The purpose of this work is to present the Computational Fluid Dynamics (CFD) transient flow simulations of a centrifugal compressor stage with cavities. The cavities are included in the CFD model for several reasons. First the leakage flows and the windage effects are important to predict the absolute level performance of the stage (efficiency). Secondly the cavities also play an important role in the generation of the aerodynamic forcing on the impeller as well as in the prediction of the aerodynamic damping. As the compressor stage contains unequal numbers of blades and vanes, the Time Transformation method, an extension of the time inclining method is used in this work. This method which requires only a few passages per row is employed to reduce computational load and accurately model the full wheel. The numerical procedure is applied at design point firstly and then extended to the off-design operating conditions, permitting to compare and understand the transient flow behavior. The ability of the method to predict the impeller/diffuser interactions and cavity unsteady pressure response is discussed. The respective contributions of the diffuser, impeller blade passages and side cavities in terms of impeller forcing amplitude and frequency content are studied. In addition to this, comparisons between the experiment, the reduced model and the equivalent part wheel periodic solution are made to demonstrate the accuracy and computational efficiency of the considered transformation method.

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Compressors installed in plants worldwide can be modified to meet new operating conditions and other specific customer needs. This basic concept is even more relevant in offshore application. Timely upgrading of the centrifugal compressors to suit the late life requirements of the reservoir means, in fact, increasing hydrocarbon recovery versus reservoir depletion. GE Oil & Gas has accumulated extensive experience in upgrading Centrifugal Compressors. Customers can benefit from implementing GE Oil & Gas new technologies and modeling methods that allow the extension of the operational envelope of the compressors beyond that previously possible in a predictable manner. This paper shows the solution adopted by applying GE Oil & Gas design, testing and offshore installation methodologies in the context of brown field constraints to extend gas field production achieving up to 5 additional years through a new re-bundle inside existing casing, obtaining +50% extra polytrophic head versus existing machine and with a new, tailor made, centrifugal compressor, same footprint, obtaining +130% polytrophic head versus original one. INTRODUCTION Centrifugal compression facilities for Natural Gas Export and Enhanced Oil Recovery are affected by substantial changes in the operating conditions, which are the consequence of the reservoir depletion. Compression unit retrofit can be applied to offset those changes by increasing the compressor unit pressure ratio and/or flow rate. At the end of the 90's GE Oil & Gas started to partner with End Users to retrofit the existing equipment by, mainly, changing centrifugal compressor internals, whose design was performed to center the machine envelope to the new operating conditions (as set by the analysis of the constrains, the goals and the thermodynamic performances of the available technology at that time). A decade of experience accumulated in retrofit jobs in both offshore and onshore fields proved that cost effective projects are deliverable by lining up the machine internal stator and rotor components (re-bundle) to new operating conditions, this way avoiding off-design operation and allowing the retrofitted compressor to run outside its original envelope with increased operational efficiency. In the early 2000 GE Oil & Gas started research activities, dedicated to retrofit applications, to develop the aerodynamic performances of the centrifugal compressor impeller stages to further enhance the existing assets value by combining the above approach (retrofit the machine to center it to new operating conditions) with the extra performances of advanced aerodynamic design developed to meet the constrains of brown field jobs.