IMPROPER PLACEMENT OF SPHERICAL WASHER :
The engine was B&W7L67GA.
One unit decarbonisation was done in the port. At full away on sailing, the cylinder head gasket started leaking. The engine was stopped, and the cylinder head was opened and taken out for replacing the gasket. While opening, it was observed that 3 of the nuts opened at 400 bar pressure even though they were tightened at 700 bar pressure as per the manual.The arrangement in this engine is that a spherical washer is placed under the nut which normally elongates to normal shape once it is tightened to the required 700 bar pressure.
The placement of washer play an important role. When the engine was opened up earlier for decarbonisation, this washer was found to be put in the opposite direction. The 3/E corrected this while boxing back as given in the manual.After renewing the gasket, the engine was retightened to 700 bar pressure uniformly. Then it was loosened to check whether it opens at correct pressure of 700 bar. This time again the three nuts opened at 400 bar. The spherical washer was renewed for one nut and again retightened. Found tightening and opening pressure to be 700 bar. The remaining two nuts opened at 400 bar. Hence for the two nuts also the spherical washer was changed. All the stud tightness checked and found satisfactory.
Lessons Learnt :
Even though the assembling is done as per the manual, it is better to check whether the tightening and opening pressure remains same. Otherwise there should be some means of check to find the undue elongation of materials, so that such stoppages can be avoided.