Magnetic Bearings As An Impeller Force Measurement Technique

Abstract

The technique of measuring steady loads on centrifugal pump impellers via magnetic bearings is demonstrated and shown to be reliable way of clarifying loads, which have been traditionally difficult to determine both analytically and experimentally. Among the most challenging geometries in this respect is the semiopen impeller in an end-suction single volute pump. Because of the uneven distribution of the pressure on the shroud of a semiopen impeller, axial and radial loads are created. In addition to the classical axial and radial loads, it is recognized that a hydraulic couple is also generated with semiopen impellers. The hydraulic couple is not discussed. In the past, experimental evaluation of the hydraulic forces and moments on impellers has been accomplished through techniques such as strain measurements and static pressures at taps in the walls adjacent to the impeller. The present experimental results are obtained using the magnetic bearing test rig that was designed to measure the forces and moments that result from the hydrodynamic interactions in a centrifugal pump under various operating conditions. The comparison is presented between the results for the single stage pump obtained by the traditional methods and those derived from magnetic bearing reactions. The results are given for the full range of flowrates from shutoff to runout and a Reynolds number effect is also examined for semiopen impellers. The information obtained will be used to design new industrial pumps.