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
“After starting the freshwater pump the flow sight glass must be emptied. if water remains in the flow sight glass, troubleshooting must be carried out.”
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
“ Apart from the air vent, all valvesmust be keptshut, whenthe plant is outof operation.”
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
“Ensure the pressure relief valve in the steam/ water heating circuit is working as same will release the excess pressure caused by condensation of steam.”
References
The Ultimate Guide to Operating Procedures of Engine Room Machinery. Anish Wankhede [2014]
