Answer EKM Question 40

Q40. With respect to Air starting systems for 2 strokes diesel engines: A. Sketch and describe Main engine starting air distributor. B. List the safety devices and interlocks incorporated in main engine air starting system and state the purpose of each.

Answer:
Sketch and operation of distributor:

The Air to the main engine starting air line passes through the automatic valve, a non-return valve, flame trap then on the staring air manifold fitted on the engine. The manifold has a connection to each cylinder air starting valve fitted in the cylinder cover. A separate air line passes from the automatic valve to the pilot air distributor. A stop valve is often fitted in this line for use when testing starting air valves for possible leakages.

Starting air pilot valves are driven by negative (inverse) cam which have recesses instead of lobes. The air is admitted forcing the pilot valve onto the cam. Thus air can floe to actuating pilot of cylinder air starting valve and opening it in sequence. The crankshaft starts rotating and thus the starting cam. The cam pushes the pilot valve upward cutting off the admission of the control air and at the same time opening a venting channel for the purpose of removing the control air pipe.
A positive cam could have resulted in reciprocating motion of the pilot valve even if starting is not required while at sea. Therefore by providing negative cam pilot valves are already off the cams unless air supply is needed.

B. The safety devices
(a). Non-return valve in the starting airline, closet to the main engine.
(b). Pressure relieving devices in the starting airline viz Bursting Disc.
(c). Relief valve In each cylinder head of the main engine.
(d). Flame trap to spark arrestor.
(e). Means to prevent Starting Air admission to a unit which is already firing.
(f). Fusible plug /relief valve on air bottles.
(g). Relief valves on each stage of main air compressors.
(h). Un-loaders on main air compressors.
(i). Bursting discs on inter/after coolers.
(j). High air temperature alarm for compressors.
(k). Overload trip for the motor for driving the compressor.

Interlocks: they are provided so that the engine can be started or reversed only when certain conditions have been fulfilled.
(a) Turning gear Interlock: This device prevents the engine from being start if the turning gear is engaged. The turning gear is provided for turning the engine. 
(b) Running direction Interlock: This prevents the fuel from being supplied if the running direction of the engine does not match the telegraph.
(c) Starting air distributor in the end position: This prevents starting if the shifting of she distributor has not been completed.
(d) Main Lube oil low pressure: piston cooling pressure jacket water, pressure and important parameters must be above the required minimum.
(e) Auxiliary Blower interlock: The auxiliary blower is provided in case of constant pressure turbo charging. This ensures, efficient air during low speed running, which Is essential for proper combustion.
(f) Air Spring Pressure Interlock: In case of the present generation of engines using exhaust valves shut by air spring.
if low air pressure then the exhaust valve may not close while running.
(j) Control air low pressure.
(h) Cam-shaft lube oil low pressure.
(i) Jacket water high temperature.