Influence of valve plate wake on the performance of centrifugal pump

Centrifugal pumps are widely used in industrial fields due to their simple structure and reliable operation. In industrial sites, valves are usually arranged in the inlet pipe of centrifugal pumps to cut off water flow during maintenance. However, the valve wake causes inflow distortion of the centrifugal pump, and the wake vortex also has a strong unsteady effect. These characteristics contradict the assumed stable and uniform inflow conditions during the design stage of the centrifugal pump, resulting in a deviation between the actual performance of the centrifugal pump and its design performance and reducing its stability. Therefore, conducting research on the impact of valve for maintenance on the performance of centrifugal pumps is of great significance for optimizing the design of inflow pipelines and improving the operational stability of centrifugal pumps. The research object of this article is a Tri-eccentric butterfly valve and a semi-open centrifugal pump, aiming to study the unsteady wake characteristics induced by the valve and its impact mechanism on the operating characteristics of centrifugal pumps under high flow conditions. The Tri-eccentric maintenance valve is placed at the position of one pipe diameter before the inlet of the centrifugal pump as the inflow distortion condition, and the situation without the valve is used as the uniform inflow condition. The external characteristics of the centrifugal pumps under uniform and distorted inflow conditions are compared through experiments. Numerical simulation is used to study the flow characteristics of valve plate wake vortices and their impact on the unsteady internal flow field of the centrifugal pump. The radial force of the impeller induced by valve plate wake vortices is analyzed. The results show that under the two conditions of uniform inflow and distorted inflow, the numerical simulation and experimental results of the external characteristics are in good agreement, with errors within 5%, and the accuracy of the numerical simulation used to reflect the flow characteristics of the centrifugal pump is verified. When water flows through the valve, complex vortices are generated, and the vortices that have a significant impact on the centrifugal pump mainly come from the boundary layer separation and suction on one side of the valve plate. This vortex results in a 9.15% decrease in efficiency and a 1.2 m decrease in head under high flow conditions. The shedding frequency of the valve plate wake vortex is 1.9 times rotational frequency, and a pressure pulsation of the same frequency is generated at the inlet of the centrifugal pump. The periodic inflow of wake vortices leads to an increase in the maximum relative flow angle of two blade leading edges from 30° to 43° and 39°, which exacerbates the flow separation of pressure surfaces of the two blades. Due to the influence of flow separation, a gradually dissipating stall vortex is generated in the impeller channel. The mismatch between the period of the stall vortex and the wake vortex results in the blade being subjected to complex non-stationary excitation forces, but the main frequency of the excitation force is still 2 times the rotational frequency, corresponding to the development period of the stall vortex. The periodic suction of wake vortices and the development of stall vortices cause the average radial force acting on the impeller to increase to about 4.5 times that of uniform inflow, and the maximum radial force to reach about 7 times that of uniform inflow, while the radial force vector shifts. The frequency spectrum of radial force fluctuation is mainly composed of the development frequency of stall vortices and the shedding frequency of wake vortices. The radial force vector shifts will exacerbate the risk of centrifugal pump shaft failure. The conclusions obtained can provide a judgment basis for the operating status of centrifugal pumps under inflow distortion conditions and a theoretical basis for improving the operational stability of centrifugal pumps.