The interaction of the flow in the impeller and the casing of centrifugal pumps leads to periodically unsteady pressure distributions in the casing. The resulting unsteady forces affect the impeller and produce radial deflections of the pump shaft. These oscillations of the rotor are periodic with the turning frequency times the blade number. Especially pumps with small blade numbers (for example single-blade and double-blade impellers) which are often used for the sewage transport have very strong pressure changes within one impeller revolution and therefore often show extreme vibration amplitudes at the blade passing frequency. The present contribution describes the calculation of the hydrodynamic excitation forces in a sewage water pump which can be alternatively equipped with a single-blade and a double-blade impeller by numerical simulation of the three-dimensional, viscous and unsteady flow for several operating points. Subsequently the resulting vibration amplitudes are determined by a transient analysis of the pump rotor system using a commercial FEM-Method. With the so known pressure field and vibration amplitudes the vibration behavior of sewage water pumps can be influenced already during the design by changing the relevant parameters.

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