The initiation of a crankcase explosion is a 'hot spot' or overheated parts within or adjacant to the crank case of an operating engine. Due to the 'hot spot', the lube oil coming in contact with it gets evaporated. The evaporated oil circulates to the cooler parts of the crankcase and there it condenses forming a white mist coming of finely divided oil particles well mixed with air. If this mist having right proportion of air circulates back to the hot spot, it will be ignited and a primary explosion occurs. The explosion may cause considerable pressure rise o blow off the crankcase doors if crankcase relief valves are not fitted and the vacuum which follows the primary explosion will draw the atmospheric air into the crankcase where it will mix with evaporated and burning oil to cause a catastrophic secondary explosion.
The Causes of Hot Spot and Explosive Oil Mist
- The frictional heat generated due to rubbing at the crankcase bearings, piston rod glands, timing chain, etc.
- Failure of lube oil supply
- Inadequate clearance between bearings and other moving parts
- Prolong overloading of engine
- Reduction of flash point of crankcase oil due to contamination with fuel
- Hot combustion gas or sparks from piston blow past
- By scavenge fire resulting in heating of diaphragm, piston rod, etc.
Actions after detecting Hot Spot
- Reduce the engine speed to reduce heat generation
- Increase lubrication to the running surfaces
- Keep clear off crankcase door to prevent personnel injury, keep fire extinguisher ready and open skylight
- Inform bridge, stop engine, open indicator cocks, engage turning gear and turn the engine with lube oil circulation continuing to prevent engine seizure
- After about 30 minutes or more, when the engine is sufficiently cooled down, stop lube oil pumps and open crank case door.
- Try to locate the hot spot by observation and feeling over
- If the hot spot is located prevent the re-occurrence by permanent repair
- Make thorough inspection of crankcase
- Start lube oil pump and check for proper lubrication of all parts
- If everything is normal, inform bridge, start the engine, and gradually increase the speed
Preventive Measures of Crankcase Explosion
- Ensure adequate cooling of the engine
- Ensure proper purification and analysis of lube oil
- Lube oil filter should be changed over and cleaned as per schedule
- Ensure proper cylinder lubrication by checking the condition of piston, piston rings and liner through scavenge or exhaust ports
- Clean scavenge spaces as per schedule and drain scavenge space regularly
- Maintain stuffing box gland in good condition
- Be alert and rectify for any abnormal noise in crankcase
- All safety alarms and trips fitted on engine to be tried out satisfactorily
- Proper watch on all running gears temperature and pressures to be maintained
- Blow through all sampling tubes of Oil Mist Detector (OMD) regularly
- Zero adjustment and sensitivity of OMD to be checked regularly
- Check for any oil leakage at crankcase relief doors and check for the operation by hand or tool
- Check flame trap for cleanliness
Protection against Crankcase Explosion
Oil Mist Detector
Warning prior to a crankcase explosion
Crankcase relief doors
Releases pressure inside crankcase due to primary explosion, prevent rupture of crankcase and entering fresh air into the crankcase.