Research On Defect Recognition Technology Based On MFL Internal Inspection Of Pipeline

With the continuous summary of the national economic level,the demand for manufacturing industry is also increasing,so the dependence on important national resources such as oil and natural gas is gradually increasing.Once oil and gas leakage occurs due to pipeline damage,the damage to personnel,property and environment is extremely serious.Therefore,regular maintenance of oil and gas pipelines is required to prevent unnecessary losses caused by pipeline defects.In a series of pipeline defect detection methods,magnetic flux leakage detection has obvious advantages in efficiency,safety and cost.Under the premise of ensuring the safe operation of the pipeline and improving the use efficiency,it is necessary to have a more accurate assessment of the pipeline defects to avoid unnecessary shutdown.Therefore,the demand for quantitative analysis of defects is increasing.In order to realize the quantitative analysis of defects,it is necessary to fully understand the causes of MFL signals and the quantitative relationship between these factors and MFL intensity.By solving the analytical solution of MFL,we can fully understand the source of each parameter in MFL signal and its influence on MFL signal,so as to realize more accurate evaluation of defect size and risk degree.In this paper,through the theoretical calculation and Research on the signal distribution of magnetic flux leakage produced by the cylinder surface defects,the quantitative relationship between the three-dimensional defect magnetic flux leakage intensity and the defect size,material permeability,detection position and the external magnetic field intensity under the condition of external magnetic field is obtained.Based on the study of the boundary conditions of the interface of different media in the electric field,the boundary conditions composed of potential ? and the application of Laplace equation in the electric field are obtained.Then,the corresponding relationship between the electric field and the magnetic field is used to apply the boundary conditions and Laplace equation to the magnetic field,and the scalar magnetic potential?_m is set up as the pending solution Quantity solves Laplace equation.The magnetic charge density of the defect surface can be calculated by solving the scalar magnetic potential?_m of the defect leakage magnetic field.Based on the magnetic dipole model,the analytical solution of the distribution of the defect leakage magnetic field in three-dimensional space can be obtained.Using MATLAB software to program,the distribution of axial magnetic flux leakage signal and radial magnetic flux leakage signal under the condition of changing the radius,height and lift off value of cylinder defect is obtained.Through COMSOL simulation software,the influence of different radius,height and lift off value on the distribution of axial magnetic flux leakage signal and radial magnetic flux leakage signal under the condition of cylinder defect is verified,and the distribution characteristics of defect magnetic flux leakage are obtained Compared with the three-dimensional leakage magnetic field distribution image obtained by MATLAB.According to the flux density mode of COMSOL result,the change rule of leakage magnetic field component of axial and radial defects is obtained,which is in good agreement with the change rule of defect radius,height and lift off value of cylinder in MATLAB result.The results show that the analytical solution of the magnetic flux leakage field is correct under the magnetic dipole model,and the relationship between the magnetic flux leakage intensity and the characteristic value of the signal of the magnetic flux leakage field and the external magnetic field intensity,material permeability,cylinder defect size and lift off value is obtained.