Effect Of Relative Journal Bearing And Honeycomb Seal Direct Stiffness On Radial Synchronous Vibrations Of High-Pressure Centrifugal Compressors

Abstract

High pressure compressors are commonly equipped with honeycomb seal on the balance piston, as an effective mean to improve the rotordynamic stability of the system. The gas leakage flow across the honeycomb seal clearance develops significant stiffness and damping effects, that have been extensively studied and experimentally measured in the past. When the differential pressure across the honeycomb seal is very high, the magnitude of the stiffness and damping synchronous coefficients associated to the seal may become comparable to those of the journal bearings. These additional coefficients cause significant variations of the rotordynamic behavior of the compressor with respect to the no-load case, altering the frequency, amplification factor and mode shape of the natural modes of the system. This has a direct impact on the radial vibrations of the rotor that needs to be evaluated during the design phase of the machine. The present study describes the above mentioned aerodynamic effects, analyzing their impact on compressor rotordynamics as well as their dependence on influencing parameters such as the stiffness ratio between honeycomb seal and journal bearings. The analysis is supported by experimental data measured on two high pressure compressors (550 bar final discharge) with similar geometry and operating conditions but different journal bearing configuration. Conclusions include some recommendations about the design of the honeycomb seal and of the journal bearings, based on the outcome of the analysis and on the presented experimental results.