Maintenance

The main problem with fresh water generators is scale formation on the heating surfaces. Chemical injection systems are fitted to allow anti-scale chemicals into the feed line to help prevent scale formation. This system is quite often neglected, however, it is important to ensure the flowmeter is working correctly and the correct amount of chemical added to the treatment tank.

Once scale forms on the heating surfaces the evaporator output will fall off rapidly. The best cleaning method is to circulate descalent chemicals through the heater tube stack.

Chemicals should be circulated for a period of time and the chemical manufacturer's instructions should be referred to. The descaling chemical usually has a built in colour indicator to show when it has been exhausted. The tube stack should then be drained and flushed with fresh water. It is impossible to determine the condition of the tubes without removing the lower cover therefore the performance of the unit should be noted after each cleaning session.

Occasionally the fresh water generator should be inspected internally. Older models may have a rubberised coating which may start to blister or crack. Any damaged areas should be cut away and the metal underneath thoroughly cleaned of rust and then allowed to dry. Only specially approved coatings should be used for repair and the instructions should be followed carefully as ventilation may also be required. Modern fresh water generators have no coatings and are constructed from corrosion resistant materials.

When reassembling after maintenance, ensure all joints and connections are in good condition and have been tightened properly.

Table 3. Fresh Water Generator Problems

PROBLEM	CAUSE	ACTION
High Salinity	Capacity too high Evaporation temperature too low Brine level too high Condenser tube leaking Defective or dirty salinometer cell	Reduce feed Reduce sea water cooling Ejector nozzle clogged or ejector pump overboard closed Find and plug leak Replace/clean cell
Low Capacity	Heater tubes fouled with scale Air Leaks (low vacuum) Defective ejector (low vacuum) Low jacket water temperature Defective distillate pump	Clean Find leaks and seal Inspect and replace Increase temperature Check pressure and load

4. Questions that could be asked at the exam with regard to

   1. Pumps

1. Why is the gear pump so called?

2. What are the general problems with the gear type pump?

3. What is the main difference between gear pumps and screw ones?

4. Into what types are positive displacement pumps categorized?

5. What special features of reciprocating pumps do you know?

6. Why do reciprocating pumps require much maintenance?

7. How are positive displacement pumps filled?

8. What does the pump maintenance include?

9. Can centrifugal pumps handle all fluids?

10. What device converts the kinetic energy of the water into the pressure energy?

11. How is the discharge rate controlled?

12. Should you always use manufacture’s recommendations to check clearances?

13. What does maintenance of the pump consist of?

14. How is it possible to provide correct alignment of the motor and pump? What measuring gauges are necessary?

15. What requirements should the pump drive belt meet?

16. How should pump glands be kept?

17. What order is recommended when replacing gland packing?

18. How can you reduce the leakage during the first running of the pump?