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As a ship normally sails on long voyages, certainly drinking water is one of the most vital elements required on board. If you are thinking that this should not be a problem since the oceans contains unlimited water, read this article on the effects of drinking sea water and especially imagine the condition in areas such as the Dead Sea. Hence a provision for converting salt water to fresh water exists on ships, and the machine is known as the fresh water generator.
We have learned about the fresh water generator and its working in our previous articles, but it is a different thing to know how and when to start these water generators on a ship.
When I was a ships engineer many years ago, we had evaporators that used steam coils to heat and evaporate the seawater before it was cooled and processed to distilled water. In those days of large water-tube boilers supplying superheated steam to the main turbine, a lot of distilled water was required to make-up the boiler feed water. Today’s motor ships require much less water than the older steamships, but water is still required for drinking and domestic use.
The modern equipment we use for this is examined in the following sections; here we will see the methods and sequence involved in start-up the various components, as well as when it is safe to run the generators. The first section deals with the regulations regarding the safe processing of drinking water from seawater.
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Regulations Regarding Safe Production of Drinking Water from Seawater
Current marine regulations regarding seawater distillation using a fresh water generator stipulate;
- The FW generator can only be started when the ship is 12 nautical miles away from the nearest coastline.
- The engine must be running at full sea speed at start of passage as advised from the bridge.
This will ensure all the main engine temperature and pressure parameters are normal, main engine is on sea speed and not in congested waters and all maneuvering completed
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The fresh water generator should be fired up once the above regulations and requisites have been achieved; using the guidelines listed below.
A sketch showing the various components is shown below and can be referred to when reading the guidelines.
1. Check the jacket cooling water temperature outlet from the main engine - it should be constant
2. Open both the ejector pump suction valve and the overboard discharge valves.
3. Close the vacuum breaker valve
4. Open main sea water feed inlet valve inlet and discharge valves to and from the generator condenser; (the seawater can also be supplied from a stand-by sea water pump; if this is to be used instead, open the pump main inlet and discharge valves and start the pump)
5. Check salinometer (salinity indicator) and distillate pump operation
6. Start the ejector seawater pump and maintain the pressure of 5kg/cm2 or higher
7. Check that the vacuum inside the shell is slowly rising as the ejector removes the air from the unit.
8. When the vacuum gauge reading reaches about 17mm of mercury, slowly open the seawater feed to heating tubes.
9. Check the seawater level inside the shell through the sight glass and adjust this water level using the feed inlet valve.
10. Once the heating coils have been covered with seawater; open the jacket water inlet valve slowly to the full open position whilst throttling back the jacket water outlet valve.
11. The effect of the jacket cooling water circulating through the heating tubes causes the shell vacuum to drop, its temperature to rise and feed water level to fall.
12. When all the above is stabilized and running normally; the seawater starts evaporating and steam can be observed rising up through the demister units to be condensed by the condenser coils.
13. Once condensed, the fresh water droplets fall downwards to be collected in the plate collector tray. This can be witnessed through the sight glass
14. When the gauge glass level on the plate collector is more than ¾ full, put the salinity indicator to 'ON' position.
15. Divert the processed water to the bilges or return it to the feed system again till the salinity level comes to the required set value of 5-10 ppm (Parts per million)
16. When the salinity level comes to set value, open the discharge valve of distillate pump to the fresh water tank through the flow meter and start the pump
17. The evaporation rate can be increased by throttling the jacket cooling water return outlet
18. Check for the tank vacuum has stabilized.
19. Observe the following gauge readings on a regular basis;
- Inlet and outlet temperature of jacket cooling water
- Condenser sea water inlet and out let temperature
- Feed water level inside the shell
- Distillate level
- The salinometer is designed to alarm, automatically shutting the freshwater discharge valve to the storage tank, and dumping the distillate to bilge when maximum salinity is exceeded. It is always best to check the diverter valves are all operating as these can stick due to heat/coating of salts.
- The distillate level in gauge glass should be monitored and always maintained at half gauge glass level when the distillate pump is running
- Remember to check the main engine jacket water cooling temperatures once the freshwater generator has settled down.