Research On The Mechanism And Prediction Method Of Multiphase Flow Erosion-cavitation Wear On The Valves With Harsh Operational Conditions In Coal Chemical Industry

The multiphase flow erosion-cavitation wear in the coal chemical valves operate with harsh working conditions are researched in this paper.The failure mechanism of coal oil slurry valve and coal water slurry valve are clarified through analyzing the technical process,operation state and micro-morphology of corroded surface.The mechanisms of cavitation erosion and particle erosion in the valves are investigated by the experiments of cavitation,cavitation erosion and particle erosion in the high temperature environment.Moreover,the cavitation model and particle erosion model are both modified.The mathematical model of multiphase flow erosion-cavitation wear that involves the vapor-liquid phase transition is constructed.In addition,the characterization parameters of core area,particle erosion and cavitation erosion are proposed.Based on the verification of mathematical model and calculation method,the erosion-cavitation wear in the coal oil slurry valve and coal water slurry valve are numerically predicted.Then the failure mechanisms of valves at different operation stages are obtained.The accurancy of numerical calculation is verified by comparing with the actual failure instances.The improved structure of valve are proposed and validated.The main content of the research can be summarized as follows:1.Failure analysis of valves in the coal chemical industry.The physical parameters of coal oil slurry and coal water slurry are obtained by the technical calculation.The variations of valve opening,structural changes of valve spool and corrosion morphologies at different stages are obtained.The failure mechanisms of the two valves mentioned above are clarified by the investigation on the technical process,macro and micro morphology of corroded surface.The results provide the basis for the subsequent design of experiment scheme,construction of mathematical model,determination of boundary conditions and verification of calculation results.2.Experiments of cavitation,cavitation erosion,particle erosion and the investigation on the failue mechanism.In the cavitation experiments,the unsteady cavitation flow characteristics and time-spatial evolution of cavitation cloud at different valve openings and flowing conditions are obtained.Besides,the relationship between the unsteady cavitation structure and cavitation erosion is analyzed.In the cavitation erosion experiments,the locations and intensities of cavitation erosion on the surface of valve spool and seat at different openings are obtained.In the high temperature erosion experiments,the erosion behaviors of substrate and coating under different impact conditions are achieved.The cavitation erosin mechanism and particle erosion mechanism of coating and substrate are revealed by analyzing the experimental results.These experiments also provide the criterion for the modification of mathematical model.3.Construction of mathematical model for calculating the multiphase flow erosion-cavitation wear.Based on the methodology of large eddy simulation,the volume of fluid(VOF)model,Zwart-Gerber-Belamri cavitation model,particle motion model,recovery coefficients model and particle erosion model,the mathematical model of multiphase flow erosion-cavitation wear is constructed.The critical parameters in the cavitation model and particle erosion model are modified by using the experimental data of cavitation flow and high temperature erosion wear.The characterization parameters of core area,cavitation erosion and particle erosion are put forward.In addition,these parameters are validated by comparing the numerial results with the cavitation experiments,cavitation erosion experiments and the actual failure instance.4.Numerical calculation of the erosion-cavitation wear in the coal oil slurry valve.The erosion-cavitation wear of the valve spool in the initial period and different damaged stages are numerically investigated.The cavitation flow features and regularities of erosion wear at these stages are obtained.The results revealed that the failure of valve spool is caused by the combined effect of particle erosion and cavitation erosion.Compared with the failure area and the damage process,the accuracy of numerical simulation is proved,and the structure of valve spool is improved.5.Numerical calculation of the erosion-cavitation wear in the coal water slurry valve.The vapor-liquid phase transition and erosion wear in the valve at the initial period and different damaged stages are numerically studied.The process of phase transition and the distribution of erosion wear at these stages are discussed.The results showed that the failure of valve components is induced by the erosion wear.Based on the verification of numerical results,the valve opening and flow channel structure are improved.The field test indicated that:because of the structural improvement,the material loss rates on the top of valve spool decreases.Therefore the service life of valve is effectively prolonged.The innovation points of this paper can be summarized as follows:the cavitation model and particle erosion model are modified by using the experimental data of cavitation,cavitation erosion,and high temperature erosion wear.Moreover,the accuracy of calculation model and method are verified;the influence of valve spool structure on the erosion-cavitation wear is studied,then the failure mechanism of valve at different operational stages is obtained;based on the analysis of failure mechanism and damage process,the improvement of valve spool and intel flow channel structure are proposed and validated;the condensation rate of vapor phase is used to characterize the intensity of cavitation erosion in the valve,and the accuracy is proved by comparing with the experiment results of cavitation erosion.