The development of a hotspot in the crankcase is a source of heat required to vaporise the oil and form an explosive mixture.

The hotspot in the crankcase is due to the following:

• Failure of lube oil to bearing, sprockets and similar parts.
• Hot gas blowing past the pistons may provide a spark sufficient to cause an explosion in the trunk-type pistons engine.
• Scavenge fire in crosshead engines
• Overloaded engine
• Hot spot temperature: 280 °C to 400 °C above the L.O flash point

The sequence of events leading to the crankcase explosion.

• Oil particles in the vicinity of the hotspot evaporate and settle in a cooler, will come in contact with it, and will be vaporised.
• The vapour will circulate to cooler parts of the crankcase and condense to form a white oil mist
• The oil droplets in this white mist are very small. If this oil mist circulates back to the hotspot in such concentration (with typical particle sizes of around 0.5 to 5 microns in diameter, density between 30 to 50 mg/L (milligrams per litre)), it will be ignited, and a primary explosion will occur.
• The explosion can cause a flame front and pressure wave to accelerate through the crankcase, vaporising further oil droplets in the path.
• The pressure shock wave may build up sufficiently to rupture crankcase doors if not relieved.
• If the relief valves do not reseal after lifting, it will cause fresh air to enter into the crankcase, resulting in another flammable mixture to be developed, leading to a secondary or major explosion.

An oil mist detector (shown below) is designed to detect the presence of an explosive mixture in the crankcase

The detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end, which generates an electric current directly proportional to the intensity of light. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tubes onto the cells, which are then in electric balance.

One tube is sealed to contain clean air and is termed the reference tube. The other, the measuring tube, has connections through which samples of the crankcase vapour are drawn by an electric extractor fan.

Sampling points should be fitted to each cylinder crankcase, and their connections are brought to the rotating selector valve, which is driven from the fan motor. This repeatedly connects each sampling point to the measuring tube in sequence.

If a concentration of oil mist is present in the sample, the light will be obstructed before reading the cell of the measuring tube. The electric balance between the two cells will be disturbed, and an alarm will be operated. The rotary valve stops to indicate which sampling point has a high concentration of oil mist.