Investigation On Cavitation Erosion Mechanism And Numerical Prediction Method In Pressure Relief Valve For Coal Liquefaction

Pressure relief valve is the critical equipment for direct coal liquefaction which is usedfor regulating the level of high-pressure separator and reducing the pressure of reactionproduct. The operating condition of the pressure relief valve is charactered with hightemperature, high pressure difference and high solid content. There are serious cavitation,cavitation erosion and particle erosion in the pressure relief valve. Material upgrade andsurface-hardening treatment of the pressure relief valve could improve its abrasion resistancein some extent, but it is still not enough to resist serious cavitation erosion. Understanding theinternal flow of the pressure relief valve is one of the important ways to analyze its failure andto optimize, but experiment research is limited by the harsh actual operating condition.Therefore, it is of great importance to research the cavitation mechanism and numericalprediction method of pressure relief valve for coal liquefaction.In this thesis, the pressure and temperature on the wall that are related to cavitationerosion and the process of the bubble collapse are obtained which are based on the bubbledynamic. According to the computational fluid dynamics methods, such as fluid controlequations, turbulent model, cavitation model and so on, combined with the actual structure andoperating conditions, the numerical prediction method for the3D cavitation flow of thepressure relief valve is established. Cavitation flow inside the pressure relief valve is analyzedand operating optimization measure is proposed. The main research contents and conclusionsof this dissertation are as follows:(1) The numerical method for deformation and collapse of a single bubble is developed.The pressure, velocity and temperature in the flow field and the process of the bubble collapseare obtained. The maximum pressure and temperature on the wall will increase with thebubble radius increase when the bubble collapse on the wall.(2) According to the computational model for cavitation flow and the actual operatingconditions, the3D cavitation flow fields of the pressure relief valve under different openingsare obtained. With the increase of the opening, the region and velocity of the backflow near the valve disc decreases and the extent of cavitation on the surface of the disc is also reduced.(3) Based on the idea of controlling the degree of decompression to reduce cavitation, thecavitation conditions inside the pressure relief valve while increasing the pressure of outletand decreasing the pressure of the inlet are numerical predicted. Operating optimizationmethod that two pressure relief valves installed in series achieving two stage decompression isproposed and the pressure on the outlet of the first valve should be controlled in the range of7.5~5MPa.The innovative works of this dissertation are as follows:(1) The numerical method for bubble collapse is developed which concerns the viscosity,compressibility, thermodynamic property of both gas and liquid. The relation of time withpressure and temperature on the wall and the process of the bubble collapse are obtained.(2) The numerical prediction method for3D cavitation flow of the pressure relief valve isestablished. The cavitation flow flieds in the pressure relief valve are analyzed combined withthe actual parameters. The reason for cavitation erosion on the disc is analyzed.(3) Based on the idea of controlling the degree of decompression to reduce cavitation,operating optimization method that two pressure relief valves installed in series achieving twostage decompression is proposed and the pressure on the outlet of the first valve is controlledin the range of7.5~5MPa.