Although common problems with well established solutions are met in all rotor systems embodying hydrodynamic bearings, a number of factors peculiar to centrifugal pumps radically affect their vibrations response. Close internal clearances retaining pressure act as powerful hybrid bearings. In high performance pumps with vaned diffusers, large hydraulic forces are exerted on the rotor at part-load: these forces depend qualitatively and quantitatively on the ratio of flow to optimum flow. A calculation method taking all relevant factors into account is described. Some typical forced whirling response and stability results for a multistage rotor system are given. The application of calculations to remedial action in a service problem is described.

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