Study On Complex Flow And Structural Characteristics During The Opening And Closing Of A Nuclear Check Valve

For the check valves used in general industrial processes,the operation performance and flow characteristics have been well understood.In comparison,the check valve that operates in environments of high temperature and high pressure has rarely been studied.In this dissertation,a check valve serving in nuclear industry was investigated with an emphasis placed on the flow inside the valve.Special attention was dedicated to the unsteady processes of valve opening and closing.Furthermore,in consideration of the safety and operation stability of the valve,structural characteristics of the valve during the transient processes of opening and closing were described.This study would explain the flow mechanism in the valve under extreme conditions.Furthermore,it was expected to provide a sound reference for the design and operation of the nuclear check valve.Major works and the conclusions drawn from the study are as follows:(1)Safety of the nuclear check valve was analyzed under high temperature and high-pressure conditions.The commercial computational fluid dynamics(CFD)code ANSYS Fluent was used to simulate the opening process of the valve.The distributions of static pressure and velocity inside the valve were obtained.The fluid-solid coupling numerical technique was used to investigate stress and deformation of major components of the valve.The operation safety of the valve was evaluated.The results indicate that at the early stage of valve opening,the inlet flow velocity is low,and the outlet pipe is featured by the coexistence of high and low flow velocities.The pressure concentration beneath the valve plug enhances the force imbalance.Remarkable vortices are formed in the inlet and outlet pipes.During the valve opening process,the flow velocity in the inlet pipe increases but is consistently lower than that in the outlet pipe.The pressure in the inlet pipe gradually decreases with the increment of the opening degree.The pressure in the outlet pipe is slightly reduced and pressure fluctuations are appreciable.A considerable distortion of the valve plug occurs at the early stage of valve opening.As the valve is opened,both deformation and distortion are mitigated.Repeated valve opening will increase the risk of fatigue failure of the valve plug.(2)Complex internal flows during the opening and closing processes of the nuclear check valve were investigated.ANSYS Fluent was used to simulate the threedimensional unsteady flows in the valve during the opening and closing processes.The obtained distributions of static pressure and velocity were analyzed and compared to describe transient flow characteristics of the valve.The results show that as the valve opening increases,the inlet and outlet pipes share a significant increase of flow velocity.A high-velocity zone is formed between the throat and the outlet pipe.The pressure in the inlet pipe decreases apparently as the opening increases,while the pressure in the outlet pipe is insensitive to the variation of the opening.However,pressure fluctuations are intense in the outlet pipe.Low-pressure areas appear in the inlet and outlet pipes.During the process of valve closing,low velocity and high pressure concentration are seen in the contact area between the inlet pipe and the valve plug.Beneath the valve plug,the high-velocity jet pattern and negative pressure are produced.As the valve opening decreases,the flow velocity in the inlet pipe decreases but the static pressure increases;in the outlet pipe,the static pressure remains constant.(3)A structural analysis of major components of the nuclear check valve was performed during the processes of valve opening and closing.The structural characteristics of the valve under various opening conditions were analyzed using the fluid-solid coupling module embedded in the code ANSYS Workbench.The pressure distribution obtained using CFD simulation was imposed on the fluid-wetted wall of the components,then the stress and deformation distribution in the components were simulated and analyzed.The results show that during the valve opening process,the maximum deformation of the valve body is concentrated at the throat part.With the variation of the opening,the deformation magnitudes of the valve body fluctuate.During the closing process,the maximum deformation of the valve body occurs in the lower part of the inlet pipe.The maximum deformation of the valve plug varies with the opening degree.The deformations of the valve body and valve plug fluctuate with the opening degree but the variation tendencies are consistent.The maximum equivalent stress of the valve plug occurs in the area adjacent to the spring during the opening and closing processes.The maximum equivalent stress is smaller than the maximum allowable stress of the material,satisfying the requirements of operation safety.