Rotor instability vibrations in compressors and turbines have caused severe failures and costly downtime for several large projects. Rotor instabilities can occur in flexible shaft units which operate above their first critical speed. The whirling instability frequency is usually near one of the shaft critical speeds and can be caused by many factors, including hydrodynamic cross coupling of bearings and seals, internal friction, aerodynamic cross coupling and lateral–torsional vibration coupling. The whirling motion can be subsynchronous or supersynchronous, and may have forward or backward precession; however, in general most problems are subsynchronous and have forward whirl.
The most sensitive elements which influence rotor stability include the following:
- hydrodynamic cross-coupling in fluid film bearings, seals and labyrinths
- aerodynamic cross-coupling forces
- hysteretic or internal friction damping
- pulsations
- pulsating torque and axial loads
- asymmetric shafting
- fluid trapped in rotor
- stick-slip rubs and chatter
- dry friction whip
To properly calculate the stability margin of a rotor, the mathematical model must be able to simulate all possible destabilizing components. The logarithmic decrement evaluation of rotor system damping is useful for predicting rotor stability. Experience shows that this technique provides proper direction in designing for stability; however, uncertainty still exists in quantitatively predicting the onset of instability and defining the contribution of individual influencing parameters.
Theoretically, when the log dec becomes negative, the unit will become unstable. In practice, EDI has found that a minimum log dec of +0.1 is desired for stable operation. Aerodynamic loading is used to simulate destabilizing forces that occur in compressors and turbines and are dependent upon speed, horsepower and flow restrictions. Forward whirl modes are most likely to occur in compressors and turbines. Backward whirl may be excited when the shaft rubs a stationary element. The stability of the compressor is affected by the clearances of the bearings and seals. The sensitivity of the stability of the first forward whirl mode to aerodynamic load for a centrifugal compressor is shown below:
