Thanks to Dylan Wheel for contributing this account of how the source of an oil mist on a MAN-B&W MC engine was traced and how the problem which caused the mist was found and corrected.
Oil Mist in the Engine Room
The oil mist detector was going into alarm when the main engine was stopped showing a high oil mist concentration . The oil mist detector was showing the mist was in compartment no.8 which is the chain case.
The main engine crankcase was opened up to see if there was any oil mist in the crankcase, which there was. All units bearings main, bottom end, crosshead, guide shoes were checked.
There was no steam on the separator oil tank, situated under the main engine; it is possible that if their had been it may have caused the oil to heat up in the crankcase and form a mist.
camshaft tank air vent was checked to ensure it was clear and that any
vapour was not building up and entering the camshaft boxes from
camshaft oil was changed to ensure the oil was clean and fresh and not
contaminated, causing the mist.
The camshaft lubricating oil pumps were turned off and the inspection plates were removed to allow the mist to clear. When clear, the pumps were turned on so the exact source of the mist could be identified. The mist was coming from the camcase around main engine units 1 and 2, but the exact location could not be identified.
the bearing was in place the area around the bearing was inspected again,
and this time air could be felt coming from area around the bearing. The
problem had been solved! It
was suspected that the exhaust valve air supply for the air spring was
leaking and finding its was into the camshaft, forming a mist.
The main engine unit 1 exhaust valve had been recently overhauled and it was since the overhaul that the problems had been experienced with oil mist in the camcase when the engine was shut down.
Air Spring Dissembled Showing Seals
This only happened when the main engine was stopped because the 7 bar air spring supply pressure was greater than the hydraulic oil pressure because the valve was not operating. When the engine was running and the valve was operating, the hydraulic oil pressure was sufficient to stop the air entering the oil above the air spring and therefore no mist was forming in the engine.