Behavior of axial thrust caused by intake sump vortices in a vertical mixed flow pump

Abstract

Axial thrust forces acting on an impeller of a diffuser type mixed flow pump and the behavior of the axial thrust generated by the vortex occurrence in a pump intake sump were studied. The thrust from distinct discontinuities, such as the hysteresis phenomenon, was measured from a pump's hydraulic performance. In pump partial load operation the axial thrust increases discontinuously because of the reverse flow at impeller inlet. Synchronous and non-synchronous components of axial thrust increase during the onset of reverse flow zones. A prediction method for static axial thrust was derived based on Oshima's theory, accounting for the pressure reduction at pump suction bell-mouth caused by a vortex. Qualitative comparison of the predicted results with the actual static axial thrust results measured during the experiment, showed reasonable similarities around pump best efficiency flow rate. The thrust was measured using a pump intake sump test apparatus in which both an air-entrained surface vortex and a submerged vortex could occur. The level of dynamic axial thrust is dependent on the pump intake sump water level and the dynamic axial thrust begins to increase when a full air core vortex occurs at the pump intake. When submerged as well as full air core continuous vortices occur, the axial thrust and the fluctuation of total pump head become abruptly large. The fluctuation of axial thrust becomes the highest when the impeller draws in a large air-drawing vortex, imposing a pressure fluctuation which generates not only a rotational speed but also nZ vane passing and low frequency components.