air, compressors, air compressors, compressed air, reservoirs, pneumatic, actuators, overhaul, crankcase, valve, cooler, lubricating oil, carbonizing, heat transfer, relief valve, intercooler, pressure

Air compressors - the heart of the starting air system

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air, compressors, air compressors, compressed air, reservoirs, pneumatic, actuators, overhaul, crankcase, valve, cooler, lubricating oil, carbonizing, heat transfer, relief valve, intercooler, pressure

Diesel generator sets are started with compressed air

Main air reservoirs for starting main engine

Reciprocating air compressors can achieve pressures up to 30 Bar

3 units of diesel generators

Synchronizing 2 or more generators running in parallel

Air compressors - the Heart of the Starting Air System

The air compressor is very important for a diesel operated ship because it is the heart of the starting air system for the diesel electrical generators and the main diesel engine.

The compressed air stored at 25 to 35 bars in air reservoirs must have the capacity to start the main engines for at least 13 times before it becomes too weak. Thus the marine engineers who are maneuvering the ship must try their best to kick start the engine without using too much of the air.

This comes with practice. Inexperienced engineers will hold the starting lever of the main engine for too long and a lot of the air will be consumed. This can be dangerous especially in narrow and congested waters of a busy port.

Another place air can be lost is the pneumatic control systems. Because of hazardous combustible atmospheres in some areas of the ship, and also because air systems do not depend on electrical supply to operate, pneumatic control systems using nozzle-flapper controllers, pneumatic positioners and actuators are often used.

Below are some practices that engineers should observe when they are operating and maintaining air compressors:

During overhaul:

  • Grit or dirt must be prevented from entering any part of the unit. As with any machines, these will give rise to numerous problems later.
  • When opening any pipes, the ends need to be covered up to prevent dirt from entering the system. Any oil pipes need to be blown through with air.
  • If crankcase oil is changed, clean the crankcase. Do not use waste that may leave lint because they can get trapped in the system.
  • Never use any inflammable liquid to clean the crankcase.
  • All air passages must be examined and cleaned, and blown through with high pressure air to remove loose material.
  • Valves must be cleaned and examined for cracks, marks, or scratches. If found, the valves must be replaced, because if any small pieces from the valve were to break they can cause severe damage to the compressor.
  • The amount of oil must be checked at intervals. Too little of it will cause wear down of the parts. If too much, carbonizing of valves can occur, resulting in loss of compressor efficiency, and loss of cooler efficiency.
  • Check on the oil condition. If unsuitable oil is used, it may emulsify with the water vapor contained in the air, resulting in loss of lubricating quality and wear down of parts. Unsuitable oil may also evaporate and condense and deposit at the cooler tubes resulting in poor heat transfer, which can lead to carbonizing of valves.
  • Keep records of the number of working hours so that oil can be changed at the correct intervals.

After overhaul.

  • Turn compressor a few revolutions by means of the turning gear to ensure that all moving parts are free to move.
  • Open cooling water inlet and outlet valves. Check for leaks
  • Start compressor. Allow to idle for a short time in order to build up ample supply of lubricating oil over all the moving parts.
  • Check operation of lubricating pump and cooling water pump.
  • Examine relief valves and check that they are free to operate
  • Stop the compressor and make a check of all the bearings so that no part is unduly hot
  • Check thermometer readings at inlet and outlet of intercooler that they are showing correctly.

Something to note regarding intercoolers and after-coolers:

  • Moisture in the hot air which have condensed into water is extracted via the drains.
  • Inter-cooler and after-cooler water spaces must be fitted with a relief valve or bursting discs so that if a tube should burst the valve will open and so prevent the casing from being subjected to excessive pressure.

Engineers must take care of the compressors diligently, especially the cooling system. If the compressor becomes overheated, usually many other problems will occur. Because of the overheating, carbonization of the valves and even tubes can take place. The latter will result in poor heat transfer and more overheating. The compressor air might become so overheated that even the valve springs may lose their elasticity and fail. In the worst case, the compressor may even explode due to heat, air and fuel present together.

Article on explosive air.


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