Keyphasor^® Based Torsional Vibration Detection and Field Applications

Torsional vibration in rotating machinery is not easily detectable, but it is very important in maintaining the rotating machinery system safety and productivity. Traditionally, the torsional vibration is detected by a phase demodulation process applied to the pulse train signal generated by a tooth wheel or an optical encoder attached to the shaft. This demodulation based method has a few unfavorable issues: the installation of the tooth wheels needs to interrupt the machinery normal operation; the installation of the optical barcode is relatively easier, however, it suffers from short term survivability in harsh industrial environments. The geometric irregularities in the tooth wheel and the end discontinuity in the optical encoder will most likely introduce overwhelming contaminations from shaft order response and its harmonics. In addition, the Hilbert Transform based phase demodulation technique has inevitable errors caused by the edge effects in FFT and IFFT analyses. Fortunately, in many industrial rotating machinery applications, the torsional vibration resonant frequency is usually low and the Keyphasor^® and/or encoder for speed monitoring is readily available. Thus, it is feasible to use existing hardware for torsional vibration detection. In this paper, we introduce our in-house developed torsional vibration measurement tool, which used the Keyphasor/encoder data digitized by a high sampling rate and high digitization resolution analog-to-digital (A/D) convertor to evaluate the torsional vibration directly. A wavelet decomposition (WD) based method was used to separate the reference for torsional vibration extraction, which extended the analysis method to variable speed applications, such as the speed-up and coast-down operations. A few successful application cases are described in detail in the end of the paper.