This thesis presents a method to improve the accuracy of detecting corrosion and cracks in metal pipes using the magnetic flux leakage method. In the presented method, a two-dimensional (2D) magnetic system is proposed for a smart pipeline intelligent gage (PIG). This PIG uses the fluid energy to move along the pipeline and inspects the inner surface of the pipe. Pigging is one of the modern and non-destructive methods of detecting corrosion and cracks in a pipe. The smart magnetic PIG detects the location of corrosion and cracks inside the pipe through the measurement of changes in leakage flux. The PIG is entered into the pipe by an appropriate mechanical system and moves along the pipe by means of fluid pressure. This allows the PIG to inspect the inside of the pipe. The PIG equipped with a magnetic leakage flux detection system has advantages such as the possibility of being used for both gas and liquid fluids, less sensitivity to fluid pressure speed changes, and faster inspection speed compared with the ultrasonic and eddy current smart PIGs. However, the application of the magnetic flux PIG is confined to ferromagnetic metal pipes. The magnetic flux leakage method consists of a ferromagnetic core, a permanent magnet and a magnetic field sensor, in which the magnetic core is in contact with the pipe body by two metal brushes. If there is a change in the reluctance of the magnetic flux path at the location of the defect due to cracks or corrosion, the magnetic flux changes are detected by a magnetic sensor. Then, the signals corresponding to the cracks/corrosion are logged for additional processing to detect the type and location of defect. A conventional magnetic smart PIG merely uses a magnetic system along the axis of the pipe. Thus, the sensor accuracy and sensitivity are drastically reduced by reducing the dimensions of the crack along the tangential axis. In the proposed method, a 2D orthogonal magnetic system is presented which has two sensors in the axial and tangential directions. Based on the magnetic dipole analytical model, it is shown that by using a 2D orthogonal magnetic field, the leakage flux due to corrosion and cracks is strengthened, and as a result, the accuracy of detection is improved within different directions. In order to verify the performance of the proposed method, the 2D orthogonal geometric structure was designed for applying the magnetic field to a crack sample with specific dimensions. The performance accuracy was eva‎luated through the numerical analysis of the field in Maxwell software tools. In this regard, the size of the leakage flux density for all types of cracks with different dimensions in the axial and tangential directions has been calculated through numerical simulation, and through that, the performance of the proposed 2D orthogonal geometric structure is compared with the conventional structure. The results of the comparison show that the use of orthogonal structure enhances the leakage flux due to the application of the field in both axial and tangential directions, and as a result, it enables detection of axial and tangential cracks with small dimensions. In order to more comprehensively examine the superiority of the proposed orthogonal structure, various cracks and corrosions with different aspect ratios are simulated on the surface of the test samples.Then, the magnetic field density created by cracks and corrosion in the conventional orthogonal structure are calculated and compared to investigate the improvement due to using 2D magnetic system.

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فرزانه شايق بروجني

محمد سعيد مهدوي , فاطمه زارع