Crankcase explosions

Crankcase explosions happily are not an everyday occurrence but when they do occur can lead to extensive damage, fire and possible loss of life. Experience indicates that such explosions can be most dangerous in the large slow speed engines. The consequences have been more pronounced in the large engines because the design must be such that the crankcase doors are light enough to handle manually and, at the same time, large enough to permit access; also the doors are not normally constructed to withstand excessive pressure effects or explosion.

There is a continuing updating of all engine designs with higher bearing pressures, increased thermal stresses and temperatures, thus making increased demands on cooling and lubricating systems. It is therefore all the more vital to ensure that when a failure does occur and may produce conditions favourable to an explosion, firstly the explosion should be averted or, in the extreme case, the effects be controlled. This leads us to a consideration of the cause of crankcase explosions and the methods used both to control their effects and to monitor conditions in a crankcase.

Oil mist in crankcases The figure gives the relationship between the air/oil ratio and the temperature for a typical crankcase oil. This shows the upper and lower limits of inflammability and indicates that spontaneous combustion can occur at a temperature of 270C with a mixture having an oil content of 13% by weight. This figure also shows the two separate regions in which ignition can take place with a band of temperature between where no ignition is possible.

The above work also established that overheating of mechanical parts and the creation of what has commonly been called a hot spot can result in the formation of an explosive mixture which, in turn, is ignited by the hot spot itself. The overheated area thus becomes both source and ignitor of the explosive medium. It is important to note that it .is the condition of the air/oil mixture in the vicinity of the overheating which is the controlling factor in determining whether an explosion will take place and not the general condition in the crankcase. In a trunk piston engine entry of flame past the piston into the crankcase could also act as ignitor.

Under normal conditions a crankcase contains a mixture of mechanically generated spray and a small amount of condensed mist resulting from the vaporizing of oil from bearings and other parts at working temperature. The larger droplets of oil do not constitute a hazard in themselves and normal crankcase breathing and circulation of the air maintain a stable condition whereby any vaporized oil is condensed in the cooler parts of the space but with the mixture remaining outside the limits of flammability. In the normal course of events, with a typical crankcase oil, the mixture becomes over rich and above the limit of flammability before the minimum ignition temperature is reached. Provided the stable conditions in the crankcase persist the presence of a hot spot will not inevitably lead to an explosion.

The dangerous situation arises when the degree of overheating leads to accelerated vaporization of the oil which may be accompanied by cracking and oxidation to form gases and vapours even more hazardous than the oil mist. An explosive mixture can then result which may be ignited by the overheated part. Any changes in the stable conditions in the crankcase can lead to hazard. These can arise from dilution of the overrich mixture caused, for instance by removal of an inspection door and changes in the air circulation patterns induced by the moving parts, including stopping and starting of the machine. Each may have the effect of bringing an explosive mixture to this hot spot area with consequent risk of ignition.

An alternative explanation of delayed ignition that it is likely that the machinery part heats up through the low temperature ignition region without producing flame because of the length of the ignition delay at low temperatures. When the temperature reaches the non-ignition band between 350C and 400C. shown in figure, there will be vaporization of the oil without ignition, which can promote formation of an explosive mixture. If now the engine is stopped, or for any other reason cooling is introduced, the temperature of the heated part will fall into the region where an explosion can occur. The rate of cooling will determine when the explosion will take place and can be some minutes after stopping the machine.

Explosion effectsThe magnitude of the explosion may vary between a mild puff, which produces a small amount of white smoke and detonation, which causes extensive damage and the emission of flame. Research has indicated that ignition of the hydrocarbon/air mixture within an enclosure, initially at atmospheric pressure, does not create a pressure greater than 7 bar. The violence of an explosion will however depend on the composition and quantity of the explosive mist, the speed of propagation of flame and of pressure rise within the crankcase.

The pressure wave of the primary explosion is followed by a negative pressure, and if air is permitted to enter the crankcase, a further mixing of oil and air occurs which may result in a secondary explosion. Figure shows that the instantaneous pressure wave is extremely short and is followed by a relatively long negative pressure during which the incoming gases air mixes thoroughly with the oil mist, with the result that the secondary explosion can be more violent than the first.In the assessments of all the reported incidences of crankcase explosions, it was apparent that the chief causes of explosions were associated with the overheating or seizure of pistons and bearings.

Prevention of explosionsIt is clear that elimination of either an explosive mixture or a source of ignition will completely remove the possibility of an explosion. For the latter, overheating of parts is an ever present possibility, while the former can only be achieved by flooding the crankcase with an inert gas to reduce the oxygen level below 10%. The provision of means to monitor conditions in the crankcase is usually made.

Design aspectsIn a small engine it is possible to design the crankcase to withstand the forces likely to be produced by an explosion. However this would be impractical in the larger engine. In these engines the maximum realizable pressure resulting from an explosion can be reduced by subdivisions since the maximum pressure is proportional to the distance of flame travel and the crankcase volume. In practice, however, only partial subdivision is feasible as access is required for assembly and maintenance and the solution has been to incorporate other means of limiting the maximum pressure. Designers have done much to reduce hazards by minimizing the risk of overheating and seizure and considerable attention has been given to the design and securing of crankcase door.

Such doors on larger engines are capable of withstanding reasonably high pressure while retaining a light weight. Crankcase doors for some small and medium size engines are in fact made from reinforced plastic materials. Construction from such materials allows easy forming of a dished shape to obtain the benefits associated with hoop stress. Similar attention is given to securing arrangements since the spacing of bolts and clamps on the door must be such as to ensure that the door is not blown off by the initial explosion or sucked in during the negative pressure period.

Crankcase breather pipes are fitted to relieve internal crankcase pressure due to expansion and these should be as small as practicable. For marine engines, classification societies stipulate that these pipes should be led to a safe space on deck. Preferably they should be fitted with flame arrestors and where there is more than one engine, the pipes should not be inter-connected.

Explosion relief valvesIt has already been pointed out that it is possible to design the crankcase of smaller engine to withstand the anticipated maximum internal pressure of 7 bar but in the case of larger engines protection is normally by means of relief valves. These must be of sufficient size, in sufficient number and be placed in positions most likely to limit the pressure. They should also have the capacity to close quickly after lifting in order to prevent ingress of air after the initial explosion. The valves are normally fitted with flame traps to prevent emission of flame. Classification Societies have rules which state that valves can only be omitted in engines having cylinder bores not exceeding 200 mm and a gross crankcase volume not exceeding 0.6m3. Above the 200mm bore size there is a progressive increase in the number of valves to be fitted until in engines with cylinder above 300mm bore at least one relief valve is to be fitted in way of each crank throw, plus a separate valve for each separate chain case, gear case, etc.

The requirements further state that the free area of each relief valve should not be less than 45 cm2 and the combined free area of the relief valves fitted to an engine should not be less than 115 cm2/m3 based on the volume of the crankcase. The quick-acting relief valves should he made as light as possible to reduce inertia effects and should be designed to open at a pressure not greater than 0.02 N/mm2. The material of the valve must be capable of withstanding the shock of contact with stoppers at the full open position.

As mentioned earlier, a crankcase explosion can generate flame and there have been many cases where the discharge of burning has caused injury to personnel. For this reason flame guards or flame traps are fitted to minimize the danger arising from the emission of flame. Research has established that an oil wetted gauze greatly increases the effectiveness of a flame trap, wetting of the gauze being achieved by arranging the gauze assembly before the valve and in the path of lubricating oil spray within the crankcase. As will be noted from the descriptions which follow later, oil wetted gauze is not, however. fitted to relief valves in all cases.

The figure illustrates an explosion relief valve, where the flame trap is arranged to form a single unit with the valve, the crankcase door being sandwiched between the two components. The internal element supplies the frame for the mild steel gauze layers and for the spider which supports the spindle on which the relief valve rides. The external part of the assembly is a, combined valve cover and deflector and is of aluminium. The deflector opening extends 120 degrees and can be turned to the direction which is safest for the issuing hot gases. The valve itself is an aluminium alloy casting having synthetic rubber oil and heat resisting seals A spring, which presses against a stop in the spider, is used to retain the valve lid in the closed position.

Crankcase monitoring systemsThere are a number of methods by which conditions in a crankcase may be monitored and these include temperature sensing of parts likely to overheat, checking the temperature of the lubricating oil itself and monitoring the metallic debris in the lubricating oil returns. Temperature sensing would require a large number of sensors to be fitted to efficiently monitor a crankcase besides presenting practical difficulties in fitting detectors to moving parts. However such sensing of main journal bearings and cylinder liners where they enter the crankcase is common practice. Monitoring of the temperature and of the metallic debris in lubricating oil has the disadvantage that there is a delay before the effects of abnormality are detected. One method which has, however, found favour is that of oil mist detection.

The formation of oil mist or condensed oil vapour is largely dependent on the relationship of oil vapour pressure and temperature the oil mist increasing in quantity as the temperature rises. Oil mist detection is based on the principle that there is a non-linear relationship between oil mist density and its opacity to light. The optical density of oil mists is fairly high and, with rising concentration, the intensity of a light beam directed towards a photocell is reduced.

Oil mist detectorsThe detector will give alarm and slow-down at a mist concentration which is only a fraction of the lower explosion limit, LEL, to gain time to stop the engine before ignition of the oil mist can lake place. A sample taken from each crank chamber is compared in turn with the combined mist from the remaining chambers. The operation follows a procedure whereby oil mist is continually drawn by a fan in the detector from sampling points on the engine.

A rotary valve passes each sample in turn to the Measuring Tube while the remaining samples (representing the average oil mist density) are simultaneously passed to the Reference Tube. The photocells in the Measuring and Reference Tubes combine to give an output proportional to the difference between the density of the oil mist in the one crank chamber as against the average. Once per cycle the rotary valve passes clear air into the Measuring Tube, the photocells then giving an output proportional to the average oil mist density. Further, as a manual daily operation, air is admitted to both tubes as a check for meter zero. If an out-of-balance is produced the unit can easily be adjusted to the zero position. In the case of increased oil mist in an individual crank chamber, the system is so arranged that an alarm sounds and simultaneously the rotary valve stops at the position which caused the alarm. A scale enables the area with excessive oil mist to be identified. An alarm condition also operates in the event of the overall oil mist density being excessive when measured against clear air.

Crankcase oil mist detector (light scatter)

The disadvantage of obscuration types is that they are generally slow to operate and suffer from inaccuracies and false alarms caused by such things as a dirty lens. Light scatter do not suffer from these problems, are faster reacting and do not need to set zero during engine operations. The relationship between the light landing on the sensor is nearly proportional to the oil mist density therefore the unit can be calibrated in mg/l. It is possible to have the sensor and a LED emitter in a single unit which may be mounted on the crankcase. Several of these can be placed on the engine each with a unique address poled by a central control unit.

The results of which may be displayed on the control room having these heads mounted on the engine removes the need for long sample tubes which add to the delay of mist detection. This makes the system much more suitable for use with medium and high speed engines were otherwise detection would be impossible.

Practical aspectsThe risk of hot spots is always present but is more likely when the moving parts may not have fully bedded in, i.e. after overhaul or on new engines. If overheating is suspected or smoke is seen issuing from the crankcase, the engine should be stopped, but the crankcase doors or other closing covers likely to allow ingress of air, should not be removed until sufficient time has been allowed for the parts to cool. It is equally important that an engine should not be re-started before a fault has been rectified since this has on occasion also led to crankcase explosions.

Measures to be taken when Oil Mist has occurredDo not stand near crankcase doors or relief v a l v e s - nor in corridors near doors to the engine room casing.1) Reduce speed/pitch to slow-down level, if not already carried out automatically,2 ) Ask the bridge for permission to stop.3) When the engine STOP order is received:stop the engineclose the fuel oil supply.4) Switch-off the auxiliary blowers.5) Open the skylight(s) 6) Leave the engine room.7) Lock the casing doors and keep away from them.8) Prepare the fire-fighting equipment. Do not open the crankcase until at least 20 minutes after stopping the engine. When opening up, keep clear of possible spurts of flame. Do not use naked lights and do not smoke.

9) Stop the circulating oil pump. Take off/open all the lowermost doors on one side of the crank case. Cut off the starting air, and engage the turning gear.10) Locate the hot spot. Use powerful lamps from the start. Feel over, by hand or with a thermo-feel, all the sliding surfaces (bearings, thrust bearing, piston rods, stuffing boxes, crossheads, telescopic pipes, chains, vibration dampers, moment compensator, etc. Look for squeezed-out bearing metal, and discolouration caused by heat (blistered paint, burnt oil, oxidized steel). Keep possible bearing metal found at bottom of oil tray for later analyzing.11) Prevent further hot spots by preferably making a permanent repair. Ensure that the respective sliding surfaces are in good condition. Take special care to check that the circulating oil supply is in order.

12) Start the circulating oil pump and turn the engine by means of the turning gear. Check the oil flow from all bearings, spray pipes and spray nozzles in the crankcase, chaincase and thrust bearing. Check for possible leakages from pistons or piston rods.13) Start the engine.Af t e r:15-30 minutes,one hour later,when full load is reached- Stop and feel over:- Look for oil mistEspecially feel over (by hand or with a thermo-feel) the sliding surfaces which caused the overheating.

Suppression of the Risk of Crankcase ExplosionsOn large diesel engines, the use of inert gas creates a new risk to the personnel, as venting of the crankcase compartments is rather difficult. The use of water mist is harmless to the personnel and, in addition, the water can be removed in the course of the normal lubricating oil cleaning process. By the inflow of a preheated liquid medium under pressure, a well-dimensioned sharp-edged nozzle design, with an appropriate length of the outlet pipe designed for the actual medium and its inlet thermo-dynamical conditions, is able to force a localized evaporation of the medium inside the nozzle, utilizing the natural proper-ties of the liquid medium and, thereby, to produce a flow at the nozzle outlet of the medium in vapour state. This outflowing vapour condensates into very small droplets, partly because of the increasing pressure, and partly because of the mix with the sur-rounding relatively cooler gases, and thereby forms a mist of the liquid medium.

The pressurized hot water (15 bar and 180o C), which is controlled by the oil mist monitoring system, is injected into the crankcase, through special sharp-edged nozzles, where it is transformed into small droplets of a size below 10 microns. The water mist system replaces the use of inert gas and, thereby, completely removes the risk of choking due to the lack of oxygen. The use of water mist is harmless to the personnel, and the water can be removed in the normal lubricating oil cleaning process. The water mist cannot prevent an explosion. However, practical tests with explosions in an air/methane mixture have shown that the pressure rise, as can be seen in Fig. 3, is considerably reduced, thereby eliminating the risk of mechanical damage caused by the pressure wave.