Fresh water generator on ships helps in converting sea water to consumable fresh water. The fresh water produced bu the fresh water generator is used for drinking, cooking, washing, and even running other important systems on ships, which use fresh water as a cooling medium.

STARTING

• First, check if the engine speed is running above 50 RPM. The reason for this is that at low RPM the temperature of jacket water is around 60 degrees, which is not sufficient for evaporation of water
• Check if the drain valve present at the bottom of the generator is in closed position
• Now open the suction and discharge valves of the sea water or ejector pump, which will provide water for evaporation, cooling, and to the eductor for creating vacuum
• Open the sea water discharge valve from where the water is sent back to the sea after circulating inside the fresh water generator
• Close the vacuum valve situated on top of the generator
• Now start the sea water pump and check the pressure of the pump. The pressure is generally 3-4 bars
• Wait for the vacuum to build up. Vacuum should be at least 90% which can be seen on the gauge present on the generator. Generally the time taken for vacuum generation is about 10 minutes
• When vacuum is achieved, open the valve for feed water treatment. This is to prevent scale formation inside the plates
• Now open the jacket water inlet and outlet valves slowly to about half. Simultaneously close the bypass valve
• Keep an eye on the jacket water pump pressure as it should not fluctuate. Always open the outlet valve first and then the inlet valve
• Slowly start to increase the opening of the valves to “full” open
• The boiling temperature would then start increasing and the vacuum would start dropping
• The vacuum drop of about 85% indicates that evaporation has started
• Open the valve from the fresh water pump to drain
• Switch on the Salinometerif it has to be started manually. Generally it is on auto start
• Now start the fresh water pump and taste the water coming out of the drain
• When the fresh water starts producing, the boiling temperature again drops slightly and the vacuum comes back to the normal value
• Check the saltiness of the water coming out of the salinometer. Also check the reading of the salinometer
• This is done to see if the salinometeris working properly or not and to prevent the generated fresh water from getting contaminated with the salt water. The value of salinometeris kept below 10ppm
• The freshwater pump pressure is normally between 1.2 and 1.60 bars
• After checking the taste of the water coming out of the salinometer, open the valve for the tank from the pump and close the drain valve

STOPPING

• Stop the hot water supply to the plant.
• Close valve for feed water treatment, if any
• Stop freshwater distillate pump
• Switch off the Salinometer
• Stop the ejector pump and open air screw / air vent
• Close valves on the suction and discharge side of the ejector pump
• Close overboard valve for combined brine/air ejector
• Close the valve to freshwater tank

Operation of FWG steam injection system:

If the heating of JCW is not sufficient for boiling water in FWG, addition steam heating through a system known as steam injection system.

The steam injection system is a closed water/steam system where the steam is injected into a closed water circuit and condensation of steam heats the water which circulates in the evaporator.

Procedure

• Set the fresh water generator system as required for starting
• Fully open the HT fresh water system evaporator by-pass and close the inlet and outlet valves of HT fresh water to evaporator
• Open steam heating valve and steam heating fresh water priming valves
• Close the priming valve once the fresh water is filled
• Open steam supply valve to steam injector
• Open the fresh water heating to the evaporator
• Control the steam supply to control the rate of evaporation

References

The Ultimate Guide to Operating Procedures of Engine Room Machinery. Anish Wankhede [2014]

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