MAINTENANCE
MAINTENANCE
As an Engineman, maintaining the refrigeration and air-conditioning plants may be one of your responsibilities. To do this, you must understand the maintenance procedures. In most instances, personnel who are assigned to maintain refrigeration plants are graduates of the Navy’s air-conditioning and refrigeration school. This school teaches most operating and maintenance procedures. However, you should refer to the manufacturer’s technical manuals for the details of the plants on your ship.
Testing Suction and Discharge Valves
Faulty compressor valves may be indicated by either a gradual or a sudden decrease in the normal compressor capacity. Either the compressor will fail to pump, or the suction pressure cannot be pumped down to the designed value, and the compressor will run for abnormally long intervals or continuously. You may get a rapid buildup of suction (crankcase) pressure during an off cycle. This causes the compressor to start after a very short off-period and indicates leaking discharge valves.
If the refrigeration plant is not operating satisfactory, you should first shift the compressors and then check the operation of the plant. If the operation of the plant is satisfactory when the compressors have been shifted, the trouble is in the compressor and not in the system.
To test the compressor discharge valves, pump down the compressor to 2 psig. Then stop the compressor and quickly close the suction and discharge line valves. If the discharge pressure drops at a rate in excess of 3 pounds in a minute and the crankcase suction pressure rises, this is evidence of compressor discharge valve leakage. If you must remove the discharge valves with the compressor pumped down, open the connection to the discharge pressure gauge to release discharge pressure on the head. Then remove the compressor top head and discharge valve plate. Be careful not to damage the gaskets.
If the discharge valves are defective, replace the entire discharge valve assembly. Any attempt to repair them would probably involve relapping and would require highly specialized equipment. Except in an emergency, such repair should never be undertaken aboard ship.
You can check the compressor internal suction
valves for leakage by following these steps:
1. Start the compressor by using the manual control switch on the motor controller.
2. Close the suction line stop valve gradually to prevent violent foaming of the compressor crankcase lubricating oil charge.
3. With this stop valve closed, pump a vacuum of approximately 20 in.Hg. If this vacuum can be readily obtained, the compressor suction valves are satisfactory.
Do not expect the vacuum to be maintained after the compressor stops, because the refrigerant is being released from the crankcase oil. Do not check the compressor suction valve efficiency of operation for at least 3 days. It may be necessary for the valves to wear in.
However, if any of the compressor suction valves are defective, you can pump down the compressor, open it, and inspect the valves. Replace defective valves or pistons with spare assemblies.
