ANS) Crankcase explosions are also the result of high operating temperatures of the engine. The main cause of crankcase explosions is the development of hot spots at various places in the crankcase. Due to the reciprocating motion of the piston the lubricating oil in the crankcase is splashed in the air. Now 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.
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.
Now, when these hot spots come in contact with the oil in the crankcase, 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 an explosion. The fire or the explosion creates immense pressure inside the crankcase and if this pressure crosses the permissible limit, crankcase explosion takes place. The explosion will rupture the crankcase doors and even cause heavy damage to the inside of the engine.
It is a bit difficult to read the early signs of crankcase explosions. This is because the indications are similar to many other emergency situations. But there are few pre-explosion signs that can be read. Crankcase explosion will lead:
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.
Prevention: -
Preventing the generation of hot spots can do prevention of crankcase explosion. It can also be prevented by the following ways:
By providing proper lubrication to the reciprocating parts, thus avoiding high temperatures.
Avoiding overloading of the engine
Using bearings with white metal material, which prevents rise in temperature.
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.
Pressure relief valves should be fixed on the crankcase for the instant release of pressure. They should be periodically pressure tested.
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
Pressure relief valves should be provided with wire mesh to prevent the release of flames inside the engine room. Safe distance should be kept from the crankcase and the
relief valves in case the indications are sighted.
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.
Fire extinguishing medium should be kept standby. In many systems, inert gas flooding system is directly connected to the crankcase.
OIL MIST DETECTOR: -
Lubricating oil is supplied to the main engine under pressure from the main lube-oil pump. It passes through the crankshaft, lubricating and cooling the main and bottom end white metal bearing, returning to the sump. It is also supplied to the crosshead guides and piston rod bearing, from which it cascades down to the main sump.
During this activity an oil mist is produced, which is to be expected, however if there is a hot component the oil mist will be increased and vaporize with the real risk of fire and explosion in the engine crankcase.
The purpose of the oil mist detector is to detect any increase in the density of the oil mist, setting off an alarm to warn the watch-keeping engineer of potential danger.
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.
CRANKCASE RELIEF DOOR: -
As a practical safeguard against explosions, which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase.
Various designs and arrangements of these valves exist where, on large slow-speed diesels, two door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used. One design of explosion relief valve is shown in Figure. A light spring holds the valve closed against its seat and a seal ring completes the joint.
A deflector is fitted on the outside of the engine to safeguard personnel from the out flowing gases, and inside the engine, over the valve opening, an oil wetted gauze acts as a flame trap to stop any flames leaving the crankcase. After operation the valve will close automatically under the action of the spring.
The Crankcase relief doors are also fitted to prevent any damage to the crankcase and ingress of fresh air inside the crankcase.
The crankcase doors are spring-loaded valves, which lift up in case there is any rise of pressure inside the crankcase. Once the pressure is released they re-seat to prevent any ingress of fresh air. This helps especially in case of any ingress of air that can lead to a secondary explosion followed by a lot of surge and damage to the crankcase.
The opening pressure and sizes of the valves are specified by different classification societies, depending on the volume of the crankcase. The number of doors to be present also depends on the bore of the cylinder.