Why is elevated vibration during surge often absent from the data collected by the online vibration monitoring software?
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Surge and rotating stall in centrifugal and axial compressors are well-understood aerodynamic phenomena, documented in literature. The mechanical vibration, both radial and axial, that occurs during surge and rotating stall has likewise been documented. Depending on where vibration sensors are mounted, and the compressor stage at which stall/surge may be occurring, it is usually possible to observe a simultaneous rise in vibration. For example, 14 separate surge tests conducted across seven separate compressor trains showed a 100% correlation between vibration and surge, observed by almost every vibration sensor on the machine (Table 1).
Yet if vibration can often be such a good precursor or corroborator of surge, why is it often missing in action when the vibration condition monitoring software is consulted following a real or suspected surge event? To understand why this happens, and what can be done to remedy it, we must first look at how vibration data collection is triggered in most online systems today.