Reliability Engineering Snapshot TM

Corrosion - Case Study No. 6: Corrosive Gas Attacks Idle Gas Compressor Bearing

Gas compressors in corrosive service which sit idle are subject to corrosion by the stagnant gas inside the compressor; allowed to sit idle for extended periods of time, the bearings are sure to fail. To the left is a disassembled ball bearing assembly showing the inner race and the ball cage, a couple of balls are shown for illustration. The service was not thought to be corrosive, yet the failure rate of the bearing shown was unacceptably high. The mechanics were being blamed for not knowing how to install a bearing. Upon closer examination, the ball bearing in the lower left picture revealed a very unusual corrosion pattern. First off, it was corroding, but more importantly it was obvious that some parts of the ball were not corroding, as if being protected by something. What was confusing at first, really turned out to be simple. The pattern couldn't be made while the compressor was running. The ball would be turning and such a pattern would be impossible to make; instead, the ball had to be stationary for such a pattern to be created. The long narrow line on the lower left hand picture, where there is very little if any corrosion taking place, is actually where the ball came into contact with the cage, as depicted in the picture to the left. With the compressor idle the ball bearing had no oil lubrication. The only oil left on this bearing was whatever was held to it from capillary action, that is, pieces touching other pieces and holding what little oil there was to the contacting surfaces. Taking the ball depicted in the lower left picture and rotating it roughly at right angles to the line where the cage contacted the ball, this is what is seen in the lower right hand picture. The new and different pattern widens out tremendously. This section of the ball was contacting the outer race.

The thin oil film was acting as a protective barrier against the corrosive gas which was stagnant in the idle compressor. This compressor was a spare unit and was not routinely rotated into service; this failure is proof that in such service it is wise to routinely rotate compressors.

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