The authors would like to thank the Rockwell Company, Milwaukee, USA, for the sponsorship of the project.Abstract-Faults in pumping systems can be caused through the changes in flow regimes, such as cavitation, that lead to impeller degradation or corruption of pumped material, and ultimately to the deterioration and breakdown of pumps themselves. Results are presented of experiments using spectral analysis of motor current to diagnose and predict specific failures in pumps. Fault signatures were established by relating spectral features to individual faults, and by analysing their behaviour in the presence of faults. Faults investigated were cavitation, blockage, and impeller damage. A fuzzy logic system was built as a final decision making module.

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Maintenance of process equipment is one of the inescapable tasks associated with the operation of process plants and, until relatively recently, it was implemented either on a routine basis or after the failure of equipment. Attitudes are changing and now many organizations are adopting methods for identifying incipient faults, so that maintenance can be scheduled before there has been a failure which would lead to loss of production and spoilage of raw material. These methods include schemes in which all the measurement signals in a plant are gathered and, in addition to providing the input to the various control systems, they are compared with model-based values for individual sections throughout the entire process, so that a warning or an alarm can be raised, according to the seriousness of the observed discrepancy. Although these methods represent a considerable advance on ‘routine maintenance’ and ‘repair after failure’, their effectiveness is limited by the quality and detail of the information that is gathered. Each process measurement should be regarded as a ‘window’ through which the operation of the process can be observed. However, the majority of existing process measurement systems have been designed to provide the input signal to a process control system, for which the primary requirement is a good average value. Typically, this is obtained by filtering out all the components of the sensor signal having frequencies greater than about 5 Hz, with the result that the ‘window’ is only translucent, rather than transparent, and much of the detailed information which is available at the interface between the process and the sensor is lost. If, on the other hand, the measurement system has a wide frequency response, then much of the information that is available at the interface between the process and the sensor can be gathered and analysed to provide a clear view of the operation and status of both the process and the measurement system. This concept involves two significant departures from the conventional approach to process measurements and to the utilization of measurement information. In the first place, the measurement systems should be designed to have the widest possible frequency response, so that the maximum information regarding the measured process parameter that is present at the interface between the process and the sensor is gathered. Secondly, instead of conditioning the primary sensor signal to provide a good average value for process control, it is analysed using well established techniques to provide not only the signal for process control but also information on which predictive maintenance procedures can be based.

Abstracl -It is well known that, although the output from a centrifugal pump operating under normal conditions is essentially steady, there are small fluctuations in the flow rate. These fluctwionsare manifest in the measurement signals and, in most applications, are classified as 'noise' which for process control purposes is regarded as a nuisance and is attenuated or removed by frlters etc. If, however, the 'noise' component is separated out and the fiequency spectrum analysed, useful information regarding the status of both the measurement system and the associated process plant can be recovered. This paper describes how the 'noise' component of signals from an orifice plate/ differential pressure measurement system in a variety of applications can beanalysed to identify changes in the operation or status of process plant as well as in the measurement system itself.