Study On The Solid Particle Erosion Mechanism Of Pipe Bend For Multiphase Flow

Sand erosion is a common problem in oil and gas field development.Continuous impacts can cause pits or scratches,and further lead to wall thinning and even leaks.The solid particle erosion mechanism of the pipe bend for gas-solid two phase flow,liquid-solid two phase flow and gas-liquid-solid multiphase flow are studied by using the theoretical analysis,experimental research and numerical simulation methods.The solid particle erosion mechanisms for multiphase flow are analyzed.And reasonable prediction methods are proposed to predict the erosion rate and the most serious erosion location of the pipe bend.(1)Two large scale erosion test loops are designed for the experimental research of the erosion regularities of pipe bend for liquid-solid two-phase flow and gas-liquid-solid multiphase flow.A detachable erosion test elbow is designed to study the erosion profiles of the specimen and further analyze the microscopic erosion mechanism of the pipe wall for multiphase flow.The results show that there are three main collision mechanisms for particles wearing the wall,namely micro-cutting,plowing and impact deformation.The erosion is dominated by micro-cutting in liquid-solid two-phase flow and slug flow,resulting in uniform wear on the surface.The erosion is dominated by micro-cutting and impact deformation in dispersed bubble flow,and the surface of some specimens will appear deep pits and erosion lips.The erosion is mainly dominated by the impact deformation in annular flow,resulting in erosion pits and raised lips around the pits and some scratches.(2)The influence of different parameters on the erosion of pipe bend for gas-solid two-phase flow is obtained by using the CFD based erosion prediction method.An erosion prediction model with multiple factors is established by using the regression analysis method.Finally,the equation for predicting the most severe erosion position is established based on particle trajectory,and the accuracy of the equation is verified by the experimental data.The results show that the particle mass flow rate is the most important parameter that influences the erosion and the pipe diameter is the least important parameter.Particle direct collision and sliding collision result in different erosion profiles.The V-shaped erosion scar in short-radius elbows is mainly caused by the combined effect of the first collision and secondary collision which all belong to the direct collision.However,the V-shaped erosion scar in long-radius elbows is mainly caused by the first collision and the grooved erosion scar behind the V-shaped erosion scar is mainly caused by the secondary collision.(3)The erosion regularities of pipe bend for the liquid-solid two-phase flow is analyzed by using the CFD based erosion prediction method.According to the relationship between the particle trajectory and the force on the particles,three particle collision forms are proposed to explain the erosion mechanism and the Stokes number is introduced for explain the dynamic changes of the erosion region.The results show that the most and the least important parameters influences the erosion are the velocity and bending angle respectively.The erosion profile varies due to the direct collision,sliding collision and secondary flow driven collision.The maximum erosion location changes with the Stokes number.When the Stokes number is less than the critical value that equal to 1.47,the maximum erosion position appears on the side wall of inner arch,and when the Stokes number is greater than the critical value,the maximum erosion location appears on the outermost side of the junction between the elbow and the downstream pipe.(4)The simplified erosion prediction method based on the particle distribution and the gas-liquid-solid multiphase transient erosion prediction method based on the multiphase flow model(VOF?Multi-Fluid VOF)and discrete phase model(DPM)are established to calculate the relationship among the particle trajectories,the distribution of gas and liquid and the three-dimensional erosion profiles of pipe bend in the dispersed bubble flow,slug flow / churn flow and annular flow.The results show that:(1)The solid particles erosion for the dispersed bubble flow can be equivalent to the erosion in the mixed fluid of the gas and liquid based on the result of the simplified erosion prediction method.According to the results of the multiphase transient calculation method,the solid particles in the dispersed bubble flow are mainly distributed in the liquid phase.The erosion of the horizontal pipe bend for dispersed bubble flow is mainly concentrated in the elbow outer arch near the bottom of the pipe,showing a uniform parabolic shape.But for the vertical pipe bend,the erosion profile is smooth and shows an elliptical shape;(2)Assuming that all of the particles distribute in the slug body,the predicted error is 40.7% when using the simplified erosion prediction method to calculate the erosion of pipe bend for slug flow/churn flow.In horizontal pipe bend,about 33.5% of the solid particles are distributed in slug body.However,this proportion comes to 82.7% in vertical pipe bend.The multiphase transient calculation method is used to analyze the erosion regularity of the pipe bend for slug flow/churn flow.The maximum erosion location for the horizontal slug flow is close to the bottom of the pipe bend.But in the vertical pipe bend,the area of the maximum erosion location is small and the erosion profile becomes narrower with the increasing of the axial angle of the elbow;(3)The simplified calculation method based on the equivalent wall roughness height is established to predict the erosion rate of the pipe bend for annular flow.The percentage of the solid particles wrapped by the droplets is 62.6% in the vertical pipe bend.The multiphase transient calculation method is used to analyze the erosion regularity of the pipe bend for annular flow.The "particle free zone" is found in the elbow area and a clear V-shaped erosion scar also appears on the surface of the elbow which means that the erosion mechanism is similar to the pipe bend for gas-sold flow.However,there is a thin layer of liquid film in the inner wall of the elbow under the annular flow condition which will provide a cushioning effect on the solid particles.The erosion profile of pipe bend for annular flow is smoother than that for gas-solid two-phase flow as mentioned in in Chapter 3.