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Horror Stories

Cracked Piston Crown - Slipped Crankshaft

 

A container ship with a 6 cylinder 700mm bore engine stopped for investigation when the piston cooling oil return on cylinder number 3 went into alarm.

The engine was stopped for inspection, but the engineers found no fault and restarted the engine.

They couldn't have looked very far because number 3 piston crown had cracked and oil was leaking into the cylinder.

When the engine was restarted the hydraulic shock caused the semi built crankshaft to slip between units 3 and 4.

The slip was so severe (300mm) that after limping to port on 3 cylinders, the crankshaft had to be renewed, as the damage to the crankshaft did not allow expansion, realignment and reshrinking.

The lessons that can be learned are:

When a crack occurs in the piston crown, when the cylinder pressure is below the cooling oil pressure, oil will leak into the cylinder. The oil will burn, causing high exhaust temperatures, smoke, and fouling. When the cylinder pressure is higher than the piston cooling oil pressure, the hot gas will leak into the cooling space, overheating the oil and causing pockets of gas which may interrupt the flow. It is possible that the mixture of hot oil and gas returning to the crankcase may set off the oil mist alarm.

It is obvious that the engineers could only have restricted their examination to the crankcase, when, with the engine stopped, all would have appeared normal. if they had opened the scavenge space up, they could have sighted the tops of the pistons through the scavenge ports. They couldn't miss the oil leak then!

However, the unforgivable sin was to try and restart the engine without turning on the gears with the indicator cocks open.

If they had done this they would have seen the oil jet from No 3 indicator cock before the turning gear stalled. This would have saved the crankshaft.

Never start an engine without taking this precaution. And don't be one of the idiots that think you can turn it over slowly enough using main start air. Unless you have a slow turning valve, you can't.

I don't know what the chief engineer of the above vessel is doing now. Wiping up somebody else's oil I expect!

 

 

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