written by: Willie Scott
• edited by: Lamar Stonecypher
• updated: 2/10/2010
Oil mist detectors offer watchkeeping engineers protection against explosion in the ship’s main diesel engine spaces. Crankcase and chain cases explosions can be caused by an oil mist contacting a hot bearing or by a scavenge fire heating the dividing plate between the scavenge space and crankcase.
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Oil Mist Detector – Purpose and Operation
This is another article in the series on marine diesel engines. This time we look at the modern oil mist detector, comparing it to the type I was used to when I was an engineer at sea many years ago. However the memory of several near-misses involving crankcase explosions preceded by the ear-splitting howl of the high oil mist alarm is still with me.
We will take a look at how the lube-oil mist is produced by recapping on a previous article of mine on lube-oil systems.
Lubricating oil is supplied to the main engine under pressure from the main lube-oil pump. It passes through the crankshaft, lubricating and cooling the main and bottom end white metal bearing, returning to the sump. It is also supplied to the crosshead guides and piston rod bearing, from which it cascades down to the main sump.
During this activity an oil mist is produced, which is to be expected, however if there is a hot component the oil mist will be increased and vaporize with the real risk of fire and explosion in the engine crankcase.
The purpose of the oil mist detector is to detect any increase in the density of the oil mist, setting off an alarm to warn the watchkeeping engineer of potential danger.
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Crankcase Explosion Relief Doors Operation
The crankcase explosion relief door should be made of as light a material as possible but still maintain its strength, and must be self-closing to prevent a backflow of air into the crankcase. This will certainly lead to further explosions, being much more severe than the initial one.
The relief door mounting frame is bolted onto the main door, the frame containing the components.
These consist of several layers of fine, medium, and course wire gauze stretched across the framework on the crankcase side, acting as a flame trap as opposed to the older type of relief door which had the flame trap on the engine room side. This has reduced the incidence of flames escaping into the engine room and being constantly sprayed with lube-oil has made it a superior flame trap.
A spring arrangement holds a light aluminum circular relief valve shut against a seat inside the frame. The valve face has an oil and heat resistant seal which prevents any ingress of oil to engine room during operation. The spring is set to lift at a few pounds. A rise in crankcase pressure above this will lift the valve, relieving the pressure, and immediately reseating to prevent any air being drawn into the crankcase. There is a deflector plate on the outer cover so that if the valve lifts any flames are directed downwards towards the engine room plates.
We shall now look at the operation of an oil mist detector…
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Crankcase Oil Mist Detector (OMD)
Modern OMD units are fitted to the crankcase doors and are designed and calibrated to compare a sample of the internal oil mist contents with a sample of air which is piped in from a clean dry source by a small fan.
The results are electronically transmitted to a unit in the control room and, if a buildup is detected an alarm will sound preceding an explosion, giving the engineer enough time to slow the engine down and let it cool.
A CO2 supply to the crankcase can be operated either by hand or automatically on a high oil mist reading. It is injected through several spray nozzles at the top of the doors forming a CO2 blanket as it falls down towards the main bearings and crankshaft smothering any fire.
If a high oil mist alarm is activated, the engine should be slowed immediately, and a close eye kept on the lube-oil temperature and pressure whilst checking for a false alarm.
If in any doubt the CO2 system should be manually operated and under no circumstances should the crankcase doors be opened until the engine has cooled sufficiently.
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Timing Chain-Case Oil Mist Detector
There can also be an oil mist detector fitted to the timing case with the same detecting and CO2 smothering system as the crankcase.
I have not witnessed any explosions in this area, but I have heard of a few and they are quite frightening. The oil mist can build up and remain relatively high due to the action of the drive chains and cogwheels which run from the crank drive. The chain is kept in tension and aligned by a couple of jockey wheels, one of which is adjustable.
As these cogs and chain rotate they require lubrication, so they throw the lube oil in all directions, creating an oil mist. If this becomes excessive and comes in contact with a hot component, the result will be an explosion. Most engine timing chain cases are now fitted with explosion relief doors.
Heat can also be transferred from the chain-case to the crankcase by a drive bearing/shaft seizing if close enough to the dividing plate into the crankcase. This can then cause a hotspot in the crankcase resulting in explosion in the crankcase as well as the chain-case.
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The modern oil mist detector system is a far cry from the type that was fitted when I was a young lad at sea. In those faraway days the oil mist detector alarm would activate, sounding like a banshee and scare the boiler suit off you. These early type consisted of a series of pipes which were fitted through the crankcase door frames and, led back through microbore pipes to an analyzer. By the time the oil sample got to the analyzer, condensation had taken place giving a false reading. But, every alarm had to be investigated and we used to slow the engine down and feel along the crankcase doors, (just right in front of an explosion relief device lifting or, a crankcase door being blown off, how naive the young are!) and wait a while to make sure it was a false alarm, then back to full ahead and full away.
The modern types are much more accurate, seldom giving a false alarm. The analyzers are in the OMD unit, bolted onto the crankcase door and the results transmitted electronically to an analyzer conveniently placed in the control room. The alarm can automatically set off the CO2 gas injection to the crankcase or this can be done manually, but either way once the gas bottles are activated, they continue to operate until empty.