Rotordynamic Stability Of A Rotor With Three Open Impellers, Validated By Magnetic Bearing Exciter During Full Load Full Pressure (Flfp) Testing

Abstract

As speed and power density increase, open (unshrouded) impellers are becoming more and more utilized in process gas compressors. Open impellers enable higher tip speeds and more aerodynamic head per stage than closed impellers. However, the industry still lacks maturity and experience of rotordynamic stability assessment with single shaft multistage compressors when open impellers are used. To accurately assess the stability of compressors with open impellers, the estimation of the destabilizing fluid force induced by open impellers is important. The American Petroleum Institute (API) uses the anticipated cross-coupling (QA) to estimate the induced rotordynamic destabilizing forces, whereas this empirical QA number was initially derived for applications with closed (shrouded) impellers. This paper presents the measured and predicted stability (log dec.) results from a full-load, full-pressure test with a magnetic bearing exciter of a 6 stage back-to-back centrifugal compressor for natural gas processing. The compressor has three open impellers in the first section, and (three) closed impellers in the second section. The unit was tested to 17,800 RPM with a shaft end horsepower of 20,500 HP (15.3 MW). The labyrinth seals at interstage and balance piston locations are equipped with swirl brakes to reduce the cross-coupled effects. The magnetic bearing exciter (MBE) test provides the rotor stability results and thus helps to confirm calculated impeller induced destabilizing forces. The comparison of the impeller-induced destabilizing forces (Kxy obtained through different methods, namely, PACC (Predicted Aerodynamic Cross-Coupling), Wachel, CFD (Computational Fluid Dynamics), and measurements, gives Copyright© 2022 by Siemens-Energy & Turbomachinery Laboratory, Texas A&M Engineering Experiment Station -- 2 the industry insight into the open impellers and their induced destabilizing forces. This paper provides additional test data in the open literature.