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Naval ships must be self-sustaining as far as the production of freshwater is concerned. The large quantities of water are required aboard ship for boiler feed, drinking, cooking, bathing, and washing make it impracticable to provide storage tanks large enough for more than a few days supply. Therefore, ships depend upon distilling plants to meet the requirements for large quantities of freshwater of extremely high chemical and biological purity. Evaporator and potable water systems produce from seawater, a supply of fresh water suitable for ship's services as may be required. Basic principles of operation will be discussed.
Submerged tube (heat recovery) evaporators(Figure-3)
1. Heat recovery units use the heat from engine cooling systems to produce potable water from seawater.
2. Heat recovery units come in various sizes which range from 1,000 gallons per day (gpd) to 12,000 (gpd).
3. All heat recovery units are constructed and function alike; the Aqua-Chem Model S-500 S/T low-pressure distilling plant will be used as a reference.
4. Major components (submerged tube)(Figure-3)
a. Seawater feed pump - An electrically driven, centrifugal pump supplies seawater as feed to the distilling plant. Firemain can be used as a backup supply source.
b. Distiller condensers - Water from the seawater feed pump passes through the distiller condensers. The distiller condensers are installed in the upper section of the shell. The distiller condensers condense the vapor produced in the flash chambers to form distillate. Incoming feed water is preheated as it absorbs the distillate's latent heat of condensation.
c. Vapor separators (demisters) - The vapor travels up the flash chamber through a vapor separator. The vapor separators are installed in the upper section of each flash chamber.
They control distillate purity by separating salt-laden moisture, allowing the distillate to pass through and the salt-laden moisture to fall into the flash chamber.
d. Distiller condenser - After passing through the vapor separators, the vapor will condense on the distiller condensers.
e. Distillate pump - An electrically driven centrifugal pump takes a suction from the final stage distiller condenser. The pump discharges distillate through a salinity cell, 3-way solenoid valve, flow meter, and a manifold which delivers the distillate to the ship's feedwater tanks, freshwater tanks, bilges, or overboard.
f. Salinity cells - Measures the electrical conductivity of the distillate. The electrical conductivity is proportional to the chloride content of the distillate. Chloride content is measured in equivalents per million (EPM). The system consists of an indicating panel and 4 cells located as follows: distillate drains from the second stage distiller condenser, drains from the first stage distiller condenser, seawater heater drain pump discharge line, and the air ejector condenser drain line. The cell in the distillate pump discharge line operates the 3-way solenoid trip valve.
g. Solenoid trip valve - The 3-way valve is installed in the distillate line on the discharge side of the distillate pump. The valve repositions automatically and sends distillate to the bilges or directly overboard if the salinity cell measures a salinity content in excess of .065 EPM. When the cause of the high salinity condition is corrected, the valve must be manually reset. Overriding an inoperative solenoid trip valve can lead to the contamination of feed or potable water in storage tanks.
h. Brine pump - An electrically driven centrifugal pump which takes a suction from the final stage shell and discharges the brine overboard.
i. Air ejectors - A two stage air ejector assembly is provided to maintain the vacuum in the distiller condensers and seawater heater. The first stage air ejector removes air and non-condensible gases from the distiller condenser and discharges to the suction side of the second stage air ejector. The second stage air ejector takes a suction from the discharge of the first stage air ejector and discharges to the air ejector condenser. In multiple stage evaporators, an orifice plate between each flash chamber stage maintains the required vacuum. Vacuum is attained in the seawater heater via a connecting line to the first stage flash chamber. Both connecting lines are equipped with orifices to restrict flow, thereby creating a vacuum differential. The air ejectors are operated by low pressure auxiliary steam.
5. System description (submerged tube)(Figure-3)
a. Plant construction
(1) Single-shell design with the following components located inside the shell:
(a) Distillate condenser
(b) Vapor separator (demister)
(c) Jacket-water tube bundle
(2) The following components are located outside the shell:
(a) Distillate cooler
(b) Feed treatment system
(c) Electric sterilizer
(d) Electric heaters
(e) Brine eductor
(f) Air ejector
b. Jacket water circuit
(1) Jacket water from the SSDG provide the heat source used in the evaporator.
(2) Jacket water leaves the engine and passes through the waste heat exchanger via the engine's fresh water pumps.
(3) The jacket water is returned to the diesel engine cooling system after being used in the evaporator.
c. Waste heat circuit
(1) Waste heat water is used to provide heat in the
waste heat exchanger circuit.
(2) Waste heat water leaves the waste heat exchanger via the waste heat pumps.
(3) The water flows through the distilling plants and returns to the waste heat exchanger.
d. Electric heater - An electric heater provides heat to the water before it enters the distilling plant.
6. Operating principles (submerged tube)
a. The shell is operated at a vacuum in order to create a lower boiling temperature for the evaporator feedwater (seawater).
b. Waste heat water is fed to a tube bundle which is submerged in seawater.
c. When the waste heat water (which is heated to 170°) passes through the unit, the seawater boils.
d. Steam rises to the top of the shell after passing through the demister pads.
e. The steam is condensed by the relatively cool incoming feedwater (seawater) flowing through the internal distillate condenser.
f. The condensed distillate is then pumped from the shell to the steam sterilizer to destroy any contaminants.
g. Distillate is then sent through the condensate cooler to the ship's potable water via a three-way dump valve that ensures only high quality water is sent to these tanks.
h. Due to the temperature of the steam sterilizer (about 250°F), it is important to keep a pressure on the distillate in the sterilizer to prevent the heating elements from burning out.
a. Quantity and heat content of the jacket water
b. Vacuum in the shell
c. Flow of feed (seawater)
d. Brine density
e. Condition of heat exchanger surfaces
f. Amount of Ameroyal (chemical treatment) injected
g. Jacket water should be supplied at approximately 170°F and leave at about 155°F to ensure proper evaporation.
h. The vacuum must be maintained at 26-27 inches of mercury; the higher the vacuum, the more evaporator output.
8. Condition of the heat exchanger surfaces (submerged tube)
a. The physical condition of the heat exchanger is important in the process of transferring heat from the jacket water to the seawater.
b. The rate of condensation regulates shell temperature and shell vacuum.
c. The brine level must be above the tube bundle to prevent excessive scaling.
9. Safety precaution while deballasting/main drainage eductor operations - To prevent inadvertently drawing oily or contaminated water into the intake for the distilling plant, do not operate the distilling plant while deballasting or pumping bilges when the distilling plant sea suctions are aft of the deballasting/eductor discharges.
10. MCM specifics
MCM SUBMERGED TUBE SYSTEM
Output: 3000 gpd
Output: 1000 gpd
Number of plants: one 3000, one 1000
Location: AMR No.1
Heating sources: 1. Jacket Water from 1A, 1B, and No. 2
SSDG
H. Chemical treatment system (all except reverse osmosis)
1. The vacuum creates a low boiling point in the evaporator shell, thus allowing the 170°F jacket water temperature to boil the seawater. The tube surface temperature will also be low, so scaling of the heat exchanger tubes is generally not a problem.
2. The minerals in seawater require the feedwater to be treated in order to prevent long-term scaling.
3. The distilling plants use a small piston-type injection pump to treat the feed systems with a chemical treatment which reduces scale build-up.
I. Operating principles (submerged tube)
1. The shell is operated at a vacuum in order to create a lower boiling temperature for the evaporator feedwater (seawater).
2. Waste heat water is fed to a tube bundle which is submerged in seawater.
3. When the waste heat water (which is heated to 170°) passes through the unit, the seawater boils.
4. Steam rises to the top of the shell after passing through the demister pads.
5. The steam is condensed by the relatively cool incoming feedwater (seawater) flowing through the internal distillate condenser.
6. The condensed distillate is then pumped from the shell to the steam sterilizer to destroy any contaminants.
7. Distillate is then sent through the condensate cooler to the ship's potable water via a three-way dump valve that ensures only high quality water is sent to these tanks.
8. Due to the temperature of the steam sterilizer (about 250°F), it is important to keep a pressure on the distillate in the sterilizer to prevent the heating elements from burning out.
9. Maximum plant output depends on the following variables:
a. Quantity and heat content of the jacket water
b. Vacuum in the shell
c. Flow of feed (seawater)
d. Brine density
e. Condition of heat exchanger surfaces
f. Amount of Ameroyal (chemical treatment) injected
10. Jacket water should be supplied at approximately 170°F and leave at about 155°F to ensure proper evaporation.
11. The vacuum must be maintained at 26-27 inches of mercury; the higher the vacuum, the more evaporator output.
12. Condition of the heat exchanger surfaces
13. The physical condition of the heat exchanger is important in the process of transferring heat from the jacket water to the seawater.
14. The rate of condensation regulates shell temperature and shell vacuum.
15. The brine level must be above the tube bundle to prevent excessive scaling.
J. Potable water system - Potable water is defined as water that is suitable for drinking. The potable water system supplies potable water throughout the ship for scuttlebutts, sinks, showers, galley and sculleries, sickbay, and the laundry.
a. Potable water tanks - Store potable water for use on board ship.
(1) Small supply tanks are normally independent of the ship structure.
(2) Ship tanks are built-in tanks formed by the ship's physical structure.
(3) All tanks are adequately stiffened and braced.
(4) A combined vent and overflow connection is installed on each tank.
(5) The tank overflow connection is fitted with an
insect screen and is directed so as not to overflow on equipment.
(a) The overflow connection is also fitted
with some type of locking device.
(b) The number and size of potable water tanks
vary with each class of ship.
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A. Principles of distilling plant operation involve anywhere between one and five flash chambers of the flash-type evaporator. The number of chambers is proportional to the amount of water the distilling plant will distill; the more chambers, the more water distilled. A flash-type distilling plant is called a single-stage flash-type distilling plant if it has one chamber, a two-stage flash-type distilling plant if it has two chambers, etc. The flash-type distilling plant has pre-heaters that heat seawater to a high temperature. The seawater is then admitted to a flash chamber that is under a vacuum where some of it flashes to vapor. The remaining seawater is directed either overboard for a single-stage flash-type evaporator or to another flash chamber maintained at an even lower vacuum. Here, more seawater flashes to vapor. At this point, the remaining seawater is pumped either overboard or other flash chambers depending on the design of the distilling plant. The vapor is condensed and routed to the ship's fresh water tanks, boiler reserve feed water tanks, or engine jacket water systems.
B. Terms and Definitions
1. Vapor - The product of the evaporation of seawater
2. Distillation - The evaporation and subsequent condensation of a liquid. The seawater is flashed to vapor and the vapor is condensed.
3. Evaporation - The first part of the process of distillation. Evaporation is the process of converting seawater into a vapor.
4. Condensation - The latter part of the process of distillation. Condensation is the process of cooling the vapor to produce usable water.
5. Feed - The seawater which is used in the distillation process
6. Distillate - The product resulting from the condensation of the vapor produced by the evaporation of seawater. Distillate is what exits the distilling plant.
7. Potable water - Distillate that has been treated with bromine or chlorine and sent to the potable water tanks. Potable water is used for drinking, bathing and laundry.
8. Feedwater - Distillate that has been sent to the reserve feed and make-up feed tanks. The feedwater is used to fill boilers.
9. Brine - Water exiting the distilling plant that has not
flashed to vapor. This water contains a highly concentrated mixture
of feed impurities.
10. Salinity - The concentration of Chloride ions in water.
B. Major components
1. Seawater feed pump - An electrically driven, centrifugal pump supplies seawater as feed to the distilling plant. Firemain can be used as a backup supply source.
2. Distiller condensers - Water from the seawater feed pump passes through the distiller condensers. The distiller condensers are installed in the upper section of the shell. The distiller condensers condense the vapor produced in the flash chambers to form distillate. Incoming feed water is preheated as it absorbs the distillate's latent heat of condensation.
3. Seawater heater - Water exits the distiller condensers and the shell of the distilling plant and enters the seawater heater. The seawater heater is a multi-pass shell-and-tube heat exchanger which raises the temperature of the feed prior to entering the first vacuum chamber. Seawater is heated by either auxiliary exhaust steam or auxiliary steam. The feed exiting the seawater heater is maintained at 170°F +/- 5°F. Regulation of feed temperature at 170°F is achieved by throttling the manually operated first stage feed inlet valve in the piping between the seawater heater and first stage flash chamber (feed inlet box). Temperature must be maintained within the 170°F range. If temperature is too low, the distilling plant will not operate at maximum efficiency and all organisms may not be killed. If the temperature is above 170°F, scaling is increased and the possibility of carryover, the salt not being separated from the vapor as it passes through the demister pad, is increased.
4. Feed inlet boxes - Water exits the seawater heater and enters the feed inlet boxes. The feed inlet boxes are located at the bottom of each flash chamber and contain an orifice plate. Spreader plates are installed above the orifice to evenly distribute incoming feed. Spreader plates cause the water to form a thin circular curtain for effective flashing to vapor. Perforated plates are installed above the spreader plates to prevent priming and carryover. Priming is defined as the distilling plant drawing the water into the flash chamber instead of having the seawater feed pump push the water into the chamber. Carryover is defined as seawater entering into the distillate chambers.
5. Flash chambers - Water exits the feed inlet boxes via the orifice and enters the flash chambers. Flash chambers are located in a rectangular shell. A division plate will separate stages if the distilling plant has more than one stage. Here, the feed will be exposed to lower than atmospheric pressure and will flash to vapor. If there is more than one chamber, the chambers are connected by a distillate line, an orifice, and a loop seal.
6. Vapor separators (demisters) - The vapor travels up the
flash chamber through a vapor separator. The vapor separators are
installed in the upper section of each flash chamber. They control
distillate purity by separating salt-laden moisture, allowing the distillate
to pass through and the salt-laden moisture to fall into the flash chamber.
7. Distillate pump - An electrically driven centrifugal pump takes a suction from the final stage distiller condenser. The pump discharges distillate through a salinity cell, 3-way solenoid valve, flow meter, and a manifold which delivers the distillate to the ship's feedwater tanks, freshwater tanks, bilges, or overboard.
8. Salinity cells - Measures the electrical conductivity of the distillate. The electrical conductivity is proportional to the chloride content of the distillate. Chloride content is measured in equivalents per million (EPM). The system consists of an indicating panel and 4 cells located as follows: distillate drains from the second stage distiller condenser, drains from the first stage distiller condenser, seawater heater drain pump discharge line, and the air ejector condenser drain line. The cell in the distillate pump discharge line operates the 3-way solenoid trip valve.
9. Solenoid trip valve - The 3-way valve is installed in the distillate line on the discharge side of the distillate pump. The valve repositions automatically and sends distillate to the bilges or directly overboard if the salinity cell measures a salinity content in excess of .065 EPM. When the cause of the high salinity condition is corrected, the valve must be manually reset. Overriding an inoperative solenoid trip valve can lead to the contamination of feed or potable water in storage tanks.
10. Brine pump - An electrically driven centrifugal pump which takes a suction from the final stage shell and discharges the brine overboard.
11. Air ejectors (two sets) - A two stage air ejector assembly is provided to maintain the vacuum in the distiller condensers and seawater heater. The first stage air ejector removes air and non-condensible gases from the distiller condenser and discharges to the suction side of the second stage air ejector. The second stage air ejector takes a suction from the discharge of the first stage air ejector and discharges to the air ejector condenser. In multiple stage evaporators, an orifice plate between each flash chamber stage maintains the required vacuum. Vacuum is attained in the seawater heater via a connecting line to the first stage flash chamber. Both connecting lines are equipped with orifices to restrict flow, thereby creating a vacuum differential. The air ejectors are operated by low pressure auxiliary steam.
12. Air ejector condenser - A multi-pass shell-and-tube heat exchanger. The condenser condenses the steam from the air ejectors. The latent heat of condensation is given up to the saltwater feed. The air ejector condenser drains by gravity to the freshwater drain system or to the bilges. Air and non-condensible gases are vented to the atmosphere.
13. Drain regulator - The drain regulator ensures that a constant suction head is maintained on the seawater heater drain pump. A level of condensate must be maintained in the final distiller condenser to prevent a loss of vacuum in the seawater heater shell.
14. Seawater heater drain pump - An electrically driven centrifugal pump which takes a suction from the bottom of the seawater heater shell and discharges to the main condensate line, fresh water drain system or the bilges via a level control valve (discharge regulator). A portion of the condensate will be delivered to the de-superheater flow nozzle in order to cool the auxiliary exhaust steam before it enters the seawater heater.
15. Seawater heater drain/regulator - Steam to the seawater heater is supplied from either an auxiliary exhaust line or a auxiliary steam line through an air operated reducing valve where steam pressure is reduced to a pressure between 15 psig and 3 psig, depending on the model of the distilling plant. Installed in the line is an orifice and de-superheater nozzle, which de-superheats the steam to prevent rapid scale formation and over-heating of the seawater heater.
C. Operation (flash-type)(Figure-1)
1. Seawater is pumped to the distilling plant by the seawater feed pump prior to entering the first stage flash chamber. The feed flows through the following components: each stage distiller condensers (heating medium for both is the fresh water vapor which is being condensed), air ejector condenser (feed is heated by condensing steam which has been discharged from the air ejectors), and the seawater heater (primary heating takes place here). The feed now enters the feed inlet box of the first stage flash chamber through the manually operated feed inlet valve.
2. Once the feed enters the 1st stage flash chamber, it goes through the following process:
a. In the 1st stage flash chamber, the feed is exposed to a lower than atmospheric pressure and flashes to vapor. The vapor rises and passes through the monel mesh demisters which separate the feed droplets from the salt free vapor. The vapor goes into the 1st stage distiller condenser where it condenses as distillate. The remaining feed drops to the bottom of the shell and is drawn to the next stage through a loop seal arrangement and the process is repeated. Distillate condensed in the first stage distiller condenser is sent to the next stage distiller condenser via a distillate line containing an orifice. The distillate is moved by a combination of gravity head pressure and the difference in vacuum between the two stages.
b. The feed that does not flash in the final stage shell is called brine and is pumped overboard by the brine pump. The distillate is pumped by the distillate pump through the three way solenoid trip valve to feedwater tanks, fresh water tanks, the bilges, or overboard.
1. An inoperative dump valve will permit water with a higher salt concentration to enter either the potable water system or reserve feed system. If water with a high salt concentration enters into the potable water system, water will be unfit for drinking. If water with a high salt concentration enters into the reserve feed system, boiler damage can occur.
2. Low auxiliary exhaust pressure causes ineffective heating
of the seawater feed and will decrease output and increase priming.
3. Condensate from the air ejector feed tank and seawater heater drains into the main condensate or fresh water drain systems and, if contaminated, may also contaminate these systems.
4. Upon a loss of electrical power, the pumps will secure.
The distilling plant is still being maintained under a vacuum and will
continue to draw in seawater. The distilling plant will flood, salt
up and cease to operate.
5. A loss of LP air pressure will affect the performance
of the seawater heater since the steam to the heater is regulated by an
air operated valve. If air pressure is lost, the valve fails shut,
eliminating the heat source to the heater. The temperature of the
feed going to the flash chambers would then fall, and the feed would not
flash in the chambers. The distilling plant floods and "salts up"
6. Flash type evaporators (ARS, LSD)
a. Aqua-Chem Model S1250FL3 detail (LSD-41)
b. Designed to produce 30,000 gpd
ARS-50 * FLASH TYPE SYSTEM Aqua-Chem S-4000
LSD-41 * FLASH TYPE SYSTEM Aqua-Chem Model S1250FL3
Output: 4,000 gpd Output: 30,000 gpd
Number of plants: two Number of plants: two
Location: AMR Location: MMR No.1, 2
Heating sources: Steam heater from boiler(s) Heating sources: Steam
heater from boiler(s)
c. The following information clarifies the LSD-41 class configuration:
(1) In the three-stage, flash-type distilling plant,
seawater is heated in
a series of heat exchangers and subsequently discharged into the
first-stage flash chamber.
(2) The hot seawater (feed) passes through the first stage flash devices (nozzles) and a portion flashes to steam because the pressure in the evaporator is lower than the saturation pressure corresponding to the temperature of the feed.
(3) The vapor that flashes off rises through the mesh separators, (demisters) where any salt-laden droplets are removed.
(4) The vapor is condensed as it passes over the tubes of the first stage condenser. From this point in the system, the condensed vapor is referred to as distillate.
(5) Distillate from the first stage joins the distillate from the remaining two stages and is pumped to the distillate cooler by the distillate pump.
(6) Feed which did not flash in the first stage flows to the second- stage flash chamber. The feed, which is now at the saturation temperature corresponding to the first stage vacuum, is flashed to steam.
(7) The flashing and condensing process is repeated in the third stage.
(8) The vacuum is greater in each stage when compared to the proceeding stages. This is a result of the physical arrangement of the plant.
7. Safety devices
a. A relief valve on the feed heater shell and a rupture
disc on the distilling plant shell are installed to protect the components
from over- pressurization.
b. These safety devices have the following settings:
(1) Feed heater relief valve 20 psig
(2) First stage shell rupture disc l5-20 psig
8. Chemical feed treatment system (Located in AMR No.1)
a. An independent assembly consisting of a proportioning pump, tank relief valve, and piping, injects a liquid chemical and fresh water solution into the feedwater line upstream of the distillate cooler.
b. This system prevents rapid scale formation on the heat
exchanger surfaces.
c. The injection rate for the feed treatment solution is set at a rate which is proportional to the rate of the feedwater entering the shell.
(1) Underfeeding will result in scale deposit foaming on the heating surfaces.
(2) Overfeeding causes a buildup of a soft deposit on the tubes (sludge).
(3) Either condition will result in a decrease in
the heat transfer rate of the exchanger.
Vapor compression distilling plants
1. Vapor compression - In this method of evaporation, seawater is boiled on one side of an exchanger's surface and the resulting vapor is passed across to the other side of the heat exchanger's surface as the primary heat source. This creates a very efficient method of freshwater distillation.
a. Developed for use where steam is not available
(1) Diesel submarines.
b. Electrically operated
c. Up to 6000 gallons per day capacity
2. Major components
a. Three stream heat exchanger - The exchanger is a shell and tube type heat exchanger with a bank of enhanced tubes (finned tubes to ensure the greatest possible heat exchange). Incoming feed is preheated in the tubes in the lower portion of the distiller. Distillate and brine from the upper shell are passed over the tubes (in separate chambers) to ensure as much heat as possible is returned to the evaporator.
b. Evaporator - The evaporator is a shell and tube type heat exchanger with enhanced surface tubes. Flashed vapor from the compressor is condensed inside the tubes and incoming feed outside the tubes absorbs the latent heat of condensation. Freshwater vapor is generated in this stage at approximately 215 degrees F.
c. Compressor - Freshwater vapor is drawn off from the evaporator by a lobe type vapor compressor (similar to the scavenging pump on a two stroke diesel engine). It raises the pressure from 1 to 3.5 psig and the temperature from 215 to 222 degrees F and returns the higher pressure and temperature vapor through the tubes of the evaporator to the shell.
d. Electric strip heaters - Electric immersion heaters or steam coils are used to replace the small amount of heat that is lost to radiation and heat not recovered by the three stream heat exchanger. The heaters are required for continuous operation of the distilling plant and are also used to generate the initial heat to start the system.
3. Operation
a. Seawater (feed) is drawn into the distiller by a centrifugal pump and passed through the three stream heat exchanger. There, it picks up sensible heat from the brine being discharged overboard and from distillate being sent to the potable water tanks.
(2) The preheated feed then enters the evaporator where it picks up the latent heat of condensation from the freshwater vapor which increases its temperature to 215 degrees F. At this point, some of the feed is flashed off to vapor. The remaining feed, now brine, is pumped through the three stream heat exchanger where it gives off much of the sensible heat it has picked up in the heat exchanger.
(3) Freshwater vapor flashed in the evaporator is drawn through the vapor compressor where it is compressed from about 1 psig to about 3.5 psig and is raised in temperature to approximately 222 degrees F. It is then pumped back through the evaporator where it is condensed into distillate and gives up the latent heat of condensation to the feed. The condensed distillate is collected at the bottom of the condenser and is drawn by a centrifugal pump through the three stream heat exchanger where it gives up additional heat (sensible heat). It is then sent to the potable water tanks for storage and treatment.
Reverse osmosis (RO) desalination plant system description(Figure-2)
1. Natural osmosis
a. If there is a differential pressure between two solutions of differing concentrations, the natural tendency is to equalize concentrations by the more concentrated solution flowing to the weaker solution.
b. The osmotic pressure of seawater is 350-400 psig.
a. A process by which high-quality water is produced by passing pressurized seawater (700-900 psig) over semipermeable membranes.
b. Approximately 30% of the feed will pass through the membrane. Rejected impurities are flushed overboard by the constant flow of feed water. This flow keeps the membrane clean.
3. Reverse osmosis components(Figure-2)
a. Duplex strainer - The duplex strainer, either furnished with the RO unit or supplied by the ship, is the first step in removing large pieces of debris from the seawater.
b. Feed booster pump - The feed booster pump is rated at a pressure of 50 psig. This ensures a positive pressure is maintained at the high pressure pump.
c. Centrifugal separator - Heavy suspended particles (sand and silt) are removed by centrifugal force. The larger particles along with a small amount of seawater is removed from the bottom of the separator through a continues drain. The centrifugal separator is designed to remove 98 percent by weight, of separator solids.
d. High pressure pump - The HP pump increases the pressure of the seawater to between 700 and 1000 psig providing the driving force for the RO process.
e. RO modules - The actual process of desalination occurs in the RO modules. The RO modules consist of a pressure vessel that contains one or two membrane elements.
f. Dump valve - The dump valve is a 3 way solenoid valve that diverts the water from the RO modules to either the bilge or the freshwater storage tank. The dump valve is designed to prevent out of specification [above 500 ppm total dissolved solids (TDS)] permeate from entering the freshwater system.
4. RO basic operation
a. Feed is pumped into the pressure vessel and flows over
a mesh flow spacer. Permeate passes through the membrane and into
the permeate support tube. Brine is flushed out the opposite end of the
pressure vessel.
b. Feedwater of less than 1% total dissolved solids will yield a permeate of less than 500 ppm salinity. This meets the BUMEDINST 6240.3C water quality requirements for drinkable water. Higher quality water can be achieved by staging membranes in series (the output of one membrane becomes the feed of another).
c. If the permeate is greater than 500 ppm total dissolved solids, a dump valve diverts the unsatisfactory water to the bilge.
5. PC-1 specifics
PC-1 * REVERSE OSMOSIS SWEET WATER
400
Output: 400 gpd
Number of Plants: (3) located in AUX-1,2,3
Heating Sources: none
6. MHC specifics:
MHC-51 * REVERSE OSMOSIS Chem Model
Output: 1,600 gpd
Number of plants: one
Location: AMR
Heating sources: none