Crankcase explosions occur when an explosive mixture of oil mist and air forms inside the crankcase due to high operating temperatures. Hot spots inside the crankcase can cause oil to vaporize and condense into a fine mist, which can then ignite and cause an explosion if pressure rises above safe levels. Prevention methods include proper lubrication to avoid hot spots, overload protection, pressure relief valves, flame traps, oil mist detectors, and inert gas flooding systems. Crankcase explosions present dangers such as structural damage and loss of life if pressures exceed thresholds.

1. CRANKCASE EXPLOSION
The burn rate (or burning rate) is a measure of the linear combustion rate of a
compound or substance such as a candle or a solid propellant. Burn rate is
measured in length over time, such as "mm/second" or "inches/second". Burning
rate typically increases with pressure and temperature.
The flame speed is the measured rate of expansion of the flame front in a
combustion reaction.
A flash fire is a sudden, intense fire caused by ignition of a mixture of air and a
dispersed flammable substance such as a solid (including dust), flammable or
combustible liquid (such as an aerosol or fine mist), or a flammable gas. It is
characterized by high temperature, short duration, and a rapidly moving flame
front
There are two types of explosion:
1. Physical explosion such as an exploded bottle due to pressure.
2. Chemical reaction such as ignition of fuel air mixture.

2. There are two types of explosions involving chemical reactions:
Deflagration
Detonation
The difference between the two is the reaction speed through the explosive. If it
moves slower than the speed of sound it is a deflagration. If it moves faster than
the speed of sound it is a detonation.

3.  To form an explosive mixture, they should be between LEL and UEL.
 Normal oil mist size in a crankcase is about 200μm, but to form an explosive
fine mist they need to be around 6μm.
 Crankcase explosion starts at 0.3m/s and if not relieved will increase to
about 300m/s and can even reach 3km/s in a long crank case.
 Experiment indicates two separate temperature regions in which ignition
can take place, i.e. 2700- 3000 C and above 4000C
 The pressure rise varies with condition, but without detonation does not
normally exceed 7 bar.
 A properly designed crank case should withstand a static 12 bar pressure
without distortion.
 Total clear area thru relief valve should be at least 9.13cm2/m3 of gross
crank case volume.
 No relief valve required for engines with a bore 150 mm or less
 Relief valves at the ends only for engines with bore not exceeding 300 mm.

4.  Lube drain pipes from sump to drain tank to be extended below the
working oil level.
 Drain or vent pipes for multiple engine installation should be separate or
isolated.
 The pressure setting on a relief door is about 1/15 bar and crank case
pressure must not be allowed to exceed 3 bars.
 Flame traps effectively reduce the explosion pressure and prevent two
stage combustion.
 The flame traps reduce the temperature of gas from 15000 C to 2500C.
 Ample oil coating of wire gauge increases the effectiveness of flame trap.
 Some engines are fitted with a system of injecting inert gas into the
crankcase.
 Alarm form oil mist detector is set at 2.5 % of LEL.
EFFECTS OF EXPLOSION DESTRUCTIVE PEAK PRESSURE (1bar= 14 psi app)
GLASS SHATTERING: 0-5 PSI
FIREFIGHTER KNOCKDOWN: 1.PSI
WOOD PARTITION COLLAPSE: 1-2 PSI
CINDERBLOCK WALL COLLAPSE: 2-3 PSI
BRICK WALL COLLAPSE: 7-8 PSI
FIREFIGHTER LUNG DAMAGE: 15 PSI
THRESHOLD FOR FATALITIES: 35 PSI
50% FATALITIES: 50 PSI
99 FATALITIES: 99 PSI
Crankcase explosions are also the result of high operating temperatures of the
engine. The main cause of crankcase explosions are the development of hot spots

5. 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 is 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 rod.
•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.
Indication of crankcase explosion
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

6. •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
Prevention of crankcase explosion can be done by preventing the generation of
hot spots. 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.

7. •Fire extinguishing medium should be kept standby . In many systems , inert gas
flooding system is directly connected to the crankcase.
CRANK CASE RELIEF DOOR

8. OIL MIST DETECTION
Fan draws air by sequence from each unit and also fresh air for comparison
through the measuring chamber and discharges to the atmosphere.
References
Diesel Engines by A J Wharton, Reeds Marine Engineering Series,