Rotor Stability Evalution for High Pressure Multi-stage Pump by Excitation Diagnosis Method

Abstract

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). Therefore, it is important to evaluate the stability limit（onset of self-excited vibration）at the design stage. However, the accuracy of the calculation tool is not sufficient at present. Thus, a special measurement tool is needed to analyze the phenomenon of actual equipments. For this reason, we aim at developing an excitation system to evaluate the stability limit by utilizing an active magnetic bearing as an exciter. First, we devised an excitation theory based on Bently-Muszynska model. According to the theory, the stability limit can be evaluated by rotor natural frequencies, and fluid average velocity within the clearance of bearings and balancing drum. In order to measure these parameters, we applied the system to excitation tests, which included the rotating test in air, and in water, respectively. The difference between the tests is whether the fluid force in the balancing drum exists or not. Through the test results, the balancing drum had a significant effect on the rotor stability, and the self-excited vibration occured at the intersection of the natural frequency and fluid average velocity in the balancing drum. Moreover, the resulting frequency（0.72×rotational speed）matched the fluid average velocity (0.73×rotational speed) in the balancing drum.