A new index to evaluate the potential damage of a surge event: The surge severity coefficient

Industrial compressors suffer from strong aerodynamicinstability that arises when low ranges of flow rate are achieved;this instability is called surge. This phenomenon creates strongvibrations and forces acting on the compressor and systemcomponents due to the fact that it produces variable timeaveraged mass flow and pressure. Therefore, surge is dangerousnot only for aerodynamic structures but also for mechanicalparts. Surge is usually prevented in industrial plants by means ofanti-surge systems which act as soon as surge occurs, howeversome rapid transients or system upsets can lead the compressorto surge anyway. Despite the fact that surge can be classified asmild, classic or deep, depending on the amplitudes and frequencyof the fluctuations, operators are used to simply referring tosurge, without making a distinction between the three mainclasses. This is one of the reasons why, when surge occurs inindustrial plants, it is common practice to stop the machine toperform inspections and check if any damage occurred.Obviously, this implies maintenance costs and time, duringwhich the machine does not operate. On the other hand, not allsurge events are dangerous in terms of damage, and they can betolerated by the mechanical structures of the compressor; thus,in these cases, inspections would not be required. Unfortunately,a method for establishing the potential damage of a surge eventis not available in literature. In order to fill this gap, this paperproposes a final formulation of a surge severity index, which wasonly preliminarily formulated by the authors in a previous work.The preliminary form of this coefficient demonstrated somelimitations which are overcome in this paper. The surge severityindex derives from an energy-force based analysis. Thecoefficient demonstration is carried out in this paper by means ofi) the application of the Buckingham's Pi-theorem, and ii) acareful analysis of the causative and restorative factors of surge.Finally, some simple practical evaluations are shown by meansof a sensitivity analysis, using simulation results of an existingmodel, to effectively further highlight the consistency of thiscoefficient for industry.