Marine Engineering Study Materials

Precautions Before Overhauling

Air compressor may be overhauled for planned maintenance or stripping down for survey purposes. First of all, make sure the spare parts are available on board. This will include joints, gaskets, major spare parts such as piston, piston rings, bearings, etc.

Unless the compressor has suffered a major failure, it would be better to leave overhauling air compressor until the vessel has berthed, especially on a reversible engine with fixed pitch propeller. This is in case there is a high demand for compressed air during manoeuvring.

For survey purpose, ensure the following papers are readily accessible: copies of previous survey reports, running hours since last survey, copies of planned maintenance reports, recorded clearances and manufacturer’s recommended tolerances.

Overhauling Air Compressor

The compressor to be isolated both electrically and mechanically and locked out. ‘MEN AT WORK’ notices are posted. The oil and cooling water is drained, compressor is stripped down removing cylinder head and valves, piston and connecting rod. The circlips, gudgeon pin and top end bearing are removed from the piston using proper tools. The piston rings are removed. The parts to be cleaned for survey, but no evidence of wear or damage is removed. The parts are gauged (ring grooves, piston pin diameter, bearing clearances) and the readings are recorded. Safety devices are checked. Hydraulic tester can be used to test the opening pressure of relief valves.

Air Compressor Assembly

Major parts such as piston, liner (replaceable type), bearings, etc. to be inspected and renewed if necessary. Piston rings would also be renewed as would the gudgeon pin and top end bearing. Before fitting new piston rings, butt and axial clearances to be checked. Ensure correct tool is used to guide the rings to the cylinder bore. All nuts to be torque tightened to correct settings and with new locking devices such as split pins. Suction and discharge valves must be renewed or overhauled. Checking bumping clearance is necessary after a major overhaul. Normal value of bumping clearance for a two stage main air compressor is about 0.5 mm. Use modern ‘squeezable plastic’ or ‘lead wire ball’ for measuring bumping clearance. After fitting cylinder cover, turn flywheel by hand to make sure it is free to turn.

Clean or renew crankcase oil suction filter and refill crankcase with new oil. Renew lube oil filters and air filter. Open cooling water valves and check for leakages. Turn the flywheel continuously by hand to prime the lube oil pump. Conduct an initial running of compressor in unloaded condition and check motor amperes, noise, etc. Then load the compressor and then check for bearing overheating. Once everything found satisfactory, test run the compressor and measure time taken to fill the air bottle from empty to full.

Keep all the records of clearances, spare parts consumed, etc. and sent to the company. Raise requisitions for necessary spares immediately. Update the work done and running hour in computer based planned maintenance system (PMS).