Marine Engineering Study Materials

General Problems of Air Compressors on Ships

Most of the problems associated with air compressors lead to increase in running hours. They are:

• Leaking automatic drains. The reason may be deposition of dirt in the valve seat.
• Leaky relief valves.
• Unloaders not operating correctly.
• Damaged or worn suction and discharge valves.
• Dirty air filter.
• Leaks in the compressed air systems, such as service air to deck, control air systems, etc.

Since the increase in compressor running hour is gradual, it will go unnoticed. Because of this reason, running hours to be daily logged. Any increase in running hour to be investigated and rectified. It is better to note down the time taken to fill the air receiver.

Clogging of air filter leads to reduction in volumetric efficiency of the compressor.

Wear on main and bottom end bearings increase the bumping clearance and reduce volumetric efficiency. Ensure correct thickness head gasket is used while overhauling.

If intercoolers are clogged air passing to the next stage will be less dense and at a higher temperature. this may cause overheating, vaporizing lube oil, increased wear down and possible explosion.

If non return valves on the discharge line of the compressor not seated properly, running hours of compressor increases.

Defective automatic drains cause oil and water to get carried to the air bottle. This leads to corrosion of the air bottle as well as service pipe lines. Again, oil film may be formed inside the pipe lines which can lead to disastrous starting air line explosions.

If planned maintenance is not carried out in time, efficiency of air compressors drastically reduces because of excessive wear in the components. Result is, compressors will not keep up with demand during manoeuvring.

Good Practices for Safe and Efficient Operation of Air Compressors on Ships

• Check oil level in sump. Record the amount topped up. Do not overfill. Excessive oil consumption or pressure build up in the crankcase to be observed, investigated and rectified as soon as possible.
• Drain all air bottles regularly.
• Ensure air to deck, service air is shut when not in use.
• Give attention to any air leakages reported and rectify.
• Daily record running hours and log it.
• Daily check automatic drains and unloader of air compressor for proper functioning.
• Weekly check the operation of the relief valves connected to the LP, HP stages by operating manually.
• Follow planned maintenance as per manufacturer’s manual.
• Air filter to be renewed every 500 hours.
• Compressor valves to be removed for inspection every 1000 hours.
• Do not reverse valve plates. There is a danger of fatigue cracking.
• Every automatic drain valves to be stripped, cleaned and checked every 2000 hours.
• Every relief valves to be stripped, cleaned and overhauled, set at correct pressure.
• Major overhaul to be carried out as per PMS.

Effect of Oil and Water Carry Over

• Water carry over in the air can lead to general corrosion and pitting.
• The air bottle is internally coated with a varnish to resist this attack.
• Main air starting lines to be regularly examined for corrosion and wastage.
• Oxidised lubricating oil in the air also cause corrosion.
• Valves in the starting air line system may jam and corrode.
• Corrosion may cause main air starting valve to stick open, which is a dangerous situation.
• Possibility of starting air line explosion.