3.6 Operation of a compressed air system
The main aim of a compressor, as the name suggests, is to compress air or any fluid in order to reduce its volume. Some of the main applications of compressors onboard ships are main air compressor, deck air compressor, AC compressor and refrigeration compressor. Failure to start or control air compressor can be inconvenient, costly and can carry risks, which need to be managed. Compressed air has many uses on board ship, ranging from diesel engine starting to the cleaning of machinery during maintenance. Ships compressed air systems are usually divided into one or more of the following:
a. Main and auxiliary engine starting air system (high pressure), b. Service air, c. Control air, d. Instrument air, e. Deck services
High Pressure System
The Main and Auxiliary Engine starting air supply is to be monitored carefully for excess oil and water. Where compressors are fitted with cylinder lubrication, then particular attention to the cylinder oil feed is required. Moisture traps and drain valves are to be blown down at regular intervals. Should the blowdown indicate excessive oil or moisture then the reason is to be immediately investigated.
All operating valves in the starting air system are to be operated slowly and with extreme caution. Do not stand directly in front of a high pressure valve when opening or closing. The Engine Room Staff are to be made fully aware by the Chief Engineer of the importance of correct operation and maintenance of the starting air compressors, and the effects of their failure during manoeuvring of the vessel. During manoeuvring of the main engine the starting air compressors are to be monitored locally for any sign of overheating or problem.
Low Pressure System
Compressed air equipment which supplies low pressure air for control or instrument purposes must be monitored constantly for moisture or other contaminants. Air, which is wet or contaminated, will have detrimental affects upon engine manoeuvring systems and process control. All filters and moisture traps are to be cleaned and serviced as per the maker’s instructions or at intervals determined by operational experience. Short cycling of compressors indicated excessive consumption (leakage) must be investigated immediately. Long periods of compressor running are also to be investigated.
Dryers
For engine room service, refrigerant tap driers are normal practice. These
units give long service with little maintenance, however, manufacturer’s
instructions must be adhered to. In some cases, absorption type dryers are
fitted, which requires strict change over routines.
Deck Service Compressors
Unwarranted use of these machines is to be avoided at all times. It is
essential that any deck services are shut down when not required. Supply from
the main starting compressors via reducing valves is not to be used for deck
general service air supply unless an emergency exists.