Review of Parameters Influencing Hydraulic Forces on Centrifugal Impellers

Both radial and axial hydraulic forces act on the impeller of a centrifugal pump. The radial forces are due to three different mechanisms: (a) radial thrust due to non-uniform pressure distribution around the impeller, (b) labyrinth forces and (c) impeller-diffuser interaction forces. The nature, size and origin of these forces are discussed. Steady radial thrust depends on the specific speed of the pump, type of casing (single or double volute, diffuser), geometric parameters and operation conditions including cavitation. The impact of these factors on radial loads is demonstrated and quantitative data are presented. Four different methods for radial thrust measurements are presented. Axial thrust on pump impellers is extremely sensitive to tolerances and to the labyrinth flowrate. Two methods for predicting axial thrust are discussed and predictions are compared to measurements on a multi-stage boiler feed pump. A number of parameters influencing axial thrust are treated.