Investigation of a Rotordynamic Instability in a High Pressure Centrifugal Compressor Due to Damper Seal Clearance Divergence

This paper documents the potential for excess distortion of a honeycomb damper seal under high differential pressure in a centrifugal compressor, the problems this can cause, and options for solution. A strong negative stiffness can result, dropping the first natural frequency into a region of negative effective damping. The paper shows the need to manage seal clearance profile and inlet swirl to avoid this condition, and to optimize damper seal contribution to stability. The paper presents predicted seal distortion, resulting dynamic characteristics, and their influence on rotor stability. Field vibration data confirm seal distortion under pressure can cause damaging, self-excited sub-synchronous vibrations, and that an appropriate seal clearance profile predictably corrects this condition. The paper shows that optimum stability requires uniquely different clearance profiles for low and high-pressure compressors.