Identification and Cure for Morton Effect Vibration Amplitude Oscillations in Overhung Compressors
Reference Number: MEMEC-07-98
by: William D. Marscher and Eric J. Olson
July 1998

ABSTRACT

A tilting pad bearing supported overhung compressor gradually had developed symptoms of very consistent cycling of high synchronous vibration levels. The vibration envelope cycled over a six minute period. There was a difference in the phasing of vibration increase at one end of the rotor versus the other, such that the two ends were not exactly in phase. Intensive site testing determined that no rotor critical speeds were close to the running speed of the compressor. However, it was observed with infrared thermography that the temperatures of the bearing pedestals were cyclic with the same period as the vibration, as also were the bearing pad and exit oil temperatures. The temperature cycle peaks were not in-phase with each other, nor with the vibration. Various process parameters were changed on an experimental basis, indicating that the main parametric influence was bearing oil inlet temperature. If the oil temperature was optimized in a certain range, the cycling stopped. It was concluded that the cyclic vibration behavior was a form of “Morton Effect”, as discussed in recent research papers. The bearing clearances and shaft straightness were being changed by bearing oil film temperature, which in turn was being affected by vibration level, which in turn was affected by the instantaneous clearances and degree of instantaneous shaft bow. This chain of events, combined with the time lag associated with transient thermal conduction, led to the cyclic but stable oscillation of the envelope of the amplitude of synchronous vibration. During this process there was a phase angle shift of the “hot spot” which varied in a deceiving, but ultimately explainable, manner. Field testing showed that maintaining an oil inlet temperature above a practical threshold temperature minimized viscous shearing forces in the bearing film enough to prevent further thermal cycling from occurring. The ability to address the problem through a bearing design change is also discussed.

This was a technical paper presented at a 2007 technical conference

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