BOG Compressor Thrust Bearing Root Cause Analysis

A Boil Off Gas (BOG) Compressor of Customer LNG plant tripped because of excessive rotor movement. This event occurred after 18 months of operating; when the expected life to inspection had been thought to be 6 years. The impact of this breakdown was the loss of facility, the need to flare off gas when B unit was not running, plus associated down time and repair costs.

When opened for inspection, the LP compressor active thrust bearings were found to have been wiped. Also, pitting damage was observed on the LP and HP journal bearings, and in addition, the HP active thrust bearing. The pitting appeared to be more extensive in the loaded region, and was accompanied with abrasive scratch marks and smearing of the bearing metal, where deep abrasive scoring had displaced surface bearing metal.

Before the involvement of MACH10, various investigations were undertaken to determine the cause of the failure but  were all inconclusive.

Therefore, the Customer decided to carry out a full RCA examination of the problem, to identify the underlying cause of the failure, and assembled an RCA team with the appropriate experience and knowledge, under MACH10 leadership. The RCA procedure used was the ‘Defect Elimination Process’; a technique which consists of 4 phases and 11 steps. The 4 phases comprise: Incident Capture, Problem Analysis, Root Cause Analysis, and Solution Development. Each step should be carried out sequentially throughout the investigation, with no stage being skipped over without a clear definition of the problem (with boundaries) and establishing the root (or proximate) causes of it.

As part of the analysis, a detailed review of the corrosion mechanisms which may occur on the white metal bearings was done, in particular:

• Corrosion of White Metal Bearings – plain bearing corrosion problems which are generally separated into the following categories: weak organic acid attack, strong mineral acids, sulphur in oil, and water in oil.
• Corrosion of Tin with VSI (vapour space corrosion inhibitor) in Oil – there are 4 possible species within the VSI that may produce corrosion of the bearings: the carrier fluid (XHVI), the organic acid additives in the VSI, amine additives in the VSI, and contaminants such as water.

The findings concluded that corrosion by water and/or contaminants in the lubricating oil produced pitting on the bearing surfaces. On the critical LP active thrust bearings, the depth of pitting, and size of area affected, achieved a level which impacted on the formation of a full hydrodynamic oil film. In turn, this allowed the thrust collar to run against marginally lubricated bearing pads causing progressive plastic flow, surface wiping and wear of the bearings, eventually leading to the machine tripping. This was highlighted by plastic flow at the peaks of the pits. If failure had resulted from a total loss of lubrication, more extensive melting of the bearing surface would be expected to have occurred with associated evidence of heat on the thrust collar. According to the timeline, failure appears to have been progressive, with  temperatures progressively increasing and shaft position progressively changing over the final 3 months of operation.