When ship carry out such a long voyages the ships are made to carry a huge amount of lube oils, fuel oils and diesel. Though they help in propulsion and maintenance of the ships but they are also the source of major fire hazards which can lead to explosion also.
In this article, we discuss about one such explosion, which is supposed to be known as the most disastrous explosion take place in the ship’s main engine. It is called crankcase explosion. An article published in marine insight explain about crankcase explosion.
Here’s an excerpt from that.
What Is Crankcase Explosion?
It is an explosion that occurs in the crankcase because of the mixture of oil mist and air in the ratio that is within the range of flammability.
Cause Of Crankcase Explosion
The cause of the crankcase explosion is hotspot or overheated part within or adjacent to the crankcase of an operating engine.
What Is Hotspot And Why It Occur?
Hot spot is the heat source produced as a result of rubbing between two metal surfaces or friction between two metals parts such as piston rod and gland, cross head guides, chain and gear drive etc.
Hot spot may arise due to
- Failure of lube oil supply
- Inadequate bearing clearance
- Hot combustion gases or sparks from piston blow past in engine where no diaphragm is fitted
- Reduction of flashpoint of crankcase lubeoil
- Scavenge fire
Primary Crankcase Explosion
If a hotspot exist, some oil will come in contact with it and will get vaporized. It will circulate to cooler parts of the crankcase and will condense to form white oil mist. The oil droplets in this white mist are very small. If this mist should now circulate back to the hotspot in such concentration, it will be ignited and primary crankcase explosion will occur.
Secondary Crankcase Explosion
The primary explosion causes a flame front and pressure wave to accelerate through the crankcase, vaporizing further oil droplets in the path. The pressure shockwave may build up sufficiently to rupture crankcase doors, if not relieved. Also, if relief valves don’t reseal after lifting, it will cause air to enter into the crankcase resulting into another flammable mixture to be developed leading to secondary or major explosion. The secondary explosion is more violent and cause crankcase doors to blow off and start fire in engine room.
Indication
Detection of hotspot can be by
- Use of temperature sensitive probes within crankcase near bearing oil return
- The white mist can be detected by Oil Mist Detector (OMD)
Action To be Taken
In case of detection of hotspot,
- Engine must be slowed down and stopped and bridge should be informed accordingly
- Keep clear off crank case relief door
- The engine must be turned on turning gear with lubrication oil supply on to prevent seizure
- Once engine cooled, stop main LO pump, the crankcase should be opened and ventilated
- Inspect all bearing surfaces for any hotspot
- Investigate the cause for the hotspot and engine to be started only after fault is rectified
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Source: Marine insight
