Main Engine start Air Systems

With reference to a main engine air start system:

state, with reasons, the purpose of the slow turning system and how it functions; state with reasons, Three safety features incorporated in an air start system;

State with reasons, why an engine may fail to turn over on air

Explain how the cause of an engine failing to turn over on air may be determined.

Ans)

The slow turning system consists of a small bore pipe complete with an automatic valve, bypassing the main air start automatic valve. When opened the slow turning automatic valve provides enough air for the engine to turn over slowly on air, but not fast enough to do any damage should water or fuel be sitting on the piston due to a leak, which would cause the engine to ‘hydraulic’, damaging head and running gear had full air throughput been used.

Two situations in which the slow turning valves is used:

On a slow speed engine, during maneuvering. If the engine has been on ‘stop’ awaiting a movement for, more than a set period (usually 20-30 minutes) then when an order is given and the engine control moved to start, instead of the main air start opening, the slow turn valve will open. If the engine satisfactorily completes a revolution then the main air start opens allowing the engine to start.

On a medium speed diesel electric installation where engines are on standby for immediate start, a slow turning valve may be timed to turn the engines every hour, an alarm sounding if they fail to complete 2 revolutions, However air used for this slow turning must be dry to prevent corrosion of liners over a period of time when the engines are not needed.

Non return valve: In case of a pressure rise in the air start manifold above that in the air bottle( for instance because of a detonation in the air start manifold) the non return valve will close.

Flame Trap: if a cylinder air start valve is jammed open, the flame from combustion could travel back down the air start manifold igniting any oil vapor in the line. To prevent this, a flame trap consisting of a brass plate with drilled holes is placed between cylinder air start valve and manifold.

Relief valve or bursting disc: placed in the air start manifold to relive any increase in pressure

Main air start valve closed: operator error, Turning gear engaged, or if disengaged, the air operated interlock switch may not be venting. If a CPP is part of the installation the pitch may not be set to zero( an interlock) .On a reversing engine, the camshaft may not be in the correct position for the direction required. The air start pilot valve for the particular unit receiving air may be sticking shut, the engine will therefore have a “dead spot”

The engine is not receiving the start signal from the bridge or control room.

Always check the simplest cause first. When preparing an engine for sea, it is usual to blow the engine over using compressed air before shutting the cocks. It is here that the problem may manifests itself.

Check that the isolators on the correct air receivers are open and that the automatic valve is in the automatic position .

Check that the turning gear is out and that no interlock warning lights are showing. on a unidirectional engine fitted with a CPP check that the pitch is at zero.

On a reversing engine check that the camshaft is in the correct position for the direction required. Now the engine should turn when the automatic valve is operated by hand, if not then the fault will probably lie with the pilot valve for a particular unit. It is not unknown, because of moisture in the air for these to corrode and stick. Note the position of the flywheel (mark with chalk)on a reversing engine try an astern start, on a unidirectional engine turn the engine so another piston has just come over TDC, disengage turning gear and try again. If the engine now turns when operating the automatic valve manually, try again from the control room.

If the engine will start locally, but not from the control room or the bridge, then the fault must lie with the air signal to he automatic valve. The investigation from here must follow a logical series of steps with the help of the manual and the air start drawings.

Engine system vary, but basically a pneumatic or solenoid valve allows an air signal to operate the main automatic valve. At the same time an air signal allows air to the air start distributor which will allow the cylinder valves to open in the correct sequence. Before the air signal can reach either of these devices, switches either electrical or pneumatic relays, must confirm that the previously mentioned interlocks are clear and that the camshaft is in the correct position. some engine builders include a fault finding system which involves checking the air start relays which are all together in a box by the engine side local controls. An indicator will show which relay valves are open and these are checked off against the fault finding list.