Maintenance and Repairs

Tracing The Cause of Oil Mist Alarm With Engine Stopped

 
 

 

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.

The chain casing cover was removed to see if the source of the mist could be traced. Oil spray nozzles were performing satisfactorily, and all seemed in good condition but mist was seen to be coming from the camshaft seal between the chain case and the chain casing.

The camshaft seal was removed to see if was damaged or broken, however it was in good condition. The mist was coming from the camshaft, and when the camshaft inspection plates removed, mist was seen in the cam boxes. 

The 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 the tank.

The 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 camshaft L.O pumps were stopped and the camshaft boxes were removed. The camshaft/bearings and cam followers were inspected and nothing unusual was found. It was decided to examine the camshaft bearing,  The fuel cam follower on to the camshaft was raised to release any pressure on the camshaft. The camshaft half bearing between units 1 and 2 was taken off using hydraulic jacks and the bearing was gently lowered for examination.

Inspection showed there was wear on one of the edges on the white metal bearing and there was scoring on one section on the bearing. There was no evidence of overheating, so the bearing was replaced and tightened to the correct pressure of 900 bar.

 

When 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.

The main engine unit 1 exhaust valve was replaced and the removed valve stripped and examined. The air spring piston seals were worn and the air spring safety valve was loose and was allowing air to leave the spring and enter the hydraulic oil space above the air spring, where it entered the cambox via a leaking hydraulic oil relief valve and from there to the chaincase and crankcase.

 

Air Spring Piston and Cylinder

Removing Exhaust Valve

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.

 

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