Overhauling Air Compressor on Ships

Onboard have different types of air compressors in use. There are main air compressors used for supplying high pressure air for starting of the main engine and auxiliary engines, control air compressor for supplying control air, topping air compressors for topping up and taking account of the leakages, and on some ships we have deck air compressors to give air for the use on decks.Generally two pressures of air are used: 30 bars high pressure air used generally for starting of the engines and 7 bars low pressure air used for control and general service air.In the choice of an air compressors for applications requiring air above 7 bars, we generally use reciprocating air compressors as it has a more positive sealing in comparison to the other types of compressors like rotary vane, centrifugal, rotary screw etc. In reciprocating compressors the compression ratio of 1:7 is the limit due to the rise in temperature, thus to get 35 bars pressure the machine has to be a double stage compressor with inter-cooling. Most of the marine compressors used are two staged reciprocating compressors with inter-cooling and after cooling.

On ships compressed air is used for a variety of uses some of which have been mentioned below:

• For automation and control of main engines, auxiliary engines, etc.
• For starting of the main engines, auxiliary engines, emergency generators, emergency fire pumps, etc.
• For heaving the life boat up by the use of an air motor.
• For use in fog horns and ships whistles.
• For use in pneumatic tools like pneumatic grinder, pneumatic chisels, pneumatic drills, pneumatic spanners, etc.
• For use in painting operations.
• For general cleaning.
• For soot blowing of the boilers.
• Used in pneumatic pumps for oil transfer.
• And used as general service air

Safety Features of Marine Compressors

Marine compressors have various safety features and cutouts installed so that they will trip when running in an unsafe mode. The compressors are reliable machines, but have to be maintained properly and in the proper intervals. There have been cases of the cylinder heads bursting and often resulting in fatalities. The various safety devices put in a marine compressor are as follows:

•  Lubricating oil low pressure trip
•  Cooling water high temperature trip
•  Motor overload trip and Motor overheating trip

Operating Procedure of a Reciprocating Compressor

• Lubrication: check the oil level in the sump and in the lubricator tank.
•  Unloading: the valves in the unloading lines must be open.
•  Pressure gauge cocks: the cocks must be slightly open to avoid excessive pressure fluctuations which can damage the pressure gauges.
•  Air filter: check that the air intake filter is clean.
•  Cooling: check that the cooling water pressure is correct and in the case of attached pumps a visual check of the pump and all the valves in the line must be open.
•  Relief valves: some relief valves have hand testing levers given for testing, the same can be done.
• Air line piping: check that all the valves in the discharge piping are open and correctly lined up.
•  Final check: the compressor can be turned a few revolutions with a turning bar to check for the free movement.

Overhauling Air Compressor on Ships – Marine Engineering

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.

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.

With any compressor, there are things that arise during the operation of the machine. Here are some of the most common cases with air compressors.

• The air compressor motor is overloaded
• Oil and water are in compressed air
• Oil flow suction valve
• The air compressor is overcurrent
• Air compressor does not reach the set pressure
• The air compressor does not start
• Low pressure in compressed air system

We, HAI HA M&S company have done a lot of works on repairing compressors in some ports in Vietnam such as:

• Hai Phong Port
• Vung Tau Port
• Ha Long Anchorage
• Campha Anchorage
• Nghi Son port
• Hochiminh Ports alongside