How Crankcase Explosion In Marine Diesel Engine? Indication & Prevention

> Crankcase explosions are mainly because of high operating temperatures of the engine

> High operating temperature and metal to metal contact between moving parts leads the development of hotspot at various places in the crankcase

> Crankcase is rich of lube oil, when these lube oil contact with the hotspot it will catch fire when leads to the explosion.

> It is necessary that the flash point of the lubricating oil be maintained at around 200 degree Celsius. If this is not done then there are high chances for the lubricating oil to catch fire.

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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 

Primary Explosion

    When the lube oil in the crankcase is in contact with the hotspot, the oil gets vaporized.  When these vaporized 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 somewhat similar to atomization. This white mist when again travels to the hot spot area, can easily catch fire, which might also lead to a primary explosion. The primary explosion might be mild and sufficient enough to lift the crankcase relief valves but there could be more severe and dangerous effects during secondary explosion

Secondary Explosion

   Primary explosion produces a shock wave which has a breaking effect which further reduces the size of oil droplets, producing more fuel for ignition. Now the air enters into the scavenge space through leaky piston gland or leaky relief valves. This new air and new supply of fuel produced after the first explosion comes in contact with hotspot, causing another explosion, which is extremely severe as the amount of fuel is high now. This explosion is known as secondary explosion and it causes very severe damage to engine plating

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Pre-Explosion Signs of crankcase explosion

> 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. 

How To Prevent Crankcase Explosion  

Preventing the generation of hot spots can prevent crankcase explosion.

> Proper lubrication which avoids metal to metal contact. 

> Avoid sudden change in engine load and overloading

> Use oil mist detector in crankcase with alarms

> Pressure Relief valve fitted on crank case should be tested Periodically

> 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.

> Crankcase doors should be made of strong and durable material.