Marine Engineering Study Materials

Extreme care must be taken during the operation of freshwater generator onboard ships. Operate all the valves gradually. Sudden opening and closing of valves may result in thermal shock to the main engine. Also make sure that distillate pump never runs dry.

Starting of Freshwater Generator

1. Make sure seawater ejector pump suction, discharge and overboard valves are open. Start the ejector pump. Seawater pressure at the air ejector must be 3 bar or more.
2. Wait for vacuum to build up inside freshwater generator shell. (About 92 % vacuum).
3. Open the feed water valve to feed seawater to the evaporator. Adjust the feed water pressure. Normally marking is provided on the pressure gauge for desired feed water pressure.
4. Open main engine jacket cooling water inlet and outlet to the evaporator gradually.
5. Open the air vent clock at the top of the evaporator to make sure the evaporator is filled with jacket cooling water. Air must be purged out if any.
6. Switch on the salinity alarm panel for measuring purity of the freshwater produced.
7. There will be a sight glass provided at the suction line for the distillate pump. Make sure condensed water is coming to the suction line. Now start the distillate pump and open discharge valve to lead generated water to specified storage tanks.
8. Checks While Running Freshwater Generator
9. Through the sight glass provided in the evaporator shell, observe flashing of water.
10. Also check for the brine level inside. It should not be too high or too low.
11. Shell temperature must be around 50 deg cel.
12. Make sure shell vacuum is more than 90% from the vacuum gauge.
13. Check seawater inlet and outlet temperature to the condenser.
14. Ensure seawater pressure at air ejector inlet more than 3 bars.
15. Check for distillate pump pressure and water flow meter.
16. Check salinity of freshwater produced.
17. Check level and flow of dosing chemical.
18. Check ampere of ejector pump and distillate pump motor.

Regulating the Capacity of Freshwater Generator

Capacity of a freshwater generator means the quantity of freshwater produced by it per day. The capacity of freshwater generator can be varied by reducing or increasing the amount of jacket cooling water to the evaporator. The quantity of jacket cooling water to the evaporator can be adjusted by adjusting the bypass valve provided. When the temperature of jacket cooling water is comparatively low, the quantity to the evaporator to be increased a bit. At the same time cooling seawater pressure to the condenser also to be regulated accordingly. Refer to the detailed line diagram for freshwater generator down below.

During very low seawater temperatures, evaporation temperature can falls to a lower value. In that case, adjust vacuum adjusting valve (as shown in the diagram) to control vacuum inside the shell. Cooling seawater quantity to the condenser also can be reduced to increase the evaporator temperature. During high seawater temperatures, evaporation temperature can go up. In that case, increase the quantity of seawater to the condenser for reducing evaporation temperature.

Too high evaporation temperature causes scale formation in the heat exchanger. On the other hand, too low evaporation temperature results in seawater carry over which increases salinity of freshwater produced.

The distillate pump discharge to be throttled so that pump should not run dry. The rate of distillate pump discharge and rate freshwater produced in the condenser should match. When distillate pump is not able to extract the freshwater at the rate of production, level of freshwater increases in the condenser and effective cooling area of the condenser reduces. This finally results in reduced evaporation quantity.

Stopping of Freshwater Generator

When ship approaches port, shallow water, etc. it is desirable to stop the freshwater generator. This is because the seawater may contain harmful bacteria which can enter the freshwater produced. Operation of freshwater should be carried out in consultation with bridge watch keeper. Following procedure may be adopted for stopping freshwater generator.

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

Precautions for Operation of Freshwater Generator

1. Seawater pressure at the inlet of air ejector must be 3 bar or more.
2. The pressure at ejector outlet should not exceed 0.8 bar.
3. Never start freshwater generator distillate pump in dry condition.
4. Operate jacket cooling water valves to the freshwater generator gradually to avoid thermal shock to the main engine.
5. Feed water to be supplied for a few minutes to cool down the evaporator before stopping.
6. Never open the drain valve of evaporator before opening vacuum breaker. Otherwise atmospheric pressure causes seawater inside to hit the deflector.