Fresh water is an
important commodity on board ships, especially on long sea voyages. Hence the
procedure for starting the fresh water generator on ships should be familiar to
the senior watch-keeping engineer as they need to be started as soon as the
ship enters open sea.
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.
·
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
·
Start-Up Procedures
The fresh water
generator should be fired up once the above regulations and requisites have
been achieved; using the guidelines listed below.
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
Notes:
·
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.
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