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 270°C 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 350°C and 400°C. 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 effects
•
The 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 explosions
• It 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 aspects
• In 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 valves
It 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 systems
• There 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 nonlinear 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 detectors
• The 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 aspects
• The 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 occurred
• Do 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 engine
• close 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 mist
Especially feel over (by hand or with a “thermo-feel”) the
sliding surfaces which caused the overheating.
Suppression of the Risk of Crankcase Explosions
• On 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 sharpedged 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 surrounding 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.