Investigations Concerning Flow Conditions in a Centrifugal Pump, and the Effect of Blade Loading on Head Slip

Abstract

The object of the paper is to examine the flow conditions observed in a specially designed 6-inch centrifugal pump; firstly, when the directions and magnitudes of velocities of flow were obtained by means of Pitot tubes, and secondly, when the flow was examined with an electronic stroboscope, through an inspection window of “Armourplate” glass in the casing. To provide uniform entry conditions a single-entry pump was chosen, preceded by a long length of straight suction-piping, but at small discharges a powerful forced vortex was found to exist with a spiral flow extending backwards for some distance from the pump inlet. This phenomenon was investigated by traverses with a direction vane and Pitot tubes, and an explanation of its cause is offered. The direction and magnitude of the absolute velocity at the impeller outlet were measured by cylindrical Pitot-tube traverses across the passage at two positions, and at small discharges a component of flow from the casing towards the suction side was found to occur through a portion of the impeller on the side nearest to the suction pipe. Similar traverses were made at two positions around the volute casing. These indicated a symmetrical, double spiral-flow at the best-efficiency point, and at other discharges this was modified to a single-direction vortex having a core at a much lower pressure than that on the outside of the casing. Pressure tappings on the casing, with an open-sided impeller and diffuser, showed the effect of shock on the inlet tips of the diffuser vanes, and inspection through the window confirmed a flow surging to and fro at small discharges. The effect of impeller blade loading on “head slip” is also investigated—two impellers showed a direct relationship—and values of the resulting “slip constant” are given. A graphical analysis shows the head slip, and head losses in the impeller and casing for the full range of discharge. Finally the performance curves of the pump, operating in reverse as a turbine, are given, from which the “runaway” speed under any head and any resisting torque may be estimated.