Rotordynamic Stability - A Simplified Approach.

LecturePg. 3-10Straightforward mechanics are used to give a physical understanding of the important parameters involved in rotor dynamic instability. A concise analysis procedure is proposed for assembling available information into a meaningful design stability analysis of between-bearing rotors. The technique combines the destabilizing effects on the rotor into an equivalent single source at the rotor mid-span, and uses an excitation "threshold value" to rate the system stability. Results of applying this design analysis technique to an unstable compressor and the benefit from bearing redesign are presented

Subsynchronous Vibration Problem And Solution In Multistage Centrifugal Compressor.

LecturePg. 65-74The investigation of a subsynchronous vibration problem encountered in a six stage centrifugal compressor is discussed. At a running speed of approximately 9000 rpm, a subsynchronous vibration (at 4200 rpm) of nearly two times the synchronous vibration level was encountered. A systematic program was undertaken to identify the problem and correct it. A detailed analysis of the floating ring annular oil seals, balance piston labyrinth seals and impeller aerodynamic cross coupling was conducted. The oil seals were identified as the primary cause of the subsynchronous vibration due to lock up, and a modified seal design incorporating circumferential grooves was developed. This radically reduced the seal cross coupled stiffness. Further, a modified bearing design was investigated to increase the rotor logarithmic decrement. Changes were implemented in the compressor with the result of no subsynchronous vibrations for the operating conditions of the compressor thus far