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Horror Stories

A Recent Crankcase Explosion

 

The story starts when the oil mist detector alarm for No 10 unit (a QMI type) on a 12 cylinder 2 stroke engine went off when the Chief Engineer was in the engine control room together with two of his staff.

The Chief Engineer thought that the alarm was due to a dirty detector head of the Oil Mist Detector and immediately sent one of the engineers down in the engine room to check the detector heads.

The Chief Engineer noticed that the oil mist level was increasing in crankthrow spaces 10 and 11, so he called the bridge and asked the bridge watchkeeper to reduce the revolutions of the engine.

5 minutes later he noticed that the oil mist level had increased further. It was at this point that he cancelled the load reduction programme and reduced  the main engine revolutions to 75 rpm. At almost the same time a heavy explosion occurred in the engine room.

The engineer sent to clean the detector heads was standing close to a relief valve and was severely burnt.

A fire started in the engine room and purifier room caused by the shockwave from the explosion bursting an oil pipe. The fire was eventually extinguished by injecting CO2. (after blocking off access doors that had been damaged in the explosion).

The Cause of the explosion was an over heated main bearing (no 13). Pieces of white metal were found in the crankcase and subsequent investigations established that some of the white metal had detached due to fatigue and poor bonding. As the metal detached, the load on the existing white metal increased, due to the now reduced area to support the crankshaft journal leading to breakdown in the lub.oil film and wiping of the bearing. The crankshaft journal was now running on the bare steel backing.

When the explosion occurred, 4 doors on the "safe side" (the side without the explosion doors) blew off, starting the fire.

The vessel had to be towed to port for repairs which took 9 weeks.

The crankshaft journal was badly scored with extensive heat cracks. It was reduced in diameter and undersize bearings fitted.

It came to light in subsequent investigations that fragments of white metal had been found in the LO filters  2 weeks before the explosion. It was assumed that this was white metal from the guide shoes and no further action was taken.

 

It is easy to be wise after the event. However the oil mist detector is designed to protect, and a sophisticated set up like the QMI, which works on the light scatter principle, should be believed when it indicates a high oil mist. The first mistake was sending someone down to "clean the lenses". A crankcase explosion is a very serious occurrence. The engine room should be evacuated of all personnel not dealing with the situation from the control room. 

A seemingly simple solution is "as soon as the alarm sounds, the engine should be stopped" But if this is the case why isn't the oil mist detector linked to the engine shut downs. On some ships it has been, but consider this: The ship may be manoeuvering in a risky situation, or there may be a shaft alternator in use.

The general procedure should be this. On a bridge control set up with the engine room in UMS mode, if alarm conditions are reached, the auxiliary generators should run up, load transferred, and the shaft alternator isolated. After a 10 second delay the engine should immediately power down to 50% full load, bypassing the load control programme. This reduction in load will reduce the likelihood of a crankcase explosion. The delay is to give the navigating officer time to override the slow down incase safety of the ship is at risk. Once the engine has slowed down it should be stopped as soon as it is safe to do so.

If the engine room is manned, and on bridge control, the watchkeeping engineer may choose to take control of the engine, reduce the load and stop as soon as it is safe to do so. Once the engine has stopped, the cooling and LO must be kept running, the turning gear should be engaged and the engine turned continuously. The engine room should be evacuated and time given (30 minutes) for the engine to cool down before opening up the crankcase for inspection. 

Purifiers are fitted in separate rooms for a reason: Fire risk. The doors should be closed and dogged shut when not passing through them.

What about the white metal found in the filters? If, instead of making assumptions, the crankcase had been opened up they may have found the white metal in the vicinity of the damaged bearing.

The fact that the oil mist detector alarmed for a unit not adjacent to the overheated bearing should be noted. The oil mist can circulate within the crankcase. This should be borne in mind when investigating the cause of a oil mist alarm.

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