M2 Fresh Water Generation Flashcards
(42 cards)
What is fresh water used for?
Domestic and sanitary uses
- Boiler feed water
- Topping up the central cooling system
What properties must the water have?
- High in purity, clean, free from contaminants
Thermal Desalination
Process by which pure water is made by evaporating and then re-condensing sea water. Sea water is boiled or “flashed” in a vacuum.
Dissolved solids are reduced to 1-2 ppm
- Device used is called an evaporator or distiller. Maybe be single effect (one pressure only, one or more evaporators in parallel) or double effect (different pressures, evaporators are in series)
Distiller Construction
Shell of the distiller is coated fabricated steel, gunmetal or non-ferrous metal. Coatings and internal components must resist corrosion, be inert, resist acid cleanings and water treatment chemicals. Shell is normally fabricated in two halves.
Heat exchangers may be shell and tube (series of brass or copper tubes held between two bronze end plates, hot water passes through the tubes and sea water flows over the tube) or plate (series of titanium plates, hot water passed on one side and sea water on the other).
A knitted monel metal wire or polypropylene demister is fitted to prevent the carryover of water droplets into the condensing section of the distiller.
All piping will be corrosion resistant such as cupro-nickel, stainless or copper. Evaporator shell will be fitted with relief valve, compound pressure gauge, sight glass, jacket water inlet and outlet, sea water cooling inlet and outlet, thermometer, sea water evaporator supply with orifice plate, brine ejector suction, air ejector suction, distillate outflow with flow indicator, vent, and a drain valve
Single Effect Evaporator Operation
Sea water is supplied via a dedicated pump from a dedicated sea chest in the vessel’s hull. The suction line from the sea chest will be fitted with an isolation valve and duplex strainer. The sea water will be supplied to the unit by the ejector pump. A separate pump for the condenser may be fitted or a line may be tapped off from the ejector pump for this purpose.
Air and non-condensable gases are removed from the distiller by means of an air ejector. The removal of the air induces a vacuum in the shell which lowers the boiling point of the sea water to approx 40C. A brine ejector removes any salt water that does not boil off. The brine is discharged overboard through a ship side valve.
A line is connected to the piping after the ejector pump and is used to send sea water to the evaporator section of the distiller. Flow is controlled by use of an orifice plate (fitted between two flanges with a set size hole drilled through it).
The sea water is heated in the evaporator section by water from the central cooling system. This water contains a high amount of latent heat and causes the sea water to boil at a very high rate. Sea water that doesn’t boil will fall to the bottom of the shell and is removed by the brine ejector. The demister separates out any entrained sea water droplets and allows them to fall back down to the evaporator.
The water vapor travels upward and is condensed at the top of the condenser section. Condensed water, aka product water, accumulates in a tray in the condenser section where it is then removed by the distillate pump. The product water passes through a salinometer which will divert the flow to bilge or overboard if the reading is too high. The high salinity reading will also cause an alarm to sound on the unit and in the control room. If product water meets the requirements then it will flow through a water meter into the vessel’s storage tank.
Double Effect Evaporator Operation
Multi-stage flash evaporator consisting of two or more identical shells made of fabricated steel with a protective internal coating. Fitted with demister screens and normally shell and tube type heat exchangers.
Sea water from the sea bay is pumped through the control valve to the second stage and then the first stage condensers where it gains latent heat by condensing the water vapor into water. From the condensers, the salt water travels to a steam heater where the temperature is raised to 70-80C.
The pressurized heated sea water flows through an orifice into the first flash chamber. The orifice is used to control flow to the evaporator however a regulating valve may be fitted instead. The first stage chamber is at a low pressured allowing the sea water to boil at a temperature below atmospheric. Some sea water evaporates and passes through the demister. The remaining sea water flows through another orifice and into the second chamber where more water boils off.
A brine pump extracts the un-evaporated water and discharges most of it overboard. Some may return to the suction side to maintain the feed sea water temperature above 30C (increasing efficiency).
The vapor and non-condensable gases in each chamber pass through demisters and then through the vapor condensers. Distillate flows from the first stage to the second through an orifice and then is extracted by the distillate pump and is delivered to ship’s storage tanks.
A salinity detector controls the distillate pump - if too high, the pump stops and the distillate is discharged overboard.
Scale and Deposits
Sea water contains dissolved minerals that are left behind in the evaporator and form scale and sludge deposits on the evaporator coils, heaters, and internal surfaces. The performance is adversely affected by scale formation, and heat transfer will fall. The evaporator must be blown down and cleaned mechanically or chemically to remove the scale.
Water treatment is available to slow the build-up of scale to a point where 90 days or more of service is the norm before chemical cleaning is required.
Cleaning of the evaporator is accompanied by using an acid solution injected into the shell and circulated until no scale remains. The acid cleaning solution is then neutralized and the evaporator is flushed using clean water.
Annual dismantling of the evaporator for manual cleaning is usually recommended.
Membrane Desalination
AKA reverse osmosis
Uses a high pressure pump, special membrane and a control valve to produce fresh water.
Osmosis
The membrane acts as a filter to allow the passage of water but not of the salt. The membrane is semi-permeable. The level in the water side drops as the solution reaches equilibrium and the difference between the two sides is known as osmotic pressure.
Reverse Osmosis
Reverse osmosis is a pressure induced reversal of the natural flow, forcing fresh water from the concentrated side to the pure water side. The membrane separates out the desirable product water and rejects the concentrated brine solution. The membrane element must be large in area and strong enough to withstand the high pump pressures. Usually made of spirally wound polyamide of sheets of polysulphonate.
Membrane Desalination Operation
A boost pump sucks from a dedicated sea bay and supplies the system through an inlet strainer. Feed lines will be stainless or cupronickel. After the pump, the flow will proceed through a media filter to remove any suspended particles or contaminants and small marine organisms. This filter is fitted with a backwash arrangement supplied from the boost pump. Polypropylene or charcoal cartridge filters are next in the feed water flow to ensure the last contaminants are removed from the feed water.
A high pressure PD piston-type pump driven by an electric motor is fitted and it pumps the water to the membrane modules. The membranes are spiral wound type fitted inside individual casings. The number of membranes depends on the amount and quality of the product water desired.
On the outlet line after the membranes is a stainless steel regulating valve. This valve is throttled after the pump is started, resulting in an increased pressure output from the PD pump, usually around 800 psi but much be sufficient to overcome the osmotic pressure and push fresh water through the membrane.
Product water will now flow from the membrane module through a salinity monitor and either overboard (if too high) or to the fresh water tanks.
The rate of flow for the feed is kept at a high rate so membranes are constantly washed and do not foul.
There can be no back pressure between the outlet product line and the domestic fresh water tanks or it will disrupt the operation of the membranes.
Membrane Desalination Maintenance
The media filter (sand filter) is regularly backflushed and the membranes are periodically cleaned with a citric acid solution. If the system is not to be used for several days, the membranes must be inhibited with a biocide.
The rate of product water flow is dependent on water treatment, salinity, and membrane age and cleanliness. Chemical dosing may be carried out to control deposits on the membranes and extend their life. The membranes have a fixed life and may show up to a 15% loss in performance after 10,000 hours.
Membrane Desalination Performance
Sea water feed temperature greatly effects the reserve osmosis performance rates. As the feed decreases in temperature, the output rate decreases. This is an important consideration when operating in cold environments. Some systems will heat inlet feed with waste steam, boiler returns or electricity.
Membrane Desalination Forms of Deterioration
- Biological growth or organisms
- Iron or ferrous particles in the feed, rust
- Chlorine or oxidizers in the feed or flushing water
- Grease, oil, pipe sealing compounds
- Feed contaminated with chemicals and pollution
- High feed temperatures
- Back pressures applied at any time
Membrane Desalination Standards
The WHO permit a maximum content of 500 ppm of total dissolved solids for potable use.
The systems may be arranged in series to give higher quality product water for boiler feed purposes. Single pass system output water is not normally suitable for high pressure boiler feed and must be treated further.
What is the purpose of a DFW system
To store and distribute fresh water on board a vessel
What are the two types of fresh water?
Potable and non potable
What is potable water?
Water that is fit for consumption, drinking, cooking, etc
What is non-potable water?
Water used for showers, washing machines, toilets, etc
Who governs potable water on ships?
TC Marine Occupational Safety and Health (MOSH) - (standards for water quality, how much is retained, storage)
Guidelines for Canadian Drinking Water Quality -Health Canada
Coast Guard Fleet Safety Manual (taking on, making, maintaining and testing potable water)
Taking on of water
Water is taken onboard by means of a connection at the fill station. The filling station is located on or above the freeboard deck and the connection (called the deck hydrant) to the tank filling lines will be clearly identified and painted blue. The valve to the fill line must be capped and locked so that no contaminants can be introduced into the water tanks. As well, water may only be taken on through hoses dedicated for this purpose and the dock hydrant that is supplying the water must be flushed for a minimum of 5 minutes at full volume to ensure that standing water in the system has been removed from the piping.
The Chlorinator
Water passes from the deck hydrant through a chlorinator and into the domestic fresh water (DFW) tanks. The chlorinator is an automatic system that measures the flow rate of the filling water and periodically injects a chlorine solution so that the free chlorine in the water is between 0.2 and 0.5 mg/L. Fitted above the chlorinator is a line coming from the vessel’s fresh water generator. Any water produced aboard the vessel must pass through the chlorinator as well.
DFW Tanks
Water is retained in two DFW tanks. These tanks are sized and located as dictated by class. The interior of the tanks will be covered with a paint or coating suitable for potable water and will be fitted with a vent and sounding pipe. Like the filling hydrant, the vent and sounding pipe must be clearly identified and the sounding pipe cover must be secured and locked in place when not in use. A sight glass is fitted to the tanks so that the water level can be periodically verified without the need for going on deck.
DFW Pumps
Two centrifugal DFW pumps are fitted and are fed from the tanks by a common line. Generally, piping from the tank will either be galvanized or cuper-nickel with flanged connections. The pumps are electric and are turned on and off by means of a pressure switch, set to start them at 32 psi and shut off at 40 psi. During normal operation, only one pump will run with the other in standby. However when there is a large demand (e.g. pressure drops to 28 psi) the second pump will start. Valves will be fitted to isolate the pumps as required.
