The Challenge for the Accurate Determination of the Axial Rotor Thrust in Centrifugal Compressors

Abstract

The axial thrust is basically generated by a pressure imbalance across the impeller. The impeller is subjected to a great force trying to move it against the incoming flow. The cumulative thrust of all impellers on the shaft, especially when the impeller arrangement is in-line, has to be compensated by a balance piston. The purpose of this paper is to describe the calculation procedure for the accurate evaluation of the thrust in centrifugal compressors for medium and high pressure applications. In this respect the paramount factors which influence the thrust are considered such as the operating conditions (surge or choke), the different combinations of operating modes inside the compressor (in the case of multi-section configuration), as well as variable labyrinth seal clearances and different roughness of rotor and stator. Different arrangements of the compressor like in-line and back-to-back configurations are analysed and compared with regard to their influence on thrust and thrust variation. An example with a medium pressure compressor is given. The paper also focuses on the main inaccuracies for the calculation of the thrust like the core rotation factor and the degree of reaction of the stages. To validate the calculations, extensive measurements of the thrust forces acting on the axial thrust bearing were performed during the full-load, full-pressure factory testing of different high-pressure centrifugal compressors. The results of the tests demonstrate the accuracy of the prediction and the high sensitivity of the thrust to the boundary conditions. The evaluation of the thrust analyses is the base for the further determination of the stability behavior of the compressor which is compared to the damping measurement carried out during factory testing. These measurements emphasize the importance of performing such thrust measurements during a full-load, fullpressure or ASME PTC10 Type 1 test. The requirements of some specifications are discussed and their consequences on the design of the compressor are evaluated with respect to their feasibility. The impact of the size of the ASV (Anti-Surge Valve) on the thrust range of the compressor is further discussed.