Diesel Engine Protection: Crankcase Mist Detector

I have a related question, I have a diesel engine with a mechanical governor, I want to replace it with an electronic governor, do you have a source for and electronic governor?

What is the engine make? Who is the manufacturer of the governor?

Answer these two questions. Contact both manufacturers and find out what you can and cannot do with the engine in question.

May i know the brand of your diesel engine? You can use Woodward EPG governor if your engine is equipped with rotary injection pump.

help!!

Good morning

My name is Mam Singui Sarr from Senegal.

I have to choose an OMD for a 18KU30B diesel engine.

i don't know whether to an OMD 3M or an OMD 9M.

i need some elucidation or documentation

email: singui4@gmail.com

All of the diesel engine crankcase mist detectors that I am familiar with do not shut down the engine, they sound an alarm in the event of an oil mist being detected.

Hopefully this would allow a ship, if in restricted waters, to be put into a position of safety before the engine is shut down and correct procedures are followed to prevent a crankcase explosion.

The detectors that I am familiar with have a fan that samples each crankcase in turn, that is on a large two stroke engine (automatically). There are two tubes, a reference tube that is sealed and the sample tube which has the crankcase sample drawn through it by the fan. Both tubes have a light source directed through the lengths of the tubes onto similar photo electric cells which are wired to an alarm circuit.

When there is no problem both photo electric cells are receiving the same amount of light, therefore there is no voltage flow through the alarm system. If a condition exists that could be building up to an explosion, the mist obscures some light from the cells in the sampling tube, then a potential difference occurs between the photo electric cells, voltage flows through the alarm circuit and wham oh! the alarm sounds.

If the mist, for some reason was not detected you would probably have a crankcase explosion, which obviously causes major, major damage to the engine. There are two explosions that occur when you have a crankcase explosion, the first is only small, the overheated component will ignite the oil mist,but, because of a very low air/fuel(oil mist) ratio the explosion is small. This causes the crankcase explosion door valves to lift off their seats, but before the spring loaded valves re-seat a large amount of air (oxygen) is drawn back into the crankcase, caused by the pressure drop of the initial explosion. We now have a much better air/fuel ratio, hence a much larger explosion, which will probably blow out explosion doors, inspection doors and large pieces of crankcase, etc.

There is some interesting stuff on this site!. I was ready to presume that a mist detector was there to ensure the lubrcation system was working and that there therefore was a mist. Where does the danger of crankcase explosion come from?

This comes from an overheated component in the crankcase, such as an overheated big end or main bearing, oil pump, etc and on large slow speed two stroke engines a fierce scavenge space fire could cause a crankcase explosion. What happens is the heated component vapourises the oil in contact with it causing it to form a mist.

I was thinking that this type of diesel scaenged like a 4-stroke, sorry.

There is a mist, but provided the surface temperature of all the components remains below the flash point of the mixture, whatever the mixture content is, there will not be a flash (errr - should not be!).The danger of crankcase explosion can come from unsuspected causes or combination of causes. About 30 years ago I was on the third sea trials of a small coastal tanker with a newish design of 6 cylinder 3000bhp two stroke direct reversing engine. The ship had failed the first two trials due to instrumentation and valve gear reversing malfunction, but thats another story. Working up speed to go to sea, tests including demonstrating the modified reversing gear, about 15 miles from port and currently on about half load there was a crankcase explosion. Being in the engine room, and turning around to see where the deep whuush was coming from, great walls of mist rush at you and envelope everyone and you follow everyone skidaddling out. Drifting in a river is not much fun especially when you know representatives of the owners, builders, insurance company. the skipper etc are jumping up and down fron the bridge. When deemed safe, an hour, the engine was restarted and the ship crawled back into port. The explosion doors had worked, relieving the crankcase explosion and shutting before air got back in, leaving the remaining mist inert. When tied up and the engine cooled down we crawled over the engine room and removed the crankcase doors. Inspection eventually found cracks in the bottom end of the cylinder liners. But why had they cracked and did this cause the explosion.The engine room lub oil system was examined. The piston cooling oil system on a 4 inch main was completely separate to the lubricating system. In this pipe system one engineer eventually queried a 4 inch diameter flanged insert in the pipe. The ship builder said it was the oil filter as specified in the drawings supplied by the engine builder. We, the engine builder, said no way, the filter was shown as massively larger, or it would have restricted oil flow, as indeed it had demonstrated in a very forceful way. The ship builder had picked up the wrong filter and assumed its location. The cooling supply to each piston had been severely restricted causing a tight fit and eventually cracking the chrome plated liner. The hot spot created caused the explosion. Oh, goody, replace with the correct much larger filter, large because of the through flow, not dirt, because it was not connected to the crankcase oil system, and all is solved - not so.

The fourth sea trial, out in the river, much further down, another crankcase explosion, limp back to port, engine cool, remove doors, cracked cylinder liners. What!!!! Dismantle engine again, remove cylinder liners, half of them cracked. The engine builder had, in the rush to rebuild the engine, put in a set of iron chrome plated liners, instead of the later much better steel chrome plated liners. Result, new steel liners, problem solved.

Why? Well, a couple of years previously, the engine builder had been bought out by a large electrical conglomerate and wanted a quick return on capital so decided to accelerate sales of the new engine - before development was complete.

So what caused the failure in operation - poor engine room building, marginal piston/liner design through incomplete development, financial greed ?

Georgee

The primary purpose of the Oil Mist Detector (OMD) is for Safety.

If an high Oil Mist is present in the crankcase compartment, and an blow-by ignites the combustible mixture, an explosion takes place. For a ship, this explosion endangers the lives of the mariners, and the International Community calls for a regulation on the safety standard to be imposed, thus the implementation of Oil Mist Detectors.

In most cases, when an increase of Oil Mist concentration surges, it is often due to the over-heating bearings due to the lack of lubricating oil. The Oil Mist Detector will either shut down the engine or slows it down to idle speed. The OMD does not help in any way to prevent the bearings from overheating, neither will it protect the engine from damage, only to prevent an explosion inside the crankcase compartment.

Various manufacturers of OMD has different approaches to solve this problem.

1. QMI measures oil mist concentration by light refraction, with Optometric principle as to that applied in the measurement of oil in ballast water discharge system. Partial forging of the optical system does not affect the measurement, and there is no need to float zero point or to do an opacity calibration. QMI boasts of an Atmosphere OMD for the Engine Room monitoring.
   
2. Schaller Automation VN215 VISATRON measures oil mist by Opacity principle, where the concentration of oil mist obscures the light beam to a photodetctor. This is prone to forging optical system. It also pipes the sample into a measuring chamber controlled by sliding valves. Over a distance of piping, the oil mist concentration is lost, and condenses into oil droplets.
3. Graviner MK 1 to 5 have the same principle of measurement by Opacity. The MK 6 has individual heads at the crankcase, instead of piping the sample to a selection valve.
4. Dr. E. Horn Crankcase Supervision Monitoring has the opacity method, but with an added crackcase pressure monitoring to detect a blow-by from the cylinder head. Their advantage is the easy installation of the OMD at the manifold, instead of the pipes or detectors at the crankcase covers or the engine side.

Modern engines are all fitted with AI system, aka OBD II Code System for vehicle engines. Wartsila, MAN Diesel, Daihatsu, Hyundai, and others have their own computerized monitoring system. Parameters such as: Lubricating oil temperature, Cylinder head pressure, Exhaust gas temperature, cooling water system, water-in-oil contamination monitoring, engine speed, and turbocharger condition monitor are connected to a computer, which constantly monitors and gives alarm or shut-down the engine if the set limits are exceeded.

tijit^{digital is our name, portable is our game}

Yes and these explosions have occurred in the past, as I remember when I was a young engineer back in the 1960's, and they took lives with them. Phew that was a long time ago.

Regards JD.

There are two principles used for Oil Mist Detection by the existing companies making these Safety Devices.

The earlier one is the Opacity method. Samples of the crankcase compartment atmosphere is sucked into a measuring chamber via port-selection valves, by an ejector system, the flowrate is maintained constant and a vacuum is formed to suck the samples. Inside the measuring chamber is a set of optical transmitter and a receiver. Any amount of oil mist will obscure the light path and detected by the photocell as a increased percentage or ppm of oil mist. This signal is a Square Root gradient in the chart oil mist concentration to output signal curve. Eg. of these systems are the SCHALLER AUTOMATION VN215, and GRAVINER MK2, Mk3, MK4 and Mk5 and Daihatsu.

The later method is by Optometric principal, which is different from the above. Optometric gives a positive signal output when the oil mist concentration is high, and the signal is linear, which gives this method an edge over the predecessor, as the linearity counts much with the resolution when at lower signal level. The latest design model has individual sensor head at various locations of the Diesel Engine's crankcase compartment. The samples are taken right on the spot, not piped in, and there is higher redundancy rate, and faster response as there are multiple measuring points. Eg. of this principle application is: QMI and Specs.

tijit

Mist detection is primary utilised for bearing failures where hot spots 'distill' the oil into a mist right on the surface hot spot and it is detected faster (in theory) than an RTD in the bearing which must heat up base metal to reach the RTD and often times the bearing surface is wiped by that time. Plus there is logistics issues of RTDs in rotating components such as big end bearings and wrist pins. Mist detectors are prone to false alarms and are often disconnected later.

How to have other mist is from the injection process. If you have a bad injector, there is little peak pressure in that cylinder to force the rings against the cylinder walls and unburned atomised fuel blows by the ring pack and charges the crankcase with an explosive mixture searching for an ignition point and oxygen. If any hot surface is higher than the auto ignition point of the mixture, and you have enough oxygen to be above the low explosion limit (LEL) you will touch off and explode.

Upon any emergency shutdown such as overspeed or operator caused ESD button where the air valves shut off inlet air at full speed, there is still some fuel in the injection pumps and this fuel is sprayed into each cylinder, without air to burn, and without combustion to seal the rings, and there is tremendous explosive mixture charged in the crankcase while the engine slowly comes to a rest. If you open the explosion doors before 1-2 hours, you let in oxygen and then it may touch off.

For the fellow that had cracked liners there was unburned combustion mix leaking past the cracks due to cylinder pressures forcing it out under combustion (emissions are caused by unburned fuel, so there is ALWAYS unburned fuel in every engine normally). Then the overheated piston crowns touched off the mixture the first time, and overheated rings faces (perhaps - and perhaps due to wrong surface on the liners that retains lube oil that lubes the ring faces) or some other normal heat source the second time.

You can use OMD MK6......

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Sir, We have demand from one Ship

CRANK CASE OIL MIST DETECTOR

MK 4

GRAVINER ENGLAND / EQUIVALENT

We have

MKS Graviner (OMD England)

Pt. No. : 53531-22

S. No. : 4876

Date : 2/98

Can i Quate this Graviner to my Cutomer.

Danish

Pkgi@cyber.net.pk

When there is bearing failure, there is friction between two metals.

This temperature above 200 Celsius vaporize the Lube Oil and bring its flash point. The crankcase compartment becomes an explosive environment, only takes a source of ignition to set an explosion in the crankcase. This posses a serious threat to the ship and the lives of the seamen at sea.

The mandate requirement by SOLAS (Safety of Life At Sea) and IMO (International Marine Organisation) is for all ships to be fitted with a device to prevent this explosion.

Various companies come up with the Oil Mist Detector, they were Graviner, Schaller Automation and Daihatsu. Later on, QMI and DR HORN GmbH.

All of them work on the same principle of detecting oil mist, but the method of sampling the environment are different.

Tijit