Refinement of Physics Based Approach Used in the Prediction of Impeller Rotordynamic Forces for Centrifugal Compressors

The accurate prediction of centrifugal compressor stability continues to be an important area of interest in the oil and gas industry. Ensuring stability is critical to the cost-effective installation and operation of these machines in remote environments where field stability problems are much more expensive to diagnose and correct. Current industry standards and tools for the prediction of impeller destabilizing forces are based on empirical methods that, to date, have served fairly well for systems with reasonable stability margins. However, as stability margins are decreased, use of a modeling method that is more physics based that can better represent the observed trends in machine behavior at low stability margins is needed. Furthermore, the development of mega-class Liquefied Natural Gas (LNG) compressors and ultra-high pressure re-injection compressors provide further motivation to improve accuracy. In this paper, a new physics based expression for the prediction of impeller cross-coupling, previously introduced by Moore et al. [1] is further investigated by analyzing several classes and scale factors of impellers ranging from 2-D designs used in re-injection up to full 3-D impellers typically used in LNG. The new expression is based on both numerical simulation (CFD) and experimental test data from a known instability. Comparisons are made for overall stability prediction as well as sensitivity to system changes. Conclusions are made regarding the applicability and limits of this new approach.