Effect of Specific Load of Bearing on the Centrifugal Compressor Rotordynamic Stability

Abstract

With the ever increasing of the centrifugal compressor capability, such as large scale LNG and CO2 re-injection, the stability margin evaluation and instability control is crucial both for the centrifugal compressor OEM and end users. An industrial scale experimental test rig with two 5-pads tilting pad journal bearings and two active magnetic bearings was introduced. Tilting pad bearings were used to support the rotor, and active magnetic bearings were used to exert static load and sub-synchronous sine load to excite the rotor. The logarithmic decrement identification method of rotor-bearing system was presented. Here, weighted instrument variable and directional frequency response function were combined to improve anti-noise performance and separate forward/ backward whirl vibration. Thermoelastohydrodynamic bearing model was used to investigate the effect of hot clearance on the stability prediction of centrifugal compressor. Also, the contribution of bearing specific load on the stability was studied numerically. Also, experimental work was carried out to investigate the damping ratio of rotor under different rotating speed and bearing specific loads. The numerical results correlate with the test results very well. The results of the investigation indicate that: (i) Hot clearance is very important for centrifugal compressor rotordynamic instability prediction; (ii) The effect of pad mechanical deformation are smaller than that of thermal deformation; (iii) With the increase of bearing specific load, the logarithmic decrement is decreasing.