High pressure multi-stage pump is one of the most important machinery in various plants. Due to the high rotational speed, there are cases where excessive vibration problems occur. Especially, the self-excited vibration arises from the fluid force, which exists within the clearance of the oil film bearings and balancing drum (annular seal). In order to evaluate the rotor system stability, we developed an excitation method based on Bently-Muszynska model, and applied this method to the air test and water test. Through the test results, the onset (threshold) of instability (self-excited vibration) can be evaluated by the rotor system natural frequency and the fluid average angular circumferential velocity within the clearance of the bearings and balancing drum. Moreover, the balancing drum had a significant effect on the rotor stability, and the resulting frequency of the self-excited vibration matched the fluid average angular circumferential velocity in the balancing drum.

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