Operating a FWG

by | Aug 17, 2023 | Aux Machinery, Mechanical

How Do I Start A Fresh Water Generator ?

Starting The Fresh Water Generator

We need to check before starting the fresh water generator that the ship is not in congested water, canals and is 20 nautical miles away from the shore.This is done because the effluents from factories and sewage are discharged into the sea near the shore which can get into the FWG.

Check whether the engine runs above 50 rpm, which is because the temperature of the jacket water at low rpm is around 60 degrees and is not sufficient for water evaporation.

  1. Check the drain valve is in close position,Which is present at the bottom of the generator.
  2. Now open the sea water pump’s suction and discharge valves which provide water for evaporation, cooling, and to the eductor for vacuum formation.
  3. Open the seawater discharge valve through which the water is sent back to the sea after , circulating inside the freshwater generator.
  4. Close the vacuum valve, which is situated on top of the generator.
  5. Now, we should Start the sea-water pump and check the pump pressure. In general, the pressure is 3-4 bar.
  6. Wait until the vacuum builds up. Vacuum should be at least 90 percent, which can be clearly seen on the generator gauge ,situated on FWG . The time taken for vacuum generation is usually around 10 minutes.
  7. When vacuum is achieved, open the valve for the treatment of feed water, this is designed to prevent the formation of a scale within the plates.
  8. Now open the inlet and outlet valves of hot water (jacket water), slowly to about half.Always,First open the outlet valve and then the inlet valve. Slowly start increasing the opening of the valves to full opening.
  9. Now we can see that the boiling temperature starts to rise and the vacuum starts to fall.
  10. The vacuum drops to about 85 percent, which is an indication that evaporation begins.
  11. Open the valve for drain from the fresh water pump.
  12. Switch ON to the salinometer if it has to be started manually. Generally, it’s on the auto start mode.
  13. Now start the fresh water pump and test the water that comes out of the drain.
  14. When fresh water starts generating, it can be seen that the boiling temperature drops slightly again and that the vacuum goes back to normal value.
  15. Check that the water, which coming out of the salinometer is not salty and also check the reading of the salinometer.This is done to see whether the salinometer is working properly or not, and to avoid contamination of the entire fresh water with salt water.Salinometer values are kept below 10ppm.
  16. Open valve for tank from the pump and close drain valve after testing the taste of the water coming out of the salinometer.

 

Fresh Water Generator Stopping Procedure

It is desirable to stop the fresh-water generator as ship approaches port, shallow water, etc.This is because the seawater may contain harmful bacteria that can enter into the produced freshwater.The operation of freshwater should be carried out in consultation with the bridge watchkeeper.

Following, procedure to stop the fresh water generator can be adopted.

  1. Open the bypass valve , slowly for main engine jacket cooling water.
  2. Ensure that the cooling water temperature of the main engine jacket is within normal limits.
  3. Close inlet and outlet valves of jacket cooling water for the freshwater generator respectively.
  4. Close the feed water chemical dosing valve.
  5. Stop the distillate pump and shut down the discharge valve.
  6. Switch off the salino meter.
  7. Close the filling valve of the freshwater tanks.
  8. Wait for the temperature of the evaporator shell to drop below 50 deg celcius.
  9. Close the evaporator feed-water valve.
  10. Stop the pump ejector. Shut down overboard valve of fresh water generator.
  11. Open the vacuum breaker valve so that the side pressure of the shell is equal to the atmospheric pressure.
  12. Open the evaporator drain valve to drain all the seawater from the freshwater generator.

Precautions For Operation Of Fresh Water Generator

  1. The pressure of seawater at the inlet of air ejector must be 3 bar or more.
  2. The ejector outlet pressure should not exceed 0.8 bar.
  3. The distillate pump of fresh water generator never start in dry condition.
  4. To prevent thermal shock to the main engine, operate the jacket cooling water valves slowly to the fresh water generator.
  5. Feed water to be supplied to cool down the evaporator for a few minute before stopping.
  6. Never open the evaporator drain valve before the vacuum breaker is opened. Otherwise, the atmospheric pressure causes seawater to hit the deflector insiders.

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