Medium speed engines. Crankcase explosions
Crankcase explosions can cause serious damage to engine room equipment, but more important is the hazard to the engine room personnel. It is necessary therefore for the engineer to completely understand the process leading to the propagation of conditions favourable to an explosion. The engineer can then maintain his engine so that those conditions should not occur.
It must first be understood that an explosion can take place in any enclosed mechanism such as a chain case, gear case, crankcase of a diesel engine or air compressor where oil is present. The magnitude of explosion is governed mainly by the available volume of explosive vapour, and it is this that would make large, slow speed main engine explosions potentially devastating, were they not adequately protected.
It has been proved that engine size does not affect the incidence of explosions (which are as likely in lifeboat engines as they are in large main propulsion engines), and that any moving part within the enclosed space can be responsible for the explosion, eg. piston rods, piston skirts, chains, gears, bearings and so on.
The sequence of events leading up to explosive conditions is as follows. The natural atmosphere in a crankcase consists of large globules of oil (100-300 mm in diameter) dispersed through the air. These globules are relatively so large that they will not ignite explosively, though they may burn under the correct conditions. A ‘hot spot’ (minimum temperature approx 360oC) can vaporise these globules. The vapour, rising to cooler parts of the crankcase, is then condensed into an oil mist. This oil mist consists of small globules of oil of approx 2-10 mm in diameter. When ignited, an accumulation of this oil mist can cause a heavy explosion. The initial vapour created by the hot spot may cause an explosion, though in most cases there would not be sufficient to cause a heavy explosion.
The oil mist may be ignited by coming in contact with a hot spot or spark at a temperature of 270oC. It may also be ignited if heated above 370oC (self-ignition temperature).
The amount of oil mist generated before ignition regulates the severity of the explosion. A small amount will create a fire; a large amount an explosion. The sooner the generation of oil mist is discovered, the smaller is the chance of an explosion, provided that the correct procedures are then followed.
The ratio of oil mist to air also governs the severity of the explosion. A weak mixture (2% or 3% by volume) will give a middle explosion causing little, if any damage. A mixture in the middle of the range (5 to 7% oil fuel vapour in air) will, if ignited, cause a heavy explosion, probably blowing off crankcase doors, causing external damage and engine room fires. A rich mixture (9-10% oil fuel vapour to air by volume) may cause a mild explosion. It should be appreciated that, following the explosion, a partial vacuum is created in the crankcase, and the engine room atmosphere flows back into it.
In the case of the rich mixture, the explosion will be followed by a period when air flowing back into the crankcase dilutes the rich mixture into the middle of explosive range. A secondary explosion at this condition could be devastating. In past cases the vacuum has been responsible for drawing off the crankcase doors of adjacent engines, laying their atmospheres open for combustion. It is to avoid this ‘chain reaction’ that crankcase explosion doors are designed to close as rapidly as possible after relieving an explosion, the closing being a way of preventing air ingress to the crankcase. For similar reasons, there should be no cross connecting pipes between the crankcase of engines. Oil return pipes to a common sump should be taken to below the surface of the oil so that an explosion in one engine cannot find its way into the second engine.
Extraction fans exhausting to atmosphere up the funnel are sometimes fitted to keep the engine clean. The fans cause a small pressure depression in the crankcase that prevents oil leakage, as air is drawn in through any small aperture that would otherwise weep oil. The fans must be shut off if conditions that could lead to a crankcase explosion are suspected. It left running, they could dilute a rich mixture to the middle of the explosive range.
Oil mist detectors are fitted to many engines today and are a particular requirement for unattended machinery space vessels. They continuously monitor the atmosphere inside the crankcase, taking samples in turn from both low and high levels along the length of the engine. The samples are compared to a reference so that a change in conditions at any one point is detected quickly. The alarm point is set very much below the lower explosive limit so that a very early warning is provided. The monitor will indicate the location of the detector head providing the alarm reading, and the engineer can check on conditions himself. It should be appreciated that these units are very sensitive and may give alarm conditions because of slight fouling of the lens inside the detector. Regular checking and cleaning of these will reduce the incidence of false alarms. The units should never usually be switched off. It is better to respond to a few false alarms to ignore a warming preceding an explosion.
An alert watchkeeper can detect rising oil temperatures quickly and respond to the dangers before conditions get too severe. Stopping the engine is by far the best thing to do but this can only be done with agreement from the bridge. Only when there are no hazards in a navigational sense should the engineer slow the engine down and stop it. Permission from the bridge watchkeeper should always be sought before slowing or stopping the engine; which may cause a collision/grounding that would otherwise have been easily avoidable. After checking with the bridge, slow, or preferably stop, the engine, and, if possible, increase the flow of lubricating oil. Never open the crankcase until adequate time for cooling has elapsed. ‘Adequate’ time is not easy to define but in most cases at least 30 min should elapse, preferably much more. In the crankcase, the hot spot will still have enough heat left in it for it to be located. Carbon dioxide flooding would inert the crankcase, approx 30% by volume being sufficient, but not many engines are fitted with such facilities. Permanent inerting is not practicable, as not only could the gas leak into the engine room atmosphere, but routine maintenance would be inhibited. Inevitable, cost would also preclude the use of permanent crankcase flooding.
Crankcase explosion doors are fitted in order to reduce the effects of an internal explosion. They have to be able to withstand the force of the explosion and the passage of high temperature gases without distortion. Equally, they must close and seal quickly to stop the ingress of air that would otherwise occur during the period of vacuum. They should be fitted along the length of the engine and positioned at high and low level to give maximum protection. Note that the further an explosive wave travels the greater its momentum, so on large engines, doors should be numerous us so that the distance travelled by the wave, before its release, is as short as possible. Medium speed engines with cylinder bore less than 12 inches only need explosion doors at the ends. Smaller engines with cylinders of less than 6 inches are not required to have explosion doors at all. Areas of doors are set down by the various governing bodies, as are the lifting pressures (nominally 0.5 bar).
Above all, good and regular attention to the maintenance of the engine, avoidance of overloading and the provision of adequate lubricating oil should mean that explosions never occur, but to protect against the unpredictable oil mist detectors and crankcase explosion doors should always be checked and maintained in satisfactory condition.