Investigation of the closed valve head of centrifugal pumps

Abstract

The ability to accurately predict the entire performance range of a centrifugal pump is commercially valuable. Pump manufacturers are reluctant to diversify their product portfolios unless they have confidence in their prediction methodologies. The prediction methodologies developed within this work are used to give Union Pump greater confidence in the prediction methods. A methodology for using Computational Fluid Dynamics is presented which, when compared with time averaged pressure readings from the Union Pump test stand, accurately predicts the closed valve head of a centrifugal pump. The work outlines the computational procedure used to achieve a successful CFD solution for a pump operating at closed valve. The procedure outlines the importance of including piping system elements within the simulation to allow the boundary conditions to be assigned correctly. The methodology is applied to diffuser and volute pumps to give an insight into the flow regime. This work details the effects of rotor-stator interaction on the closed valve head. Both diffuser and volute pumps exhibit a similar flow regime. At pump inlet the impeller experiences a strong steady outflow from the blade tip. The flow spirals out of the impeller eye into the pump suction channel. This spiralling flow is accommodated within the computational solution. A new mental model, for use by designers, is proposed based on rotor-stator interaction analysis. The annular gap between the impeller and diffuser vane is filled with a pulsating the frequency of which is determined by the number of vanes within the impeller. The nature of this pulsation passed through to the stator passage influences the pressure at closed valve. This disagrees with the current mental model used by designers. The minimum pressure fluctuation falls dramatically when the impeller vane number equals the diffuser vane number. This fall in minimum pressure is linked to the A2BB impeller dimension. This relationship is used to develop criteria for stability based on the pressure pulsation propagation through the diffuser. This closed valve prediction method facilitates the redesign of existing unstable products to incorporate stable performance characteristics. Stable machines are more commercially acceptable as their performance is more reliable when they are operated in parallel with a similar machine. Unstable performance can be eradicated from pump performance characteristics through impeller redesign. The methodology also provides a roadmap for performance prediction of machines at off-design conditions for different pump configurations.