Thursday, January 9, 2020

Fresh water generator

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.
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
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
Parts of Fresh water generator:

1 ) Heat Exchanger

A heat exchanger allows for the heat to move from high temperature medium towards the low temperature medium; without actual contact in between. In the process a medium loose heat while the other gained it. A heat exchanger basically consists of a number of segregated elements; in form of plates or tubes with high thermal conductivity

2 ) Demister

A demister is a thickened layer of mesh structure; fitted in between the evaporator and the condenser element. This is used to separate sea water from the steam vapour. A demister can be made of nickel, monel metals, copper, stainless steel and synthetic fibers; such as Polypropylene and PVC. Typically; demisters made of monel metal are used for fresh water generator.

3 ) Ejector Pump

Both Brine and air ejector in combined are called as ejector pump in general. In maritime industry specially with marine engineers; the sea water pump supplying operating water to these ejector are many a times also considered in ejector pump. An ejector is similar to that of eductors with no moving parts.

4 ) Distillate Pump

A distillate pump is a normal centrifugal pump located at the lower most part of the fresh water generator. It takes suction from the steam condensate; and discharge to the drinking water tank on ship. The output pass through a salinometer which checks for the salt content in the output water.

5 ) Salinometer

A salinometer is the device installed on fresh water generator; capable of detecting even the slightest of salt content in the sampling water. It is connected to the distillate output just before the solenoid operated three way valve. The salinometer output is feed to the control panel; which then based on required salt limit will send fresh water for storage or back to generator.

6 ) Sea Water Pump

Since the both ejectors works on the principle of venturi effect; you need something to provide them with the operating water. This is where the sea water pump came into play; it not only pass through the ejector creating suction pressure but also provide feed water for the fresh water generator to produce steam

7 ) Control Panel

A control panel is what that makes it easier and possible to control and operate any machinery including fresh water generator. It is what that automate the starting and stop procedures; letting us just to monitor and see if its all well. It just makes it easier for us to start, stop, test alarms, test salinometer and set desired ppm level.

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