Question

(a) Sketch and describe Main Engine starting air distributor. (8)

(b) List the safety devices and interlocks incorporated in main engine air starting system and state the purpose of each. (8)

Answer 1

Part (a)

The starting air valve is pneumatically operated by the air distributor shown above in the sketch.
When the engine starting lever is operated, air is admitted to the distributor, forcing all pilot valves against the spring, onto the cam.
The pilot valve of the cylinder unit, which is in the correct position for admitting air, will be pushed into the depression of the cam.
In this position, ports 1 and 4 will be connected, and control air will act on top of the starting air valve to open it, admitting starting air to the cylinder. At the same time, ports 3 and 5 will be connected, and air below the starting air valve piston will be vented.
At the end of the starting air admission period in the cylinder, the pilot valve will come out of the cam depression, due to which ports 4 & 2 got connected & the opening air to the starting air valve is vented. Also, port 1 & 5 is connected, so closing air will keep the starting air valve in the closed position.

Part (b)

Flame Trap/Flame Arrestor: Prevents flames from entering the airlines and reaching the air bottles in case leaking air start valve
Bursting Disc: Releases excessive pressure in the starting airline
Relief Valve: Fitted on the starting air manifold to release excessive pressure.
Non-Return Valve: Prevents hot gases, flames, or sparks from travelling back towards the air bottles in case of a faulty air start valve, minimising the risk of explosion.
Turning Gear Interlock: Prevents the engine from starting if the turning gear is engaged.
Running Direction Interlock: Ensures the engine will not receive fuel if its running direction does not match the specified direction on the telegraph.
Starting Air Distributor End Position Interlock: Prevents the engine from starting if the distributor has not reached its correct end position.
Lube Oil Pressure Interlock: Prevents the engine from starting if the lube oil pressure is low
Auxiliary Blower Interlock: Ensures the engine will not start if the auxiliary blower is not in automatic mode.

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