Field Experience And Solution Of A Fractional Frequency Vibration Problem On A High Speed Centrifugal Compressor

PaperPg. 47-54.This paper describes how a serious field vibration problem on a high speed centrifugal compressor using tilting pad bearings was identified, following the introduction in the plant of simple methods of vibration recording and analysis, as being due to fractional frequency whirl at around 40% of the running speed. A clear correlation was discovered between the vibration behavior and the bearing clearance. This correlation is shown to match closely the predictions of a theoretical study of the effect of bearing geometry (clearance and preload) on the stability of the particular rotor/bearing system, when a commonly used empirical criterion for stability is employed, namely that the logarithmic decrement of the system should be at least 0.25 for instability to be avoided. The study shows that adequate stability could only be achieved in this particular system for bearings lying within a limited range of clearance and preload. Tight clearance, high preload bearings or loose clearance, negative preload bearings both give instability, and variations in bearing dimensions between nominally identical units are shown to be capable of moving the system from the stable to the unstable zone. The vibration problems have now been overcome by proper pad selection aimed at achieving an optimum bearing clearance and preload of about 7 mils and 0.15, respectively. The marginal stability in this machine is associated with the fact that the shaft first critical speed is about 44% of the full running speed, while machines of the same type, but with stiffer shafts giving a critical speed of about 55% of the running speed, have been free of instability problems. To provide a long term solution to the problem, and to cater for some increase of the maximum running speed in the future, stiffer rotors are being manufactured for the machines to raise the rigid bearing first critical speed to 57% of the current maximum running speed. In future purchase specifications for high pressure compressors a stability analysis will be requested for machines having a rigid bearing critical less than 60% of the maximum running speed, which will be required to demonstrate a logarithmic decrement of not less than 0.4 over the full possible range of bearing geometries, allowing for shaft, housing and pad tolerances, and wear. A main recommendation of the paper is that a refined rule of this type should be developed and incorporated in API 617