Study On The Mechanism And Prediction Method Of Erosion Wear And Cavitation In The Cage-typed Control Valve For Direct Coal Liquefaction Industry

As a typical representative of valves in the coal chemical industry,the cage-typed control valve for the direct coal liquefaction industry has severe working conditions.Under the combined action of particle erosion and cavitation wear,the valve generally exposes problems such as low adjustment accuracy and short service life,which pose a serious threat to the safety and stability of coal chemical production.In-depth study of the erosion wear and cavitation mechanism and prediction method in the cage-typed control valve for direct coal liquefaction industry is not only beneficial to alleviate the above problems,but also helps to improve the design theoretical system of cage-typed control valve and similar valve with severe working conditions.Supported by National Natural Science Foundation of China through No.52176046 and No.51875514,the mechanism and prediction method of erosion wear and cavitation in the cage-typed control valve for direct coal liquefaction industry are studied in this dissertation.The main contents and achievements are as follows:(1)Numerical model for flow characteristics of the cage-typed control valve in direct coal liquefaction industry based on the Reynolds time-average equation and the Realizable k-εturbulence model was constructed.The experiment on the flow characteristics of the whole valve was carried out.The flow characteristics in the control valve were investigated,and the influence of the failure defect of the valve on the flow performance was explored.The results show that the ideal flow characteristics of the valve are typical linear flow characteristics.The relative deviation between the numerical simulation value and the experimental measurement value of the flow coefficient under different opening degrees is less than 10%,which verifies the accuracy of the constructed numerical model.Based on the preset defects of the actual failure mode,the multilayer diversion grooves at the bottom of the cage are connected to each other,so that the fluid flowing from the outer orifice directly converges,and the flow performance of the valve is improved at a small opening.As the valve opening increases,the flow performance of valve with the preset defects decreases,while the influence of the preset defect on the flow performance of the valve gradually weakens.(2)A multi-phase erosion-cavitation wear mathematical model with gas-liquid phase transition in the cage-typed control valve for direct coal liquefaction industry was constructed.The erosion wear and cavitation characteristics in the valve was analyzed using the experimentally verified erosion wear rate calculation model and cavitation characterization parameters.Combined with the cage failure photos,the analysis results are verified.The results show that the main areas of cavitation in the valve are located in the inner orifice of the valve cage.With the increase of the valve opening,the the vapor volume increases,the number of orifices through which the fluid flows increase,the cavitation range and intensity also increases.The pit-like defects around the outlet of the orifice in the inner wall of the cage are mainly caused by the collapse of the cavitation bubbles and the impact on the wall.Due to the high-speed impact of particles on the inner wall of the diversion groove,there are obvious deformation and penetration defects in the center and surrounding areas directly impacted by the particles.The areas with the highest erosion wear rate under multiple openings are located on the inner wall of the guide groove away from the valve inlet side.(3)Based on the constructed mathematical model of erosion-cavitation wear in multiphase flow with gas-liquid phase transition,the erosion and cavitation wear in the valve under the interaction of a single structural parameter and multiple structural parameters were studied using response surface methodology.An optimization scheme for the structural parameters of the cage is proposed.The results show that the increase of the diversion groove width can effectively reduce the erosion wear of the inner wall of the diversion groove,but the excessive width of the diversion groove will lead to an increase in the cavitation intensity in the orifice of the cage inner and middle layer.The increase in the height of the diversion groove can reduce the erosion wear rate in the area away from the inlet side of the inner wall of the diversion groove,and has little effect on the cavitation intensity in the inner layer orifice and the middle layer orifice.The enlarged diameter of the orifice in the buffer stage can effectively prevent the pressure drop of the orifice in the middle layer from being too large,and alleviate cavitation intensity in the orifice of the middle layer.The optimization scheme of cage structure parameters reduces the maximum erosion wear rate in the valve by 74.4% compared with the original structure,the vapor phase volume is reduced by 45.7%.Effective control of erosion and cavitation wear in cage-typed control valve for the coal direct liquefaction industry was realized.(4)T-shaped structure and stepped structure were extracted based on the failure characteristics of the cage,the influence mechanism of the structural parameters and boundary conditions on the erosion wear and cavitation intensity of the basical structures were studied.The results show that the distribution of erosion wear on the inner wall of the diversion groove with the T-shaped structure shows the characteristics of low center and high sides,which is determined by the high impact frequency,high impact velocity and low impact angle of particles in this region.With the increase of the inlet particle concentration,the impact frequency of the inner wall of the diversion groove increases,resulting in an increase in the erosion wear rate on both sides of the center of the direct impact area,the change in particle velocity has little effect on it.The area with more serious erosion wear in the stepped structure is located at the edge of the inner diameter side of the rounded surface.With the increase of inlet particle concentration and velocity,the maximum erosion wear rate on the rounded surface gradually increases.Based on this,the relationship between erosion wear rate,vapor phase condensation rate and structural parameters,boundary conditions were fitted.The rapid prediction calculation of the erosion wear rate and vapor phase condensation rate in the key area of the cage is realized.It also has certain reference significance for the prediction of erosion wear in other multi-stage cage control valves with diversion groove.