Vibrations in a centrifugal compressor at less-than-rotor-speed (sub-synchronous) may have multiple causes. They addon to the overall vibration and hence need to be resolved to ensure safe and optimal operation of the compressor. Lube oil / bearing related vibrations are relatively easily identifiable since they occur generally within known range of sub-sync speeds. However, another common source of such vibrations, especially in a centrifugal compressor, can be the presence of an aerodynamic instability. This may manifest itself at any speed depending upon its characteristics; however, most of the times at speeds less then the rotor speed. This paper documents results of a case study where a centrifugal compressor was reported to be occasionally exhibiting unacceptably high overall vibration levels. Since this was causing the machine to shutdown, it had started affecting the production and reliability of the machine. Initial investigation revealed that these vibrations were observed only at certain operating conditions where the discharge pressure of the machine was high. In order to further diagnose the issue, a continuous monitoring system was installed on the unit. This system helped in gathering continuous vibrations as well as process data and provided an insight into how the two were correlated. The results revealed that the primary reason behind the elevated levels of overall vibrations was a previously non-existent peak in the spectrum at a frequency corresponding to 0.38X (approx.). Other frequencies with smaller amplitudes were also observed in the spectrum of the increased vibrations. By means of this paper, the authors intend to present the detailed investigation and explanation of this issue.
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