Rotor Instability Problems In An Integrally Geared Compressor Supported By Tilting Pad Bearings.

Abstract

The appearance of a strong subsynchronous instability in an overhung compressor highlights the fact that rotordynamic instabilities are not restricted to high-pressure multistage between bearing compressors with overhung rotors. Another important point is that many users mistakenly believe that tilting pad bearings eliminate all cross-coupling forces that cause subsynchronous instability. While this is true of the cross-coupling coefficients of the bearings themselves, a substantial cross-coupling coefficients of the bearings themselves, a substantial cross, coupling force from labyrinth seals can overcome the beneficial characteristics of tilting pad bearings. This paper describes the installation of an integrally geared compressor that initially had problems with high bearing pad temperatures. The manufacturer attempted to modify the bearings to reduce the pad temperatures but this was unsuccessful due to unacceptable high vibration amplitudes caused by rotordynamic instability. The rotordynamic instability was caused by a strong subsynchronous excitation of a cantilevered rotor natural frequency located at 47 percent of running speed. The subsynchronous excitation was due to labyrinth seal cross coupling effects. Based on the recommendation of the authors, the manufacturer installed swirl brakes to decrease the subsynchronous vibration excitation, which eliminated the rotordynamic instability and allowed the bearings to be modified in order to reduce bearing pad temperatures.