is allowed to enter a line with condensed water. The steam condenses and partial vacuum occurs and moves back the water along the pipe with very high velocity, and the water will strike at the vent or valves. To prevent the water hammering; 1.Install steam trap in the line 2.Open the drain first before allowing the steam into the line. 3.Crack open steam valve at first and open slowly in increments as oppose to fully opening it quickly

Answer 43

Slowly open the salinometer cock until clean hot water is coming out. Then collect the boiler sample with copper jar if available or tin. Copper is used if possible, rather than steel (an alloy of iron) because copper does not react with either water or steam, whereas iron reacts with steam

Answer 44

It can be check by opening of ship side blow down valve. Banging noise will appear, scum blow down valve is in open position. It can also detect, over heating of scum blow down pipe.

Answer 45

one is fitted at steam side and other one at water side.

Answer 46

1.Can cause water hummer 2.Can cause contamination and scaling. 3.Can cause fluctuation of working water level.

Answer 47

It occurs when lighting up with explosive gas and oil droplet (Oil residue) inside the furnace without pre-purging sufficiently. The ignition results in a large flue gas inside the furnace and these gases blow out with increase high pressure through the furnace opening.

Answer 48

Hydraulic pressure test is 1.25 times working pressure, ( 10 minutes maintain) 1.Close all openings. 2.Open air vent cock. 3.Fill up boiler water fully 4.Close the air vent cock 5.Place hydraulic jack to feed water line. 6.Fit standard pressure gauge. 7.Applied hydraulic pressure 1.25 times of working pressure and maintain 10minutes.

Answer 49

It has more molecular weight, colourless solution, containing sodium aluminate, starch, terming, and resin. Once being dose into the boiler water floating solid particles and suspended solid are settled to the bottom of the boiler and easily remove by blowing down.

Answer 50

1.Shut steam and water cocks and open drain cock. 2.Check any leakage from the respective cock. 3.Slacken the gland nuts. 4.Open the cap and remove gauge glass. 5.Fit new gauge glass, correct size and length into its space. 6.Give the expansion allowance 1/8 in vertical clearance. 7.Use new packing. 8.Close the cap. 9.After fitting, warm the glass by steam. 10.Tighten the gland nut. 11.Then steam and water pressure should be tested whether they clear. 12.Close the drain and open the water and steam side cocks. 13.Check the leakage; all are satisfactory put back into operation.

Answer 51

It is a mechanical attack on the metal surface which may be due to a disturbance in the flow of the fluid over the metal surfaces, resulting in a loss of metal. Suspended abrasive matters in the fluid can increase the rate of metal losses.

Answer 52

It is the deterioration of metals due to oxidation. The presence of water in an acidic condition provides the electrolyte required for corrosion action. there are two forms of corrosion Direct chemical attack It occurs when metal at high temperature comes into contact with air or other gases, resulting in oxidation of the metal. Electro-chemical action Galvanic action, this being set up when two dissimilar metals are placed in an electrolyte. The noble of the two metals form a cathode to the base metal which, forming the anode, is wasted away.

Answer 53

Caustic embattlement is a form of inter crystalline cracking, which results from a solution of sodium hydroxide or caustic soda, becoming more and more concentrated at the bottom of a crack or fissure (narrow opening) which may be the result of fatigue, in the boiler plate or furnace. The plate must be stressed, so that wastage take place at the bottom of crack, the plate weakens, the crack extends to expose new metal to the caustic action and thus it proceed. Caustic embattlement will only occur when there is a high caustic alkalinity that is when the ratio of NaOH to the alkalinity is high. To keep this ratio at the safe level the sodium sulphate to sodium hydroxide should be maintained above 2:5. The sodium sulphate comes out of solution in high sodium hydroxide concentration and by doing forms a protective layer on the surface of the plate. Also keep the lowest T.D.S level. (Total dissolved suspended & solid)

Answer 54

1.Clean the rotating cup. 2.Check and adjust the belt tension between motor and rotating shaft. 3.Clear the pilot burner nozzle and fitter. 4.Clean carbon deposit on electrodes ( igniter ) and adjust the gap. 5.Check the fuel valve and air register linkages and joints. 6.Check and clean the flame eye cover glass. 7.Check and clean inspection peep hole glass cover. 8.Clean fuel oil fitter. 9.Check the fuel oil pressure.

Answer 55

Every boiler survey and After safety valve overhaul.

Answer 56

To prevent pressure build up in blow down line and a pipe burst, which can cause injury to watch keeper.

Answer 57

Reduction in boiler efficiency due to poor heat transfer Reduction in tensile strength Reduction in factor of safety Overheating of metal resulting distortion and eventual failure Increase in fuel consumption Excess concentration of NaOH (caustic soda) may cause caustic embrittlement on boiler metal and tend to failure of boiler metal Corrosion Scale formation Foaming, Priming, carry over (due to increase T.D.S level, foaming is present and tend to carried over and priming)

Answer 58

It’s the process of removing soot, to prevent EGE fire, soot fire and or boiler fire. It maintains steam generating efficiency and increases heat transfer efficiency. Its done on daily basis at sea and or in accordance with manufacturer and C/E guidance.

Answer 59

1.Inform to bridge 2.Check wind direction, good if transverse direction 3.Raise boiler pressure 4.Open drain cock until soot blow pipe drain clear 5.Open soot blower valve 6.Increase the air to boiler to more than 50 % 7.Open steam valve & carried out by turning wheel 20 -30 sec 8.Then close steam valve, soot blower valve 9.Open drain valve

Answer 60

1.Insufficient purging time 2.Accumulation of oil in furnace from the leaky burners 3.Boiler tubes and uptake have full of soot deposits. 4.Air registers control not operating for the high flame mode. 5.Too little air 6.Insufficient oil temperature

Answer 61

To prevent scale formation ( Trisodium phosphates is used) To remove trace of oxygen (sodium sulphide or hydrazine is used ) To give alkalinity and minimize corrosion.(sodium hydroxide is used ) To reduce risk of caustic cracking ( sodium sulphate or sodium nitrate is used)

Answer 62

This test is carried out a new boiler or new safety valve. 1.Shut off feed water 2.Closed main steam stop valve. 3.Increase cut off pressure of boiler. 4.Bypass high pressure cut off of the boiler. 5.Arrange the boiler fire rate to a maximum. 6.Safety valve will be lift during the test. 7.The test is carried out as long as the water permits in the boiler. 8.Accumulation pressure should not exceed 10% of working pressure in the specified time. 9.Specified time is 15 mins for a smoke tube boiler and 7 mins for water tube boiler.

Answer 63

To know if the safety valve is suitable for this boiler or not. To limit the rise in boiler pressure under full fire condition.

Answer 64

It is condition in which large amount of water are carried along with the steam into the steam line. It is caused by: 1.Excessive foaming 2.Improper amount of steam space 3.By a sudden rush of steam such as is produced when steam stop valve is suddenly opened. To prevent priming, never keep the water level too high. Open steam stop valve slowly.

Answer 65

1.Scum blow down 2.Reduce boiler fire rate 3.Check whether boiler chemical added are in excess 4.Detect the source of contamination 5.In case of bad priming the boiler may have to be taken out of service, shut down. Note: For contamination due to oil, auxiliary boiler have to be chemically cleaned.

Answer 66

The boiler is carried out firing from cold condition to normal working pressure condition very slowly to avoid thermal stress. 1.Check the boiler blow down valve in close position. 2.Shut the main steam stop v/v. 3.Open the air vent cocks. 4.Open the feed check valve and pumping up to ¼ of gauge glass level. 5.Start the force draft fan with dampers, open correctly to purge the furnace and combustion space of any foul gas. 6.Light the burner after closing the recirculating valve. 7.Normally allow the fire to burn for 5-minutes and stop for 15 mins. This step continues until steam come out. 8.When steam coming out from the air vent close the air vent (at 1 to 2 bar pressure). 9.Rise up the working pressure step by step slowly. (Allow the fire to burn for 30 mins and stop the fire for 10 min.) 10.When the steam pressure reached is working pressure drain the steam line. (to avoid water hammer) 11.Main steam stop valve open slowly (crack opening)

Answer 67

1.To remove top man hole door, slacken the dog holding nuts but do not remove them until first broken the joint 2.Remove nuts and dogs and take out the door. 3.The bottom door can be removed after warning personnel to keep clear of the top door. 4.Make ventilation before entering. Do not allow naked light near the boiler. 5.Preliminary internal inspection carried out before cleaning, to check scale deposits and any special points. 6.Plug orifice to blow down valve to prevent choke, place guards over the manholes landings to prevent damage. 7.Carried out cleaning and internal works. 8.When all works completed, a full internal examination must be carried out 9.Cleanliness, all openings are clear, water level gauge connection clears from deposits 10.All internal pipes and fittings have been replaced correctly and securely attached, 11.Remove plug from the blow down valve orifice 12.The face of manhole doors and landings inspect to clean and undamaged. 13.Replace manhole doors by using new joints. 14.Operate all boiler mountings. Open air vent cock and fill the boiler with water to sufficient level.

Answer 68

It is the formation of thick layer of steam bubbles on the top of the water surface inside the boiler due to: High concentration of impurities. By the animal or vegetable fats in feed water carry over from the oil heaters. Increase in level of dissolved & suspended solid TDS level. Increase in water level. To prevent foaming, surface or scum blow down should be done frequently to expel any floating impurities for the boiler and no lube oil should be allowed to enter the boiler.

Answer 69

For fire tube boiler out of service for short period The boiler must be completely filled with alkaline water. The boiler must be topped up periodically and any air in the system must be got rid off. Regularly test the boiler water and keep the alkalinity in the range of recommended value. If the boiler is to be taken out of service for long period It should be drained completely and open up. Dried out by means of heater units. Then the trays of quick lime (moisture absorbent material) should be place internally in suitable positions. Blanks should be fitted to the pipe connections in the event of steam being maintained in other boiler and blow down line. The quick lime should be renewed at least once every two months.

Answer 70

1.Change the fuel oil burning system from HO to DO and then shut down the burning system. 2.Stop feed p/p close feed check valve. 3.Drain down the boiler after allowing it to cool down. 4.If no sufficient time to do this, lower the boiler pressure to 3 to 4 bar. 5.Shut the main steam stop valve. 6.Open the ship side valve then open the blow down valve. 7.Banging noise will appear when boiler is empty. 8.Close the boiler blow down valve and ship side valve. 9.Then release the steam pressure through safety v/v by means of easing gear. 10.When pressure is off, open the air vent and allow boiler to cool down.

Answer 71

Steam and water cocks and passages in the gauge can be cleared while the boiler is still steaming. To do this, shut the steam and water cocks and open the drain cock. Remove the check plug opposite the obstruction. Insert the cleaning plug. Screw in the plug with small hole about 5 mm diameter, drilled through it in place of the cleaning plug. Insert into this hole a rod of such a size that held by a gloved hand, it can be easily moved without being stuck. Then open the chocked and push the rod through to clear the blockage. When clear, open drain to prevent a build up of pressure and only a small amount of steam will blow past the rod, the glove protecting the operator from injury. Then close the cock and replace the normal cleaning plug. The gauge glass can be tested now and if satisfactory return to service. Do not carry out this operation on a plate type glass on a high pressure boiler.

Answer 72

Accumulation pressure is the rise in boiler pressure which take place when the spring loaded safety valve lift due to the increase loading caused by further compression of the spring.

Answer 73

If exhaust gas and oil fired can be used the same time, it is termed the composite boiler. In this type a separate tube nest of tube for exhaust gas is provided, situated above the return tubes from the fire furnace. Uptake from the tube nest are separated.

Answer 74

It is the logarithm of reciprocal of hydrogen iron concentration.

Answer 75

Blow down frequently Reduce the boiler load to minimum. If highly contaminant shut down completely and wash out. Trace and find out the fault and remedies.

Answer 76

1.Take standard pressure gauge (approved by surveyor) for accuracy. 2.Fill up water up to ¼ of gauge glass level, and shut main steam stop valve, feed check valve. 3.Without compression rings, hoods and easing gears, reassembled the safety valves with spring compression less than previous setting. 4.Raise the boiler pressure to desired blow off pressure. 5.Screw-down spring compression nuts of any lifting valves, until all are quite. 6.Arrange to have the desired steam pressure 7.Adjust each valve in turn: Slacken compression nut until the valve lifts. Screw-down compression nut sufficiently enough, so that when the valve spindle is lightly tapped, valve return to its seat and remain seated. Measure gap between compression nut and spring casing. Make a compression ring equal to this gap, and insert under compression nut. Gag the spindle of this safety valve, to prevent opening, while remaining valve is being set. 8.Remaining valve is again set and insert compression ring. 9.Remove gag and retest both valve to lift and close together. 10.Cap, cotter and easing gear to be refitted 11.Caps and cotter pins padlocked to prevent accidentally altering the setting. 12.When the surveyor satisfied the setting pressure, easing gear should be tested. 13.All safety valves set to lift at not greater than 3% above approved working pressure (design pressure).

Answer 77

Check valve and valve seat for wear, cavity corrosion and any fault. They should be grounded properly but maker’s limit must be maintained such as width of seating, clearance between valve lip and seat. Valve chest must be cleaned condition and drain line clear. The spring should be hammer tested for any fracture and check for corrosion. Free length is limited to 0.5 % of original free length. The spindle should be hammer tested for any crack and its straightness. The guide plates and bushes are checked for uneven wear and have sufficient clearance to allow free movement of spindle. The compression nut and cover bush’s threads are carefully checked it for any sign of wear and tear. The easing gear should be checked in good order including bearings, cable pulley, and connecting links. Connecting pin should be a free fit in the lid and pin should not bent or pinhole gone out of shape. All safety valves are to be set to operate under steam a little above working pressure not greater than 3% above the approve working pressure of the boiler.

Answer 78

Steam traps are special types of valves which prevent the passage of steam but allow condensate through. Work automatically and are used in steam heating lines to drain condensate without passing any steam. The benefit gained with a steam trap, is that steam is contained in the heating line until it condenses, thus giving up all of its latent heat. Thermodynamic steam trap use pressure energy of the steam to close the valve which consists of a simple metal disc. The rate of operation of a steam trap depends upon he steam pressure and ambient air temperature. In practice, the trap will usually open after 15 – 25 seconds; the length of time open depends on the amount of condensate to be discharged. If no condensate have been formed, then the trap snaps shut immediately. From the foregoing it will be seen that the trap is never closed for more than 15 – 25 seconds, so condensate is removed virtually as soon as it is formed.

Answer 79

In (i), disc A is raised from seat rings C by incoming pressure allowing discharge of air and condensate through outlet B. As the condensate approaches steam temperature it flashes to steam at the trap orifice. This means that the rate of fluid flow radially outwards under the disc is greatly increased. There is thus an increase in velocity and a reduction in static pressure. The disc is therefore drawn towards the seat. Due to this alone the disc will never seat. However, steam can flow round the edge of the disc resulting in a pressure build up in the control chamber D as shown in (ii). When the steam pressure in chamber D acting over the full area of disc (iii) exceeds the incoming condensate / steam pressure acting on the much smaller inlet area, the disc snaps shut over the orifice. This snap action is important. It eliminates any possibility of wire-drawing the seat, while the seating itself is tight, ensuring no leakage. As shown in (iv) the incoming pressure will eventually exceed the control chamber pressure and the disc will be raised, starting the cycle all over again.

Answer 80

Can be used to reduce steam or air pressure. When steam passes through the valve no work in required since this process involves throttling so total heat before and after pressure reduction is almost the same. When air passes through valve pressure is reduced but as no work is done only throttling temperature remains constant. Reducing valve body is made up of a body of cast steel or iron, a valve, valve seat and spindle of steel or bronze, dependant upon operation conditions. Fitted on the discharge side of valve is pressure gauge to measure reduced pressure and relief valve to prevent damage to low pressure side of system in event of reducing valve failing. Valve has to be in a state of equilibrium due to action of forces. If supply pressure is kept constant discharge pressure can be reduced or increased by rotating adjustment screw.

Answer 81

Most common system for supplying heat is steam system, which uses a boiler to increase temperature. When generating steam to achieve heating temperatures required its necessary to have a pressurised system due to potential problems that can occur when dealing with high pressure. Water increases the potential for corrosion and if steam condenses in pipeline that is then exposed it possible for freezing to occur at low temperatures. In this thermal fluid system an oil fired heater is used a well as pipes to circulate heated thermal fluid to various systems on ship. Advantage of this thermal fluid system is it does not need to condense fluid for it to be pumped which must occur for boiler steam system.

Answer 82

> Boiler gauge glass > boiler feed check valve > boiler water sampling valve > boiler Pressure gauge connection > boiler blow down valve > boiler drum vent valve > main steam stop valve > safety relief valve

Answer 83

a.) > Boiler cascade tank is empty so feed pump cannot produce suction > boiler tube leakage or damaged so boiler water is being evaporated and water level is being lost > problem with boiler feed pump e.g. pump coupling is damaged so motor running but pump cannot deliver any water to boiler > boiler water controller has failed so feed pump cannot supply water b.) Upon rectification check boiler water level. this can be done by checking the water level indicators (boiler gauge glass) Then check boiler water temperature if its too hot it means cold feed water cannot be supplied to boiler to avoid thermal stress. instead hot feed water must be used Check the fuel oil temperature and pressure to ensure correct atomization of fuel can be achieved for combustion and refer to manual for guidance on the optimum parameters and or consult with C/E Shut main steam stop valve to ensure steam is not being fed to system as this could again cause thermal stress start the boiler on low auto fire mode to slowly increase steam pressure ensure force draught fan runs 5-10mins to complete purge cycle and that control damper is correctly positioned. then electrode switch should turn on to create spark for ignition this should detected by PEC. ensure pilot burner and main burner are working and that flame is detected and fuel solenoid valves open in the correct order. ensure to monitor the flame by using the flame eye and then continue to monitor boiler parameters until normal steam pressure is reached.

Answer 84

a.) acid corrosion - Failure to treat boiler water can lead to increased chance of boiler water becoming acidic or alkaline which leads to corrosion. if PH level isn't maintained at recommended level 9.5 boiler water can become acidic, causing boiler water chamber failure and tube corrosion. scale formation - boiler water contains chloride and sulphate and if level isn't maintained correctly this will lead to solid deposition in water. solid deposition meaning chloride and sulphate are changing states to hardening salts when the boiler temperature is increased. this accumulation in boiler tubes will cause blockage and reduce the amount of water being circulated. additionally phosphate and calcium enter the boiler through the feedwater and again if no treatment is carried out salt deposition level will increase and cause thermal stressing and corrosion and reduce boiler performance. b.) Total Alkalinity test: This test is preformed to measure alkalinity in order to determine precipitation (hardening salts settling to the bottom of solution) level of hardness salts test result will indicates if boiler water requires chemical dosing or boiler needs blow down. to preform test - fill a vial with boiler water sample - add 5 drops of total alkalinity indicator so solution goes clear. - Add sulphuric acid (N/10) until sample goes pink. check level in burette and convert ml in burette to ppm using the manufacturer chart e.g. (drew chemicals) and chemically treat using manufactures guidance. Chloride test: This test measures the amount of chloride in the water. it reveals the amount of dissolved salts in boiler water. an increase in chloride level could be an indication of contamination by seawater. if chloride level is too high this causes foaming or scale formation. to preform the test fill the plastic vial with boiler water sample to 10ml and add 3 drops of phenolphthalein indicator to neutralise the sample. Add N/10 until sample turns clear and add one more drop, add 6 drops of potassium chromate to turn yellow. Add silver nitrate until orange. Multiply drops of silver nitrate by 10 to convert to ppm. Chloride level shouldn't exceed 200ppm, if it does preform a boiler blow down procedure.

Answer 85

a.) at first the force draught fan will start and will run to purge the combustion space (furnace) of any unburnt gases. Force draught fan will run for 5-10mins to remove the explosive gases. This is know as the purging cycle, when purging is complete and correct pressurised air obtained by having control dampers in correct position, electrode switch is activated to create a spark for ignition, the spark will then be detected by the photo electric cell (PEC). Diesel oil pilot flame solenoid valve will then open and pilot burner switches on, then electrode switch goes off. At a predetermined pressure value the main burner is started as long as water level in boiler is correct and pilot flame is stable the main burner fuel solenoid valve will open and main burner switches on. once main burner ignition commences, pilot flame will extinguish after a time delay and pilot burner switches off. Control of burners is carried out by a series of sensors, electronics and mechanical equipment. main burner is started to to produce more steam. once steam pressure reaches a working pressure set point power to main burner cuts off at the same time the PEC continues to monitor the flame. if there is no flame after 5secs fuel solenoid valves close to cut off fuel and then boiler stops. if boiler doesn’t light on first try , go to the boiler and try to remedy the situation ensure everything is working before starting boiler again because If the sequence is started again once more and for whatever reason boiler doesn’t start sequence will be aborted and boiler goes into lockout mode. Lockout mechanism initiates an alarm and engineer has to attend to boiler and rectify fault before boiler can be restarted. b.) if there is an extra low water level in boiler then it would automatically stop to protect he tubes from overheating. if flame isn't detected by PEC (photo electric cell) then boiler would automatically stop to prevent oil accumulation in furnace if FO pressure is low then boiler burner atomizer will not create proper atomization which is essential for firing and therefore boiler would stop.

Answer 86

see EOOW ORAL Sketch Pack for drawing Feedwater Header tank allows for expansion Auxiliary feed pump allows for leakage This system feeds Water to boiler. This system contains feed pumps, feed check valves, feed water pipes, valves and tanks. The modern version of this system is classed as a closed system, where the water is kept away from the air as much as possible because without oxygen system corrosion is kept to a minimum. Feed system for boiler must have a means of checking the water level, this can be done manually by visual water level indicator or a sight glass but because boiler has to be automatic there must also be another means of control. This can be done by float valves, capacitance probes or conductivity probes. Its important for boiler efficiency that feed water is as warm as possible. Feed water is only cooled enough so that it can be pumped into the boiler again. Feed water pump is located as low as possible in system so that pressure of water is kept at a point where there is no gassing up. By this I mean steam bubbles will start to appear in water and in extreme conditions this will cause pump to stop working. At this point water isn't entering boiler and water level is dropping. This will lead to a low water cut out and alarm being activated causing boiler to stop working.

Answer 87

A flame failure alarm sounds if boiler has tried but failed to light. This can be for any one of following various reasons or a combination of them: No fuel due to * Blocked filter or burner nozzle * Poor quality fuel * Faulty fuel supply pump * Fuel tank empty No spark due to * High voltage leads becoming disconnected * Dirty electrical leads or connections * Failed transformer * Burnt electrodes * Poorly adjusted electrodes Not enough air due to * Fan stopped and not running any more * Air register (pipeline) blocked * Drive between motor and fan is broken * Fuses have blown * Electric Motor has overheated Another common fault is the ignition system, especially in older boilers. This system includes electrodes that are set at a specific position in reference to burner tip so that fuel can be atomised just as it comes into contact with igniter (heat source). Due to to force from air blowing past and combustion taking place electrodes can become misaligned and so fuel fails to ignite. To rectify fault all you have to do is realign the electrodes but as well as this you have to check all other parts of boiler system are working like for instance purge cycle can be done and fan is working. Restarting a boiler after a lock out needs extra attention and the engineer should be very clear about why the boiler has stopped working and the faulty condition rectified. When ready to restart the boiler you must ensure there are no unburnt gases in the combustion space as if ignited could lead to more fuel igniting than boiler can cope with. if flame inspection holes are provided don't use them during ignition sequence as there could be blow back through this hole leading to injury. Just as important is boiler water level. This is because metal parts close to very hot products of combustion rely on water level being correct to stay cool and not fail. Loss of water also leads to overheating of furnace which will then rupture. This situation can quickly escalate because water is under pressure while its in boiler and as its released to atmospheric pressure it turns into steam. Boilers that run at 7 bar are classed as auxiliary boilers and will cause a lot of damage if they are ruptured due to overheating of furnace.

Answer 88

PH test - PH of boiler water needs to be maintained between 9.5 to 11.0ppm to prevent acidic corrosion attack on boiler metal. below 9.5 causes acidic corrosion to boiler metal surface and tubes and therefore chemical dosage needs to be adjusted using manufacturer guidance and chemicals then added to boiler water or preform a boiler blow down procedure. P-alkalinity test - phenolphthalein alkalinity test indicates alkaline level in boiler water. alkaline level must be measured to determine precipitation level in hardness salts. test will indicate if boiler requires chemical dosage or a blow down. recommended range for P-alkalinity test is 100-150ppm phosphates test - this test indicates hardness salts in boiler water. if phosphate residue goes above recommended conditions this shows dirty boiler water and can cause caustic corrosion. bottom boiler blow down is done to remove this. if low then recommended range (20-50ppm) feed water needs to be dosed with chemical to increase phosphate level and avoid acidic corrosion. chloride test - this test will reveal any dissolved salt in the boiler water. any increase in chlorides is an indication of contamination by seawater. if chloride level exceeds 200ppm this will cause foaming or scale formation. preform blow down to reduce chloride level.

Answer 89

There are two types of component used for a boiler, boiler mountings and boiler accessories. mountings are attached with boiler shell at pressure side to ensure safe operation of boiler. boiler accessories aren't attached with boiler shell and are used to improve boiler efficiency. Accessories - super HTR, feed p/p, injector, steam separator and air pre heater Mountings - This is equipment that is attached to pressure parts of boiler. Below is a description of the main boiler mountings

Answer 90

Safety valve Main steam stop valve Air vent cock Two gauge glass level indicators Feed check valve Scum blow down valve Bottom blow down valve Salinity cock or test cock Man hole doors.

Answer 91

Safety valves Low / high water level alarm Too low water level alarm Water level indicators Pressure gauge Low fuel oil pressure alarm Low / high fuel oil temperature alarm Flame failure alarm Smoke density alarm Easy gear arrangement Air vent Force draught fan stop alarm Low / high steam pressure alarm

Answer 92

access doors Access to the furnace and smoke outlet box are possible through the access doors placed at the bottom of the furnace and at the smoke outlet box respectively. Both access doors enable inspection of the generating tube bank. Sampling valve A sample valve is installed enabling connection to a sample cooler for taking test samples to perform boiler water analyses. Scum valve The scum valve is mounted at the top of the boiler body. In the event of scum in the boiler, this scum can be blown off from the water surface by opening this valve. If connected to a separate drainage system the valve is of the ball type. When connected to a common drainage system two valves are provided, one shut-off valve and one shut-off/non-return valve. Inspection hole Two small inspection holes are provided in the furnace wall to enable inspection of the burner flame. A proportion of the air supply is bled off from the burner fan to cool the window of the inspection hole, and prevent soot deposits. Manhole Two manholes placed at the boiler top and boiler bottom allow inside inspection of the steam/water drum. Feed check valves Stop feed water returning back down feed water pipe while feed water pump is not running. Boiler feed check valves are normally the double shut off arrangement. This allows the non return feed check valve to be overhauled while boiler is running. As if screw don valve is closed non return is isolated from boiler. However best time do these valve overhauls is when boiler is being fully examined. Non return valve is needed because if a feed line fractures or a joint in line blows, then boiler contents cant be discharged out of the feed line. This double shut off arrangement also reduces risk of any leakage into the feed line whilst its under repair and boiler is running. Main steam stop valve This valve allows steam passage from the boiler to the steam services. These type of valves not designed as control valves and are either kept fully open or fully closed. They need to be strong to withstand steam pressure and so their made of cast steel or bronze. Open these valve slowly as sudden rise in pressure and steam flow can cause water hammer in steam service pipes. Air vents and vacuum breakers When filling the boiler after maintenance the air is released through air vent. As boiler is allowed to cool down and steam condenses back into water, a vacuum is created inside boiler. Vacuum on inside means pressure is applied on outside on boiler from outside. To ensure no damage occurs vacuum breakers are fitted to boiler shell to allow air into boiler and remove the vacuum. By-pass valve The by-pass valve is a shut off valve. The purpose of the valve is to equalise the pressure between the boiler and the steam system when the main steam valve is closed. The by-pass valve is only supplied for larger boilers. Water level gauges Two local water level gauges are connected to the front of the boiler, each gauge being provided with two shut-off valves and a drain valve. The shut-off valves, fitted at the top and bottom of the sight glass, have a quick-closing mechanism to be used in case of broken glass. The pipes from the drain cocks on the water level gauge must lead to an open drain, visible for inspection Blow-down valve Two blow-down valves are mounted at the bottom of the boiler body. If connected to a separate drainage system the valves are of the ball type. When connected to a common drainage system two valves are provided in each group, one shut-off valve and one shut-off/non-return valve. The shut-off function is for security and the non-return function prevents steam/water from flowing into an empty boiler by mistake. These valve are used to release some of the water and sediment from the bottom of the boiler so that fresh feed water can be introduced into the system, which increases the quality of the water

Answer 93

see EOOW ORAL Sketch Pack for drawing Soot blower Between periodic boiler cleaning, tube surfaces of boiler must be kept as clean as possible. To enable this soot blowers which are steam or air operated are fitted. These allow tube surfaces to be cleaned of loose sooty deposits rapidly without stopping boiler. With steam supplied to blower and steam supply line drained rotating soot blower hand wheel causes supply tube and nozzle to move towards combustion chamber. Nozzle and tubes are rotated as they move inwards by means of a scroll cut in the nut and a stationery pin in the body assembly that runs in the scroll. Steam comes in through stem inlet pipe and through the steam ports. When you rotate the hand wheel your directing high pressure jets of steam that are discharged to the tube plates steam ports. When not being used the retractable nozzle of blower is at a safe distance within the housing tube and is protected from overheating, which would cause burning and misalignment or damage to the nozzle. Too frequent use of blower should be avoided as this causes wastage of tube plate. Operate blower regularly even if boiler tubes are clean but without the steam supply to blower to make sure soot blower is operating fine. The most modern soot blowers use ultrasonic wave technology which when applied will dislodge soot from tubes without using steam thereby saving energy. Soot blowing is necessary especially if engine has been operated at a lower load than maximum continuous rating (MCR).

Answer 94

After boiler repairs a hydraulic test is done. In preparation of the test all equipment and foreign matter is removed from the water space of the boiler and everything is examined. Any permanent welding should be done by a professional welder and welds are subjected to non destructive testing techniques. Test is preformed to hold the safety valves and boiler mountings closed while equipment is subjected to a hydrostatic test. With the exception of feed check valve and air cock. To avoid damage to the spindle and/or seat, care is required so the gag screw is not tightened. Boiler is then filled with clean fresh water and purged of air. Using a hydraulic pump unit connected by a small bore pipe to boiler directly or to feed line and then pressure is gradually applied. Testing pressure used is 1.5 times the working pressure applied for at least 30mins. When boiler is under pressure its examined for leakages and faults. Weld repairs should be checked repeatedly with blows by a hammer.

Answer 95

If an examination of boiler has to be done boiler has to be emptied of water. Its always better if time is available to allow boiler to cool down for a period of time after shut down and then you pump the water out. Avoid sudden shock of cooling due to complete blow down. Allow as much time for blow down as possible after shut down and before starting if blowing down to the sea. Ship side blow down cock must be opened first and then blow down valve for boiler is gradually opened. This way engineer has better control of operation. For example if external blow down pipe between boiler and ship side is in a corroded condition and engineer opens boiler blow down valve first this leads to rupturing of blow down pipe. As a safety measure when handle is removed valve must be in closed position this way the valve cannot be accidentally left open. When blow down process starts noise level falls and pressure is low. Ensure no cold sea water can enter boiler. When boiler is empty of water it still contains steam, which can condense and cause a vacuum condition. This increases risk of entry of cold sea water but to prevent entry boiler blow down valve is non return type with some water tube boilers having a double shut off. Even with these arrangements its recommended to start closing boiler blow down valve when pressure is low enough and down to the desired value, the blow down valve is then closed tightly as well as ship side cock. After this allow time for boiler to cool down and lose all its pressure. When pressure is atmospheric, open air cock and gauge glass drain to ensure pressure inside boiler is atmospheric. Then boiler door is unbolted and manholes knocked in top or bottom but not both. If top door secure rope to eye bolt then secure other end of rope. Loosen the retaining nuts but don’t take them off yet. Then take a long plank of wood stand back and knock door down. When door open remove retaining nuts. Do not then immediately open bottom manhole because if boiler is hot this will lead cool air passing through as well as thermal shock.

Answer 96

Boiler blow down is made to reduce the density of salt and to remove the dissolved and suspended solids, also the floating solid impurities in the boiler system. If there are not removed from the boiler water system, foaming, priming, corrosion will occur in the boiler steam space and feed water system.

Answer 97

To remove accumulated, suspended and floating solids, impurities and also remove dissolved solid concentration, they hinder the formation of steam.

Answer 98

1.Reduce the density of the boiler water by reducing the water level and refilling it with distilled water. 2.Remove the sediments from boiler water. 3.Remove oil, grease etc. from top surface of boiler water. 4.Empty the boiler prior to entry. 5.Reduce the water level in an emergency.

Answer 99

see EOOW ORAL Sketch Pack for drawing Open valves in order (1), (2), (3), and close in reverse. The blow down valve adjacent to the boiler (2) should be fully open to prevent cutting of the valve seating faces. The rate of blow down is controlled by the second blow down valve (3). Never leave the boiler unattended when blowing down and ensure all valves are fully closed after blowing down the boiler. A hot drain pipe indicates a leaking blow down valve. Never open the blow down valves before ship side valve. It result in unwanted pressure build up in the blow down line. The scum valve is used to remove floating, foam and oil from the surface of the water.

Answer 100

Deformation This caused by scaling, oil, sludge or poor circulation, which results in overheating of furnace. It can be seen visually along the furnace or by furnace gauging. Deformation can be repaired by cutting back through the bulge, heating and pressing back the material into its original shape and then welding. Cutting through bulge before heating and pressing allows flow of metal during pressing. Alternatively the defective portion could be cut out completely and a patch welded in its place. Wastage Causes of wastage are corrosion and erosion. Can be dealt with by cutting out defective portions of furnace and welding in a new piece of material. Cracks Can be identified circumferentially around lower part of connecting necks. Cracks are caused by mechanical straining of furnace and defect is referred to as grooving. If the groove is shallow compared to plate thickness, depth can be found by drilling or ultrasonic detection. Usually you cut groove out and weld. If grooving is deep material has to be cut right through and welded from both sides. Cracks caused by overheating can be seen where deformation has occurred.

Answer 101

Deformation In addition to the causes of deformation provided above for a furnace, combustion chamber top would first place to suffer overheating due to water shortage. Deformations are repaired by cutting out defective portion of plate, through line of stay or tubes and then welding in a new piece of plate. Cutting the plating through line of stay or tubes avoids continuous welding and reduces risk of defects that can happen due to contraction stresses. Slight distortion of combustion chamber and smoke box plating could occur due to boiler being used in a dirty condition. This is common and if no leakage past stays or tubes no repair is necessary but its essential to keep surfaces of plating clean to stop further distortion. If too much distortion combustion chamber plating needs to be renewed. Another cause of combustion chamber plates bulging could be corrosion of stays or tubes, in this case renew stays or tubes. Wastage Leakages past tubes, stays and through riveted seams cause wastage. If there isn't too much defective portion of plating can be built up by welding or the tubes and stays renewed. Cracks Cracks happen due to overheating and mechanical straining. Likely place to find these is on the edges of the combustion chamber seams on the fire side because of doubling of plate thickness (riveted only) and impairing of heat transfer, and around tubes and stays due to straining of boiler and or scale build up around necks of tubes or stays. If there isn't too many cracks and they are dry they can be left, if there's too many cut them out and fill in by welding and renewing rivets. Radial grooving around stays or tubes if not extensive can be repaired by cutting out crack and fill by welding. If grooving extensive defective part of plate to be cut out and a new part welded in its place.

Answer 102

Wastage normally occurs at the parts where leaks can happen like for example riveted seams and boiler mountings. Leakages at seams and between boiler mountings and shell in water area of the boiler lead to salt deposition due to water flashing off to steam, leaving behind any salt it contained. These deposits of salt must be cleaned away and plating can the be inspected for wastage and cracking. Cracking occurs due to caustic embrittlement. If there isn't too much wastage defects then it can be dealt with by welding or renewing defective part of plate if there is a lot of wastage. If there is slight leakage of seam and it its found early after examining material it can be repaired by re-caulking the seam. Ensure that over caulking doesn’t happen as it can lead to lifting of plates and deposits can accumulate between plates. In addition to cracking by caustic embrittlement, grooving of flanged end plating can occur, especially when furnace front plating is flanged inwards. Repairs for grooves can be done the same as the ones for the furnace. Cracks that happen from embrittlement generally means you have to renew the affected part of plate. If there is extensive caustic cracking on main seam of boiler then you would have to have a boiler renewal.

Answer 103

Causes of Marine Boiler Burner Misfiring Insufficient Fuel Supply: Clogged or blocked fuel filters – check and clean if required. Incorrect fuel pressure or flow rate –The correct fuel pressure or flow rate on a boiler burner is essential for ensuring proper combustion and efficiency. If the fuel pressure is too low, the burner will not ignite properly or will produce a weak flame. If the fuel pressure is too high, the burner may ignite with a loud bang or produce a smoky flame. Malfunctioning fuel pumps – A malfunctioning fuel pump can affect the firing process into the boiler burner in a number of ways, including: insufficient fuel delivery, unstable flame, clogged nozzle, damaged pump etc. Ignition System Issues: Faulty ignition electrodes or ignition transformer – Faulty ignition electrodes or ignition transformer can affect boiler burner firing in a number of ways, including: no spark, weak spark, misfire, carbon monoxide poisoning etc. Improper spark plug gap or electrode alignment – The spark plug gap and electrode alignment are essential for the proper firing of a boiler burner. If the gap is too wide, the spark will be weak and will not have enough energy to ignite the fuel. If the gap is too narrow, the spark will be too hot and can damage the spark plug. The electrode alignment is also important, as the spark must jump between the electrodes in order to ignite the fuel. If the spark plug gap or electrode alignment is incorrect, the boiler burner may not ignite at all, or it may ignite with a weak or unstable flame. This can lead to a number of problems, including: low heat output, smoky flame, damage to the burner. Combustion Air Supply: Inadequate combustion air intake due to blockages or restrictions – this can affect boiler’s burner firing in a number of ways, including: incomplete combustion, smoky flame, low heat output, damage to the burner. Malfunctioning air dampers or fans – Air dampers and fans are essential for controlling the flow of air to a boiler burner. If the air dampers or fans malfunction, it can affect the boiler’s burner firing in a number of ways, including: incomplete combustion, smoky flame, low heat output, damage to the burner, fire hazard. Air leaks in the combustion air system Fuel Quality: Contaminated or degraded fuel – Contaminated or degraded fuel can affect boiler’s burner firing in a number of ways, including: incomplete combustion, smoky flame, low heat output, damage to the burner. Incorrect fuel viscosity or flashpoint – Same as above. Inconsistent fuel composition – Same as above.

Answer 104

Line filter at the inlet of the fuel line for burner is choked: If the system runs in heavy oil then there are chances of filters in the line getting choke. To avoid this, boiler system are normally built for changeover from diesel to heavy oil during starting and heavy to diesel during stopping. This keeps the filter and the fuel line clean. Boiler fuel supply pump is not running: There are two main reasons for fuel pump not running. Normally when the pumps are in pairs, the changeover auto system is kept in manual position, and if the operating pump trips, the stand by pump will not start automatically. Another reason is tripping of pump due to short circuit in the system etc. Solenoid valve in the fuel supply line is malfunctioning Nowadays most of the system adopts advance automation, but there can be a possibility wherein the solenoid in the fuel supply line is malfunctioning and not opening. Air or Steam ratio setting is not proper For proper and efficient combustion, air fuel ration is very important, if the supply of air is excess then there will be excess of smoke, and if it exceeds more than normal level the combustion will burn off causing flame failure. Forced draft fan flaps malfunctioning For removing excess gases trapped inside the combustion chamber forced draft fan (FDF) are used for pre purging and post purging operation and are connected with a timer to shut the fan flaps. If the flaps are malfunctioning then continuous forced air will go inside the chamber, preventing the burner to produce flame causing flame failure of the boiler. Any contractor switch inside Control panel is malfunctioning Boiler control panel consist of several contractors and PLC cards. Even one contractor malfunctioning may result in trouble for boiler starting. Trip not reset If any previous trips like low water level, flame failure, emergency stop etc. has not been reset than boiler will not start. Main Burner atomiser is clogged Main burners consist of atomizer for efficient burning of fuel. If the atomizer is clogged by sludge and fuel deposits then burner may not produce flame and trip the boiler.. Electrodes are not generating spark Initial spark for generating a flame is produced by electrode which may be due to carbon deposits on them or fault in the circuit of electrodes etc.

Answer 105

An atom is composed of a nucleus with an electron or electrons in orbit around it. There is a nucleus which consists of protons and neutrons. Protons have a positive electrical charge and neutrons have no charge. Electrons have a negative charge. Number of proton present is equal to number of electrons. Therefore resultant electrical charge will be zero so atom remains electrically balanced. If atom gains or loses an electron there is then either a positive or negative electrical charge and its then refereed to as a positive or negative ion

Answer 106

An atom of hydrogen which has lost its electron, normally written as H+ to indicate positive electric charge or H - ɛ (loss of electron)

Answer 107

Compound of oxygen and hydrogen that’s gained an electron. Normally written as OH- indicating negative electric charge or OH+ ɛ indicating gain of electron.

Answer 108

pH value is an indicator of acidity, neutrality, or alkalinity. Water contains hydrogen and hydroxyl ions. The concentration of these ions is important because it indicates whether its acidic, alkaline or neutral. Hydrogen and hydroxyl ion concentration in water must always equal 10-14 gm ion/l (gram ion expressing weight of one mole of ions in grams) of solution. When hydrogen ion concentration exceeds hydroxyl concentration water becomes acidic, if ion concentrations are equal water is neutral and if hydroxyl ion concentration is greater than hydrogen ion concentration that water is alkaline. Ion concentrations always equal to 10-14. pH values range from 0 to 14. 0 being very acidic and 14 being very alkaline. When water temperature is increased, hydrogen ion concentration increases and there could be increase in acidity or decrease in alkalinity. When chemicals are added to water this changes the hydrogen ion concentration as well as pH value. If chemicals are acidic this lower pH value and if alkaline pH value increases. For electro-chemical corrosion of metals pH value is important because it controls the degree of corrosion to metal.

Answer 109

ANODE - the metal or site on the metal where oxidation occurs (loss of electrons). CATHODE - the metal or site on the metal where reduction occurs (gain of electrons).

Answer 110

When there's an electric field between two electrodes with opposite charges, ions can move because they are attracted to the opposite charge. for example the positive electrode pulls the negatively charged ions towards it, while the negative electrode pulls the positively charged ions towards it. This movement of ions is what we call "ion migration" and it happens because of the attraction between opposite charges. So, in the presence of an electric field created between two electrodes with opposite charges, positive ions move towards the negative electrode, and negative ions move towards the positive electrode. the positive electrode is an anode, and the negative electrode is a cathode.

Answer 111

this corrosion occurs when iron is in contact with water which contains hydrogen ion. The hydrogen ions in contact with the metal surface become hydrogen atoms by taking an electron from the metal. Due to loss of electrons this produces metal ion which combines with hydroxyl ions in contact with the metal surface and then this forms a metallic hydroxide. This is able to be dissolved in water depending on pH value so metal is corroded. This corrosion is similar to batteries where current is cause to flow from anodic to cathodic regions. The migrating ions in the electrolyte (water) and the electrons in the metal form the circuit. Hydrogen which has formed on the surface of metal due to combination of hydrogen ion and metal electron, can form a polarising layer upon metals surface. This prevents further corrosion. If dissolved oxygen is present in water, it will combine with hydrogen to form water and no polarisation will occur and corrosion will continue. If water is acidic enough hydrogen can leave the surface of the metal in the form of hydrogen gas, which again prevents polarisation and corrosion continues. Therefore its vital to keep boiler water alkaline with little or no dissolved oxygen content.

Answer 112

Oils Lubricating oils can contaminate the feed system and find their way into the boiler. This could be caused due to over lubrication of machinery and inefficient filtering of feed water. Oils such as animal and vegetable oils can decompose in boiler liberating their fatty acids and its these acids that cause corrosion. Therefore only use pure mineral oil for lubrication of machinery parts. Oil of any description should never be allowed to enter the boiler as it can attach to heating surfaces and causes overheating. It can also cause priming due to excessive ebullition. Mechanical straining Not a corrosive agent but due to breakdown of the surface of the metal. Pitting type corrosion can occur due to differential aeration (when a portion of metal becomes partially inaccessible to oxygen it becomes anodic and corrosion may occur). Mechanical straining can occur due to not operating boiler in the correct way. For instance raising steam too rapidly from cold condition, missing or poorly connected internal feed pipes, high variations in feed temperature and steam condition. Grooving is caused by mechanical straining of boiler plates and where a groove is present there is always danger of corrosion. Galvanic action When two different metals are present in a saline solution galvanic action can occur, which results in corrosion of the more base metal. Zinc for example would serve as an anode to iron and iron would serve as an anode to copper. Sacrificial anodes are often used to provide cathodic protection. In scotch boilers, zinc plates are sometimes fixed to furnaces and suspended between tube nests. These act as sacrificial anodes giving cathodic protection to the steel plating for a boiler. Corrosion of non ferrous metals in steam and condensate system may result in deposits of copper on boiler tube surfaces (known as copper pick-up) which because of galvanic action can cause boiler corrosion.

Answer 113

This is caused by high concentrations of sodium carbonate, which undergoes hydrolysis which then forms sodium hydroxide (NaOH), which dissolves the iron in boiler components. Stress corrosion cracks will follow the the grain or crystal boundaries of the material. Concentrations of sodium hydroxide required for embrittlement to occur are dependant on operating conditions but a rough guide is 1300 grains/litre at 300degrees Celsius. Normally this doesn’t occur in a boiler but any slight leakage can cause the water to flash off into steam which can then leave behind enough solids that can cause an increase in concentration. Sodium hydroxide depresses solubility of sodium sulphate, although sodium sulphate can be made to precipitate and provide protection against caustic embrittlement. Ratios of sodium sulphate to caustic soda ,dependant on pressure will provide protection for boiler parts. Therefore its recommended that ratio doesn’t fall below 2:5. other substances that are used as inhibitors against caustic embrittlement are quebracho tannin and sodium nitrate. Caustic corrosion can be in high pressure boiler can be indicated by gouging of the tubes and is caused by excessive sodium hydroxide. This corrosion causes destruction of the protective magnetic oxide of iron film and base metal is attacked by concentrated sodium hydroxide.

Answer 114

As well as internal corrosion, external corrosion can occur in a boiler. The cause of this is sooty deposits in the uptakes in the presence of moisture that then form sulphuric acid, which can corrode a boiler which is not producing steam with damp lagging and acidulated bilge vapours.

Answer 115

1. force fan isn't working so pre-purging of boiler combustion space hasn't been carried out so explosive gases haven't been removed so fuel supply is automatically cut off. 2. electrode fails to create a spark due to carbon deposits or Pilot Burner nozzle is choked. A Pilot burner nozzle is very small and can be blocked by carbon deposits and sludge resulting in flame failure. Some pilot burners consist of small filter which can be clogged after continuous operation resulting in flame failure because of carbon accumulation. 3. photo electric cell is dirty so it cant detect a spark so pilot burner cant ignite a pilot flame and so fuel supply is cut off. Flame eye is malfunctioning: A Flame eye is a photocell operated flame sensor fitted to detect weather the burner is firing or not. If the flame eye unit is malfunctioning, then it will give a trip signal even before the burner starts firing. 4. fuel pressure has dropped due to a blocked filter or fuel inlet valve to the burner is in close position. The fuel line for boiler’s burner consists of several valves located at fuel tank, pumps suction, discharge valve, or valve before the boiler burner. Any of these can be in closed position resulting in starvation of fuel.

Answer 116

High Water Level alarm, using the manual feed valve slowly over fill the boiler until the alarm sounds and the feed pumps cut-off. Low fuel oil temperature - turn off the fuel oil heaters and allow the fuel to cool tripping the burners. Low-low water level - blow the boiler down until the alarm sounds followed by the tripping of the burners. Ignition failure alarm - isolate the ignition transformer and begin start sequence, the burner will lock out after purge. Safety Valve: They are set to lift at the blow-off pressure and shut when the pressure reduces to the safe limit. to test use easing gear. The easing gear is attached to the boiler safety valve. Every individual safety valve is provided with its own easing gear, which is a pulley and wire arrangement (connected to the lever of the safety valve) with an accessible handle at the lower operating boiler platform. It is used to lift the boiler safety valve in case of an emergency (without getting near to the safety valve) and to regularly test the operation of the safety valves.

Answer 117

a) this could be because there is low air pressure. when force draught fan is running the correct pressurised air has to be obtained for boiler operation. check that the fan is running and no obstruction to the air flow check the pressure sensor is operational b) electrode igniter tips could be dirty from carbon soot accumulation or sludge build up so you need to check they're clean. if not then stop boiler and clean with perhaps diesel oil but consult with chief engineer ad manual for guidance. alternatively ignitors might not be getting correct supply voltage so you need to also check this. c) Check that the burner atomizer tips are clean from carbon soot or and sludge so that they're able to spray a fine mist of fuel and achieve the correct combustion for fuel to air ratio. check fuel is being supplied fine and that they're are no fuel leaks in fuel line and fuel solenoid valves are working. also check fuel temperature and pressure this might give an indication as to why they're is poor combustion d) Check flame condition and fuel and air ratio is correct. This can be done by checking parameters but also when boiler is firing you should monitor the photo electric as this monitor the flame and has a flame sensor fitted to it. if this isn't working then boiler will lock out. the five second time delay is also when pilot burner switches off so check that main burner is working and atomizer tips are clean.

Answer 118

a) scaling forms due to High concentrations of calcium and magnesium salts within water. The bicarbonates of calcium and magnesium are decomposed by heat and come out of the solution as scale forming carbonates. it also caused impurities being precipitated out of the water directly on heat transfer surfaces and water settling out on the metal and becoming hard b) less heat will pass through the boiler tubes. as well as cause overheating of boiler tubes. Even light accumulation of scale in a boiler causes boiler efficiency to drop. to maintain steam production and satisfy steam demand boiler will use more fuel which results in higher fuel costs due to increased fuel consumption in order to satisfy steam demand c) Proper treatment of make-up waste and boiler water to prevent scale, or other deposits, and corrosion within the boiler. By using the correct amount of chemical treatment and preforming boiler blow down your keeping the hardness salts in suspension in the boiler water. this stops any suspended salts and impurities from sticking to the heat transfer surfaces. Also installing water softeners can help in preventing limescale (CaCO3) in the steam boiler. Blow down will reduce amount of salts by discharging the water overboard.

Answer 119

a.) boiler water level gauge glass b.) to enable boiler water level to still be monitored in the event that one gauge glass should fail (redundancy). also to ensure boiler water level can still be monitored in the event of severe rolling required by SOLAS. if one reading is wrong can use other gauge glass to compare and get a reading. c.) boiler safety valve boiler main steam stop valve blow down valve drum vent valve feed check valve pressure gauge connections water sampling valve

Answer 120

a.) i.) if there is a high water level boiler water can carry over and cause damage to heating coil for FO tank ii.) if there is a low water level then boiler tubes are damaged which will lead to thermal stress and cracking iii.) if there is an extra low water level there will be a very high temperature and cracking. you must stop the boiler burner and identify the problem.

Answer 121

a.) boiler load must be reduced and stopped to prevent water in tubes from heating up and causing tubes to overheat or burst. when filling up boiler don't fill with cold feedwater or there will be thermal stress. when rectifying problem check other gauge glass to ensure its not a faulty reading and that the gauge glass isn't broken. if there is no water level in gauge glass shut down boiler immediately, but first inform C/E and bridge in order to change the main engine from HFO to DO. Done because main engine cannot run on HFO without boiler being operational the boiler must be shut down and feed pump stopped when there is low water level to avoid thermal stress and cracking. b.) the first action would be to stop the boiler firing as there is excessive pressure in boiler steam drum, indicated by the safety valve lifting. check the steam outlet valve to ensure its open. if closed open to reduce steam pressure. test the safety valve as well by operating the easing gear to ensure its not faulty or stuck in closed position. c.) stop the boiler to avoid air pollution into the atmosphere. to rectify the situation adjust the air to fuel ratio. if air is high and fuel is low than this produces white smoke. if fuel is high and air is low than this produces black smoke. the fuel and air ratio should be adjusted to avoid boiler melt down or a boiler fire. d.) stop the boiler firing due to high corrosion indicated by the high chloride content. high chloride content will cause acidic corrosion and scale formation on the tubes. to rectify the problem find the source of sea water leakage and preform maintenance to stop leakage. preform a boiler blow down to reduce chloride content in the boiler feedwater, and minimise scale formation. then rectify condenser.

Answer 122

Good combustion is essential for the efficient running of the boiler because it gives the best possible heat release and there is a minimum amount of deposits upon the heating surfaces. To find out if the combustion is good measure the % of CO2 content and for some boilers the % of O2 content and then observe the appearance of the gases. If the % CO2 content is correct (or the % O2 content low and the gases are in a non smoky condition then the combustion of fuel is correct. With the correct % CO2 content the % of excess air required for combustion will be low and this means better boiler efficiency since less heat is needed for burning fuel due to small amount of excess air. If the excess air supply is increased then the % CO2 content of the gases will fall.

Answer 123

Burners - if these are dirty or the sprayer plates are damaged then atomisation of fuel will be effected. Types include pressure jet, rotary cup, steam jet and ultrasonic. Oil - if this dirty it can foul up the burners. Filters will be provided in oil supply lines to remove most of the dirt particles but theses can get damaged and blocked. Ideally the mesh in the last filter should be smaller than the holes in the burner sprayer plate. Water - if there is water in oil it will affect combustion an possibly lead to burners becoming extinguished and a dangerous situation arising. It could also produce panting, which can cause structural defects. Oil temperature - if this is too low oil doesn’t properly atomise due to high viscosity. This can cause flame impingement, overheating, tube and refractory failure. If oil temperature is too high the burner tips become too hot and there will be excessive carbon deposits, which will form on the tips causing spray defects. This could again lead to flame impingement on adjacent refractory and damage could also occur to the air swirlers. Oil pressure - important because it affect atomisation and lengths of spray jets Air registers - good mixing with fuel particles with air is essential for proper atomisation and so the condition of the air swirlers and air registers is important. If these are damaged mechanically or due to corrosion then air flow is affected. Pressure drops over the venturi of 25mm water gauge give air speeds of about 20m/s. Modern swirler type stabiliser designs give more efficient mixing with pressure drops up to 300mm water gauge and air speeds up to 70m/s. Air - excess air supply is controlled mainly by air pressure and if this is incorrect there is improper or no combustion.

Answer 124

There is a number of ways to produce good quality feed water for boiler plants. Methods such as e.g. reverse osmosis plants or ion exchange plants produce good quality distillate. Also evaporators generally produce good distillate. The important thing is that the distillate used should be clean and without foreign salt contamination. In practice most distillates used contain minor parts of various salt combinations which can and must be chemically treated away. Furthermore, the distillate may contain dissolved gases like for example oxygen (O2) and carbon dioxide (CO2) which may lead to corrosion in the boiler, steam, and condensate system. Boiler and feed water must be chemically treated in order to avoid corrosion and scaling in the boiler. Hydrazine can be used as a Oxygen Scavenger in High Pressure and Low Pressure Boiler it removes any dissolved oxygen. If hydrazine (N2H4) is not used, sodium sulphate (Na2SO4) can be used instead, and the excess should be 30 – 60 ppm. In cases where other kinds of oxygen binding agents are used, it is recommended that oxygen binding agents can be measured and that measurement always indicates that no oxygen has been dissolved in the boiler water to avoid corrosion. If instead you measure the content of dissolved oxygen directly, then it is recommended to keep the value < 0.02 ppm. In addition to the above values, the various water treatment companies will add further demands, depending on the method used for treatment of feed and boiler water. However, the most important point is that the above values or their equivalents are observed and that a regular (daily) test of feed and boiler water is carried out. Its important when carrying out boiler testing to check that all joints and connections in the chemical dosing equipment is secure and there no chance of air leaking into the main system. The other important reason for conducting boiler test is to stop the formation of scale. Salts present in the boiler water will precipitate out of the solution and deposit on to the heating surfaces within the boiler. Scale is very poor at conducting heat then a steel surface so the boilers ability to produce steam will be severely reduced.

Answer 125

Daily Analysis of feed and boiler water. Weekly Skimming (surface blow down) according to analysis, but at least once per week (2 minutes with fully open valve). Blow down (bottom blow down) according to analysis, but at least once per week (each blow down valve 1 minute in low load condition). Monthly Check the functions for salinity and oil detection systems. Every six months The boiler water side (interior) must be carefully inspected at least twice a year. Yearly * Check of the water side of the boiler and hot well/de-aerator for corrosion and scaling. * Check the chemical pump unit

Answer 126

1.Prevents the scale formation in the boiler and feed system 2.Prevents corrosion in boiler and feed system. 3.Control the sludge formation and prevention of carry over with the steam. 4.Maintains the boiler water in alkaline condition and free from dissolved gases. 5.prevents entry of foreign matter such as oil, waste, mill scale, FeO, Sand, etc. into the boiler

Answer 127

To ensure that proper chemical treatment are maintained at all time. To detect the present of contaminants in the water that may be injurious to boiler and system.

Answer 128

1.Alkalinity tests [Total alkalinity or P- alkalinity (phenolphthalein alkalinity) test and Caustic alkalinity test] 2.Chloride test 3.Condensate PH test 4.Amerzine test ( Hydrazine test ) 5.Phosphate test 6.Conductivity test (Total dissolve solids test) 7.Hardness test 8. sulphite test 9. pH value tests - Litmus paper - Colour-metrically - Electrolytically 10. dissolved oxygen test

Answer 129

Each test explained Tests for alkalinity Done to prevent corrosion by neutralization of acidic gases. 1.Alkalinity to phenolphthalein test How to preform test Take 100ml sample of boiler water Add 1 drop of phenolphthalein Add drops of sulphuric acid to clear sample (counting the drops) Multiply number of drops of sulphuric acid by 10 to covert to ppm Phenolphthalein is less alkaline then hydroxides or carbonates and when its added to the sample containing hydroxides and or carbonates the solution will turn pink. The sulphuric acid used will at first neutralise the hydroxides, forming salts. It then reacts with the carbonate molecules present, forming bicarbonate molecules. Bicarbonate molecules are less alkaline then phenolphthalein so the pink colour will disappear once all hydroxides and carbonates have been dealt with by the sulphuric acid. One bicarbonate molecule is formed from two carbonate molecules, therefore in the test the quantity of acid used is a measure of alkalinity. 2.Total alkalinity or P- alkalinity (phenolphthalein alkalinity) test How to preform test Take 100ml sample of boiler water Add 5 drops of total alkalinity indicator so solution goes clear. Add sulphuric acid (N/10) until sample goes pink. Check level in burette and convert ml in burette to ppm using the manufacturer chart e.g. (drew chemicals) and chemically treat using manufactures guidance. Methyl orange indicator is less alkaline then phenolphthalein and bicarbonates. It can be used instead of phenolphthalein. If there is no yellow colour when methyl orange or phenolphthalein is added then no bicarbonates are present. Hydroxides and carbonates can coexist in a solution but hydroxides and bicarbonates cant. This test is preformed to measure alkalinity in order to determine precipitation (hardening salts settling to the bottom of solution) level of hardness salts test result will indicates if boiler water requires chemical dosing or boiler needs blow down. 3.Caustic alkalinity test How to preform test Take 100ml of sample of boiler water Add 10ml of barium chloride Add 10 drops of phenolphthalein to turn sample pink Add sulphuric acid to clear sample Note the amount of sulphuric acid added in ml and multiply it by 10 to convert it to ppm In this test barium chloride is first added to boiler water in order to precipitate all carbonates that are present. Chloride test How to preform the test Take 10ml of boiler water sample and add 3 drops of phenolphthalein indicator to neutralise the sample. Add N/10 until sample turns clear and add one more drop, add 6 drops of potassium chromate to turn yellow. Add silver nitrate until orange. Multiply drops of silver nitrate by 10 to convert to ppm. Chloride level shouldn't exceed 200ppm, if it does preform a boiler blow down procedure. This test measures the amount of chloride in the water. it reveals the amount of dissolved salts in boiler water. an increase in chloride level could be an indication of contamination by seawater. if chloride level is too high this causes foaming or scale formation. Chlorides may be present in sample and its essential they are measured as this is an indicator of salt water leakage into the feed system or a leaky condenser or primed evaporator. Silver nitrate has an affinity for potassium chromate and chlorides. It neutralises the chlorides present in sample and then reacts with the potassium chromate. As the silver nitrate is added a reddish brown colour results, which will then disappear quickly if chlorides are present. Sulphite test How to preform test Take 100ml of boiler water sample Add 2ml of sulphuric acid, add 1ml of starch solution Add potassium iodide until sample turn blue Covert ml of iodide solution used into ppm by multiplying by 12.5 The boiler water sample is made slightly acidic by the sulphuric acid to speed up the chemical reactions that are going to take place. The potassium iodide reacts with sulphite and then with starch solution turning sample to a blue colour. The amount of potassium iodide used is a measure of the sulphite content present in the boiler water sample. As far as possible the atmosphere should be excluded from this test otherwise an incorrect result will occur. If test indicates that an adequate reserve of sodium sulphite is present in boiler water then there no need to carry out the dissolved oxygen test. Phosphate test How to preform test Take 25ml of filtered boiler water sample Add 25ml vanadomolybdate reagent Fill comparator tube with solution and place in right hand compartment of comparator In left hand compartment place a blank prepared by ,mixing equal volumes of vanadomolybdate reagent and deionised water. Allow colour to develop for at least 3mins and then compare with the disc Hardness test How to preform test Take 100ml of filtered boiler water sample Add 2ml of ammonia buffer solution Add 0.2 g of mordant black 11 indicator and stir until dissolved If hardness salts are present solution will turn wine red Multiply ml of EDTA solution used by 10 to covert to ppm pH value A boiler waters pH can be obtained by three methods 1.Litmus paper 2.Colour-metrically 3.Electrolytically Test 1 doesn’t give accurate result by test 2 and 3 give more accurate pH value 1.Litmus papers Used to roughly find out the degree of alkalinity and or acidity of the water. A litmus paper is inserted into a boiler water sample an then changes colour if it turns blue the water is alkaline and if red then water is acidic. This is a very rough indicator of pH value for boiler water. The salinometer and litmus papers are therefore inadequate and cant be used for accurate boiler water test. This is why the generally aren't used any more and instead more accurate boiler water test are carried out. 2.Colour-metric method How to preform test Take boiler water sample Place one thymol blue tablet in a 50ml Nessler cylinder Add 50ml of sample into Nessler cylinder and ensure tablet dissolves Put 50ml of sample into other Nessler cylinder Place sample 1 in right hand compartment of Nessleriser Place sample 2 in left hand compartment of Nessleriser Place appropriate disc in Nessleriser and match the colours, then read the pH value from the right hand window 3.Electrolytic method An electric cell is used and then the boiler water is used as an electrolyte and two special electrodes both made of glass are used. The potential difference between the electrodes is dependant upon the the hydrogen ion content of the electrolyte (boiler water). its measured by a sensitive voltmeter connected into the external circuit of the cell and calibrated to read pH values. Dissolved oxygen test How to preform test Take 500ml of boiler water sample Add 0.3 ml of manganese chloride Add 0.3ml of potassium hydroxide Add 1ml of hydrochloric acid Add 2ml of ortho-tolidine In this test its essential that the atmosphere is excluded from the sample or it will interfere with results. To achieve this a sampling flask can be used. After adding various chemicals to sample, the solution is compared colour metrically with a colour chart or a series of indicator solutions whose dissolved oxygen oxygen content is known. Where the colours of the sample and indicator match, that is the value of the dissolved oxygen content and its read from the indicator. Total dissolved solids test Preformed by using a hydrometer or electrical conductivity meter. Hydrometer Graduated in grains per imperial gallon (to convert grains per imperial gallon to ppm multiply by 14.3). care should be taken when using the hydrometer to account for the water meniscus and to obtain a accurate temperature. Temperature correction tables for the hydrometer will also be supplied Conductivity meter A portable battery operated electrical conductivity meter is used for the test. The removable conductivity cell is washed out and filled with a treated boiler water sample (treatment consists of cooling to 15-20degrees Celsius, adding phenolphthalein and removing pink sample colour with an acid.) the filled cell is plugged into the meter, its temperature checked and the temperature control set to correspond. A range switch is set to approximate range of reading expected then a central control is operated until “null” balance of the electrical bridge circuit is achieved. Position of the central control indicates the total dissolved solids in the boiler water usual in ppm. It can be in micromhos to convert to ppm multiply value by 0.7). Hydrazine test How to preform test Take 250ml of boiler water sample, excluding air and cool to 16-25 degrees Celsius Add 15ml of hydrochloric acid to each of two Nessler cylinders Add 25ml of boiler water sample and 10ml of 4-dimethylaminobenzaldehyde to one cylinder Add 35ml of boiler water sample to other cylinder Place sample 1 in right hand compartment of Nessleriser Place sample 2 in left hand compartment of Nessleriser Place appropriate disc in Nessleriser and match samples against the disc colours To covert value to ppm divide the disc reading by 25] Hydrazine reserve in boiler water should be between 0.1 and 1ppm

Answer 130

Cavitation is the process whereby pressure variations in a liquid can in a short period of time cause countless small cavities to form and then implode. The cavities fill with liquid vapor and the gases are present in the liquid. Cavitation, which is also the formation of vapour bubbles within a liquid at low-pressure regions, occurs in places where the liquid has been accelerated to high velocities, for example centrifugal pumps, water turbines, and marine propellers.

Answer 131

chemical compound used to control pH in water treatment, neutralising acidity and eliminating issues with corrosion

Answer 132

Alkalinity test (a) Phenolphthalein or ‘p’ alkalinity This test is carried out to prevent acidic corrosion To test for presence of all of the hydroxide, one half of the carbonate and one third of phosphate present in a water sample. (b) Total ( T ) alkalinity test To determine the amount all of hydroxide, all of the carbonate, and two thirds of the phosphates Note: Hydroxides and carbonates can co-exist together in a solution but hydroxides and bi-carbonates cannot. Chloride test To know the amount of salt in boiler water. To minimize chloride level and to adjust the blow down. Condensate PH test To control condensate PH value within a limit. To minimize corrosion in steam and condensate system. Amerzine test ( Hydrazine test ) To test for dissolved Oxygen content To know reserve hydrazine (N2H4 ) ppm and to prevent corrosion and aeration. To minimize oxygen pitting and corrosion in boiler, steam and condensate system. Phosphate test To control the scale formation due to hardness (presence of Phosphate in sample means no hardness salts) A reserve of phosphate should be maintained in the boiler water ready to neutralize any hardness salts which may enter. Conductivity test Measure of the total amount of dissolved solids (T.D.S) including the treatment chemicals. (Excessive density leads to priming and or deposits) To remove dissolved and suspended solid by blowing down. Hardness test To check for salt causing “hardness” Note: Hardness test of boiler water are not necessary when the phosphate is above the lower limit of the control range.

Answer 133

Used to measure the proportion of dissolved salts in boiler water .if sea water is used as feed water for low pressure boilers its recommended that boiler density should maintained as close as possible to 4/32 or approx. 125,000. This can be achieved by carrying out a blow down when required. If sea water has to be used as feed water protection can be provided by using soda ash. For low pressure fire tube boilers the salinometer and litmus paper are very rarely used as part of testing procedures but its just as important to keep feed water for these boilers in good condition as it is for water tube boilers.

Answer 134

Part of the reason why corrosion occurs in boilers is dissolved oxygen enters the feed water for boiler. The closed feed systems however reduce the chances of water coming into contact with oxygen in the air which is why its important to keep this system in good working order to minimise corrosion. Maintaining the temperature of the feed water higher than 85degrees Celsius reduces the waters ability to mix with and carry oxygen around the system. However you must ensure that feed water temperature isn't to high as this can lead to the feed pump being affected due to cavitation or steam starting to form and producing the effect known as gassing up of pump. Good watch keeping actions should therefore be maintained such as regular inspection of closed feed system and associated equipment, checking and maintaining feed water temperature , carrying out boiler water test and using chemical treatment or boiler blow down when required. The best chemicals used to reduce dissolved oxygen is the oxygen scavengers such as sodium sulphite and bisulphite.

Answer 135

Steam Safety System: The steam system in the boiler is a high pressure, high-temperature area. To safeguard the operator and the boiler itself, it is fitted with the following safety features: Pressure gauge: Multiple pressure gauges are fitted to ensure the operator has an idea of the current value of pressure inside the boiler. Usually, two pressure gauges are fitted on the boiler and one line is taken from the steam drum to the engine control room, to display the steam pressure remotely. The pressure gauges are also incorporated with cut-in and cut-out automation systems, i.e. the input from the pressure gauges are used to operate the boiler burner. When the pressure reaches the set value, the boiler burner will stop firing and when the pressure drops to a lower set value, the burner will be switched ON to raise the boiler pressure. Steam Pressure Alarm and cut-out: An audio-visual alarm is also provided for the steam pressure system to remind the operator about the steam pressure. Once the alarm activates and the pressure continues to rise (or decrease), the cut-out will get activated and it will shut off the fuel burner. The cut-out functioning is different and independent of the automation which operates the burner. The low-pressure cut-out has an option to override it, but the high-pressure cut-out will stop the burner and should never be overridden in any case Boiler Vent: Vent on the boiler drum is required to ensure boiler does not implode once it is shut down. It is normally opened when the pressure gauge shows the reading below 0.5 bars.

Answer 136

Water Safety System: The water system is a high-temperature system and the level and quality of the water inside the water drum plays a crucial role in the safe operation of the boiler. Following are the equipment/system fitted on the water side of the marine boiler: Low / high water level alarm and cut-out: The boiler water drum is fitted with a level sensor, which will continuously monitor the level of water inside the drum. A full drum will carry over the water or will have no space to generate steam, thus reducing the efficiency of the boiler; whereas low or no water level in the drum will lead to over-heating of tubes and can lead to fire or meltdown of the complete boiler. The low/ high water level provides an early warning to the operator for taking appropriate action to manage the water level inside the boiler water drum. Too low water level alarm and shut down: The initial warning provided by the above arrangement (low/high water level alarm), may not be sufficient for the operator as there can be a major leak in the tubes, leading to a reduction in the water level. A secondary safety is therefore provided i.e. Too low water level alarm and shut down, which will stop the burner firing to control the overheating of the boiler internal parts. Water level indicators: The boiler is fitted with water level indicators to make it easy for the operator to see the water drum level and ensure operational safety of the boiler. Local gauge glasses are provided on the boiler drum to ensure at least one gauge glass is operational in case one stops showing the level. Remote water level indicators such as a differential pressure water level sensor, probe level sensor etc. are also provided to indicate the current level in the drum at a remote position such as the engine control room. Salinity Sensor: The boiler drum is fitted with a salinity sensor, which continuously monitors the dissolved solids content in the water. If the solid (e.g. salt) content exceeds the set value, it trips the boiler to ensure the tubes and boiler internals does not get affected due to the contamination. The operator should either blow down the boiler and feed fresh water to the drum to eliminate the cause which is resulting in high salinity (for e.g. leakage in the condenser)

Answer 137

Fuel Safety System: The boiler is provided with heavy or marine gas fuel oil for generating the heat in the furnace. To ensure the fuel system is operating efficiently, it is fitted with the following boiler safety features: Low fuel oil pressure alarm: The fuel to the burner is provided using a fuel oil pump. Two pumps are installed (one kept as standby) to ensure there is no operational hindrance in case of failure of one pump. If the fuel supply pressure is less than required, the atomization of fuel will not happen, leading to dripping of fuel inside the furnace. This can lead to blow-back of the burner and can seriously injure the operator. Once the low-pressure alarm is sounded, the operator must ensure to eliminate the cause behind it. Low / high fuel oil temperature alarm: Modern marine boilers are meant to operate in different grades of fuel due to the port / ECA regulations for minimizing the air pollution from the ship. The oil temperature is an important factor as it controls the viscosity of the fuel which is directly related to atomization and efficient combustion inside the furnace. If the fuel temperature is not at its set value (which will vary for different grades), the alarm will sound. The operator must stop the alarm and the oil temperature should be brought to normal before restarting the boiler. Flame failure alarm: The burner unit which is a photocell (also known as the flame eye) is used to detect the flame inside the furnace. If the burner is abruptly stopped, or during starting the main burner is not producing flame, the photocell will detect the absence of the flame and sound an audio-visual alarm. Smoke Density alarm: With more stringent rules coming up for environmental protection, the boiler exhaust is fitted with a smoke density sensor which detects the post-combustion product, especially during starting of a boiler and at low loads. If the smoke density is higher than the required value, it will sound an alarm to which the operator needs to check the combustion of the boiler. Force draught fan stop alarm: To have an efficient combustion, a proper mixture of air and fuel is needed. The air is provided to the burner assembly using a forced draught fan (FDF). If the fan is not operational due to any reason, it will generate an alarm.

Answer 138

1.To prevent scale formation in boiler and feed system by using distilled water and precipitating all scale forming salts into the form of non adherent sludge 2.To prevent corrosion in boiler and feed system by maintaining the boiler water in an alkaline condition and free from dissolved gases 3.Control the amount of sludge forming and prevents steam being carried over 4. Stops foreign matter entering the boiler by heating up oil, carrying out effective cleaning and maintaining steam and condensate systems in non corrosive conditions. Foreign matter could include things like oil waste, mill scale, iron oxides, copper particles, sand or weld spatter.

Answer 139

Hydrazine (60% hydrazine and 40% water) is at the moment the most popular oxygen scavenging chemical used because it reacts under boiler conditions with oxygen and forms water and therefore has the advantage of not increasing boiler water density. The excess dosage amount of hydrazine has to be controlled because it can lead to steam and condensate line corrosion due to ammonia being produced as the excess dosage of hydrazine decomposes. A controlled excess dosage of hydrazine will also be beneficial to steam and condensate system because it will counteract the effects of carbon dioxide corrosion. Hydrazine should be stored in a cool well ventilated place because it’s a fire hazard. When being handled PPE should be worn. The hydrazine solution should be injected into the de-aerated feed.

Answer 140

The following points are important when using this chemical: pH value is important to reaction rate with oxygen. At pH 7 it’s at maximum so sodium sulphite should be injected into the system before any alkaline ingredients In high pressure boilers sulphite will breakdown and form hydrogen sulphide and possibly sulphur dioxide which will attack steel, brass and copper It will increase total dissolved solids content. this is because Sulphate is a constituent of TDS and may form salts with sodium, potassium, magnesium and other cations. Use limited to low pressure boilers due to increasing TDS and reducing alkalinity by its action * Takes the form of a soft white powder * Slightly alkaline * Will react with oxygen to form Sodium Sulphate at about 8ppm Sodium to 1ppm Oxygen

Answer 141

Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts, principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulphates and some small amounts of organic matter that are dissolved in water. total dissolved solids (TDS) measures the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized, or micro-granular suspended form. This includes not only salts, but also minerals, metals, and other dissolved substances.

Answer 142

Cations are positively-charged ions (atoms or groups of atoms that have more protons than electrons due to having lost one or more electrons). Anions are negatively-charged ions (meaning they have more electrons than protons due to having gained one or more electrons)

Answer 143

1.Boil out the boiler at atmospheric pressure with an alkaline solution to remove traces of oil and dirt 2.Wash out the boiler with a heated acid solution to remove rust and mill scale 3.Rinse the boiler with a weak acid solution 4.Flush the boiler put repeatedly to remove debris 5.Subject the boiler to a passivation process. This is carried out under pressure with hydrazine The purpose of chemical cleaning is mainly to remove surface rust and mill scale that occur during boiler erection ( erection means assembling of boiler parts by welding, riveting, or other fabrication processes.) and manufacture and also dirt and traces of oil. The feed system is subjected to same process except the alkaline boil out would be omitted and the passivation would be carried out at atmospheric pressure with hydrazine.

Answer 144

When a boiler is not in service it must be protected from corrosion. In the case of water tube boilers not in service for a short period of time(e.g. 2days) the boiler can just be fired at intervals to keep boiler pressure above approximately 3.5bar and the boiler must be maintained in composition as required for the boiler when under normal steaming operation. Alternatively the boiler can be filled while hot, with hot de-aerated alkaline feed water and about 0.5kg of anhydrous sodium sulphite added for each ton of water in boiler and boiler must be topped up periodically and any air in system must be drained. With fire tube boilers out of service for short periods the only action to be carried out is to ensure that alkalinity to phenolphthalein is not less than recommended value, or just completely fill the boiler with alkaline water. If boiler is taken out of service for a longer period of time it be drained completely and then dried out using heater units. Then trays of quick lime to be placed internally in suitable positions throughout the boiler before its sealed up. Blanks should be fitted to the pipe connections in the event of steam being maintained in other boilers and the blow down should be blanked in any case. The lime should be renewed at least once every two months.

Answer 145

Sodium sulphate is used for prevention of caustic embrittlement. The ratio of sodium sulphate to caustic soda should be kept at or above recommended value of 2:5. Sodium nitrate can also be used, the ratio of sodium nitrate to caustic soda shouldn’t fall below 0.4:1 at all times.

Answer 146

When steam is wet and its in the condensate system, corrosion can occur due to the presence of carbon dioxide which is carried over with the steam. To ensure alkalinity in this part of the system a volatile alkaliser may be injected into the steam line. These alkalisers can be ammonia or cyclo-hexylamine. They combine with the steam as it condenses to form carbonates and bicarbonates, which decompose in the boiler back into CO2 and the alkaliser, some of which then returns to the steam system. If the pH value of condensate is maintained at approx 9 this will ensure no corrosion in the low temperature steam and condensate parts of the system. Filming amines, the most common of which is octadecylamine, are insoluble in water at room temperature but volatile in steam. Filming amines prevent corrosion by forming a protective adsorbed layer on metal surfaces. Neutralising amines are colourless, volatile liquids that can burn and whose fumes are toxic. Monocyclohexylamines or morpholine in solution is supplied in sealed containers and must be stored in a cool place. Anti foams are complex organic compounds of high molecular weight. They're used to control the the foam in the boiler drum and thereby preventing carry over. They're included in the boiler chemical mixtures.

Answer 147

volatile = capable of readily changing from a solid or liquid form to a vapour Alkaliser = a substance that makes something alkaline or less acidic. The acids which undergo vaporization rapidly are termed volatile acids.

Answer 148

In order for corrosion to take place oxygen has to present to achieve the formation of metal oxides. Therefore if air is removed from feed water then oxygen is removed and so no corrosion can take place which is why its necessary to carry out de-aeration. De-aeration is achieved mechanically or chemically or a combination of both. Its standard to have a reserve of chemicals in the boiler water to deal with any ingress of dissolved oxygen that can occur due to incorrect operation of de-aerating equipment or another circumstance. The oxygen scavenging chemicals used for de-aerating the water are sodium sulphite or hydrazine. Sodium sulphate which forms through the use of sodium sulphite to de-aerate, remains in the solution in the boiler water under normal operations.

Answer 149

Scale prevention can be achieved by using coagulants. The reason for using coagulants in the boiler is to condition the precipitates, converting them into the form of sludge that is non adherent and can easily be blown out of boiler. Calcium phosphate, magnesium hydroxide and calcium carbonate do form scale but through the use of coagulants they can be converted into relatively harmless non adherent sludge. Coagulants which are used to achieve this are polyelectrolytes (these are synthetic organic polymers of high molecular weight, like sodium polycrylate which can be present in boiler chemical mixtures), sodium aluminate, starch, tannin, gels and casein. Sodium aluminate can be used in conjunction with lime and soda treatment. It can breakdown and form aluminium hydroxide, which combines with the magnesium hydroxide in a flocculent form, which turns into a scum usually on the top of the water. Other precipitates can combine with flowing floc (scum) and be blown out of boiler using the scum valve. Floc also combines with any traces of oil that may be present, rendering them harmless. Coagulants form colloidal suspensions in the boiler water. Colloids consists of sub microscopic particles (cluster of atoms or molecules) with electrical charge which repel each other and prevent the formation of larger particles. They combine with precipitates of opposite electrical charge to produce a floc or scum.

Answer 150

For precipitation of scale forming salts into sludge and to give alkalinity phosphates are used. Phosphates mix with calcium in boiler water forming tricalcium phosphate which will precipitate as it has low solubility to form sludge or porous scale. Phosphates will also mix with magnesium compounds forming magnesium phosphate which also precipitates into the form of sludge. Using phosphates instead of sodium carbonate for conditioning high concentrations of caustic soda is avoided because at high temperatures sodium carbonate and water breaks down into sodium hydroxide and carbon dioxide. Through the use of trisodium phosphate sodium carbonate is formed. The sodium hydroxide amount that is formed due to the breakdown of sodium carbonate isn't normally excessive. It should give the requisite hydroxyl ions necessary to maintain moderate alkalinity. Phosphates that are normally used are sodium hexametaphosphate, sodium metaphosphate, disodium phosphate and trisodium phosphate. The metaphosphates are put into the feed system because they react slowly and don’t produce scale or sludge in the feed system. Disodium and trisodium phosphate is normally pumped directly into boiler because they react quicker and can form sludge or scale in the feed system. Due to the presence sodium hydroxide in the boiler water, metaphosphate, monosodium and disodium phosphate is converted into trisodium phosphate. Depending upon requisite alkalinity the phosphate that’s required is chosen. Due to the removal of calcium by the phosphates, the chance for silicates present in the water to form scale is greatly reduced. Instead the silicates just remain in the solution in the boiler water.

Answer 151

Can be used instead of lime and soda treatment. Caustic soda (sodium hydroxide (NaOH)) reacts with the alkaline and non alkaline magnesium compounds, the alkaline calcium compounds and forms sodium carbonate. This then reacts with the non alkaline calcium compounds. The sodium carbonate that is formed as a result of using sodium hydroxide will be in sufficient quantity to deal with non alkaline calcium compounds. If for some reason there isn't sufficient quantity of sodium carbonate to deal effectively with non alkaline calcium compounds then you have to use it in conjunction with sodium hydroxide. When adding the chemical caustic soda you have to be careful as heavy concentrations can cause skin burns. Caustic soda and lime soda treatment don’t get used as much any more but can be useful in emergency situations.

Answer 152

Lime (calcium hydroxide, Ca(OH2) and soda ash (sodium carbonate, Na2CO3) are used deal with calcium and magnesium compounds in boiler water. Calcium hydroxide reacts with alkaline hardness salts and magnesium compounds. Sodium carbonate reacts with the calcium compounds in the feed water including the ones that form due to using calcium hydroxide. Using this lime and soda treatment gives zero hardness salts and alkaline feed water. Boiler water is treated with lime and soda prior to entry into system, due to the fact that boiler water contains some form of alkaline hardness salts which would otherwise precipitate and form soft sludge or scale when water is heated by feed heater, boiler or economizer.

Answer 153

Feed water for boilers can be unevaporated freshwater, evaporated freshwater or evaporated salt water. Unevaporated freshwater and evaporated salt water is normally only used as feed water for low pressure boilers like scotch boilers. Evaporated freshwater is normally used and evaporated salt water for water tube boilers. Anyone of these types of feed water will contain salts that can harm the boiler due to scale formation and corrosion. Evaporated freshwater and salt water should be low in solids content to be less harmful. Feed systems can of course become contaminated by seawater from a leaking condenser or an evaporator priming.

Answer 154

Sodium chloride This is a common salt whose heavy concentration can cause foaming and priming. Under boiler conditions, density at which sodium chloride comes out of the solution increases as pressure and temperature increases. This means its ability to be dissolved is variable. Each of these salts present in boiler water will have variable solubility due to temperature and pressure variations. Solubility is also increased when a salt is in the presence of another salt. Sodium chlorides solubility is high and normally it doesn’t come out of the solution under normal boiler operation but if it comes into contact with another salt like magnesium sulphate then solubility is increased. Magnesium chloride Magnesium chloride will dissolve in boiler water under normal operation but its possible for it to be broken down forming hydrochloric acid and magnesium hydroxide. Magnesium hydroxide has low solubility so it can deposit and form scale but with suitable treatment its possible for it to be precipitated into the form of non adherent sludge that can be blown out of boiler. Hydrochloric acid can cause corrosion as a result of attack of the acid upon iron which then produces a chloride of iron. This breaks down to form iron hydroxide with regeneration of hydrochloric acid so corrosive cycle continues, it therefore required to use suitable treatment to prevent this corrosion. Magnesium sulphate This is soluble under normal boiler operation and if reaches too high density it may deposit and form scale. It can also combine with sodium chloride, forming magnesium chloride and sodium sulphate. Calicum sulphate This salt is the most dangerous scale former in boiler water. It can deposit and form a hard scale that greatly effects heat transfer and cause failure of the heating surface and overheating. Calcium bicarbonate This salt is decomposed when heated, liberating carbon dioxide and permitting precipitation of calcium carbonate. Calcium carbonate has low solubility and this solubility decreases when there an increase in temperature so no scale is formed. Scale formed is also soft and porous and doesn't effect heat transfer too much because its not a poor conductor of heat like calcium sulphate scale

Answer 155

This occurs when a steam bubble forms upon heating surface (plate) and the are area under the bubble becomes overheated because its insulated for a brief moment from water. The water which contains salts, is in contact with the plate around the periphery of the bubble and also becomes overheated. If the salts In the water are those whose solubility decreases with temperature (calcium sulphate being an example) they will be deposited in the form of a crystal ring. This happens because the water has become supersaturated with the salts. Then at some point the bubble can burst and the water comes into contact with the overheated plate and causes more salt deposition. Salts whose solubility decreases with an increase temperature are the ones that form scale upon the heating surfaces and then sludge upon cooling surfaces. Then the salts whose solubility increases with temperature will not form scale upon the heating surfaces but a sludge can be formed if saturation point is reached.

Answer 156

Unevaporated fresh water isn't generally used as feed water for boilers because it contains some or all the salts present in seawater as well as other salts but in small amounts. Water is classified as salt or fresh water based on whether its potable (drinkable) or not.

Answer 157

Hardness salts hardness salts are hydroxides , carbonates and bicarbonates of calcium and magnesium. The bicarbonates of calcium and magnesium are called temporary hardness salts because they decompose when heated, liberating carbon dioxide and leaving carbonates. Non alkaline or permanent hardness salts are the chlorides, sulphates, nitrates and silicates of calcium and magnesium. Hardness due to these salts is not removed by boiling or heating but can be removed with chemical treatment. Total hardness is the sum of alkaline and non alkaline hardness salts present in the water. Due to these salts being scale forming the total hardness has to be known to know when chemical treatment is added. Silicates Found in water and In low pressure boilers, silica combines with calcium and magnesium forming silicates which precipitate and form hard scale, in high pressure boilers silica combines with other elements to form silica scale, which is glassy and extremely hard and difficult to remove. If silica content of boiler water is in excess of 20ppm (amount decreases as boiler pressure increases) it will volatilise and deposit on turbine blades. Carbon dioxide If water contains dissolved carbon dioxide, carbonic acid will form which causes corrosion. The carbon dioxide is absorbed into the feed water due to contact with atmosphere. It can also form due to breakdown of bicarbonates and carbonates which are present in feed water. Carbonic acid dissociates into hydrogen ions and bicarbonate ions. Bicarbonate ions combine with ferrous metal and form ferrous bicarbonate, which dissociates into ferrous carbonate and carbonic acid, which is re-dissolved into the water. If there is a supply of dissolved oxygen in the water the ferrous carbonate is then converted into ferric hydroxide with regeneration of carbon dioxide. This can be a continuous corrosion cycle if there's a continuous supply of dissolved oxygen in the water. Hydrogen When acid corrosion is rapid for example when acid is concentrated under a deposit, damage due to newly formed hydrogen molecules at the cathode can occur. These hydrogen molecules penetrate the boiler tube metal and reacts with carbon and produces methane. This carbon loss weakens the metal and the methane gas exerts a pressure that separates the grains of steel. Hydrogen damage can also occur when hydrogen is released by caustic corrosion.

Answer 158

The condition of the gases leaving the funnel is best indicator of combustion conditions. For example black smoke due to insufficient air, white smoke due to excessive air, blue smoke due to burning of lube oil (in IC engines), yellow smoke due to high sulphur fuels. CO2 content is often required to give efficient combustion for a plant. Each plant will have its own optimum figure and this varies for boilers and is dependant on many variables.

Answer 159

net current means the sum or total of the individual currents flowing into (positive current) and out of (negative current) the node. Net current means the sum of all currents, bearing in mind current direction. (KCL)

Answer 160

There are two different types of polarization, that is, two ways that electrochemical reactions of corrosion are reduced. These are referred to as activation and concentration polarizations. Activation polarization is used to indicate retarding factors inherent in the reaction itself. Thus, for example, with a metal corroding in acid, an amount time is required to form hydrogen gas at the cathode regions even when a good supply of electrons exist. Concentration polarization refers to the retardation of an electrochemical reaction of corrosion resulting from concentration changes in the solution next to the metal surface. A good example is provided by the cathode region of a metal corroding in seawater, that is the hydroxyl reaction. Because of low solubility in seawater, the amount of oxygen in contact with the cathode areas is small and readily consumed by the reaction. For the reaction to continue, additional oxygen must therefore diffuse to these areas. The diffusion of oxygen is a relatively slow process but will increase the to concentration polarization of the cathode. To reduce or slowdown the effect of corrosion you change the concentration of polarization for metal resulting in the slow diffusion of oxygen to the cathode areas. any situation which would increase the rate of the oxygen diffusion would increase the rate of corrosion. Thus, for example, water flowing past a steel pipe is more corrosive than still water because of the increased oxygen supply. de-oxygenated water is far less corrosive than ordinary. This is the prime reason that steam plant feed water and coolant water is "de-aerated" prior to use. The larger the cathode, relative to the anode, the faster the corrosion since more surface area on the cathode is in contact with seawater and thus in contact with the oxygen.

Answer 161

Polarization can be defined as the shift in electrode potential which happens due the effects of current flow. When net current to or from an electrode exists, the electrode is no longer in equilibrium and the measured voltage difference (potential) changes. This change in voltage opposes the current. The potential change caused by the current is known as polarization. This change is caused by various physical and chemical factors at the electrodes.

Answer 162

A vessel steam system cascade tank, also known as a hot well, is a part of the boiler feed water system. It is a tank that receives the condensate (water) from the steam heating system and filters out any foreign substances such as rust, oil, or dirt. The cascade tank also treats the boiler water with chemicals to prevent corrosion and scale formation in the boiler tubes. By doing so, it ensures the efficient reuse of water and minimizes heat and energy loss, making it an essential component for energy conservation and cost-effectiveness on a vessel. The cascade tank is important because it ensures the quality and quantity of the feed water for the boiler, which affects the efficiency and safety of the steam generation process.

Answer 163

Operation of a Cascade Tank A vessel steam system cascade tank works by using a cascading method to separate the clean condensate from the dirty condensate. The dirty condensate is the water that returns from the steam heaters that are used for various purposes on board, such as fuel, lube, purifier, cargo, etc. The dirty condensate may contain oil or other contaminants due to leakage or failure of the heating coils. The dirty condensate enters the cascade tank through an oil sensing probe. The oil sensing probe detects any oil particles in the condensate and triggers an alarm and the contaminated water should be diverted to the bilge. The clean condensate passes through the condenser outlet pipe and enters the observation area of the cascade tank. The observation area is a transparent section of the cascade tank that allows visual inspection of the condensate. The observation area has a sight glass with light and a skimming valve. The sight glass with light helps to see the level and clarity of the condensate. The skimming valve is used to remove any oil film or foam that may form on the surface of the condensate. The condensate overflows from the observation area to the settling chamber. The settling chamber is a section of the cascade tank that allows gravity separation of any solid particles or sludge from the condensate. The settling chamber has a drain valve at the bottom to remove any accumulated sludge periodically. The condensate flows from the settling chamber to the oleophilic filtration chamber. The oleophilic filtration chamber is a section of the cascade tank that uses oleophilic filters to remove any remaining oil traces from the condensate. Oleophilic filters are filters that attract oil and repel water. The oleophilic filters are arranged in layers and have a large surface area to increase their efficiency. The oleophilic filtration chamber has a differential pressure gauge to monitor the pressure drop across the filters. If the pressure drop exceeds a certain limit, it indicates that the filters are clogged and need to be replaced or cleaned. The condensate passes through the oleophilic filters and enters the hot well or main chamber. The hot well or main chamber is a section of the cascade tank that stores and heats up the clean condensate before sending it to the boiler. The hot well has a heating element that generates heat by supplying power. The heating element is controlled by a temperature sensor and a control unit that maintain the temperature of the hot well at a desired level. The hot well also has a level gauge and a level controller that regulate the amount of feed water for the boiler. The feed water pump draws water from the hot well and sends it to the boiler through an economizer.

Answer 164

Maintenance of a Cascade Tank A vessel steam system cascade tank requires regular maintenance and troubleshooting to ensure its proper and safe operation. Some of the maintenance tasks are: Regular Inspections: Routine inspections are necessary to check for leaks, corrosion, and any damaged components, by: Inspecting for any leaks, corrosion, erosion, cracks, etc. in the cascade tank and its piping. Replacing or repairing any defective or damaged parts. Checking and cleaning the oil sensing probe, diverter valve, sight glass, skimming valve, drain valve, oleophilic filters, differential pressure gauge, heating element, temperature sensor, level gauge, level controller, etc. Testing and calibrating the alarm and control systems. Water Treatment: Ensuring the quality of the condensate and feed water is essential to prevent scale, corrosion, and fouling in the system by: Sampling and analysing the feed water quality for pH, conductivity, hardness, dissolved oxygen, etc. Adding chemicals such as oxygen scavengers, alkalinity builders, anti-scalants, etc. to treat the feed water. Steam Traps: Maintaining and regularly checking steam traps is crucial. Faulty or stuck traps can cause water logging, inefficiencies, and damage to equipment. Safety Valves on Boiler and Steam Condenser: Ensuring that safety valves are working correctly is paramount to avoid over-pressurization, which can be a safety hazard. Efficient Heat Exchange: Monitoring the heat exchange efficiency and insulation of the tank can help in reducing energy losses.

Answer 165

It is important to keep the temperature of the cascade tank as high as possible for several reasons: Higher temperature reduces dissolved oxygen in water which can cause corrosion in boiler tubes. Higher temperature increases thermal efficiency by reducing heat loss in feed water. Higher temperature prevents thermal shock in boiler tubes due to sudden change in temperature. Higher temperature reduces the risk of microbial growth in water which can cause fouling and contamination. The optimal temperature of the cascade tank depends on the design and operation of the boiler and the steam system. Generally, it is recommended to keep the temperature of the cascade tank between 80°C and 90°C.

Answer 166

The feed system completes the cycle between boiler and turbine to enable the exhausted steam to return to the boiler as feedwater. The feed system is made up of four basic items: the boiler, the turbine, the condenser and the feed pump. The boiler produces steam which is supplied to the turbine and finally exhausted as low-energy steam to the condenser. The condenser condenses the steam to water (condensate) which is then pumped into the boiler by the feed pump. Other items are incorporated into all practical feed systems, such as a drain tank to collect the condensate from the condenser and provide a suction head for the feed pump. A make-up feed tank will provide additional feedwater to supplement losses or store surplus feed from the drain tank. In a system associated with an auxiliary boiler, as on a motor ship, the drain tank or hotwell will be open to the atmosphere. Such a feed system is therefore referred to as 'open feed'. In high-pressure water tube boiler installations no part of the feed system is open to the atmosphere and it is known as 'closed feed'.

Answer 167

An open feed system for an auxiliary boiler is shown in Figure 5.1. The exhaust steam from the various services is condensed in the condenser. The condenser is circulated by sea water and may operate at atmospheric pressure or under a small amount of vacuum. The condensate then drains under the action of gravity to the hot well and feed filter tank. Where the condenser is under an amount of vacuum, extraction pumps will be used to transfer the condensate to the hot well. The hot well will also receive drains from possibly contaminated systems, e.g. fuel oil heating system, oil tank heating, etc. These may arrive from a drains cooler or from an observation tank. An observation tank, where fitted, permits inspection of the drains and their discharge to the oily bilge if contaminated. The feed filter and hot well tank is arranged with internal baffles to bring about preliminary oil separation from any contaminated feed or drains. The feedwater is then passed through charcoal or cloth filters to complete the cleaning process. Any overflow from the hot well passes to the feedwater tank which provides additional feedwater to the system when required. The hot well provides feedwater to the main and auxiliary feed pump suctions. A feed heater may be fitted into the main feed line. This heater may be of the surface type, providing only heating, or may be of the direct contact type which will de-aerate in addition. De-aeration is the removal of oxygen in feedwater which can cause corrosion problems in the boiler. A feed regulator will control the feedwater input to the boiler and maintain the correct water level in the drum. The system described above can only be said to be typical and numerous variations will no doubt be found, depending upon particular plant requirements.

Answer 168

A closed feed system for a high pressure water tube boiler supplying a main propulsion steam turbine is shown in Figure 5.2. The steam turbine will exhaust into the condenser which will be at a high vacuum. A regenerative type of condenser will be used which allows condensing of the steam with the minimum drop in temperature. The condensate is removed by an extraction pump and circulates through an air ejector. The condensate is heated in passing through the air ejector. The ejector removes air from the condenser using steam-operated ejectors. The condensate is now circulated through a gland steam condenser where it is further heated. In this heat exchanger the turbine gland steam is condensed and drains to the atmospheric drain tank. The condensate is now passed through a low-pressure heater which is supplied with bled steam from the turbine. All these various heat exchangers improve the plant efficiency by recovering heat, and the increased feedwater temperature assists in the de-aeration process. The de-aerator is a direct contact feed heater, i.e. the feedwater and the heating steam actually mix. In addition to heating, any dissolved gases, particularly oxygen, are released from the feedwater. The lower part of the de-aerator is a storage tank which supplies feedwater to the main feed pumps, one of which will supply the boiler's requirements. The feedwater passes to a high-pressure feed heater and then to the economiser and the boiler water drum. An atmospheric drain tank and a feed tank are present in the system to store surplus feedwater and supply it when required. The drain tank collects the many drains in the system such as gland steam, air ejector steam, etc. A recirculating feed line is provided for low load and manoeuvring operation to ensure an adequate flow of feedwater through the air ejector and gland steam condenser. The system described is only typical and variations to meet particular conditions will no doubt be found.

Answer 169

The arrangements for steam recovery from auxiliaries and ship services may form separate open or closed feed systems or be a part of the main feed system. Where, for instance, steam-driven deck auxiliaries are in use, a separate auxiliary condenser operating at about atmospheric pressure will condense the incoming steam (Figure 5.3). An extraction pump will supply the condensate to an air ejector which will return the feedwater to the main system at a point between the gland steam condenser and the drains cooler. A recirculating line is provided for low-load operation and a level controller will maintain a condensate level in the condenser. Where contamination of the feedwater may be a problem, a separate feed system for a steam-to-steam generator can be used (Figure 5.4). Low-pressure steam from the generator is supplied to the various services, such as fuel oil heating, and the drains are returned to the hot well. Feed pumps supply the feed to a feed heater, which also acts as a drains cooler for the heating steam supplied to the generator. From the feed heater, the feedwater passes into the steam-to-steam generator. Packaged feed systems are also available from a number of manufacturers. With this arrangement the various system items are mounted on a common base or bedplate. The complete feed system may be packaged or a number of the items.

Answer 170

The condenser is a heat exchanger which removes the latent heat from exhaust steam so that it condenses and can be pumped back into the boiler. This condensing should be achieved with the minimum of under-cooling, i.e. reduction of condensate temperature below the steam temperature. A condenser is also arranged so that gases and vapours from the condensing steam are removed. An auxiliary condenser is shown in Figure 5.5. The circular cross-section shell is provided with end covers which are arranged for a two-pass flow of sea water. Sacrificial corrosion plates are provided in the water boxes. The steam enters centrally at the top and divides into two paths passing through ports in the casing below the steam inlet hood. Sea water passing through the banks of tubes provides the cooling surface for condensing the steam. The central diaphragm plate supports the tubes and a number of stay rods in turn support the diaphragm plate. The condensate is collected in a sump tank below the tube banks. An air suction is provided on the condenser shell for the withdrawal of gases and vapours released by the condensing steam. Main condensers associated with steam turbine propulsion machinery are of the regenerative type. In this arrangement some of the steam bypasses the tubes and enters the condensate sump as steam. The condensate is thus reheated to the same temperature as the steam, which increases the efficiency of the condenser. One design of regenerative condenser is shown in Figure 5.6. A central passage enables some of the steam to pass to the sump, where it condenses and heats the condensate. A baffle plate is arranged to direct the gases and vapours towards the air ejector. The many tubes are fitted between the tube plates at each end and tube support plates are arranged between. The tubes are circulated in two passes by sea water

Answer 171

The extraction pump is used to draw water from a condenser which is under vacuum. The pump also provides the pressure to deliver the feed water to the de-aerator or feed pump inlet. Extraction pumps are usually of the vertical shaft, two stage, centrifugal type, as described in Chapter 6. These pumps require a specified minimum suction head to operate and, usually, some condensate level control system in the condenser. The first-stage impeller receives water which is almost boiling at the high vacuum conditions present in the suction pipe. The water is then discharged at a slight positive pressure to the second-stage impeller which provides the necessary system pressure at outlet. Where the condenser sump level is allowed to vary or maintained almost dry, a self-regulating extraction pump must be used. This regulation takes the form of cavitation which occurs when the suction head falls to a very low value. Cavitation is the formation and collapse of vapour bubbles which results in a fall in the pump discharge rate to zero. As the suction head improves the cavitation gradually ceases and the pump begins to discharge again. Cavitation is usually associated with damage (see Chapter 11 with reference to propellers) but at the low-pressure conditions in the pump no damage occurs. Also, the impeller may be designed so that the bubble collapse occurs away from the impeller, i.e. super-cavitating.

Answer 172

The de-aerator completes the air and vapour removal process begun in the condenser. It also functions as a feed heater, but in this case operates by direct contact. The feedwater is heated almost to the point of boiling, which releases all the dissolved gases which can then be vented off. One type of de-aerator is shown in Figure 5.9. The incoming feedwater passes through a number of spray valves or nozzles: the water spray thus provides a large surface area for contact with the heating system. Most of the feedwater will then fall onto the upper surface of the de-aerating cone where it is further heated by the incoming steam. The feedwater then enters the central passage and leaves through a narrow opening which acts as an eductor or ejector to draw steam through with the feed. The feedwater and condensed steam collect in the storage tank which forms the base of the de-aerator. The heating steam enters the de-aerator and circulates throughout, heating the feedwater and being condensed in its turn to combine with the feedwater. The released gases leave through a vent connection and pass to a vent condenser or devaporiser. Any water vapour will be condensed and returned. The devaporiser is circulated by the feedwater before it enters the de-aerator. The de-aerator feedwater is very close to the steam temperature at the same pressure and will, if subjected to any pressure drop, 'flash-off into steam. This can result in 'gassing', i.e. vapour forming in the feed pump suction. To avoid this problem, the de-aerator is mounted high up in the machinery space to give a positive suction head to the feed pumps. Alternatively a booster or extraction pump may be fitted at the de-aerator outlet.

Answer 173

The air ejector draws out the air and vapours which are released from the condensing steam in the condenser. If the air were not removed from the system it could cause corrosion problems in the boiler. Also, air present in the condenser would affect the condensing process and cause a back pressure in the condenser. The back pressure would increase the exhaust steam pressure and reduce the thermal efficiency of the plant. A two-stage twin-element air ejector is shown in Figure 5.7. In the first stage a steam-operated air ejector acts as a pump to draw in the air and vapours from the condenser. The mixture then passes into a condensing unit which is circulated by feedwater. The feedwater is heated and the steam and gases are mostly condensed. The condensed vapours and steam are returned to the main condenser via a drain and the remaining air and gases pass to the second stage where the process is repeated. Any remaining air and gases are released to the atmosphere via a vacuum-retaining valve. The feed water is circulated through U-tubes in each of the two stages. A pair of ejectors are fitted to each stage, although only one of each is required for satisfactory operation of the unit.

Answer 174

The gland steam condenser, drains cooler and low-pressure feed heater are all heat exchangers of the shell and tube type. Each is used in some particular way to recover heat from exhaust steam by heating the feedwater which is circulated through the units. The gland steam condenser collects steam, vapour and air from the turbine gland steam system. These returns are cooled by the circulating feed water and the steam is condensed. The condensate is returned to the system via a loop seal or some form of steam trap and any air present Is discharged into the atmosphere. The feedwater passes through U-tubes within the shell of the unit. The drains cooler receives the exhaust drains from various auxiliary services and condenses them: the condensate is returned to the feed system. The circulating feedwater passes through straight tubes arranged in tube plates in the drains cooler. Baffles or diaphragm plates are fitted to support the tubes and also direct the flow of the exhaust drains over the outside surface of the tubes. The low-pressure feed heater is supplied with steam usually bled from the low-pressure turbine casing. The circulating feed is heated to assist in the de-aeration process. The bleeding-off of steam from the turbine improves plant thermal efficiency as well as reducing turbine blade heights in the final rows because of the reduced mass of steam flowing. Either straight or U-tube construction may be used with single or multiple passes of feedwater.

Answer 175

The feed pump raises the feedwater to a pressure high enough for it to enter the boiler. For auxiliary boilers, where small amounts of feedwater are pumped, a steam-driven reciprocating positive displacement pump may be used. Another type of feed pump often used on package boiler installations is known as an 'electro feeder'. This is a multi-stage centrifugal pump driven by a constant speed electric motor. The number of stages is determined by the feed quantity and discharge pressure. Steam turbine-driven feed pumps are usual with high-pressure water tube boiler installations. The two-stage horizontal centrifugal pump is driven by an impulse turbine, the complete assembly being fitted into a common casing. The turbine is supplied with steam directly from the boiler and exhausts into a back-pressure line which can be used for feed heating. The pump bearings are lubricated by filtered water which is tapped off from the first-stage impeller. The feed discharge pressure is maintained by a governor, and overspeed protection trips are also provided

Answer 176

The high-pressure feed heater is a heat exchanger of the shell and tube type which further heats the feedwater before entry to the boiler. Further heat may be added to the feedwater without its becoming steam since its pressure has now been raised by the feed pump. The incoming feedwater circulates through U-tubes with the heating steam passing over the outside of the tubes. Diaphragm plates serve to support the tubes and direct the steam through the heater. A steam trap ensures that all the heating steam is condensed before it leaves the heater. Bled steam from the turbine will be used for heating

Answer 177

During operation the feed system must maintain a balance between feed input and steam output, together with a normal water level in the boiler. The condenser sea water boxes are protected by sacrificial mild steel plates which must be renewed regularly. The tube plates should be examined at the same time to ensure no erosion has taken place as a result of too high a circulating water speed. Any leaking tubes will cause feedwater contamination, and where this is suspected the condenser must be tested. Extraction pumps should be checked regularly to ensure that the sealing arrangements are preventing air from entering the system. It is usual with most types of glands to permit a slight leakage of water to ensure lubrication of the shaft and the gland. Air ejectors will operate inefficiently if the ejector nozzles are coated or eroded. They should be inspected and cleaned or replaced regularly. The vacuum retaining valve should be checked for air tightness and also the ejector casing. The various heat exchangers should be checked regularly for tube leakages and also the cleanliness of the heat-exchange surfaces. Turbo-feed pumps are started with the discharge valve closed in order to build up pressure rapidly and bring the hydraulic balance into operation. The turbine driving the pump will require warming through with the drains open before running up to speed and then closing the drains. The turbine overspeed trip should be checked regularly for correct operation and axial clearances should be measured, usually with a special gauge.

Answer 178

A boiler in one form or another will be found on every type of ship. Where the main machinery is steam powered, one or more large water tube boilers will be fitted to produce steam at very high temperatures and pressures. On a diesel main machinery vessel, a smaller (usually firetube type) boiler will be fitted to provide steam for the various ship services. Even within the two basic design types, water tube and firetube, a variety of designs and variations exist. A boiler is used to heat feed water in order to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water. A drum must be provided where steam and water can separate. There must also be a variety of fittings and controls to ensure that fuel oil, air and feedwater supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler. In the steam generation process the feedwater enters the boiler where it is heated and becomes steam. The feedwater circulates from the steam drum to the water drum and is heated in the process. Some of the feedwater passes through tubes surrounding the furnace, i.e. waterwall and floor tubes, where it is heated and returned to the steam drum. Large-bore downcomer tubes are used to circulate feedwater between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums. The steam is produced in a steam drum and may be drawn off for use from here. It is known as 'wet' or saturated steam in this condition because it will contain small quantities of water, Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and 'dried', i.e. all traces of water are converted into steam. This superheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. An 'attemperator', i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature. The hot gases produced in the furnace are used to heat the feedwater to produce steam and also to superheat the steam from the boiler drum. The gases then pass over an economiser through which the feedwater passes before it enters the boiler. The exhaust gases may also pass over an air heater which warms the combustion air before it enters the furnace. In this way a large proportion of the heat energy from the hot gases is used before they are exhausted from the funnel. The arrangement is shown in Figure 4.1.

Answer 179

Two basically different types of boiler exist, namely the water tube and the firetube. In the water tube the feedwater is passed through the tubes and the hot gases pass over them. In the firetube boiler the hot gases pass through the tubes and the feedwater surrounds them. The water tube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships. Firetube boilers The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. The name 'tank boiler* is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler* are also in use.

Answer 180

The construction of water tube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising process is much quicker. Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the water tube boiler has replaced the firetube boiler as the major steam producer.

Answer 181

This boiler has two drums, an integral furnace and is often referred to as the 'D' type because of its shape. The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These waterwall tubes are connected either to upper and lower headers or a lower header and the steam drum. Upper headers are connected by return tubes to the steam drum. Between the steam drum and the smaller water drum below, large numbers of smaller-diameter generating tubes are fitted. These provide the main heat transfer surfaces for steam generation. Large-bore pipes or downcomers are fitted between the steam and water drum to ensure good natural circulation of the water. In the arrangement shown, the superheater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brickwork is used on the furnace floor, the burner wall and also behind the waterwalls. The double casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner, The need for a wider range of superheated steam temperature control led to other boiler arrangements being used. The original External Superheater 'D' (ESD) type of boiler used a primary and secondary superheater located after the main generating tube bank (Figure 4.3). An attemperator located in the combustion air path was used to control the steam temperature. The later ESD II type boiler was similar in construction to the ESD I but used a control unit (an additional economiser) between the primary and secondary superheaters. Linked dampers directed the hot gases over the control unit or the superheater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low temperature superheating was required. In the ESD III boiler the burners are located in the furnace roof, which provides a long flame path and even heat transfer throughout the furnace. In the boiler shown in Figure 4.4, the furnace is fully water-cooled and of monowali construction, which is produced from finned tubes welded together to form a gaslight casing. With monowali construction no refractory material is necessary in the furnace. The furnace side, floor and roof tubes are welded into the steam and water drums. The front and rear walls are connected at either end to upper and lower water-wall headers. The lower water-wall headers are connected by external downcomers from the steam drum and the upper water-wall headers are connected to the steam drum by riser tubes. The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over the primary superheater and the economiser before passing to exhaust. The dry pipe is located in the steam drum to obtain reasonably dry saturated steam from the boiler. This is then passed to the primary superheater and then to the secondary superheater. Steam temperature control is achieved by the use of an attemperator, located in the steam drum, operating between the primary and secondary superheaters. Radiant-type boilers are a more recent development, in which the radiant heat of combustion is absorbed to raise steam, being transmitted by infra-red radiation. This usually requires roof firing and a considerable height in order to function efficiently. Both the furnace and the outer chamber are fully water cooled. There is no conventional bank of generating tubes. The hot gases leave the furnace through an opening at the lower end of the screen wall and pass to the outer chamber. The outer chamber contains the convection heating surfaces which include the primary and secondary superheaters. Superheat temperature control is by means of an attemperator in the steam drum. The hot gases, after leaving the primary superheater, pass over a steaming economises This is a heat exchanger in which the steam—water mixture is flowing parallel to the gas. The furnace gases finally pass over a conventional economiser on their way to the funnel.

Answer 182

The problems associated with furnace refractory materials, particularly on vertical walls, have resulted in two water-wall arrangements without exposed refractory. These are known as 'tangent tube' and 'monowall' or 'membrane wall'. In the tangent tube arrangement closely pitched tubes are backed by refractory, insulation and the boiler casing. In the monowall or membrane wall arrangement the tubes have a steel strip welded between them to form a completely gas-tight enclosure. The monowall construction eliminates the problems of refractory and expanded joints. However, in the event of tube failure, a welded repair must be carried out. Alternatively the tube can be plugged at either end, but refractory material must be placed over the failed tube to protect the insulation behind it. With tangent tube construction a failed tube can be plugged and the boiler operated normally without further attention.

Answer 183

Most firetube boilers are now supplied as a completely packaged unit. This will include the oil burner, fuel pump, forced-draught fan, feed pumps and automatic controls for the system. The boiler will be fitted with all the appropriate boiler mountings. There is no combustion chamber refractory lining other than a lining to the combustion chamber access door and the primary and secondary quart. Fully automatic controls are provided and located in a control panel at the side of the boiler.

Answer 184

The modern vertical Cochran boiler has a fully spherical furnace and is known as the 'spheroid' (Figure 4.8). The furnace is surrounded by water and therefore requires no refractory lining. The hot gases make a single pass through the horizontal tube bank before passing away to exhaust. The use of small-bore tubes fitted with retarders ensures better heat transfer and cleaner tubes as a result of the turbulent gas flow.

Answer 185

other steam raising equipment is in use, e.g. the steam-to-steam generator, the double evaporation boiler and various exhaust gas boiler arrangements. The steam-to-steam generator Steam-to-steam generators produce low-pressure saturated steam for domestic and other services. They are used in conjunction with water tube boilers to provide a secondary steam circuit which avoids any possible contamination of the primary-circuit feedwater. The arrangement may be horizontal or vertical with coils within the shell which heat the feedwater. The coils are supplied with high-pressure, high temperature steam from the main boiler. A horizontal steam-to-steam generator is shown in Figure 4.9.

Answer 186

A double evaporation boiler uses two independent systems for steam generation and therefore avoids any contamination between the primary and secondary feedwater. The primary circuit is in effect a conventional water tube boiler which provides steam to the heating coils of a steam-to-steam generator, which is the secondary system. The complete boiler is enclosed in a pressurised casing.

Answer 187

An exhaust gas heat exchanger is shown in Figure 4.10. It is simply a row of tube banks circulated by feedwater over which the exhaust gases flow. Individual banks may be arranged to provide feed heating, steam generation and superheating. A boiler drum is required for steam generation and separation to take place and use is usually made of the drum of an auxiliary boiler. Exhaust gas boilers recover the heat from the exhaust gas of auxiliary diesel engines to generate steam and/or hot water, or useful heat for process heating. Depending on system design, these boilers can enhance the efficiency of the auxiliary engine system by up to 20%, leading to lower overall process costs. With this arrangement the boiler acts as the heat exchanger and raises steam in its own drum.

Answer 188

The water level gauge provides a visible indication of the water level in the boiler in the region of the correct working level. If the water level were too high then water might pass out of the boiler and do serious damage to any equipment designed to accept steam. If the water level were too low then the heat transfer surfaces might become exposed to excessive temperatures and fail. Constant attention to the boiler water level is therefore essential. Due to the motion of the ship it is necessary to have a water level gauge at each end of the boiler to correctly observe the level. Depending upon the boiler operating pressure, one of two basically different types of water level gauge will be fitted. For boiler pressures up to a maximum of 17 bar a round glass tube type of water level gauge is used. The glass tube is connected to the boiler shell by cocks and pipes.. Packing rings are positioned at the tube ends to give a tight seal and prevent leaks. A guard is usually placed around the tube to protect it from accidental damage and to avoid injury to any personnel in the vicinity if the tube shatters. The water level gauge is usually connected directly to the boiler. Isolating cocks are fitted in the steam and water passages, and a drain cock is also present. A ball valve is fitted below the tube to shut off the water should the tube break and water attempt to rush out. For boiler pressures above 17 bar a plate-glass-type water level gauge is used. The glass tube is replaced by an assembly made up of glass plates within a metal housing. The assembly is made up as a 'sandwich' of front and back metal plates with the glass plates and a centre metal plate between. Joints are placed between the glass and the metal plate and a mica sheet placed over the glass surface facing the water and steam. The mica sheet is an effective insulation to prevent the glass breaking at the very high temperature. When bolting up this assembly, care must be taken to ensure even all-round tightening of the bolts. Failure to do this will result in a leaking assembly and possibly shattered glass plates. In addition to the direct-reading water level gauges, remote-reading level indicators are usually led to machinery control rooms. It is possible for the small water or steam passages to block with scale or dirt and the gauge will give an incorrect reading. To check that passages are dear a 'blowing through' procedure should be followed.

Answer 189

Safety valves are fitted in pairs, usually on a single valve chest. Each valve must be able to release all the steam the boiler can produce without the pressure rising by more than 10% over a set period. Spring-loaded valves are always fitted on board ship because of their positive action at any inclination. They are positioned on the boiler drum in the steam space. The valve is held closed by the helical spring pressure, once set, is fixed and sealed by a Surveyor. When the steam exceeds this pressure the valve is opened and the spring compressed. The escaping steam is then led through a waste pipe up the funnel and out to atmosphere. The compression of the spring by the initial valve opening results in more pressure being necessary to compress the spring and open the valve further. To some extent this is countered by a lip arrangement on the valve lid which gives a greater area for the steam to act on once the valve is open. A manually operated easing gear enables the valve to be opened in an emergency. Various refinements to the ordinary spring-loaded safety valve have been designed to give a higher lift to the valve. The improved high-lift safety valve has a modified arrangement around the lower spring carrier. The lower spring carrier is arranged as a piston for the steam to act on its underside. A loose ring around the piston acts as a containing cylinder for the steam. Steam ports or access holes are provided in the guide plate. Waste steam released as the valve opens acts on the piston underside to give increased force against the spring, causing the valve to open further. Once the overpressure has been relieved, the spring force will quickly close the valve. The valve seats are usually shaped to trap some steam to 'cushion' the closing of the valve. A drain pipe is fitted on the outlet side of the safety valve to remove any condensed steam which might otherwise collect above the valve and stop it opening at the correct pressure.

Answer 190

Combustion is the burning of fuel in air in order to release heat energy. For complete and efficient combustion the correct quantities of fuel and air must be supplied to the furnace and ignited. About 14 times as much air as fuel is required for complete combustion. The air and fuel must be intimately mixed and a small percentage of excess air is usually supplied to ensure that all the fuel is burnt. When the air supply is insufficient the fuel is not completely burnt and black exhaust gases will result.

Answer 191

The flow of air through a boiler furnace is known as 'draught'. Marine boilers are arranged for forced draught, i.e. fans which force the air through the furnace. Several arrangements of forced draught are possible. The usual forced draught arrangement is a large fan which supplies air along ducting to the furnace front. The furnace front has an enclosed box arrangement, known as an 'air register', which can control the air supply. The air ducting normally passes through the boiler exhaust where some air heating can take place. The induced draught arrangement has a fan in the exhaust uptake which draws the air through the furnace. The balanced draught arrangement has matched forced draught and induced draught fans which results in atmospheric pressure in the furnace.

Answer 192

Marine boilers currently burn residual low-grade fuels. This fuel is stored in double-bottom tanks from which it is drawn by a transfer pump up to settling tanks (Figure 4.15). Here any water in the fuel may settle out and be drained away. The oil from the settling tank is filtered and pumped to a heater and then through a fine filter. Heating the oil reduces its viscosity and makes it easier to pump and filter. This heating must be carefully controlled otherwise 'cracking' or breakdown of the fuel may take place. A supply of diesel fuel may be available to the burners for initial firing or low-power operation of the boiler. From the fine filter the oil passes to the burner where it is 'atomised', i.e. broken into tiny droplets, as it enters the furnace. A recirculating line is provided to enable initial heating of the oil.

Answer 193

The high-pressure fuel is supplied to a burner which it leaves as an atomised spray (Figure 4.16). The burner also rotates the fuel droplets by the use of a swirl plate. A rotating cone of tiny oil droplets thus leaves the burner and passes into the furnace. Various designs of burner exist, the one just described being known as a 'pressure jet burner' (Figure 4.16(a». The 'rotating cup burner' (Figure 4.14(b)) atomises and swirls the fuel by throwing it off the edge of a rotating tapered cup. The 'steam blast jet burner', shown in Figure 4.16(c), atomises and swirls the fuel by spraying it into a high-velocity jet of steam. The steam is supplied down a central inner barrel in the burner. The air register is a collection of flaps, vanes, etc., which surrounds each burner and is fitted between the boiler casings. The register provides an entry section through which air is admitted from the wind box. Air shut-off is achieved by means of a sliding sleeve or check. Air flows through parallel to the burner, and a swirler provides it with a rotating motion. The air is swirled in an opposite direction to the fuel to ensure adequate mixing (Figure 4.17(a)). High-pressure, marine water tube boilers are roof fired (Figure 4.17(b)). This enables a long flame path and even heat transfer throughout the furnace.

Answer 194

Modern high-pressure, high-temperature boilers with their large steam output require very pure feedwater. Most 'pure* water will contain some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer, which can result in local overheating and failure of the tubes. Other salts remain in solution and may produce acids which will attack the metal of the boiler. An excess of alkaline salts in a boiler, together with the effects of operating stresses, will produce a condition known as 'caustic cracking'. This is actual cracking of the metal which may lead to serious failure. The presence of dissolved oxygen and carbon dioxide in boiler feedwater can cause considerable corrosion of the boiler and feed systems. When boiler water is contaminated by suspended matter, an excess of salts or oil then 'foaming' may occur. This is a foam or froth which collects on the water surface in the boiler drum. Foaming leads to 'priming' which is the carry-over of water with the steam leaving the boiler drum. Any water present in the steam entering a turbine will do considerable damage.

Answer 195

Various amounts of different salts can be found in feedwater. These include the chlorides, sulphates and bicarbonates of calcium, magnesium and, to some extent, sulphur. These dissolved salts in water make up what is called the 'hardness' of the water. Calcium and magnesium salts are the main causes of hardness, The bicarbonates of calcium arid magnesium are decomposed by heat and come out of solution as scale-forming carbonates. These alkaline salts are known as 'temporary hardness'. The chlorides, sulphates and nitrates are not decomposed by boiling and are known as 'permanent hardness’. Total hardness is the sum of temporary and permanent hardness and gives a measure of the scale-forming salts present in the boiler feedwater.

Answer 196

This is achieved as follows: 1.By keeping the hardness salts in a suspension in the solution to prevent scale formation. 2.By stopping any suspended salts and impurities from sticking to the heat transfer surfaces. 3.By providing anti-foam protection to stop water carry-over. 4.By eliminating dissolved gases and providing some degree of alkalinity which will prevent corrosion. The actual treatment involves adding various chemicals into the feedwater system and then testing samples of boiler water with a test kit. The test kit is usually supplied by the treatment chemical manufacturer with simple instructions for its use. For auxiliary boilers the chemicals added might be lime (calcium hydroxide) and soda (sodium carbonate). Alternatively caustic soda (sodium hydroxide) may be used on its own. For high-pressure water tube boilers various phosphate salts are used, such as trisodium phosphate, disodium phosphate and sodium metaphosphate. Coagulants are also used which combine the scale forming salts into a sludge and stop it sticking to the boiler surfaces. Sodium aluminate, starch and tannin are used as coagulants. Final de-aeration of the boiler water is achieved by chemicals, such as hydrazine, which combine with any oxygen present.

Answer 197

The uptakes should be checked to ensure a clear path for the exhaust gases through the boiler; any dampers should be operated and then correctly positioned. All vents, alarm, water and pressure gauge connections should be opened. The superheater circulating valves or drains should be opened to ensure a flow of steam through the superheater. All the other boiler drains and blow-down valves should be checked to ensure that they are closed. The boiler should then be filled to slightly below the working level with hot de-aerated water. The various header vents should be closed as water is seen to flow from them. The economiser should be checked to ensure that it is full of water and all air vented off. The operation of the forced draught fan should be checked and where exhaust gas air heaters are fitted they should be bypassed. The fuel oil system should be checked for the correct positioning of valves, etc. The fuel oil should then be circulated and heated.

Answer 198

The forced draught fan should be started and air passed through the furnace for several minutes to 'purge' it of any exhaust gas or oil vapours. The air slides (checks) at every register, except the lighting up' burner, should then be closed. The operating burner can now be lit and adjusted to provide a low firing rate with good combustion. The fuel oil pressure and forced draught pressure should be matched to ensure good combustion with a full steady flame. The superheater header vents may be closed once steam issues from them. When a drum pressure of about 210kPa (2.1 bar) has been reached the drum air vent may be closed. The boiler must be brought slowly up to working pressure in order to ensure gradual expansion and to avoid overheating the superheater elements and damaging any refractory material. Boiler manufacturers usually provide a steam raising diagram in the form of a graph of drum pressure against hours after flashing up. The main and auxiliary steam lines should now be warmed through and then the drains closed. In addition the water level gauges should be blown through and checked for correct reading. When the steam pressure is about 300 kPa (3 bar) below the normal operating value the safety valves should be lifted and released using the easing gear. Once at operating pressure the boiler may be put on load and the superheater circulating valves closed. All other vents, drains and bypasses should then be closed. The water level in the boiler should be carefully checked and the automatic water regulating arrangements observed for correct operation

Answer 199

When a boiler fire causes the metal itself to burn at about 700 °C, if steam smothering soot blowing system or water jetting system have been attempted, hydrogen fire can occur. This is because the applied steam dissociates (breaks into smaller particles) into hydrogen and oxygen and accelerates the fire. Once such a fire has started, resulting two kind of fires may take place simultaneously, one kind, iron burning in steam, and the other, the hydrogen burning in an air exothermic (energy being released in the form of heat) way. This combined fire is self supporting and lasts until the supply of steam is exhausted or the metal stop burning. The primary objective of dealing with the fire is to cool the surface and burning material as quickly as possible and stop supply of steam.

Answer 200

A flame appear at the economizer coil during running of main engine at sea while the EGE is put into service with circulation water passing through the coils, it is called economizer fire. It is actually cause by soot fire at the economizer coils. For the economizer coils fire, heat is already presented due to passing the gases of main engine. Air is available due to excess supply of scavenging air into the unit combustion chamber. Thick deposit of unburnt fuel, carbon residue (soot) are sticking at the economizer coils . Soot formed from incomplete combustion and use of low grade fuel and high carbon content fuel. At the manoeuvring time, the more incomplete combustion may occur and at that time leaving flue gas velocity is very low, thus unburnt fuel can be adhering on the economizer coils. A flame is produced when air and fuel are proportionally mixed in the sufficient heating temperature.

Answer 201

1.smoke smell from the economizer. 2.Over heat at economizer body ( external casing of uptake.) 3.Heavy smoke and sparks will emit from the funnel. 4.Sudden unexpected increase in uptake gas temperature ( Abnormally high stack pyrometer reading) 5.Flame visible in the smoke indicator. 6. High and rising exhaust gas temps and Black exhaust smoke or sparks from funnel 7. Discolouration and blistering of paint is an indicator that heat being generated possibly due to a large fire 8. Safety valves lifting indicating high pressure

Answer 202

1.Slow down the engine. 2.Shut off oil burners, draught fans, dampers & air register. 3.Raise water level full and blow down continuously so as to maintain good flow of water. 4.Reduce boiler pressure by easing gear. 5.Spray water on the external casing of the uptake to cool the affected area. 6.A few times starting and stopping of main engine should be done to blow out collected soot at the uptake.

Answer 203

1.Cool down to ambient temperature. 2.Open the economizer cover. 3.Clean inside parts of the economizer. 4.Check the any defective coil and repair for temporary used by welding or plugging the tubes.

Answer 204

1.proper complete combustion of fuel, maintain the main engine (ME) in optimum combustion condition. ( governor, fuel injection pump, fuel injection valve, fuel injection timing, fuel condition, air cooler, turbocharger, do not run too long ME with slow speed) 2.Regular open up & cleaning smoke side of economiser depending upon soot accumulation. (water washing) 3.Regular soot blowing operation. Check & maintain soot blowing equipment. 4.Regular overhaul boiler burner, correct air fuel ratio & damper.

Answer 205

a.) overtime an economizer shell and tubes get dirty and blocked from carbon soot accumulation which is a by product from economizer being operated. Soot blowing should therefore be conducted on a daily basis using steam lances or compressed air. Approximately once a month a thorough water wash should be done to remove the rest of the build up of soot particles and unburnt fuel. if this isn't done economizer will get blocked from carbon soot accumulation and heat transfer and overall efficiency will be reduced. This accumulation will also begin to increase the exhaust gas temperature and steam pressure will begin to drop. this will then lead to a small economizer fire which if rises above 1000 degrees tubes will burst and hydrogen will be produced from the steam. This will raise temperature further to about 1100 - 1200 classed as a large fire. b.) its important to maintain the exhaust gas temperature above the dewpoint because exhaust gas has about 10% water vapour and sulphur products. sulphur acid has a dewpoint of 140°C so if exhaust gas temperature drops below this value then water and sulphur vapour will condense into a liquid forming sulphuric acid, leading to acidic corrosion in exhaust space and therefore attacking exhaust components due to wash down. The gas temperature outlet is also kept above 180°C to prevent low temperature corrosion occurring.

Answer 206

* Raise alarm, call the bridge and Chief Engineer, request engine slowdown and stop when navigationally safe to do so * Initiate fixed smothering fire extinguishing system if installed and monitor temperatures from ECR. * If there is any indication that a hydrogen fire may take place, fixed steam/water firefighting systems should be stopped otherwise your providing fuel for the fire. an indicator could be exhaust gas temperatures of 1100 to 1200 degrees Celsius * When support arrives, fire party should prepare the hoses and line up firefighting system so that boundary cooling can be used. an engineering officer should examine economizer casing to locate hotspots * fire party then should use Boundary cooling to cool any hotpots on the inside of casing of EGE but use only minimal water by doing short bursts from hose to prevent stability issues. Beware of running water entering the ER deck due to possible electrical shorts and stability issues. * Once engine is stopped cover Turbocharger air intake with Cover to stop supply of oxygen to economizer which would feed the fire. * Then open exhaust drains and turbocharger drains to prevent water entering cylinder liners * sudden drop in boiler water level is and indicator that may indicate water tubes are damaged. Therefore Stop circulating pumps, close the isolating valves and use easing gear to hold open safety valves and relieve pressure.

Answer 207

1.By regulating the amount of gas flowing over the extended surface, i.e. by damper regulation. 2.By dividing the unit into sections so that each section is controlled by an inlet valve. 3.By passing steam through an automatic pressure controlled surplus valve to a dump condenser. 4.By a system designed for a higher pressure than that it is to operate.

Answer 208

In this system a separate exhaust gas economizer is connected to an oil fired auxiliary boiler or an accumulator by means of piping and a set of circulation pump. Exhaust gas is used as heating medium and it has no steam space.

Answer 209

If an exhaust gas economiser has not been sufficiently cleaned to allow a proper examination of pressure parts, the survey cannot be regarded as complete until this has been done. Where the construction of an exhaust gas economiser does not allow direct visual internal examination of the headers, the attending Surveyor(s) are to be satisfied that the exhaust gas economiser is in a safe working condition by resorting to remote viewing instruments, Ultrasonic Testing (UT), or hydraulic testing to determine a safe working pressure. The mountings (including all valves, fittings, burner units, drains and filling valves) are to be opened up for internal examination and rectification as necessary. The attending Surveyor(s) are to ensure that all mountings are securely attached and are to have insulation cut away where this prevents full examination of studs and nuts. Stub piece welds are to be examined in way of attachment to shell, shell plating in way of nozzles penetrating the shell (feed, etc.) and pads attached to the shell should be carefully examined for cracks emanating from the welds. It is important that all pressure gauge connections should be free from obstructions and their shut-off fittings clearly and correctly marked to indicate when they are in the open position. A thorough external examination of exhaust gas economisers should be undertaken. It should be verified that exhaust gas economiser supports, rolling stays and uptakes are in satisfactory condition. Rolling stays, expansion/sliding feet arrangements and freedom of pins and holding down bolts are to be specially examined in way of attachments to the shell or casings and freedom of pins is to be verified.

Answer 210

see motor sketch pack figure 3.1 for drawing The steam turbine is a device for obtaining mechanical work from the energy stored in steam. Steam enters the turbine with a high energy content and leaves after giving up most of it. The high-pressure steam from the boiler is expanded in nozzles to create a high-velocity jet of steam. The nozzle acts to convert heat energy in the steam into kinetic energy. This jet is directed into blades mounted on the periphery of a wheel or disc (Figure 3.1). The steam does not 'blow the wheel around'. The shaping of the blades causes a change in direction and hence velocity of the steam jet. Now a change in velocity for a given mass flow of steam will produce a force which acts to turn the turbine wheel. This is the operating principle of all steam turbines, although the arrangements may vary considerably. The steam from the first set of blades then passes to another set of nozzles and then blades and so on along the rotor shaft until it is finally exhausted. Each set comprising nozzle and blades is called a stage. Its advantages of little or no vibration, low weight, minimal space requirements and low maintenance costs are considerable. Furthermore a turbine can be provided for any power rating likely to be required for marine propulsion. However, the higher specific fuel consumption when compared with a diesel engine offsets these advantages, although refinements such as reheat have narrowed the gap.

Answer 211

There are two main types of turbine, the 'impulse' and the 'reaction'. The names refer to the type of force which acts on the blades to turn the turbine wheel.

Answer 212

see motor sketch pack figure 3.2 and 3.3 for drawings Impulse The impulse arrangement is made up of a ring of nozzles followed by a ring of blades. The high-pressure, high-energy steam is expanded in the nozzle to a lower-pressure, high-velocity jet of steam. This jet of steam is directed into the impulse blades and leaves in a different direction (Figure 3.2). The changing direction and therefore velocity produces an impulsive force which mainly acts in the direction of rotation of the turbine blades. There is only a very small end thrust on the turbine shaft. Reaction The reaction arrangement is made up of a ring of fixed blades attached to the casing, and a row of similar blades mounted on the rotor, i.e. moving blades (Figure 3.3). The blades are mounted and shaped to produce a narrowing passage which, like a nozzle, increases the steam velocity. This increase in velocity over the blade produces a reaction force which has components in the direction of blade rotation and also along the turbine axis. There is also a change in velocity of the steam as a result of a change in direction and an impulsive force is also produced with this type of blading. The more correct term for this blade arrangement is 'impulse-reaction'.

Answer 213

Compounding Compounding is the splitting up, into two or more stages, of the steam pressure or velocity change through a turbine. Pressure compounding of an impulse turbine is the use of a number of stages of nozzle and blade to reduce progressively the steam pressure. This results in lower or more acceptable steam flow speeds and a better turbine efficiency. Velocity compounding of an impulse turbine is the use of a single nozzle with an arrangement of several moving blades on a single disc. Between the moving blades are fitted guide blades which are connected to the turbine casing. This arrangement produces a short lightweight turbine with a poorer efficiency which would be acceptable in, for example, an astern turbine. The two arrangements may be combined to give what is called 'pressure-velocity compounding'. The reaction turbine as a result of its blade arrangement changes the steam velocity in both fixed and moving blades with consequent gradual steam pressure reduction. Its basic arrangement therefore provides compounding. Reheat Reheating is a means of improving the thermal efficiency of the complete turbine plant. Steam, after expansion in the high-pressure turbine, is returned to the boiler to be reheated to the original superheat temperature. It is then returned to the turbine and further expanded through any remaining stages of the high-pressure turbine and then the low-pressure turbine

Answer 214

The amount of heat actually recovered from the exhaust gases depends upon various factors such as steam pressure, temperature, evaporation rate required, mass flow of gas, condition of heating surfaces. Waste heat boilers can recover up to about 60% of the loss to atmosphere in exhaust gases.

Answer 215

During low engine power conditions or when the main engine is not in use the boiler has to combust fuel to provide the heat source. It is therefore appropriate to repeat some very general remarks on combustion with details of the typical boiler equipment in use on-board ship. Good combustion is essential for the efficient running of the boiler as it gives the best possible heat release and the minimum amount of deposits upon the heating surfaces. To ascertain if the combustion is good we measure the % CO2 content (and in some installations the % O2 content) and observe the appearance of the gases.If the % CO2 content is high (or the % O2 content is low) and the gases are in a non-smokey condition then the combustion of the fuel is correct. With a high % CO2 content the % excess air required for combustion will be low and this results in improved boiler efficiency since less heat is taken from the burning fuel by the small amount of excess air. If the excess air supply is increased then the % CO2 content of the gases will fall. Condition of burners, oil condition pressure and temperature, condition of air registers, air supply pressure and temperature are all factors which can influence combustion.

Answer 216

There are two basic types of burners, the pressure jet and the rotary cup. The pressure jet as its name suggests relies on the fuel oil supply pressure to force the fuel through a series of small nozzles in the end of a long tube. The holes are set at an angle and will therefore give a spin to the fuel as it exits from the burner. This spin or swirl gives the fuel the right action to mix thoroughly with the air delivered by the air register and therefore when the mixture hits the flame front it is ignited. The rotary or spinning cup type of burner does not rely on the fuel pressure to give atomisation. The low-pressure fuel oil is released into the centre of the rotary cup that is spinning at about 5,000 rev/min. As the fuel follows the conical cup it eventually comes to the rim where centrifugal force makes it fly from the edge into the path of the primary air flow. The air hits the fuel atomising it as it does so. The primary and secondary air provides the oxygen for combustion as well as the shape of the flame propagation. If any of these components are dirty or the sprayer plates damaged then effective atomisation will not be achieved, resulting in poor combustion.

Answer 217

Auxiliary boilers on modern ships are usually fire tube boilers operating with a working steam pressure of about 7 bar. This is enough pressure to supply all the necessary heating required on-board the vessel. However, because the fire tube boiler has a relatively large amount of water, for the size of boiler, it also has a greater potential for causing a lot of damage if there was a structural failure. It must be remembered that if the steam is at 7 bar pressure then any parts in contact with the steam are also at 7 bar pressure. If the pressure on the water within the boiler was suddenly reduced to atmospheric pressure, due to some form of structural failure, then the water would flash off into steam. Steam requires 1,600 times the volume of water; therefore, when the pressure is released and the water flashes into steam a considerable force is released and large sections of the boiler can be moved at considerable speed. One of the most important dangers to guard against is loss of water. The metal furnace close to the burner relies upon the cooling effect of water on the other side of the furnace to ensure that it does not overheat and fail. There are a number of reasons for a loss of feed water and the motor engineer will need to understand his/her system and be able to explain to the examiner how to guard against a loss of water in the auxiliary boiler. The gauge glass is the primary source of information about the water level in the boiler. There are always two gauge glasses just in case one becomes blocked, however on a marine boiler they are situated on opposite sides of the boiler. The reason for this is that if the vessel is rolling and one glass is empty then the other should be showing a high level and vice versa. Fire is another major concern with boilers. Fuel oil is led to the boiler where it is burnt after passing through the burner and associated filters and pumps. As the burners are sometimes temperamental in their operation they have to be opened, cleaned and adjusted from time to time. Oil builds up around the furnace front and can be the cause of a fire if engineers are not careful. The watchkeeper will be responsible for the safe operation of the auxiliary boiler during his/her watchkeeping duty period; therefore, it is essential that she/he understands the following safety-related start-up and operating procedure. Safe start-up procedure involves a purge cycle. This means that the boiler will run the forced draught fan for a few seconds before trying to light the boiler. This is to ensure that any unburnt hydrocarbons from the previous cycle are taken away from the burner that will be lighting up soon. With the air operating correctly the next step is to introduce the heat source. This is usually in the form of a high voltage passing from one electrode to another via an air gap (a bit like the spark in a car). When the heat and the oxygen are both in place it will then be okay to introduce the fuel and start the burn. Feedback that to the boiler controls saying that the boiler is alight is provided by a photoelectric cell. Using this sequence there is very little chance of unburnt gasses accumulating in the furnace and causing a violent start-up or an explosion. The watchkeeping engineer could be called upon to start a boiler that has ‘locked out’. It would only do this if it had failed to light for some reason. It is very important that the watchkeeper carries out some basic checks before trying to relight the boiler. The first and most important check is to look at the water level in the boiler and make sure that the boiler has not ‘locked out’ due to low water level. Make sure that you look at both gauge glasses and if you are unsure you need to follow the gauge glass check procedure described in Chapter 3 of Volume 8 in this book series. If a low water level is suspected then it is very important that you DO NOT start the burner until the correct level is restored. The problems here could range from failure of the feed water pump to deliver a sufficient quantity of water to a malfunction of the feed water control float that dictates when and how much water is sent to the boiler. The feed water pump operates under difficult conditions because not only does it have to pump water that is close to its boiling condition but it also has to pump it at sufficient pressure to overcome the boiler’s working pressure. Sometime these pumps 'gas up’, in other words, the water’s vaporisation condition is met and it starts to turn to steam inside the pump stopping the flow of water. The other, not uncommon, problem is with the float sticking in the feed water controller. I have seen some of these corroded so much that the float has broken away from the activating arm and therefore was not capable of working. If the water level has been the problem then the boiler can be started again following the restoration of the feed water to the boiler. The watchkeeper should monitor the boiler as it works through its safety purge cycle and then the starting sequence described earlier. Some boilers are fitted with a burner viewing port which is to be treated with great respect. These should never be used when the boiler is starting up as serious injury has been caused in the past due to ‘blow back’ as some unburnt fuel has caught alight. A malfunction in the burner starting sequence is another reason for the boiler’ locking out’. If the boiler fails to light – some control systems may allow two cycles before giving an alarm – then the watchkeeper will need to identify the reason and rectify the fault. If the ‘lock out’ is due to the burner then there are three conditions to check: 1.Boiler fan working to give the correct amount of air to the burner 2.Igniter working and in the correct position 3.Fuel supply to the burner nozzles or spinning cup. Conditions 1 and 3 are relatively easy to check because the fan not working or the air register blocked will be easy to spot as will be the lack of fuel. Igniter faults and subsequent set-up however is generally more difficult to deal with but is a more common fault than the other two. Sometimes there could be quite a bit of pressure to get the boiler back online for example. If the heavy fuel oil to the generator was cooling down due to lack of steam there might not be much time in which to have the boiler up and running again or to change the generator over to a light distillate fuel. The problem with the igniter is that it has to sit right in the most turbulent area of the air/fuel mix and the forces involved are enough to knock it out of alignment occasionally and therefore it stops working. It has to be reset to the correct position which is just where the air/fuel ratio is correct for combustion. If it is placed too close to the burner the mixture is too rich for ignition and if it is placed too far away the mixture could be too lean or it might miss the spray of fuel entering the combustion space. Then, the burner will cut-out due to the length of time trying to light the fuel. Generally the engineers will know the settings and be able to work away at getting the boiler going again but late at night, by yourself and under pressure makes the job much more difficult.

Answer 218

Sunrod vertical boiler This boiler utilises a water-cooled furnace incorporating membrane-walled construction. The membrane water wall is backed by low temperature insulation (figure 10.2a). The water wall tubes are joined at the lower end to a circular header and at their upper ends to the steam chamber. Good circulation is assured by the arrangement of a number of downcomers as shown in the diagram. The steam chamber has a number of smoke tubes each fitted with a ‘Sunrod element’. The purpose of the Sunrod element is to increase the heating surface area of the boiler. This is accomplished by welding pins onto the element as shown in figure 10.2b. In some Sunrod designs the firetube is also water-cooled. This design is manufactured in sizes ranging from 700 kg/h to 35,000 kg/h with pressures up to 18 bar. The boiler is usually fitted with automatic start up/shut down and combustion control. Due to the absence of furnace refractory lining this type of boiler is extremely robust and easy to operate. Cleaning the boiler is also relatively easy and is accomplished, when the boiler is shut down, by simply removing the cleaning doors, opening the drain and spraying with high-pressure fresh water. Pressure control of the steam is accomplished by flashing the boiler when pressure drops below a pre-set level during periods of high steam load and, dumping steam to the condenser when the pressure rises due to low steam load. Vapour vertical boiler (coiled-tube) Figure 10.3 shows in a simplified diagrammatic form a coiled-tube boiler of the stone-vapour type. It is compact, space saving, designed for UMS operation, and is supplied ready for connecting to the ships services. A power supply, depicted here by a motor, is required for the feed pump, fuel pump (if fitted), fan and controls. Feed water is force circulated through the generation coil wherein about 90% is evaporated. The un-evaporated water travelling at high velocity carries sludge and scale into the separator, which can be blown out at intervals manually or automatically. Steam at about 99% dry is taken from the separator for shipboard use. The boiler is completely automatic in operation. If, for example, the steam demand is increased, the pressure drop in the separator is sensed and a signal, transmitted to the feed controller, demands increased feed, which in turn increases air and fuel supply. With such a small water content explosion due to coil failure is virtually impossible and a steam temperature limit control protects the coil against abnormally high temperatures. In addition the servo-fuel control protects the boiler in the event of failure of water supply.

Answer 219

Optimum pressure This depends on the system adopted but in general the range is from 6 bar to 11 bar. The lower pressures give a cheaper unit with near maximum heat recovery. However, higher pressures allow more flexibility in supply with perhaps more useful steam for certain auxiliary functions together with reserve steam capacity to meet variations in demand. Low feed inlet temperatures reduce pressure and evaporative rate. Temperature A minimum temperature differential obviously applies for heat transfer. Temperature difference, fouling, gas velocity, gas distribution, metal surface resistance, etc., are all important factors. Reduction in service engine revolutions will cause reduced gas mass and temperature increase if the power is maintained constant. A similar effect will be apparent under operation in tropical conditions. The effect of increased back pressure will be to raise the gas temperature for a given air inlet temperature. Figure 10.6 illustrates: (a) typical heat transfer diagram, and (b) gas temperature/mass–power curves. A common temperature differential is about 40°C, that is, water inlet 120°C and gas exit 160°C. Corrosion The acid dew point expected is about 110°C with a 3% sulphur fuel and a high rate of conversion from SO2 to SO3 is possible. Minimum metal temperatures of 120°C for mild steel are required. Exhaust system The arrangement must offer unrestricted flow for gases so that back pressure is not increased. Good access is required for inspection and cleaning. On designs with alternate gas-oil firing provision must be made for quick and fool proof change-over with no possibility of closure to atmosphere and waste heat system at the same time

Answer 220

Simple These boilers are not very common as they operate on waste heat only. Single- or two-pass types are available, the latter being the most efficient. Small units of this type have been fitted to auxiliary oil engine exhaust systems, operating mainly as economisers, in conjunction with another boiler. A gas change valve to direct flow to the boiler or atmosphere is usually fitted as described below. Alternate This type is a compromise between the other two. It is arranged to give alternate gas and oil firing with either single- or double-pass gas flow. It is particularly important to arrange the piping system so that oil fuel firing is prevented when exhaust gas is passing through the boiler. A large butterfly type of change-over valve is fitted before the boiler so as to direct exhaust gas to the boiler or to the atmosphere. The valve is so arranged that gas flow will not be obstructed in that as the valve is closing one outlet the other outlet is being opened. The operating mechanism, usually a large external square thread, should be arranged so that with the valve directed to the boiler, fuel oil is shut off. A mechanical system using an extension piece can be arranged to push a fork lever into the operating handwheel of the oil fuel supply valve. When the exhaust valve is fully operated to direct the gas to atmosphere the fork lever then clears the oil fuel valve handwheel after changeover travel is completed. It is also very important to ensure full fan venting and proper fuel heating-circulation procedures before lighting the oil fuel burners. Composite Such boilers are arranged for simultaneous operation on waste heat and oil fuel. The oil fuel section is usually only single pass. Early designs utilised Scotch boilers, with, say, a three-furnace boiler, it may mean retaining the centre or the wing furnaces for oil fuel firing. The gas unit would often have a lower tube bank in place of the furnace, with access to the chamber from the boiler back, thus giving double pass. Alternative single pass could be arranged with gas entry at the boiler back. Exhaust and oil fuel sections would have separate uptakes and an inlet change-over valve was required. In general Scotch boilers as described are nearly obsolete and vertical boilers are used. As good representative, and more up-to-date, common practice, two types of such boilers will be considered.

Answer 221

see reeds motor EK book waste heat recovery systems figure 10.7 and 10.8 Cochran boiler The Cochran boiler whose working pressure is normally of the order of 8 bar is available in various types and arrangements, some of which are as follows: single-pass composite, that is, one pass for the exhaust gases and two uptakes, one for the oil fired system and another for exhaust system; double-pass composite, that is, two passes for the exhaust (figure 10.7) gases and two uptakes, one for the oil fired system and one for the exhaust system. (Double-pass exhaust gas, no oil fired furnace and a single uptake, is available as a simple type. Or, double pass alternatively fired, that is, two passes from the furnace for either exhaust gases or oil fired system with one common uptake.)The boiler is made from good quality low carbon open hearth mild steel plate. The furnace is pressed out of a single plate and is therefore seamless. Connecting the bottom of the furnace to the boiler shell plating is a seamless ‘Ogee’ ring. This ring is pressed out of thicker plating than the furnace, greater thickness is necessary since circulation in its vicinity is not as good as elsewhere in the boiler and deposits can accumulate between it and the boiler shell plating. Hand hole cleaning doors are provided around the circumference of the boiler in the region of the ’Ogee’ ring. The tube plates are supported by means of tube stays and by gusset stays, the gusset stays supporting the flat top of the tube plating. Tubes fitted, are usually of special design (Sinuflo), being smoothly sinuous in order to increase heat transfer by promoting turbulence. The wave formation of the tubes lies in a horizontal plane when the tubes are fitted, this ensures that no troughs are available for the collection of dirt or moisture. This wave formation does not in any way affect cleaning or fitting of the tubes Thimble tube boiler There are various designs of thimble tube boiler, these include: oil fired, exhaust gas, alternatively fired and composite types. The basic principle with which the thimble tube operates was discovered by Thomas Clarkson. He found that a horizontally arranged tapered thimble tube, when heated externally, could cause rapid ebullitions of a spasmodic nature to occur to water within the tube, with subsequent steam generation. Figure 10.8 shows diagrammatically an alternatively fired boiler of the Clarkson thimble tube type capable of generating steam with a working pressure of 8 bar. The cylindrical outer shell encloses a cylindrical combustion chamber, from which, radially arranged thimble tubes project inwards. The combustion chamber is attached to the bottom of the shell by an ‘Ogee’ ring and to the top of the shell by a cylindrical uptake. Centrally arranged in the combustion chamber is an adjustable gas baffle tube.

Answer 222

see motor sketch pack for drawings Natural circulation multi-boiler system It is possible to have a single-exhaust gas boiler located high up in the funnel, operating on natural circulation whereby a limited amount of steam is available for power supply while the vessel is at sea. In port or during excessive load conditions, the main boiler or boilers are brought into operation to supply steam to the same steam range by suitable cross-connecting steam stop valves (figure 10.9). In port, the exhaust gas boiler is secured and all steam is supplied by the oil-fired main boilers. This system is suitable for use on vessels such as tankers where a comparatively large port steaming capacity may be required for operation of cargo pumps, but suffers from the disadvantage that the main boilers must either be warmed through at regular intervals or must be warmed through prior to reaching port. Further to this the main boilers are not immediately ready for use in event of an emergency stop at sea unless the continuous warming through procedure has been followed. Forced circulation multi-boiler system In order to improve the heat transfer efficiency and to overcome the shortcomings of the previous example a simple forced circulation system may be employed. The exhaust gas boiler is arranged to be a drowned heat exchanger which, due to the action of a circulating pump, discharges its steam and water emulsion to the steam drum of a water-tube boiler. The forced circulation pump draws from near the bottom of the main boiler water drum and circulates water at almost 10 times the steam production rate thus giving good heat transfer. The steam/water emulsion on being discharged into the water space of the main boiler drum separates out exactly in the same way as if the boiler were being oil fired. This arrangement ensures that the main boiler is always warm and capable of being immediately fired by manual operation or supplementary pilot operated automatic fuel burning equipment (figure 10.10). Feed passes to the main boiler and becomes neutralised by chemical water treatment. Surface scaling is thus largely precluded and settled out impurities can be removed at the main boiler blow-down. If feed flow only is passed through an economiser type unit parallel flow reduces risks of vapour locking. Unsteady feed flow at normal gas conditions can result in water flash over to steam and rapid metal temperature variations. Steam, hot water and cold water conditions can cause thermal shock and water hammer. Contra flow designs are generally more efficient from a heat transfer viewpoint giving gas temperatures nearer steam temperature and are certainly preferred for economisers if circulation rate is a multiple of feed flow. The generation section is normally parallel flow and the superheat section is contra flow. Output control could be arranged by output valves at two different levels thus varying the effective heat transfer surface utilised. In addition a circulating pump by-pass arrangement gives an effective control method (figure 10.11).

Answer 223

see reeds motor EK book waste heat recovery systems figure 10.12 and 10.13 Dual pressure forced circulation multi-boiler system This concept has been incorporated in the latest waste heat circuits and the sketch illustrates how the general principle can be applied in conjunction with a waste heat exchanger to supply superheated steam. By this means every precaution has been taken to minimise the effect of contamination of the water-tube boiler. Steam generated in the water-tube boiler by either oil firing or waste heat exchanger passes through a submerged tube nest in the steam/steam generator to give lower grade steam which is subsequently passed to the superheater. A water-tube boiler, steam/steam generator and feed heater may be designed as a packaged unit with the feed heater incorporated in the steam/steam generator. The high-pressure high-temperature system at say 10 bar will supply a turbo generator for all electrical services while the low-pressure system at say 21/2 bar would provide all heating services. Obviously the dual system is more costly. Numerous designs are possible including separate low-pressure and high-pressure boilers, either natural or forced circulation, indirect systems with single or double feed heating, etc. (figure 10.12). It is very important that students are very familiar with the current thinking in boiler design and operational practice. This is an important safety issue and will be considered very carefully by the Flag State examiner. Students must understand the watchkeeping issues with boilers identified at the start of this chapter. Feed heating The advantages of pre-heating feed water are obvious. Three methods will be considered, namely: economisers, mixture and indirect. Economiser types have been included in previous discussion and sketches. It is sufficient to repeat that such systems require a careful design to cope with fluctuations of steam demand and that particular attention is necessary to ensure protection against corrosive attack. Mixture systems employ parallel feeding with circulating pump and feed pump to the economiser inlet. Such circuits require careful matching of the two pumps and control has to be very effective to prevent cold water surges leading to reducing metal temperatures and causing corrosion. Indirect systems require a water/water exchanger feed heater.This design reduces the risk of solid deposit in the economiser and maintains steady conditions of economiser water flow thus protecting the economiser against corrosive attack. A typical system is shown in figure 10.13. If boiler pressure tends to rise too high the circulation by-pass will be opened. The effect will be twofold, that is, feed water will enter the boiler at a lower temperature and water temperature entering the economiser is at a higher temperature. These two effects serve to reduce boiler pressure and thus control the system. Obviously this system is more costly but is very flexible

Answer 224

Water hammer occurs when steam is admitted into a cold pipeline. The steam condenses producing both water, and a vacuum (when the water seals the pipeline from the steam supply). This vacuum causes the water plug to be drawn into the closed end of the pipe with increasing velocity producing high impact forces on the pipework. This impact force can be high enough to rupture the pipeline.

Answer 225

Water hammer is avoided by slowly admitting the steam into the cold line, and draining/venting the cold line to minimise the vacuum forming, and assist in draining the condensate.

Answer 226

Testing of the boiler water is important to determine that: The level of chlorides is within acceptable limits (below 200 ppm) to prevent hard scale and pitting The reserve of boiler chemicals within the boiler water is accepted (P-Alk above 100 ppm, and Phosphate above 20ppm) Treatment of the boiler water with chemicals will ensure: Alkalinity reserve against boiler water space corrosion Scale build-up is minimised Enough coagulant levels exist to minimise sludge build-up within the boiler The strength of the boiler drum is maintained

Answer 227

Boiler problems would be minimised by: Reducing the steam load and demand on the auxiliary boiler to reduce firing temperatures Ensuring hotwell feed temperatures are at the recommended 95°C to minimise oxygen levels in the feed Minimise blowdown of boiler water, whilst ensuring chloride level is not excessive, to avoid loss of the boiler chemicals

Boilers / Exhaust gas Economizers Flashcards

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