In a nuclear reactor, pumps are employed as key components for various applications, such as in the cooling system. The steady running of the pump relies greatly on the stability of the rotor. This paper analyses the non-damping natural vibration, critical speed and unbalance response of rotor in the multistage centrifugal pump. The calculation results indicate: (1) The supporting stiffness of either electromagnetic bearing or water film has a large influence on critical speed of first and second order rigid mode, but shows a minor influence on the critical speed of first and second order bending mode. In the case that the electromagnetic bearing has a supporting stiffness of 0.5 × 106 N/m, and the supporting stiffness of water film bearing takes twice of the electromagnetic bearing stiffness, the simulated frequency of first order stiffness will be larger than 50 Hz, which is the rated frequency of the rotor. The stiffness of electromagnetic bearing in our experiment was in the range of 1.3–1.5 × 107 N/m. Thus, the working speed of this rotor will be lower than the first-order rigid critical speed. (2) The unbalance responses of the rotor under four unbalance conditions show that the displacement amplitude of major rotor components (including electromagnetic bearing, water film bearing, first and final impeller) are all in the allowed range for engineering design.

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