Marine Engine Question & Answer-Crankcase Explosion

Outline the probable events leading to an explosion, for an engine where a hotspot has developed in the crankcase.
Compare the methods of operation and the performance characteristics of BOTH obscuration and light scatter type oil mist detectors.
Explain how the severity of a crankcase explosion may be limited.
Explain why stopping rather than slowing down in the event of an overheated crankshaft bearing is preferable.

a.
For an explosion to occur there must be oxygen, fuel, and a source of ignition. The oxygen will be present in the air in the crankcase, The lubricating oil is the fuel, and the source of ignition is usually an overheated bearing (although it can also be anywhere two metals are rubbing together, or blowby on a trunk piston engine)

The mixture of oil and air must be in a ratio that is within the range of inflammability; the splashing of the lubricating oil inside the crankcase breaks it up into droplets or globules of widely varying size distributed in varying density throughout the crank chamber. The overall mixture strength is usually very weak and will not support combustion. However, if a hot spot exists, some oil will come into contact with it and will be vaporised, circulate to cooler parts of the crankcase and there condense to form a white mist of finely divided oil particles well mixed with air. This mist is combustible within certain concentrations. If the mist should now circulate back to the hot spot in such concentrations, it will be ignited and a primary or minor crankcase explosion will occur. This explosion causes a flame front and pressure wave to accelerate through the crankcase, vaporising further oil droplets in its path.

The pressure shockwave may build up sufficiently by the time it reaches the crankcase casing to rupture crankcase doors or panels, unless otherwise relieved. If the pressure wave reaches an opening through which it can escape to the atmosphere the pressure pulse is immediately followed by a suction pulse of lower magnitude but greater duration. This suction pulse can be responsible for drawing in a charge of fresh air to take the place of that which has been burned by the initial explosion. A secondary explosion or major explosion of such intensity as to cause widespread damage then follows.

b.


Light Scatter

Obscuration

In the light scatter type of crankcase oil mist detector individual sensors are placed at each monitoring point - each crank throw space and chain case (where applicable). A suction fan draws the sample through each detector. Light is transmitted at one end of the head where the sample flows through. Directly opposite the transmitter is a compensating receiver. This adjusts the light intensity by feeding back a signal to the transmitter. A measurement sensor picks up the scattered light produced by the oil mist particles. The result is transmitted as an analogue signal back to the monitor twice per second. The monitor compares this signal against a set point, and an average of the other readings. When the scattered light picked up by the sensor reaches a pre determined point an alarm condition will be reached.

Advantages claimed for this system are:

Sampling points fitted close to crankcase - no long runs of piping.

Continuous parallel sampling - no high maintenance selector valves.

Fast response time - may save the engine from bearing failure.

The obscuration type of 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 the light falling on its surface. Lenses are fitted to seal the ends of each tube but allow light to pass. 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 electrical balance.

The samples drawn from the crankcase are drawn in turn along the measuring tube by means of a selector valve. If a concentration of oil mist is present in the sample, the light will be obscured in the measuring tube: electrical balance between the two cells will be disturbed and an alarm will be operated.

The model shown measures total mist concentration with respect to clean air. An alternative model draws samples through both reference and measuring tubes. A mixture from all cylinder crankcases is passed through the reference tube while comparison is made with samples from each cylinder crankcase and also from the atmosphere. In this manner a general sample of all cylinders is compared with the normal atmosphere and each individual sample is also compared against the average.

This type of detector has the advantage of simplicity of operation. The disadvantages are that comparatively long runs of pipework from sampling points to detector are required, only one sample point is measured at a time, and regular maintenance is required if false alarms are to be avoided.

c.
The severity of the crankcase explosion can be limited by reducing the pressure shockwave travelling through the crankcase; This can be achieved by subdivision of crankcase which will inhibit the build up of high velocities and pressures of flame propagation through the crankcase from a primary explosion. The pressure build up must be relieved without allowing air to be drawn back into the crankcase thus preventing a secondary explosion. To do this, the crankcase is fitted with explosion doors, which are in effect spring loaded, low inertia non return valves of sufficient size to relieve any build up of pressure (115 cm2 per cubic metre of crankcase volume) The doors are fitted with a gauze flame trap, wetted by crankcase oil, and a deflector on the exterior to prevent harm to personnel.
d.
An overheated bearing will be the cause of the generation of the vaporised oil and the source of ignition. If bearing temperature probes are fitted it may be possible to prevent the vaporisation of the oil, and running of the bearing white metal by stopping the engine before the bearing has become too hot; However the main worry besides the formation of the oil vapour and the possible danger of explosion is the damage to the crankshaft by scoring or overheating which can mean the scrapping of the crankshaft. In the case of a high oil mist alarm, the cause of which may well be an overheated bearing, or a bearing high temperature alarm, it is best to stop the engine immediately. However safe navigation of the vessel, may cause the Master/OOW to overide this, in which case reducing the load to 50% may well limit the damage caused until the engine can be safely stopped.