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Crankcase oil mist detector of a marine diesel engine

Consequence of oil mist in the crankcase
: The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.



The presence of an oil mist in the crankcase is the result of oil vaporisation caused by a hot spot. Explosive conditions can result if a build up of oil mist is allowed. The oil mist detector uses photoelectric cells to measure small increases in oil mist density. A motor driven fan continuously draws samples of crankcase oil mist through a measuring tube. An increased meter reading and alarm will result if any crankcase sample contains excessive mist when compared to either clean air or the other crankcase compartments.

The rotary valve which draws the sample then stops to indicate the suspect crankcase. The comparator model tests one crankcase mist sample against all the others and once a cycle against clean air. The level model tests each crankcase in turn against a reference tube sealed with clean air. The comparator model is used for crosshead type engines and the level model for trunk piston engines.

Crankcase Explosions countermeasures

Early detection of over-heating and the immediate stopping or slowing down of the main (or auxiliary) engine, as circumstances permit, will avoid conditions which might form an explosive atmosphere in the crankcase.

Oil mist detectors and / or bearing temperature monitors are fitted to engine crankcases to detect any build-up of a potentially explosive oil mist.

All alarms from oil mist detectors and bearing temperature monitors, (no matter how suspect the instrument may be), are to be treated as an emergency situation and the engine stopped.

Operation of an engine with a positive alarm from a mist detector or bearing temperature alarm is only allowed under the jurisdiction of the Master in extreme navigational emergency and in any even, must be run as slowly as possible for as short a period of time as possible with no personnel in the engine room. Under no circumstances is the alarm to be reset until the main engine has been stopped and the cause positively identified.

The most likely cause of such an alarm will be an overheating bearing, but could also be caused by the possibility of heavily leaking piston rings and / or piston rod stuffing boxes. Great care must be taken if the main engine has been stopped due ot a positive alarm from an oil mist detector or bearing monitor. Under no circumstances should anyone approach the engine or open a crankcase door until sufficient time has been allowed for cooling and mist dispersal.

If no manufacturer’s plate or label is attached to the crankcase, then the following instructions regarding the prevention of explosions and fires in crankcases are to be prominently displayed on the engine and all Engineer Officers are ot be familiar with them.

All alarms from oil mist detectors no matter how suspect the instrument may be, are to be treated as an emergency situation and the engine stopped. Refer to section, Crankcase Explosions. All Oil Mist detectors must be serviced on an annual basis Alarms proven to be false must be reported to the Management Office immediately.



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