Evaluation of Bearing Designs for a Multistage Centrifugal Compressor Using a Magnetic Exciter
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The amount of damping in the rotor/bearing system is the key to stability of centrifugal gas compressors, especially for rotors with high slenderness ratio. A full pressure performance test can certainly demonstrate whether a rotor is stable or not. But the drawback is that it cannot quantify the amount of damping in the system prior to the instability threshold. Alternatively, the rotor can be subjected to disturbances by applying a force acting on the rotor and the response from the rotor allows us to measure the amount of damping through vibration amplitude changes. In this paper, we will discuss a magnetic exciter system that was designed to inject a known force at one end of the rotor while the rotor is operating at different loaded conditions. The compressor rotor was excited by magnetic forces and its response was observed for evaluating the damping of the system. Different techniques were used to estimate rotor stability and were evaluated in this study. This test arrangement can also be used to evaluate different journal bearing designs. Data from a centrifugal compressor test using three different tilting pad bearings are compared and discussed. The test data shows that for the compressor investigated, the 5-pad tilting pad bearing design is better than the two 4-pad bearings. The stability threshold as a function of discharge pressure is also presented in this paper. A good agreement between prediction and test data was achieved when taking into account certain aerodynamic excitation mechanisms. The system provides a technically sound and economic enhancement to the design process: validation of full-pressure stability margin, back-to-back comparison of components such as bearings as well as valuable insight into rotor – and aerodynamic interactions.
Zhang, Donghui; Faller, Wolfgang; Lee, Chester (2012). Evaluation of Bearing Designs for a Multistage Centrifugal Compressor Using a Magnetic Exciter. Texas A&M University. Turbomachinery Laboratories. Available electronically from