A centrifugal compressor for wet chlorine gas had a history of problematic vibration behaviour. The rotor vibrations, which were synchronous with running speed, continuously did increase slowly over time. By regular water wash at the compressor suction line it was possible in most cases to bring the vibration levels down to acceptable values. In a few cases however, during a water wash the vibration levels did increase such that a machine trip was generated. The initial, most obvious, conclusion was that the vibration increase over time was caused by building up of deposits on the impeller in an uneven manner. The water wash partly removes these deposits. Vibration measurements were carried out on the compressor. From the vibration analysis and history it appeared that this rotor is very sensitive to unbalance at the impeller. Besides unbalance from deposits at the impeller, there is a thermal effect at the impeller-end bearing that amplifies the rotor response due to unbalance. The root cause of the unpredictable vibration behaviour was due to thermal shaft bending at the impeller-end bearing. Recommendations are given to take appropriate measures to improve the vibration behaviour of this compressor rotor. One of these recommendations was successfully implemented.
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