Reducing risks of crankcase explosions

Crankcase explosions which are violent and cause the maximum damage are thought to be caused by turbulent flame fronts having enough room to accelerate to sonic velocities thus raising the crankcase pressure high enough in a very short time to have serious consequences. The large two stroke engines currently being built have a crankcase volume approaching 600m3 with overall lengths of 23 metres. Experience has shown that in two similar engines having crankcase explosions, one where the ignition source was close to the centre of the engine caused minimum damage, whilst the other, where the ignition source was at one end of the engine suffered a severe explosion in which crankcase doors were blown off.

Therefore it should be possible to limit the consequences of an explosion which travels the length of the crankcase by subdividing the crankcase into separate compartments. However this might not be as simple as it first sounds. To begin with the subdividing bulkhead would have to be removable for maintenance purposes. And if it was removable it would have to be put back correctly to maintain adequate strength and subdivision. ( People have enough trouble putting double skinned fuel pipes back correctly; how do you think they would manage with crankcase subdivisions?) Lastly, no engine builder is going to carry out this form of modification unless he is forced to by regulation. It would make his engine more costly and therefore less competitive.

Once an oil mist has built up due to an overheated bearing etc, it may be possible to prevent ignition of the oil mist. MAN B&W have done experiments and taken out a patent on injecting water mist with a flame retardant chemical, the droplet size the same as that of the oil mist. The idea being that the energy from the flame would be absorbed by the fully mixed water droplets, and that the water droplets would also displace any oxygen present. Tests carried out have shown that while the water mist principle functions well at low revolutions, when engine revolutions are increased to normal working speed, the water mist is destroyed. This gives limited functionality, but could be used in conjunction with engine slow down on a high oil mist level being detected in the crankcase.

There has been an idea put forward that once an oil mist has been detected of injecting an inert gas fast enough and at high enough pressure to displace the air in the crankcase. However, the risk here is that not only may the relief valves lift allowing the oil mist/air to be displaced into the engine room, but also any static charge releases with the gas, could ignite the oil mist/air mixture in the crankcase.

Another idea which has been under discussion is the possibility of inerting the crankcase using nitrogen on a permanent basis. This is an idea which the Royal Navy has been investigating for the past few years. The RN has suffered several oil mist explosions in their turbine gearboxes, which have caused extensive damage and loss of life.

The nitrogen would be provided by nitrogen generators: - these are already in use for inerting cargo tanks on chemical tankers

The nitrogen generator is based on hollow fibre membranes. The system works by passing compressed air through a bundle of hollow fibre, semipermeable membranes. The membranes divide the air into two streams, one is essentially nitrogen and the other oxygen plus carbon dioxide, moisture and other trace gases. The capacity of the membrane generators will be dependent on the required purity, air pressure feed and air temperature feed.

The crankcase cannot be completely filled with nitrogen, excluding all oxygen, because lubricating oil e.p. additives require oxygen to work. Instead about 5% oxygen will allow the lubricating oil to maintain its properties, but keep the oxygen content below that necessary for an explosion to take place.

The main arguments against inerting the crankcase are the cost and space required for a nitrogen generator, which for a large 2 stroke engine may take up the space of 2 container units, and it must be also borne in mind that before any maintenance can be carried out, the crankcase must be vented which will add to any down time. Nitrogen is only slightly lighter than air, and so venting the crankcase would have to be done using fans.


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