When the engine is running, the air in the crankcase has the same types and amounts of gases as the air around it. However, there is a heavy shower of coarse oil droplets that are thrown all over the crankcase when the engine is running.
If there is too much friction between the surfaces that slide against each other or if heat is transferred to the crankcase in some other way (like from a scavenge air fire through the piston rod/stuffing box or through the intermediate bottom), “hot spots” can form on the heated surfaces, which makes the oil droplets that land on them evaporate.
When the vapour condenses again, it forms many tiny droplets that float in the air. This creates a “milky-white oil mist” that can feed and spread a flame if it gets started. The same “hot spot” that caused the oil mist can also start the fire.
Oil mists are formed at temperatures of approximately 350°C and ignition can start at a temperature <500°C.
If there was a lot of oil mist before the spark, the burning can cause a huge increase in pressure in the crankcase (an explosion), which forces the relief valves to open for a short time. In rare cases, when the whole crankcase has probably been filled with oil mist, the resulting explosion blows off the crankcase doors and starts fires in the engine room. The chain case and the scavenge air box can also explode in the same way.
So, it’s important to have a way to find out early on if oil mist is building up, hence the need of the oil mist detector device (OMD).
The oil mist detector is a device which constantly checks the amount of oil mist in the main engine crankcase. It does this with a high-sensitivity light scatter optical sensing system. This makes sure that an alarm goes off before the amount of oil mist gets to the lower explosion limit. Each sensor on the engine sends an electrical signal to a control unit, which checks each sensor in turn to see what its oil mist density value is.
As part of the detector readings, the system also checks itself to see if there are any problems inside.
On the ECR console is the control unit.
These units scan the signals from the detector heads one at a time, and they do this every 1.2 seconds for all of the engine detector heads. The average density of the oil mist is worked out and saved. Then, each detector signal is put up against the average that has been saved.
Then, a positive difference or deviation is compared to a set value. If the set value is higher, an alarm will go off. The stored average is also compared to a pre-set reference. If the reference value is exceeded, an average alarm is set off.
The system has a priority for alarms, so that when an alarm goes off at any detector head, it is dealt with right away.
On the Graviner type OMD, all of the detectors work on their own, so if one breaks or needs to be cleaned, it doesn’t affect how the rest of the system works.
Some sensors can be taken out of the system for maintenance while the rest are still working. Because the system is made up of separate parts, a broken detector can be changed in just a few minutes.
On the Visatron type the engine must be stopped for testing and maintenance.
It is very important that the oil mist detector system is kept in good working condition and that any alarms are dealt with right away, as this instrument is an important safety measure against a crankcase explosion. When the oil mist detector goes off, the engine slows down.
Every day, the duty engineer should check and test, if possible, how well the oil mist detector works. On the Graviner type, the unit is tested at the control panel, but each detector head has an LED indicator that must be checked every day to make sure it is working. If a detector head stops working or sends a strange signal, an alarm goes off.
In the event of an oil mist alarm on the engine, certain actions need to be taken in order to prevent further problems development. The actions that has to be taken you can find them in you follow this link: FIND OUT MORE…(You will need to Register in order to have access to all information as is for subscribers only).
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Source and Bibliography:
- Youtube video source and credit: Marine Eng Video; Schaller Automation