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Ships procedure for fresh water production & use low pressure evaporator

Service piping and pumping arrangement

Pumps and substantial piping systems are provided in ships to supply the essential services of hot and cold fresh water for personal use, sanitary and salt water for fire fighting purposes.

Modern cargo ships are provided with a large low-pressure distilling plant for producing fresh water during the voyage, as the capacities required would otherwise need considerable tank space. This space is better utilized to carry oil fuel, improving the ship’s range.

Independent tanks supplying the fresh water required for drinking and culinary purposes, and fresh washing water, etc., may be taken from the double bottom tanks, the pumps for each supply being independent also. Hot fresh water is supplied initially from the cold fresh water system, through a non-return valve into a storage type hot water heater fitted with heater coils, and then the heated water is pumped to the outlet.

A considerable amount of fresh water is consumed in a ship. The crew uses on average about 70 litre/person/day and in a passenger ship, consumption can be as high as 225 litre/person/day. Water used in the machinery spaces as make up for cooling system losses may be fresh or distilled but distilled water is essential for steam plant where there is a water tube boiler. Steamship consumption for the propulsion plant and hotel services can be as high as 50 tonnes/day.

It is now common practice to take on only a minimal supply of potable water in port and to make up the rest by distillation of sea water. The saved storage capacity for water, is available for cargo and increases the earning power of the ship, A vessel which carries sufficient potable water for normal requirements is required, if ocean-going, to carry distillation plant for emergency use.

Modem low pressure evaporators and reverse osmosis systems give relatively trouble-free operation particularly in comparison with the types that were fitted in older ships. They are sufficiently reliable to provide, during continuous and unattended operation, the water needed for the engine room and domestic comsumption.

An advantage of low pressure evaporators is that they enable otherwise wasted heat from diesel engine jacket cooling water to be put to good use. Reverse osmosis systems were installed to give instant water production capacity without extensive modifications. They are used to advantage on some passenger cruise vessels and are fitted in ships which may remain stopped at sea for various reasons (tankers awaiting orders — outside 20 mile limit).

Warning is given in M Notice M620 that evaporators must not be operated within 20 miles of a coastline and that this distance should be greater in some circumstances. Pollution is present in inshore waters from sewage outfalls, disposal of chemical wastes from industry, drainage of fertilizers from the land and isolated cases of pollution from grounding or collision of ships and spillage of cargo.

Low pressure evaporators

The main object of distillation is to produce water essentially free of salts. Potable water should contain less than 500 mg/litre of suspended solids. Good quality boiler feed will contain less than 2.5 mg/litre. Sea water has a total dissolved solids content in the range 30 000—42 000 mg/litre, depending on its origin but the figure is usually given as 32 000 mg/litre.

Low pressure evaporators for the production of water can be adapted for steamships but operate to greatest advantage with engine cooling water on motorships. The relatively low temperature jacket water entering at about 65 degC and leaving at about 60 degC will produce evaporation because vacuum conditions reduce the boiling temperature of sea water from 100 degC to less than 45 degC The single effect, high vacuum, submerged tube evaporator shown in Figure 3.8 is supplied with diesel engine cooling water as the heating medium.

Vapour evolved at a very rapid rate by boiling of the sea-water feed, tends to carry with it small droplets of salt water which must be removed to avoid contamination of the product. The demister of knitted monel metal wire or polypropylene collects the salt-filled water droplets as they are carried through by the air. These coalesce forming drops large enough to fall back against the vapour flow.

Evaporation of part of the sea water leaves a brine the density of which must be controlled by continual removal through a brine ejector or pump. Air and other gases released by heating of the sea water, but which will not condense, are removed by the air ejector. The evaporator shown has a single combined ejector for extraction of both brine and air. One of the gases liberated is CO2 from calcium bi-carbonate in the sea water.

Loss of carbon dioxide from calcium bi-carbonate, leaves plain calcium carbonate which has poor solubility and a tendency to form soft, white scale. Other potential scale-forming salts are calcium sulphate and magnesium compounds. Scale is not a major problem where submerged heating coils reach a temperature of only 60degC. This heat is too low for formation of magnesium scales and provided brine density is controlled, calcium sulphate will not cause problems.

Continuous removal of the brine by the brine pump or ejector, limits density. Approximately half of the sea-water feed is converted into distilled water, the quantity of brine extracted is equivalent to the remainder of the feed delivered. The level of water in the evaporator is maintained constant by means of a brine weir, over which excess passes to the ejector.

The small quantity of soft calcium carbonate scale can be removed by periodic cleaning with a commercially available agent or the evaporator can be continually dosed with synthetic polymer to bind the scale-forming salts into a 'flocc' which mostly discharges with the brine, Use of continuous treatment will defer acid cleaning to make it an annual exercise.

Without continuous treatment, cleaning may be necessary after perhaps two months. Steam heated evaporators with their higher heating surface temperature, benefit more from chemical dosing, because magnesium scales form when surfaces are at 80 deg C or more.

Corrosion protection

The shell of the evaporator may be of cupro-nickel or other corrosion resistant material but more commonly, is of steel. The steel shell of evaporators is prone to corrosion. Protection is provided in the form of natural rubber, rolled and bonded to the previously shot-blasted steel. The adhesive is heat cured and the integrity of the rubber checked by spark test.

Summarized below some of the basic procedure of machinery service systems and equipment :
  1. Ballast arrangements

  2. The ballasting of a vessel which is to proceed without cargo to the loading port is necessary for a safe voyage, sometimes in heavy weather conditions. On arrival at the port the large amount of ballast must be discharged rapidly in readiness for loading....

  3. Cargo ships bilge systems

  4. The essential purpose of a bilge system, is to clear water from the ship's 'dry' compartments, in emergency. The major uses of the system, are for clearing water and oil which accumulates in machinery space bilges as the result of leakage or draining, and when washing down dry cargo holds. The bilge main in the engine room, has connections from dry cargo holds, tunnel and machinery spaces.....

  5. Bilge system layout details

  6. All bilge suctions have screw down non-return valves with strainers or mud boxes at the bilge wells. Oily bilges and purifier sludge tanks have suitable connections for discharge to the oily water separator or ashore. The system is tailored to suit the particular ship......

  7. Domestic water system

  8. Systems using gravity tanks to provide a head for domestic fresh and sanitary water, have long been superseded by schemes where supply pressure is maintained by a cushion of compressed air in the service tanks....

  9. Reverse osmosis

  10. Osmosis is the term used to describe the natural migration of water from one side of a semi-permeable membrane into a solution on the other side. The phenomenon occurs when moisture from the soil passes through the membrane covering of the roots of plants,....
  11. Salinometer features

  12. The condensate or product, if of acceptable quality, is delivered to the appropriate tanks by the distilled water pump. Quality is continuously tested by the salinometer both at start up and during operation. If the device registers an excess of salinity it will dump the product and activate the alarm using its solenoid valves. The product is recirculated in some installations......

  13. Sewage systems

  14. The exact amount of sewage and waste water flow generated on board ship is difficult to quantify. European designers tend to work on the basis of 70 litres/person/day of toilet waste (including flushing water) and about 130-150 litres/person/day of washing water (including baths, laundries, etc.). US authorities suggest that the flow from toilet discharges is as high as 114 litres/person/day with twice this amount of washing water......

  15. Sewage zero discharge system

  16. A retention or holding tank is required where no discharge of treated or untreated sewage is allowed in a port area. The sewage is pumped out to shore reception facilities or overboard when the vessel is proceeding on passage at sea, usually beyond the 12 nautical mile limit. ...

  17. Biological sewage treatment

  18. A number of biological sewage treatment plant types are in use at sea but nearly all work on what is called the extended aeration process. Basically this consists of oxygenating by bubbling air through or by agitating the surface. ....

  19. Sterilization system

  20. Sterilization by the addition of chlorine, is recommended in Merchant Shipping Notice M1214. A later notice, M1401, states that the Electro-Katadyn process in use since the 1960s, has also been approved. Another problem with distilled water is that having none of the dissolved solids common in fresh water it tastes flat. It also tends to be slightly acidic due to its ready absorption of carbon dioxide (CO2). .....

  21. Treatment of water from shore

  22. There is a risk that water supplied from ashore may contain harmful organisms which can multiply and infect drinking or washing water storage tanks. All water from ashore, whether for drinking or washing purposes, is to be sterilized. When chlorine is used, the dose must be such as to give a concentration of 0.2 ppm....

  23. Water production low pressure evaporator

  24. A considerable amount of fresh water is consumed in a ship. The crew uses on average about 70 litre/person/day and in a passenger ship, consumption can be as high as 225 litre/person/day. Water used in the machinery spaces as make up for cooling system losses may be fresh or distilled but distilled water is essential for steam plant where there is a water tube boiler. Steamship consumption for the propulsion plant and hotel services can be as high as 50 tonnes/day.....

  25. Flash evaporator system

  26. The evaporator , boils sea water at the saturation temperature corresponding to the uniform pressure through the evaporation and condensing chambers. With flash evaporators the water is heated in one compartment before being released into a second chamber in which the pressure is substantially lower......

  27. Oil content monitor system

  28. In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer......

  29. Oily water separator

  30. Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard mainly when pumping out bilges. They also find service when deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation....

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