The Effect of Tilting Pad Journal Bearing Dynamic Models on the Linear Stability Analysis of an 8-Stage Compressor

Rotordynamic instability, a self-excited vibration phenomenon, can lead to equipment failures, reduced production rates, and expensive redesign. Properly characterizing the stability response of flexible rotors on tilting pad bearings is therefore vital. Typically, this must be first considered during the design stage with computer modeling. Previous studies have demonstrated that non-synchronous bearing coefficients give a lower estimate of bearing stability than the eight synchronously reduced tiling pad bearing coefficients. However, a study of a reduced order non-synchronous stiffness-damping-mass (KCM) model and the effect on stability estimates has not been done previously for the same rotor model. In this paper, four load-between-pad tilting pad bearing designs, two four-pad and two five-pad, are considered. The stability margin for an eight-stage gas reinjection compressor is estimated for the four bearings, using a full KC representation, the KCM representation, and synchronously reduced bearing coefficients. The full KC representation gave the lowest estimate of stability margin, with up to 18% difference between full KC and KCM and up to 109% difference between full KC and synchronously reduced bearing coefficients. The results indicate that the KCM bearing representation does not necessarily result in the lowest estimate of rotordynamic stability margin, which is of significant interest to rotating machinery designers.