Axial Thrust in High Pressure Centrifugal Compressors: Description of a Calculation Model Validated by Experimental Data from Full Load Test

摘要

The residual axial thrust acting on the rotor of a centrifugal compressor is the result of the non-uniform pressure distribution on the surfaces in contact with the process gas, plus the differential pressure acting on the faces of the balance piston(s) and the contribution due to the momentum variation of the process gas. During the design phase the axial load shall be verified to remain safely lower than the thrust bearing capacity, under all possible operating conditions; this requires a high degree of accuracy in the calculation model used to evaluate each thrust component. Errors in this calculation may lead to high bearing pad temperature during operation, to early wearing of the pad surfaces and ultimately to the damage or failure of the thrust bearing (Moll and Postill, 2011), thus jeopardizing the integrity of the whole compressor. The main difficulty of axial thrust calculation lies in the correct prediction of the static pressure distribution over the external surface of the impeller hub and shroud. This distribution depends on a large set of parameters, including rotor geometry, operating conditions, properties of the process gas, leakages flows across the rotor-stator seals. A detailed fluid-dynamic model of the gas in the cavities between impeller and diaphragm was developed and applied first to stage model tests and then to high-pressure centrifugal compressors, and its predictability was assessed by direct comparison with experimental data. The compressors were tested in full load conditions, with thrust bearing pads equipped with load cells, and the thrust values were recorded for several points across the operating envelope.