Pitfalls and Analysis of the Pre-Load Conditions of a Journal Sleeve Bearing

by Joe McCollum


In this short case history the author stresses the benefits of gathering dual-channel .vs. single-channel vibration data from proximity probes. A vibration problem with a high speed centrifugal compressor could have been mis-diagnosed if only single-channel data were gathered. The orbits of vibration data from the compressors were particularly helpful in identifying the pre-loaded condition present.


“This article is concerned with the analysis of displacement data in mils peak to peak taken from a permanently installed multi-channel monitoring system. The proximity probes on a multi-stage compressor train are monitored weekly using a single-channel portable data collection system. The probes are mounted at approximate X-Y positions at each bearing (Figure 1). Many of the probes are not at the typical X-Y position because of piping location, mechanical design, and reversed probe wiring. The data show the effects of this reverse convention and non-traditional orientation. Data are collected on each proximity probe from 0 to 15x fm… One parameter monitors synchronous amplitude vs phase to detect phase shifts.
The normal operating amplitude of the horizontal proximity probe at the high-pressure (HP) compressor drive end is 0.5 mil peak to peak. Over a period of two months the amplitude increased to 2.67 mils peak to peak (Figure 2). During this period the unit was shut down for about 12 hours as a result of auxiliary equipment problems.
The HP compressor typically experiences buildup of by-product, especially when shut down. Maintenance personnel involved in evaluating the problem assumed that the elevated amplitude was a result of mass unbalance forces on the rotor assembly. The reasons were that the primary source of the vibration was at the operating speed; a substantial phase shift occurred, from 298° to 358°, and the discovery that by-product was fouling the process gas.”

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