Theoretical Model And Method Of Non-Ferromagnetic Metallic Pipes With Claddings Using Pulsed Eddy Current Testing

Non-ferromagnetic pipes such as austenitic stainless steel pipes are widely used in key equipment in the petrochemical and energy industries due to their good anti-corrosion properties.For industrial equipment needs,the pipeline is often covered with insulation and cladding.It is of great significance to detect these pipes without removing the insulation.Pulsed eddy current testing(PECT)technology has been applied to detect corrosion of ferromagnetic materials because of its high penetration power and the ability to achieve inline testing of pressurized components.However,magnetic field is not focused spontaneously in non-ferromagnetic pipes like in ferromagnetic materials,the eddy current diffusion is much faster and the electromagnetic energy loss is higher,thus bringing a new challenge for PECT.Aiming at the difficult problem of corrosion detection of nonferromagnetic metal pipes,this dissertation conducts research on the theoretical model,sensor optimization method and thickness measurement method of PECT.Firstly,for the large area corrosion caused by the scouring and electrochemistry of the inner and outer pipe wall,an analytical model for vertical coils near a non-ferromagnetic metallic pipe with cladding in PECT is proposed and solved.A hypothesis of treating conductors as partition interfaces is constructed to overcome the problem of the singularity of the transfer matrix between the fluctuation equation and the diffusion equation.Thus the electromagnetic analytical solution in the multilayer pipeline structure is obtained.The model effectively improves the accuracy of the analytical calculation of PECT signals for small diameter non-ferromagnetic pipes,expanding the scope of application of the analytical model.Secondly,for the problem of large electromagnetic energy loss in the small diameter non-ferromagnetic pipes,methods are proposed to optimize the structure and size of PECT sensor for pipelines.A sensor structure with runway-type excitation coil and receiving coil is designed to change the electromagnetic energy distribution on non-ferromagnetic pipes,which can be used for pipe thickness measurement under small lift-off.Based on the the incident electromagnetic field decoupled from the analytical model,an excitation coil optimization theory is proposed to increase the effective field strength in the footprint of the excitation coil,which can adapt to different lift-off.Based on the optimization method,a sensor is developed to solve the engineering problem of unfocused magnetic field and improve the wall thickness resolution of non-ferromagnetic pipes.Thirdly,the electromagnetic wave modulation theory is proposed based on the hypothesis of treating conductors as partition interfaces.And the transmission coefficient and transmittance of the hypothetical conductor partition interface are solved by the electromagnetic wave reflection coefficient to reveal the energy loss and time delay effect of the metal cladding in the testing.A thickness measurement method for non-ferromagnetic pipes with cladding is proposed for pipes with different diameters,which significantly reduces the influence of lift-off and cladding on the evaluation of pipe wall thickness.Finally,based on the above work,PECT platform for non-ferromagnetic metallic pipes with cladding is established,PECT software with the function of signal simulation,sensor optimization and signal processing is developed,and industrial field testing is carried out.The study in this dissertation enriches the theory of PECT and its application scope,supports the development of the national standard “Non-destructive testing Pulsed eddy current testing of non-ferromagnetic metallic materials”,provides a new approach to the safety assessment of non-ferromagnetic pipeline equipment,and promotes the further development and application of PECT technology in high and low temperature industrial pipeline inspection.