Crankcase Explosion
One of the most frightening phenomenon for operators of diesel
engines is the crankcase explosion. Although many
force-lubricated, totally enclosed, reciprocating machines like
diesel engines, steam engines, air compressors and many others
had been running for many hours without any such problems, still
the problem of crankcase explosion is ever present and we cannot
forget about it.
In order to understand the phenomenon of crankcase explosion, we
have to understand the nature of fire, because, after all, an
explosion is a very rapid propagation of fire.
A fire will only start whenever three conditions are met: fuel,
oxygen, and heat.
A fire can very well start inside the crankcase of a diesel
engine when the conditions are just right. For all the recorded
occurrences of crankcase explosions, one factor is common, namely
a hot spot. That is the heat source.
Now, let us take a look inside the crankcase. It houses the
crankshaft, connecting rod, piston rod, cylinder liner, piston,
drive gears or chains and the lubrication oil. To prevent
lubrication oil loss, protect persons from the moving mechanical
parts, prevent contamination of the oil, and many other reasons
the crankcase is totally enclosed and separated from the external
environment.
First, let us look at the moving parts. There is the reciprocating
mechanism consisting of the crankshaft, journal bearings, and
connecting rod. If it is a crosshead engine, the crosshead sliding
and reciprocating mechanism. For the trunk type engine, there is
the piston and the small end bearings.
There are also gears or chains to drive the camshaft. For
reversible engines, the reversing drive mechanisms. There is also
the thrust bearings at the end of the crankshaft to take up the
driven load. There may also be cooling oil pipes, or cooling water
pipes sliding inside the crankcase for piston cooling. The piston
rod itself may also be sliding through a gland assembly at the
under piston space.
All the sliding parts can become a hotspot whenever there is
insufficient lubrication to reduce the friction, probably during
starting of the engine or any abnormal interruption in the force
lubrication oil system.
Next, the use of fuel oil in the engine. Dripping fuel injectors
will result in incomplete combustion of the fuel above the piston
top. If the dripping is serious, the liquid fuel can find their way
through the piston rings aided by the high pressure of the
combustion process to eventually find its way into the crankcase.
For trunk type engines, this is a direct path to the crankcase. It
can become a source of low flash point fuel when mixed with the
lubricating oil!
For dripping fuel injectors, it is common to find unburnt carbon that
will cause the piston rings to stick within their grooves, thus
destroying the sealing function of the piston rings. This will not
only cause the fuel to drip through, but can also cause fire from the
combustion space to blow past through the gaps between the piston
rings and the cylinder bore. This can be a source of heat!
There are other conditions favorable to cause an explosion, for
example the quality of the oil and the effects of oxidation after a
long period of use, the generation of oil mist due to agitation of
the oil spray, the property of the oil, etc.
It is impossible to guarantee that any engine will operate for the
whole of its life without, somewhere, at some time, a hot spot
appearing. Therefore it is essential that should a hot spot occur and
if it goes undetected, an explosion will not result.
That could be the subject of another article... Meanwhile procuring
hard-to-find tools is just as important for making engine adjustments.
About the Author
Many years of working experience in Marine, Facilities,
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