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Crankcase Explosion

The Maritime Industry is extremely safety diligent and is assiduous in ensuring that all the prospects of an accident or an explosion is negated. However, there are some explosions that are often overlooked due to their rare chance of occurrence but are equally ruinous. One such explosion is the crankcase explosion which has always been and is still a severe threat to the operation of large 2-stroke and 4-stroke engines. As the name suggests, it is an explosion of high intensity that occurs inside the engine’s crankcase and can even lead to the complete loss of propulsion of the ship.

What is a Primary and Secondary crankcase explosion?

A Crankcase Explosion consists of two major stages:

  • Primary Explosion: This occurs when the three components required for a fire combine in the crankcase . In this type of explosion, the oil mist formed from the lubricating oil gets ignited by the hotspots created by friction between the moving components in the presence of oxygen to initiate an explosion. This only occurs when the ratio of oil mist to oxygen in the crankcase exceeds the lower explosion limit, and the oil mist comes in contact with a hotspot with an adequate temperature. 
  • Secondary Explosion: The Secondary Explosion follows the Primary Explosion and is far more devastating than its predecessor. The flame front of the primary explosion travels down the crankcase with a shock wave in front of it. The turbulence caused by this shockwave leads to further breakdown of the oil droplets. The pressure inside the crankcase increases substantially and causes the explosion to be vented via the relief valves. This leads to a drop in the crankcase pressure which invites more air into the crankcase via relief valves and leaky piston glands. This leads to the formation of another flammable mixture, resulting in the secondary explosion. This explosion is far more ravaging than the primary explosion and causes heavy damage to the engine, people, and all other objects near it.

How is oil mist formed in the crankcase?

Under normal working conditions, the size of the lube oil present in the crankcase is too large for its rate of incineration to be adequate to initiate an explosion. However, when a mechanical default is formed in the crankcase due to friction between moving parts, a localized hotspot will be developed. When the lube oil splashed in the crankcase comes in contact with this hotspot which is at a temperature of around 200 ⁰c, it gets vaporized. These oil vapours condense to form a white oil mist of 5-10 micron diameter when it travels to the cold regions of the crankcase.

What happens in the crankcase?

The crankcase of a marine internal combustion engine houses the engine crankshaft. It consists of a lot of lubricating oil, which is splashed around in it. As discussed above, this lube oil vaporizes to form an oil mist, and when the concentration of the oil mist is sufficient (50mg/l), it reaches its lower explosive limit and gets ignited by the hotspot. The explosion causes an increase in pressure inside the crankcase, and more air is sucked in, rendering a more intense explosion.

How is a crankcase explosion prevented?

Crankcase Explosion, although very ruinous, can be averted by adopting a few preventive features. The two main features provided to prevent a crankcase explosion are:

Oil Mist Detector: It is a safety device that is fitted on the main engine to monitor the presence of oil mist in the crankcase environment. This device inspects whether the concentration of the oil mist in the crankcase is below the lower explosive limit and provides an early indication which helps prevent the primary explosion. This is done by collecting continuous samples from the crankcase through sampling points with the help of an extraction fan and comparing it with a sample of fresh air in the reference tube using a light source and photoelectric cell. Once the deviation reaches a specific limit, an alarm and engine slowdown is initiated.

Oil Mist Detector
Oil Mist Detector

Crankcase Relief Doors: All engines with a bore of more than 200mm are fitted with this spring-loaded valve, which helps in preventing any damage to the crankcase and also prevents the ingress of fresh air from outside. They lift up to release the built-up pressure inside the crankcase and closes instantly as the pressure drops. They are closed instantaneously to prevent the ingress of fresh air, which can lead to a secondary explosion.

Crankcase Relief Door

Some other preventive measures that can be followed by the crew onboard are:

  • Check for proper lubrication between rubbing surfaces like crosshead guides.
  • Frequent inspections of the bearing clearances to check whether they are in the acceptable range.
  • Regular inspections of the quality of lube oil and lubrication passages.

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