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Loose Piston Securing Nut Causes Explosion

 

 

This abstract was taken from a paper titled "Some Marine Machinery Failures and Their Causes" by RF Munro and PE Haynes Published by the Institute of Marine Engineers

During a normal sea passage, a tapping sound was heard in phase with the engine revolutions but the engine was not stopped for investigation. The sound gradually became louder and eventually the engineer on watch telephoned the Chief Engineer.

Figure 1 shows the assembly of piston rod, skirt and head, and attention is drawn to the securing arrangement for the long tubular nuts. To prevent the two set screws from turning, a soft iron tab washer is provided; a tried and trusted device used by mechanical engineers for generations. It is, however, important to remember that these tab washers should be used once and thrown away, otherwise, through fatigue, the tabs are liable to break off with disastrous results, which could have been the cause in this case.

The noise was being caused by the repeated impact of one of the tubular nuts upon the cast iron diaphragm dividing the scavenge space from the crankcase (see Fig. 2).

As the nut continued to turn back, the blows eventually smashed the casting. Pieces were struck by the whirling crank throw, causing sparks, and when the scavenge air blasted into the hot oil-rich crankcase a violent explosion occurred which blew the doors off, not only the engine, but also the engine room.

The Chief Engineer, who was descending the engine room ladders to investigate, died.

End of abstract

Although this accident happened more than 20 years ago, it illustrates a point. How many of you can, hand on heart, say that you have never re-used a tab washer or similar locking device?

The amazing thing, I find, is that the watchkeeping engineer kept the engine running. Any unusual noise or vibration should be investigated. The fact that the knocking was in time with the engine should have given cause for concern.

A similar incident happened on an engine of a similar type in 1993. But however disaster was avoided by stopping the engine in time. However the vessel was was transiting the St. Lawrence Seaway at the time and ended up colliding with the wall of the canal.

When the engine was stopped and inspected, a piston stud securing nut was found to have backed off. It was hitting against the piston rod gland diaphragm plate whenever it reached bottom dead centre. The piston rod gland diaphragm plate fractured as a result of this repeated hammering by the loosened nut.

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