Low Frequency Instability In A Process Compressor.

Abstract

A catalytic gas process compressor experienced low frequency vibrations in excess of trip levels following installation of a honeycomb balance piston. After 12 years of operation, low seal oil differential forced an unplanned unit shutdown. The source was identified as caustic corrosion of the aluminum balance piston. A stainless steel (SS) honeycomb seal was installed in place of the aluminum labyrinth seal. During the next two weeks, several trips of the compressor train were experienced caused by high levels of subsynchronous vibrations. Ties of the vibration problem to process conditions were studied and tested. Rotordynamic analysis identified the combination of damper bearing and honeycomb balance piston seal as the source of the low frequency vibrations. This paper describes the operating history of compressor, the vibration data recorded during a vibration event, teardown inspection results, and the subsequent rotordynamic analysis. The combination of low support stiffness with negative honeycomb stiffness lowered the frequency of the first natural frequency to a point that the honeycomb seal became destabilizing. To minimize unplanned capacity loss of the unit, an aluminum labyrinth seal was reinstalled, the caustic source corrected, and the unit restarted. The high levels of low frequency vibration have been eliminated and the unit has operated without vibration incidents for over 12 months.