A major factor influencing rotor stability of multistage high pressure centrifugal turbocompressors are the forces caused by the fluid-rotor interaction in the interstage and balance piston gas seals. An example of a fully 3-D labyrinth flow solution shows that using time averaged Navier-Stokes solutions are still excessive in computer time and hence cannot be used for practical applications. The paper gives a theory based on a two-volume bulk flow model for the prediction of the rotor-dynamic coefficients used for rotor stability calculation by complex eigenvalue analysis. Comparisons of predicted coefficients with measurements carried out on a high pressure compressor and on labyrinth seal test stands are given. The theoretically predicted phenomenon that labyrinth damping is retained even at zero inlet swirl of the leakage flow is confirmed. An example of a stability analysis of a high pressure natural gas compressor is given, where the stability margin increases with fluid pressure, i.e., density, provided seal swirl is controlled.

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