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What is crankcase explosion?
Having learnt about one class of fire danger on a ship namely scavenge fires, we will now talk about another possibility of fire rather explosion of the engine and this could occur within the crankcase of the marine diesel engine. Crankcase explosions are also the result of high operating temperatures of the engine. The main cause of crankcase explosions are the development of hot spots at various places in the crankcase. Due to the reciprocating motion of the piston the lubricating oil in the crankcase is splashed in the air. Now it is necessary that the flash point of the lubricating oil is maintained at around 200 degree Celsius. If this is not done then there are high chances for the lubricating oil to catch fire.
Hot spots are created in the crankcase as a result of
- high temperature due to the reciprocating movement of the piston,
- increase in bearing temperatures,
- sparks entering the crankcase due to leaky piston rings or piston blow past,
- fires in the adjacent scavenge trunks.
Now, when these hot spots come in contact with the oil in the crankcase, the oil gets vaporised. When these vaporised particles travel to the cooler part of the crankcase they get condensed into a white mist which has oil particles properly dispensed in it. The process that takes place is some what similar to atomization. This white mist when again travels to the hot spot area, can easily catch fire, which might also lead to an explosion. The fire or the explosion create immense pressure inside the crankcase and if this pressure crosses the permissible limit, crankcase explosion takes place. The explosion will rupture the crankcase doors and even cause heavy damage to the inside of the engine.
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Indication of crankcase explosion
It is a bit difficult to read the early signs of a crankcase explosions. This is because the indications are similar to many other emergency situations. But there are few pre-explosion signs that can be read. Crankcase explosion will lead:
- Sudden increase in the exhaust temperature
- Sudden increase in the load on the engine
- Irregular running of the engine
- incongruous noise of the engine
- smell of the white mist.
In case of these indications, engine speed should be brought down immediately and the supply of fuel and air should be stopped. The system should then be allowed to cool down by opening the indicator cocks and turning on the internal cooling system.
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Prevention of crankcase explosion can be done by preventing the generation of hot spots. It can also be prevented by the following ways:
- By providing proper lubrication to the reciprocating parts,thus avoiding high temperatures.
- Avoiding overloading of the engine
- Using bearings with white metal material which prevents rise in temperature.
- Using oil mist detector in the crankcase with proper visual and audible alarm.Oil mist detectors raise an alarm if the concentration of oil mist rises above the permissible limit.
- Pressure relief valves should be fixed on the crankcase for the instant release of pressure. They should be periodically pressure tested.
- Crankcase doors should be made of strong and durable material. Vent pipes shouldn't be too large and should be checked for any choke up
- Pressure relief valves should be provided with wire mesh to prevent the release of flames inside the engine room.
- Safe distance should be kept from the crankcase and the relief valves in case the indications are sighted.
- In case of indication, the crankcase doors should never be opened till the time the system has totally cooled down. Once the system has cooled down, proper inspection and maintenance should be carried out.
- Fire extinguishing medium should be kept standby . In many systems , inert gas flooding system is directly connected to the crankcase.