Surge Model: Predicting Damping and Excitation Frequencies of Complex Pumping Systems

Frequency domain analysis is presented to define susceptibility of complex pumping systems to surge oscillations. In the analysis, piping spans are considered as distributed elements, compressors or pumps, orifices, valves, and junctions as lumped acoustic four pole elements. The piping elements are connected into one network through the use of appropriate matching boundary conditions. The analysis is most readily applicable to single stage units, typical of gas transmission stations. The surge model is based on predicting damping (logarithmic decrement values) and mode shapes for pressure and flow pulsations of the interactive dynamic system at its various natural acoustic frequencies. The system design is optimized by assuring that all operating points are sufficiently removed from surge, i.e., logarithmic decrements are above certain specified value. The effects of mean flow on acoustic wave propagation, pipe friction, viscothermal dissipation, and pressure losses at various piping locations (junctions, valves, orifices) are accounted for in the model. Results of the computational modeling of a complex centrifugal compressor system is presented. The analytical model is also verified by comparing the analytical results with experimental data.