Marine Diesel Engines - Oil Mist Detectors

Fabrication

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.

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…

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.

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.

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.

www.globmaritime.com

www.oilmist.com