Differentiating Benign and Unstable Vibrations in Integral Geared Centrifugal Compressors

Industrial turbomachinery pose several rotordynamic challenges with the ever expanding envelope of design parameters. Monitoring and understanding vibration signatures of rotating equipment remains to be more of an art, despite the availability of state-of-the-art design and diagnostic techniques. Vibration characteristics of any machine can broadly be classified as synchronous or non-synchronous spectra. While synchronous vibrations can be fairly easily mitigated by balancing, non-synchronous vibrations typically necessitate a more involved treatment and analysis based on the nature of the problem. A rotor-bearing system is said to be unstable if the amplitude of vibration increases with time in an outward spiraling orbit, until the rotor contacts the stator. In the rotordynamics literature, a sub-synchronous vibration is typically associated with rotor-bearing instability. However, although it seems counter intuitive, not all non-synchronous vibrations are necessarily unstable (i.e., of the self-excited type). This paper focuses on classifying and differentiating benign sub-synchronous vibrations from potentially unstable ones, based on vibration data collected on integral geared centrifugal compressors. Guidelines have been suggested to appropriately correlate sub-synchronous vibrations with the health of a turbomachine. Both design techniques and measured vibration spectra have been presented to augment the discussions.