In order to solve the rotating unbalance problem of the rotor in a high pressure multistage centrifugal pump, the modals of its rotor system supported by two water lubricated bearings were calculated by using FEM method, the mass unbalance location and distribution on the rotor were analyzed and optimized. The results show that this rotor system is rigid based on the 1st critical speed predicted. Under the design rotating speed, when a rotating unbalance is applied on the rotor at different positions, the vibration amplitude of the bearing in the suction chamber side exhibits substantial changes, indicating that bearing is very sensitive to unbalance; the vibration amplitude of the bearing in the discharge chamber side, however, has a little change and is insensitive to unbalance. Importantly, the residual unbalance mass distributed in the first three stages has a greater effect on shafting unbalance response. The case with smaller vibration amplitude in the two bearings has been secured by changing balance quality grades of the impellers and balance disk. When the balance quality grade of the first two-stage impellers is G2.5, but the grade of the rest impellers and the balance disk is G6.3, the bearing in the suction chamber side shows a significantly reduced vibration amplitude and the bearing in the discharge chamber side still keeps an acceptable small vibration amplitude. Under the other rotating speeds, the bearing in the suction chamber side can be even more sensitive to an unbalance mass than that in the discharge chamber side, thus it is suggested that the former should use a material with better wear-resistant property. These results can provide a theoretical basis and reference for stable operation of rotor system in high-pressure multistage centrifugal pumps.
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