Study On Transient Characteristics Of Startup Process For Centrifugal Pump Systems

The start-up process of a centrifugal pump system involves the collaborative operation of the pump and the control valve,which has an important influence on the stability and safety of the system.The double-suction centrifugal pump,butterfly valve and self-adaptive hydraulic control valve commonly used in large-scale pumping stations are taken as research objects in this study.The transient flow problems involved in start-up process of a centrifugal pump system,especially with the evolution of the radial force of the impeller,the changing of the control valve's hydraulic torque,and the flow field evolution of the self-adaptive hydraulic control valve are investigated.For the double-suction centrifugal pump start-up process,a transient calculation model based on variable speed sliding mesh for the impeller domain is established,the radial force evolution is analyzed and formulated,and the correlation between the radial force and axial vortex in the blade channel is found.The results show that the increasing of radial force lags behind the increasing of impeller speed.The overall radial force direction is proximate to the radial line located 25° from the volute tongue along circumferential direction.The shape of the axial vortex in the impeller channel has a close relation with the radial force.In the process of axial vortex stretching,changing shape and eventually developing to spiral vortices,the impeller outlet pressure rapidly increases and becomes stable,but its non-axisymmetric distribution gradually becomes more obvious,radial force increases and eventually tends to be stable,but the pulsation of radial force continues to increase.Combining with the requirements of the large-scale pumping stations for speed regulation,the evolution law of radial force during the start-up process at different stable speeds is clarified.Combining with practical application,the influences of the typical structure types of double-suction centrifugal pumps such as staggered impellers and double volutes on the radial force evolution are quantified.This study provides a scientific basis for the centrifugal pump start-up control.For the opening process of the butterfly valve in the water delivery system of the centrifugal pumping station,a three-dimensional calculation model of the butterfly valve opening process based on the Bernoulli equation,upstream and downstream water levels,the pump flow-head curve and the dynamic mesh technology is established.This model introduces the dynamic boundary conditions and considers the changes of the pressure and flow rate with the opening on both sides of the butterfly valve in the water delivery system.For midline,eccentric and rib butterfly valves,the pressure on the front and back of the butterfly disc,the separation vortex in the valve chamber and the hydraulic torque evolution are obtained.In terms of the hydraulic torque,the midline butterfly valve shows the trend of rapid increase first and then slow decrease,the maximum value occurs when the valve disc angle is 20°;the eccentric butterfly valve and the rib butterfly valve have "U" shape torque characteristics,the fluctuations in the earlier stage is relatively large and the latter period is relatively flat.In terms of pressure pulsation,the rib butterfly valve has a lower pressure pulsation.This achievement provides technical support for butterfly valve opening and closing transient analysis and butterfly valve design.For the start-up operation of the centrifugal pumping station water delivery system equipped with large-diameter self-adaptive hydraulic control valve,the dynamic balance relationship and transient force calculation model of the control valve are proposed.By means of the coupling and iteration of the flow field calculation and rigid body motion calculation,together with the dynamic mesh technology updating the position of the valve disc,a self-adaptive hydraulic control valve opening process calculation model with determined initial condition,implicit solution of valve disc motion and dynamic update of boundary conditions is established.The influences of diaphragm control chamber inner diameter and flow resistance coefficient of the micro valve groups on the movement of the valve disc are analyzed.The matching characteristics of self-adaptive control valve started at different water levels are studied.The relationship between the start head changes and the critical opening time and opening duration of the control valve is established.It lays a foundation for further improving the self-adaptive hydraulic control valve and strengthening its water hammer protection effect.The research results in this paper provide important theoretical and engineering guidances for the start-up control and hydraulic design of large-scale pumping stations.