Neale Consulting Engineers - Services - Plain Bearing Failure Investigation - Chemical Effects

PLAIN BEARING FAILURE

Plain bearing failure index

6.0 Chemical effects

Chemical effects Index

6.2 Lubricant Degradation Deposits

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	Deposit of ammonium succinate in bearing of ammonia synthesis gas compressor.
Main Characteristics	This is clearly a deposit from the lubricant that has been laid over the whole bearing surface apart from the loaded lands on the bottom half (right), where the high shear rate in the oilfilm has prevented settling. Diagnosis depends on chemical identification of the deposit.
Cause	This occurred in the bearings of the synthesis gas compressor on an ammonia manufacturing plant; this was the first such plant using a centrifugal compressor rather than reciprocating ones. The bearings and shaft seals on this machine were lubricated by a turbine oil from a common system. Chemical analysis showed the deposit to be ammonium succinate. Succinic acid is widely used as the corrosion inhibitor in turbine oils. Ammonium succinate was formed by reaction between ammonia leaking across the oil barrier shaft seals into the lubrication systems and the corrosion inhibitor in the oil. Ammonium succinate is soluble in water, but not in mineral oil. The lubricant that was also used for the steam turbine driver was slightly wet, the water dissolved the succinate which was then deposited in the hot bearing as the water boiled off.
Note	Note also the small patch of fatigue damage (see Section 3: Fatigue) on the right hand land of the upper half (left} and the signs of wiping (see Section 1: Lubrication breakdown) on both lands on the bottom half (right); the latter is probably a consequence of the deposit formation.
Possible Confusion with Other Types of Damage	Other deposits described in this Section tend to look superficially the same. The final diagnosis requires chemical analysis.
Comment	It was not possible with this system to prevent minor contamination of the lubricating oil with ammonia. The solution was to reformulate the oil with a corrosion inhibitor that did not react with ammonia. Such oils are now standard on ammonia synthesis plants.