Influence of Reynolds number on net positive suction head of centrifugal pumps in relation to disc friction losses

Abstract

Determination of Net Positive Suction Head (NPSH) value in the range of best efficiency is important in order to ensure correct operation of centrifugal pumps in networks. Empirical formulas of NPSH determination include the specific speed, the head and coefficients taking into account the pump impeller type. Using recent formulas on disc frictions and correction factors including the viscosity, a new formula of NPSH correction factor is suggested in this study, taking into account the specific speed, the disc friction losses and the flow velocities at impeller inlet. NPSH evaluated for water by Stepanoff formula is divided in kinetic and disc friction heads. The formulation taking into account disc frictions including viscosity, diameters and velocities enables to determine the NPSH correction factor during the pumping of viscous fluids. Simulations on different centrifugal pump models of specific speeds between 20 and 45 min-1 are conducted. Obtained results show that the NPSH correction factor depends on the specific speed. It is near the unity when Reynolds number is greater than 105. It increases appreciably with Reynolds number values less than 105. It reaches 2.5 for a Reynolds number of 104 and a specific speed of 25 min-1, and 5 for a Reynolds number of 103 and a specific speed of 45 min-1. The established formula is related to those of Stepanoff for water. To validate this formula, a centrifugal pump with a specific speed of 20 min-1 is tested for water and a viscous fluid with a viscosity of 75 cSt. NPSH correction factor is also compared with those obtained by other authors. This comparison enables the validation of this new formula for the examined range of centrifugal pump models. This new formula facilitates manufacturers and users the NPSH prediction of standard centrifugal pumps whatever the viscosity of the fluid.