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Crankcase explosion

The atmosphere inside a crankcase is stable and will not allow combustion or an 
explosion to occur as there is no ignition or fuel source. 
Hence the first event is the production of an explosive mixture. This will occur when 
the lube oil in the crankcase is heated by a “hot spot” and lube oil coming into contact 
with this will be evaporated. The evaporated oil then rises within the crankcase, and 
then condenses in a cooler part of the crankcase. The resultant white mist is within the 
explosive envelope, and is thus flammable. 

The second event is the ignition of this white mist by either the same or another hot 
spot within the crankcase. When the oil mist is ignited, a crankcase explosion will 
occur, which will raise the pressure within the crankcase. 
One of the common areas of overheating is the various bearings within the crankcase. 
Hence bearing temperature monitors could be used to indicate that a bearing is 
overheating and could be oil mist generation site. 

The rapid pressure rise within the crankcase can cause the engine structure to be 
blown apart, causing physical damage, and the resultant flame travelling across the 
engine room space causing personnel injury. This pressure rise is limited by the 
statutory use of relief doors fitted to the crankcase. These doors will open when the 
pressure rises above 0.02 – 0.1 bar, and prevent the over-pressure of the engine 
structure. The doors also perform the added function of preventing fresh air ingress 
into the crankcase where hot burning gases are present, by the quick closing action of 
the relief door. 

As the explosion is an uncontrolled event, then great care must be taken to ensure the 
safety of the engineers within the engine room. MAN B&W recommend that 
1. Move away from the crankcase doors immediately 
2. Reduce speed to slow, and ask the bridge to stop 
3. When the engine has stopped, close the fuel supply 
4. Stop the auxy blowers 
5. Open the skylight and/or stores hatch 
6. Leave the engine room 
7. Lock engine room entry doors and keep away from them 
8. Prepare the fire fighting equipment. 
9. Do not open the crankcase for at least 20 minutes after stopping the engine, and 
ensure that the oil mist detector alarm (or bearing temperature monitor) has reset 
10. Stop the LO circulating pump. Shut the starting air, and engage the turning gear. 
11. Locate the “hot spot” (source of the oil overheating) 
12. Make a permanent repair to the fault

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