Chapter 3 Water Tube Boilers

Question 1

1-Where are the tubes flared in a water tube boiler?

Answer 1

To add to the holding power of the tubes after rolling and to prevent the tubes from pulling out of the tube holes if the holes should become enlarged from overheating caused by low water or other reasons.
Page 109 chapter 3

Question 2

2-What is the difference between a fire side boiler and a waster tube boiler?

Answer 2

In the firetube boiler, the product of combustion pass through the tubes and the water surrounds them.
In a water side boiler the water passes in the tube and the product of combustion surrounds them. Page 81 chapter 3

Question 3

3-How is new tube placed in position for installation in a vertical water tube boiler?

Answer 3

Through one of the handholes provided for this purpose in the top head of the steam drum. Page 84 chapter 3

Question 4

4-What type of firing door is required on water tube boilers?

Answer 4

The inward opening type or a type provided with self locking door latches of a style omitting springs or friction contact, so that the door will not be blown open in case of tube rupture or furnace explosion and possibly injure a person standing nearby.
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Question 5

5-Are tubes beaded or flared in the straight tube watertube boiler?

Answer 5

They are flared, for the ends are not contacted by hot gases and nothing would be gained by beading. Page chapter 3

Question 6

6-To what points are water column connections made in most watertube boilers?

Answer 6

To the upper and lower parts of one head of the main steam drum.
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Question 7

7-How many gas passes are there in most standard type watertube boilers?

Answer 7

Three

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Question 8

8-Explain the method to be use in checking the operating condition of a water column and gauge glass when there are shutoff valves in the column connecting to the drum. What causes water to return slowly into the gauge glass?

Answer 8

First close upper valve and blow column and gauge glass through the drain valve with lower column connection valve open. The water should slowly rise in gauge glass from open bottom connection. Then reverse the procedure by closing the bottom connection with the top water column connection open. Watch water rising in gauge glass. If water rises in gauge glass during both procedures, it indicates connections are not obstructed. A slow return of water to the gauge glass indicates obstruction in the column connection or the bottom of the gauge glass is obstructed. This can be checked by using the gauge glass cocks. Page 110 chapter 3

Question 9

9-What is the maximum allowable working pressure and temperature on a water column made of cast iron and malleable iron per code?

Answer 9

Cast iron: 250 psi and 450degrees F. Malleable iron: 350 psi and 450 degrees F.
Page 111 chapter 3

Question 10

10-What is the minimum and max tube projection from a drum that a tube may have if it is to be flared?

Answer 10

Per code: 1/8 in minimum and ¾ maximum

Page 109 chapter 3

Question 11

11-How would you define a steam drum in a watertube boiler used for electrical power generation?

Answer 11

This is a pressure vessel in which steam is separated from the steam-water mixture in order to be directed to the superheater section of the steam generator for an increase in steam temperature.
Page 105 chapter 3

Question 12

12-What is a reheater as used in utility boilers?

Answer 12

A reheater receives steam from a steam turbine after it has given up some of its original heat in doing work in the turbines high pressure stages and is reheated to a higher temperature in the steam generator, then returned to the lower pressure stages of the turbine to do more work in driving (usually) an electric generator. The steam line carrying the steam from the turbine to the reheater is called the cold reheat line, while the steam line carrying the reheated steam from the reheater to the turbine is called the hot reheat steam line. Page 106 chapter 3

Question 13

13-Define the work economizer?

Answer 13

This is a heat absorbing section of a large watertube boiler that preheats the incoming feedwater to the boiler, or steam generator, by absorbing heat from the outgoing flue gases. Page 105 chapter 3

Question 14

14-What are the advantages of using tangent inner furnace walls in a watertube boiler?

Answer 14

By using tangent tubes, no tile, refractory, fins, or strip are used with the tangent tubes enclosing the furnace walls to ensure maximum setting cooling and uniform heat absorption in the furnace area.
chapter 3

Question 15

15-What important safety device must be evaluated when changing fuels in a boiler?

Answer 15

Fuels have different Btus; therefore, the capacity of the safety valves must be checked to make sure they exceed the potential heat release rate of the new fuel.
Page 89 chapter 3

Question 16

16-Why is it necessary to defer draining a steam generator until it has cooled sufficiently?

Answer 16

Cooling to a safe temperature before draining will prevent sludge baking on the internal surfaces, and also will make it safer for personnel to enter parts of the steam generator for inspection and maintenance.
Page 101 chapter 3

Question 17

17-Why should positive furnace pressure operation be avoided on those units not designed for positive pressure operation?

Answer 17

Positive furnace pressure will cause hot gases to escape from crevices and openings in the setting, thus causing boiler room pollution, unsafe conditions for personnel, an deterioration of the boiler setting or enclosure.
chapter 3

Question 18

18-What safety practice should be followed when blowing down a boiler when the gauge glass is not visible?

Answer 18

A person should be stationed by the gauge glass who is able to signal to the person blowing down in the event the water levels reaches an unsafe level, so that the blowdown procedures can be immediately stopped.
Page chapter 3

Question 19

19-Name 2 precautions in using steam sootblowers?

Answer 19

To avoid tube erosion, the steam should be dry, and condensate should be drained from the sootblowers before they are used in blowing deposits off tubes.
Page 112 chapter 3

Question 20

20-What would happen if a baffle broke down?

Answer 20

Gases would short circuit one or more passes, excessive flue gas temperatures and a loss in efficiency and capacity thus resulting. Overheating and damage might result in parts of the boiler designed for low gas temperatures.
chapter 3

Question 21

21-Is the volumetric capacity of each gas pass in a watertube boiler the same?

Answer 21

No, it decreases in each succeeding pass. The gases contact as they cool, and in order to maintain the high gas velocity necessary to sweep off stagnant gas films to effect good heat transfer, the cross sectional area of the passes must decrease as the gas require less space. Page 82 chapter 3

Question 22

22-In what type of watertube boiler are stay bolts used?

Answer 22

The box header types.

Page 83 chapter 3

Question 23

23- What is a downcomer nipple?

Answer 23

It is a short length of boiler tube between the steam drum and header carrying downward circulation of boiler water.
Page 107 chapter 3

Question 24

24-Where are interdeck nipples used?

Answer 24

In double deck boilers where two sets of headers are installed one above the other. Interdeck nipples connect them vertically.
Page 103 chapter 3

Question 25

25-Name the two main sheets of a box header.

Answer 25

The tube sheet and the handhole(or tube cap) sheet

Page 83 chapter 3

Question 26

26-What is the narrow plate sometimes used to form the bottom of a box header called?

Answer 26

The header trough

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Question 27

27-How many courses are there in a cross drum boiler?

Answer 27

Usually one, for a girth seam might unnecessarily interfere with the tube ligament.
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Question 28

28-It is more difficult to withdraw a tube from a waterboiler or firetube boiler?

Answer 28

It is usually more difficult in a firetube boiler, for any scale will be on the outside of the tube. Page 81 chapter 3

Question 29

29-How many drums are there in a Sterling bent tube boiler?

Answer 29

Usually 2 to 4

Question 30

30-What is an advantage of the multidrum type? What may be a disadvantage?

Answer 30

The greater the steam and water storage capacities enable the multi drum type to meet peaks in load fluctuations with less pressure drop.
The added cost of the additional drums is the disadvantage.
chapter 3

Question 31

31-Which is the shortest drum in a Stirling boiler?

Answer 31

The lower mud drum is the shortest. It is suspended by the tubes, and the heads of the drum are inside the boiler sidewalls.
Page 86 chapter 3

Question 32

32-Is the water level equal in the steam drums of a bent tube boiler of a type having the drums at the same level?

Answer 32

It is when the boiler is idle or operating at low ratings. At high ratings, the water level in the drum supplying the downcomer tubes is often lower than that in the other drum chapter 3

Question 33

33-For what reasons are bent tubes used in watertube boilers?

Answer 33

To allow for expansion and contractions; to permit tube replacement; to allow tubes to enter drums perpendicular to surface tangent; to allow flexibility in design in regard to pressure and capacity.

Question 34

34-What is the purpose of a bridge wall in a stirling boiler?

Answer 34

It protects the mud drum form exposure to the direct heat of the fire.
Page 87 chapter 3

Question 35

35-What might happen if the stirling bridge wall collapsed?

Answer 35

The longitudinal seam might be overheated and damaged.

Page 97 chapter 3

Question 36

36-Where is the manhole in bent tube boiler drums?

Answer 36

In at least one head of the drum.

Page110 chapter 3

Question 37

37-What is the purpose of circulating and equalizing tubes in bent tube boilers?

Answer 37

Circulating tubes connect the water space of adjacent steam drums (usually at the same level) and aid in equalization of water level. Equalizing tubes connect the steam space of such drums to equalize the steam pressure.
Page 94 chapter 3

Question 38

38-What is the difference between a mud drum in a stirling and in a straight tube type of boiler?

Answer 38

Stirling mud drum is one of the main drums in the circulatory system.
In a straight tube boilers, the mud drum may be a trough in the steam drum(Heine boiler) or an external box at the bottom of the headers(as in the babock and Wilcox boiler)
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Question 39

39-Where at the blowdown connections on bent tube boilers, and how many are there?

Answer 39

In the bottom of the lowest, or mud, drum; one or more, depending on the length of the drum.
chapter 3

Question 40

40-What is the difference between the tube sheet in a bent tube boiler and in an HRT boiler?

Answer 40

In the bent tube boiler, the tube sheet is that portion of the drum into which the tubes are rolled.
In the HRT boiler, the tube sheet is a flat head.
Page 54-hrt chapter 2, chapter 3 bent tube

Question 41

41-Define a superheater as used with a steam boiler.

Answer 41

A superheater is a form of heater used to raise the temperature of saturated steam above the temperature due to its pressure. It usually consists of an inlet and an outlet header with interconnecting tubes.
Page chapter

Question 42

Name two separate ways of producing superheated steam.

Answer 42

One separately fired and the other ones located in the boiler setting or enclosures and sometimes in the walls of the furnace.
Page 103 chapter 3

Question 43

42-Define an air preheater and mention some advantages gained in using it as part of the steam generator unit.

Answer 43

An air preheater is a heat exchanger usually installed in the smoke stack or in the boiler breeching. The cool incoming air is heated by heat in the gases that would otherwise be wasted, and the heated air is used in the furnace again, saving fuel or heat needed to heat the air.

Question 44

Define air cooled walls used in boiler installation and what is gained by their use
Air cooled walls are in the furnace setting.

Answer 44

Cool air keeps the furnace brickwork form burning, and the air acts as an insulating blanket to prevent heat loss through the brickwork of the setting.
chapter 3

Question 45

44-How do water and steam circulate in a straight tube waterboiler?

Answer 45

This type of boiler has natural circulation with the water and steam rising along the inclined tubes to the higher front header, then through the headers to the drum. The water then circulates through the downcomers to the rear header to the inclined tubes to complete the cycle.
Page 94 chapter 3

Question 46

45-Name two classifications used to describe furnace enclosures for watertube boilers?

Answer 46

2 classifications are furnace enclosures consisting primarily of refractory to keep the heat within the furnace and furnaces with water cooled walls.
chapter 3

Question 47

46-What is usually the weakest par t(from the viewpoint of construction) of a bent tube boiler?

Answer 47

The tube ligament.

chapter 3; chapter 9 strength calculations.

Question 48

47-What are the types of tubes that are used in water tube boilers?

Answer 48

Operating conditions have encouraged manufactures to seek new solutions to boiler operating problems, such as when burning abrasive or corrosive fuels in the furnace. Tubes made for these special operating conditions include:
a-Seamless tubes of ordinary service made of carbon steel, hot or cold drawn. Thickness range varies from 0.049-in minimum to up.
b-Welded carbon steel tubes per code requirements with thickness also starting at 0.049-in minimum.
c-alloy and stainless steels, hot or cold drawn, starting at 0.049-in minimum thickness.
d-Bimetallic tubing. which has a stainless steel outer tube bonded by rolling to a carbon steel tube, which is sized to resist design pressure, while the ss outer tube provides corrosion and erosion resistance. These tubes were developed to be used in recovery boilers in the papermaking industry, but are also being use din trash burning facilities, and similar corrosive firing services.
e-Ribbed tubes, promote circulation and thus avoid DNB, or departure from nucleate boiling.
f-external finned tubes are extensively used for membrane walls in watertube boilers. This construction eliminates welding bars to tubes to make a membrane wall. Only the fins are welded together to form an usually airtight furnace wall.
Page 46,201 early chapters

Question 49

48-Explain the advantages of drainable superheaters and reheaters.

Answer 49

The drainable type superheater and reheater permits establishing flow throught the tubes on start up of large boilers, and thus avoids possible tube overheating. The drainable type also permits better chemical cleaning of superheater and reheater tubes by ensuring that the chemical solution is circulated uniformly through all the tube circuits. This also permits better neutralization and improves the flushing out of chemical and scale residue when the chemical cleaning is finished.
Page 106 chapter 3

Question 50

49-What does the term “ramp rate” on boilers mean?

Answer 50

This is the manufactures recommended rate of permissible pressure and temperature increase per unit load increase. This is especially applicable to utility boilers.
Excessive changes in loading with corresponding pressure and temperature changes will place additional stressed on boiler components. Metal temperature increases due to rapid ramp rates may increase superheater and reheater metal temperatures above design, and this can cause tube failure.
chapter 3

Question 51

50-What is the difference between a “once through” and supercritical steam generator?

Answer 51

The two terms apply to supercritical steam generators where the water goes through the water tubes only once in contrast to natural circulation where the liquid may circulate through the tubes more than once before it is converted to steam. The fluid in a once through steam generator is under forced circulation by means of a circulating pump in contrast to the natural circulation in a subcritical steam generator. The feedwater in a supercritical steam generator is above the critical pressure of water, but below the critical temperature.
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Question 52

51-what are the pressures and temperatures that define a supercritical steam generator?

Answer 52

An operating pressure above 3206.2 psia and an operating temperature above 705.6 degrees f.
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Question 53

52-What is the purpose of the downcomers in large watertube boilers?

Answer 53

Downcomers are pipes that carry water from the top of the boiler to the bottom of the boiler, external of any steam generating zones, in order to ensure the delivery of steam free water to the lower headers or drum to promote natural circulation in the boiler.
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Question 54

53-What limitation does the code place on hot water coil boilers that do not have a steam space?

Answer 54

The code does not cover this type of boiler if the following restrictions are complied with:
1-tubing of ¾-in diameter with no drums or headers attached.
2-maximum 6-gal water containing capacity and
3-no steam generated in the coil.
chapter 3

Question 55

54-name 2 reasons why a coil type boiler may develop excessive coil temperature?

Answer 55

Low water and insufficient water being circulated through the coil because of partial obstructions or pump failure. For this reason, coil type boilers should have a high temperature cutout and possibly a flow switch that will shut off the firing mechanism when high coil temperature is developed. This is usually done by a thermostatic switch.
Page 97 chapter 3

Question 56

55-How is feed water rate controlled in a clayton type coil boiler?

Answer 56

The feedwater is controlled by the water level control which in turn responds to the liquid level existing in the accumulator. When the accumulator water level drops, the water pump solenoid deenergizes to allow the feed water pump to operate.
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Question 57

56-What procedure is recommended in checking a coil type for leaks?

Answer 57

Most manufacturers recommend the boiler be hydrostatically tested; if failure is small, repair the leak by an approved welding process. The coil should be replaced if difficulties are encountered in the repair.
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Question 58

57-Does a cast iron boiler require a bottom blowoff pipe and valve?

Answer 58

Yes. The ASME code requires each boiler to have a blowoff pipe connection fitted with a valve or cork, of not less than ¾ in pipe size. It must be connected with the lowest water space practicable.
chapter 3

Question 59

58-What is the minimum size of pipe required for connecting a water column to a steam heating boiler?

Answer 59

The minimum size of ferrous or nonferrous pipe must be of 1-in diameter.
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Question 60

59-What hydrostatic test is required on hot water or hot water supply boilers built of cast iron and operating over a pressure of 30 psi?

Answer 60

Each section of a cast iron boiler must be subjected to a hydrostatic test of 2-1/2 times the maximum allowable pressure at the shop where it is built. Cast iron boilers marked for working pressure over 40 psi must be subjected to a hydrostatic test of 1-1/2 times the maximum allowable pressure in the field(when erected and ready for service) After the boiler is in service and a hydrostatic test is required, the test shall be at 1-1/2 times the max allowable pressure.
chapter 3

Question 61

60-Name the two pressure controls required on a steam heating boiler and the temperature controls required on a hot water heating boiler.

Answer 61

Automatically fired steam heating boilers require an operating pressure cutout switch set lower than the allowable pressure and an upper limit switch set at a pressure ho higher than 15psi for hot water heating boilers, an operating temperature cutout switch set less than the maximum allowable temperature is required, as well as an upper limit temperature cutout switch set no higher than 250 degrees F.
Page 22 chapter 1

Question 62

61-When does the heating boiler code require a low water fuel cutout for hot water boiler?

Answer 62

When the boiler is automatically fired with heat input greater than 400,000 BTU/hr
Page 25 chapter 1

Question 63

62-What hydrostatic tests are required on sections of a steam heating boiler made of cast iron?

Answer 63

The individual sections must be tested with a hydrostatic pressure not less than 60 psi The assembled boiler is tested at a hydrostatic pressure not less than 45 psi.
chapter 3

Question 64

63-What hydrostatic tests are required for a cast iron boiler to be used for hot water heating?

Answer 64

For boilers with working pressure not over 30 psi, the hydrostatic test pressure must be at least 60 psi for each individual section.
Those with working pressure over 30 psi require a test pressure 2-1/2 times the max allowable pressure on each section. In both of the above, the assembled boiler requires another hydrostatic test not less than 1-1/2 times the max allowable working pressure. These requirements apply to the manufacturer of the cast iron boilers. The test pressure required must be controlled within a 10-psi range.
chapter 3

Question 65

64-Name the three methods or types of cast iron boilers?

Answer 65

Horizontal sectional, vertical sectional, and one piece type.
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Question 66

65-What stamping does the ASME require on cast iron boilers?

Answer 66

The marking must consist of the following: manufacturer’s name , maximum allowable pressure in pounds per square inch, and capacity in pounds per hour for steam or BTU per hour for water service.
Page 112 chapter 3 also page 28 chapter 1

Question 67

66-What is the minimum size of safety valve allowed on a cast iron boiler to be used for steam heating or hot water heating service?

Answer 67

ASME rules require a minimum ¾-in safety valve for both steam heating and hot water heating cast iron boilers.
Page 113 chapter 3

Question 68

67-What pressure rise is permitted on a boiler before the safety valve capacity may be considered inadequate for heating boilers?

Answer 68

For steam heating boilers, the safety valve capacity shall be sufficient at maximum firing rate that the pressure does not rise more than5 psi. For hot water heating boilers, the relief valve capacity must be sufficient to prevent the pressure from rising more than 10 psi with the burners operating at the maximum firing rate.
Page 23 chapter 1