An integrally geared compressor, utilizing white metal tilting pad bearings came down for scheduled maintenance. The unit was overhauled and the bearings changed as they were close to minumum spesification. The compresor was started up again and ran fine for five days after which sporadic vibration started showing at sub shynchronous frequencies. Further detailed analysis were conducted which included oil analysis, bearing temperatures, process conditions, etc. The plant as a whole was not stable during this period and it was assumed that unstable gas flows were the cause of the vibration. Several changes to process conditions were made in an attempt to operate further away from the compressor surge point, all to no avail. The compressor operated like this for another week or two, before an opportunity became avalaible to inspect the compressor. The reason for the vibration came as a surprise and the root cause thereoff as even more of a surprise.

Jan Rautenbach has been the senior manager for the Rotating Equipment group at Sasol (Sasolburg) for the last 13 years. Prior to this he worked in the steel industry for 12 years as a mechanical technician responsible for hydraulics, maintenance on centrifugal fans, as well as condition monitoring. His areas of expertise include condition monitoring (handheld & online systems, tribology, balancing and laser/geometrical alignments) , maintenance on low and high speed rotating equipment, as well as management of the mentioned. Mr. Rautenbach has a B-Tec degree in Mechanical engineering from the Vaal Institute of Technology, as well as a certificate in Middle management development from University of Stellenbosch.