The initiation of a crankcase explosion is a ‘hot spot’ or overheated part within or adjacent 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 mist 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 in the crankcase causing secondary explosion and 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 explosion will rupture the crankcase doors and even cause heavy damage to the inside of the engine.
Hot spots are created in the crankcase as a result of the following:
1. Hot combustion gas or sparks from piston blow past entering crankcase.
2. Fires in the adjacent scavenge trunks resulting in heating of diaphragm, piston rod, etc.
3. In crease in temperature of parts due to frictional heat generated at the crankcase bearings, piston rod glands, timing chain, etc. due to inadequate or failure of L.O. supply.
4. Inadequate clearance between bearings and other moving parts.
5. Prolonged overloading of engine.
6. Reduction of flash point of crankcase oil due to contamination with fuel.
INDICATION OF CRANKCASE EXPLOSION
It is difficult to read the early signs of crankcase explosions but there are few pre-explosion signs that can be read by an alert watch keeper in engine room. Crankcase explosion will lead to:
· 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 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.
ACTIONS AFTER DETECTING HOT SPOT
1) Reduce the engine speed to reduce heat generation.
2) Increase lubrication to the running surfaces.
3) Keep clear off crankcase doors to prevent personnel injury, keep fire extinguisher ready and open skylight.
4) Inform bridge, stop engine, open indicator cocks, engage turning gear and turn the engine with lube oil circulation continuing to prevent engine seizure.
5) After about 30 minutes or more, when the engine is sufficiently cooled down, stop lube oil pumps and open crank case door.
6) Try to locate the hot spot by observation and feeling over.
7) Once the hot spot is located, permanent repair must be carried out in order to prevent recurrence.
8) Inspect crankcase thoroughly and record crankshaft deflection.
9) Start lube oil pump and check for proper lubrication of all parts.
10) If everything is normal, inform bridge, start the engine, and gradually increase the speed.
PREVENTION
Prevention of crankcase explosion can be done by preventing the generation of hot spots and other checks as stated below:
1) By providing proper lubrication to the reciprocating parts, thus avoiding high temperatures.
2) Avoiding overloading of the engine.
3) Using bearings with white metal material which prevents rise in temperature.
4) Ensure adequate cooling of the engine.
5) Ensure proper purification and analysis of lube oil.
6) Lube oil filter should be changed over and cleaned as per schedule.
7) Ensure proper cylinder lubrication by checking the condition of piston, piston rings and liner through scavenge or exhaust ports.
8) Clean scavenge spaces as per schedule and drain scavenge space regularly.
9) Maintain stuffing box gland in good condition.
10) 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.
11) Pressure relief valves should be fixed on the crankcase for the instant release of pressure. They should be periodically pressure tested.
12) Pressure relief valves should be provided with wire mesh to prevent the release of flames inside the engine room.
13) 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.
14) Safe distance should be kept from the crankcase and the relief valves in case the indications are sighted.
15) Fire extinguishing medium should be kept standby. In many systems, inert gas flooding system is directly connected to the crankcase.
16) Be alert and rectify for any abnormal noise in crankcase.
17) All safety alarms and trips fitted on engine to be tried out satisfactorily.
18) Proper watch on all running gears temperature and pressures to be maintained.
19) Blow through all sampling tubes of Oil Mist Detector (OMD) regularly as per maker’s instructions.
20) Zero adjustment and sensitivity of OMD to be checked regularly.
21) Check for any oil leakage at crankcase relief doors and check for the operation by hand or tool (if provided).
22) Check flame trap for cleanliness.
