Reciprocating Compressor Diagnostics, Detecting Abnormal Conditions from Measured Indicator Cards

Modern compressor analyzers enable plant operators to monitor compressors by acquiring and storing pressure changes measured inside the working cylinder, in suction and discharge chambers, furthermore ultrasonic signals and temperatures. The paper focuses on pressure changes inside the cylinder and the possibilities of detecting abnormal operating conditions, in particular valve leakage, by analyzing deviations of recorded pressure volume cards from ideal diagrams. 1.Introduction. The indicator diagram, introduced by James Watt some 200 years ago, is still a valuable tool to assess what is going on inside a working cylinder. However, although modem equipment is available, many errors are still possible and even small errors can seriously affect the outcome of an analysis. This assessment of a compressor is normally done by recording cylinder pressure, pressures in suction and discharge chambers, a number of vibration signals in frequency ranges from 6 to 45 kHz and finally crank shaft rotation over time together with the position of the top dead centre. Furthermore, it is helpful to measure mean values of suction and discharge temperatures in the suction and discharge chambers, as close to the valves as possible. The composition of the gas sample has to be known so that real gas behaviour can be predicted by means of some equation of state. This enables the analyst to establish the diagrams that would be obtained if the same gas were compressed in an ideal compressor. It is hoped that the comparison of the measured, real indicator diagrams with these ideal diagrams provides valuable information as to the state of wear of the compressor. The present paper limits its scope to the information that can be collected from the different pressure recordings, the conclusions that can be drawn and the type of errors that have to be avoided and how they can be recognized. In this context, it is helpful to distinguish between three kinds of faults or errors: • Real compressor faults due to wear of sealing elements, only leaking valves will be considered. • Errors in adjusting the measuring set up. • Errors in evaluating the recorded data. In order to produce all kinds of faults under identical conditions, the diagrams presented in this paper were not measured but calculated by the method described in [1]. 2.Real compressor faults due to wear of sealing elements. Ideally, the compression and expansion events are processes of constant entropy, provided the compression chamber is perfectly gas-tight and heat-insulated. With P as the absolute pressure, Vas the volume and m" as the exponent of the volume isentrope which is assumed to be constant, the corresponding process will follow the law p_ vmv = c or log p =log c-m, .log v. This is a straight line relationship