Oil Mist Detection

History

9/11 is remembered for largest terrorist attack However there was another 9/11, 11th

September 1947, when a crankcase explosion on the Riena Del Pacifico  killed 28 men and injured 23 and led to the development of  crankcase relief valves and oil mist detectors. Of course there had been crankcase explosions before this, but none which had such devastating consequences.

Cause of formation

For explosion to occur there should be 3 components oxygen, fuel & ignition.

Oxygen is present in the crankcase, but the lubricating oil splashing around in the crankcase is in too large droplets to start burning at the speed needed to cause an explosion, and the oil/air concentration is too weak.

Cause of formation

If, however a mechanical fault develops with the consequent rubbing of moving parts, then a hot spot will occur

This could happen in the crankcase, chaincase, or camcase

When the temperature of the hot spot reaches 200°C the lubricating oil splashing on to this hot spot vaporizes. The vapour then circulates to a cooler part of the crankcase where it condenses into a white oil mist

Cause of formation

The oil droplets in this oil mist are very small - 5 to 10 microns in diameter. When the concentration of oil mist reaches 50mg/l (about 13% oil mist - air ratio), it is at its lower explosive limit

If this oil mist is now ignited by the hot spot - and tests have shown that it is necessary for a temperature of about 850°C to ignite oil mist in a crankcase under operating conditions - then an explosion will occur.

Cause of formation

Although the most common cause of a localized hotspot is due to friction, it is not the only cause of a crankcase explosion. A cracked piston crown, blow-by or an external fire have caused crankcase explosions in the past.

Severity of explosions vary between a puff which may lift a relief valve to a violent explosions which causes major damage and may injure personnel and cause a fire

Cause of formation

Evidence indicates that the longer the combustion path, the more violent the explosion. This has become an area of concern with the large two strokes of today which may have a crankcase volume of 500m3 +.

When an explosion occurs a flame front travels down the  crankcase with a pressure wave in front of it.

Cause of formation

The turbulence caused by moving engine components causing churning and mixing of vapours increase the speed of the flame front and its area, which contribute to the increase in pressure.

Turbulence caused by venting of the pressure through relief valves can also influence the explosion.

Cause of formation

Following the venting of the explosion through the relief valves, there is a drop in crankcase pressure to below atmospheric pressure.

This can cause air to enter the crankcase resulting in another flammable mixture to be developed resulting in a secondary explosion to occur.

Cause of formation

The secondary explosion is more violent and can result in crankcase doors being blown off the engine, and fires starting in the engine room. If the relief valves do not reseal after lifting, or if they do not lift at all in the primary explosion ( due to lack of maintenance etc), then door(s) may be blown off in the primary explosion, giving a ready path for the ingress of air, which will make a secondary explosion more likely. Air can also be sucked in via the crankcase vent, although rules state that this must be as small as practicable and new installations must have a non return valve fitted.

Cause of formation

If a primary explosion occurs, the pressure wave may send a large amount of oil mist out into the engine room. Although the flame arrestors on the relief valves should prevent ignition of this oil mist by the flame front, the mist will be sucked up towards the turbocharger where it may be ignited by an unlagged hot exhaust manifold. This ignition of oil mist can cause severe damage to plant and personnel.

Causes of failure

Oil mist formation can generally occur in 2 condition:

During starting of engine During running of engine

Types of alarms in OMD

There are 2 type of faults which occur in OMD

Average alarm

Deviation alarm

Types of alarms in OMD

Average alarm usually occur when there is a deterioration of lube oil like mixing of water, fuel etc. which give rise to oil mist in the entire crankcase hence increasing the average OMD%.

Deviation alarm usually occur when there is development of a hot spot or there is blow-by from some piston.

Types of alarms in OMD

There is also a flow fault associated with OMD which basically occurs due to choking of tubes of OMD and which is a system fault and does not signify any fault in the engine per se.

Occurrence of OMD

OMD generally occurs during the starting of the engine in the winter season. The alarm is of average type.

The alarm appears when the temperature of the lube oil is low & when engine is started it comes in contact with hot surfaces causing the formation of mist.

Also during the running of engine ,average alarm appears when there is a dilution/deterioration of the lube oil

Occurrence of OMD

Deviation alarm generally occurs when there is development of hot spot in the bearings or there is leakage of charge from the pistons, signifying the sticking of piston ring for that particular piston.

Check list for Shift Engineer in case engine trips in OMD alarm Immediately open lube oil pouring point cover and observe

mist /white smoke, if any. Do not reset OMD panel and UCP panel. Record OMD % in the panel and note the cylinder no. Physically touch/take temperatures with non contact

thermometers of the crank case door and note if any particular door is more heated-up than others,

Record in log book if any alarm is appearing in UCP panel. Record whether OMD alarm is ‘average alarm’ or ‘deviation

alarm’. Record in log book whether CO2 has purged or not. Record the time of occurring OMD alarm in log book so that

maintenance crew can know whether OMD occurred immediately after starting of engine or during running of engine.

Check list for maintenance engineers In case of deviation OMD alarm in a particular cylinder:

Physically touch the crank case door immediately and note if any particular door is more heated-up than others

Check the observations made by shift engineer as mentioned above and concentrate on that particular cylinder in which OMD deviation alarm occurred.

Open all crank case doors and check the big end bearing & main bearing temperature (using temperature laser gun) in that cylinder and compare with other cylinders and there shall not be more than 5 deg difference between temp of any two cylinders. Also check temperature of liner, connecting rod (small end) in that cylinder and compare with other cylinders.

In case temperature gun is not available, check by hand whether any particular liner/connecting rod is extremely hot.

Check list for maintenance engineers In case of deviation OMD alarm in a particular cylinder:

Check for water leak from cylinder head or liner O-rings in that cylinder Check for free float of connecting rod at big end side (as we measure end-

float), Check for water% in engine lube oil. In case, lube test kit is not showing

reliable reading, send sample for outside testing Check for foreign particles in lube oil in engine sump by putting hand in the

lube oil once it cools down. In case no abnormality is observed from above, engine trial may be taken in

NO load under close observation by continuously monitoring temp of big end bearings and main bearings with the help of laser gun as done. If no abnormality is observed, then load trial may be taken.

Check list for maintenance engineers In case of average OMD alarm :

Check all the big end bearings, main bearings, liners, connecting rods (small end) for high temperature and there shall not be more than 5 deg differences between temperatures of any two cylinders.

Check for water leak from cylinder head or liner o rings in all cylinders

Check for free float of connecting rod at big end side in all cylinders

Check for water% in engine lube oil Check for foreign particles in lube oil in engine sump by

putting hand in the lube oil.

Check list for maintenance engineers In case of average OMD alarm :

It may please be noted that during winter season OMD can occur if cold lube oil falls on hot engine bodies like crank web etc during cold starting of Engine. Hence it is advisable to start the engine during winter after sufficient lube oil priming (min 15 minutes). During starting, radiators to be stopped and radiators to be started only after lube oil temp at engine crosses 50 deg c. This will minimize mist generation due to cold lube oil coming in contact with hot engine bodies.

In case no abnormality is observed from above, engine trial may be taken in NO load and then on LOAD under close observation by continuously monitoring temp of big end bearings and main bearings with the help of laser gun.

Thank You