Study On Magnetic Flux Leakage Testing Probe And Device For Tubes

Magnetic flux leakage inspection equipments for tubes were manufactured by several large-scale foreign companies, such as FORSTER and TUBOSCOPE. Their equipments detect transverse injury with through mode and detect longitudinal wound with rotary probe. Although the rotating probe has fast detection speed and high sensitivity, but high-speed rotation requires rigid design and manufacture of mechanical system, which is difficult to meet with the level of the domestic machinery. Beijing Institute of Iron and Steel Research Institute studied on working method of probe fixed/ steel spiral forward, pointed out that detect flaws of tubes is entirely possible as long as the parameters of the testing device were set properly and effectively, it was able to meet the request of the performance index in the national standard of steel magnetic flux leakage testing, this pipe leakage technique is feasible and suitable for China's national conditions. This paper developed MFL testing equipment with method of steel tubes whirling in situ/probe scanning linearly, the purpose is to study AC and DC magnetic flux leakage probe defecting vertical and horizontal flaws of steel tube, the feasibility of detection method and the reliability of the magnetic flux leakage testing device, to study the impact of magnetic flux leakage factor, and make preparation for the development of practical magnetic flux leakage testing equipment.The research approach of this task is from the study of MFL theory, combined with model calculations, to the experimental test after the design of device. First of all, the paper analyzes the main flaws of tubes, makes an overview of the basic principles of magnetic flux leakage, the formation mechanism of the magnetic field and the process of magnetic flux leakage inspection of tubes, introduces the main techniques of the magnetic flux leakage, studies status and trends of magnetic flux leakage technique and general situation of magnetic flux leakage testing equipment at home and abroad, finalizes the magnetic flux leakage testing device technology programs and the primary coverage of research. Secondly, the paper calculates the magnetic field Leakage test loop model, after that the air gap magnetic field, magnetic field leakage through joints and limited deep slit in the leakage magnetic field is calculated, then it analyzes the Magnetization methods of the magnetic flux leakage testing method, pointed out that the magnetic approach, magnetization intensity and the magnetization direction impact the magnetic flux leakage testing, and then the paper analyzes the principle of electromagnetic induction sensors and scanning factors. Finally, the analysis is magnetic circuit of AC and DC magnetic flux leakage testing, and magnetic flux leakage probe magnetic circuit model was analyzed and calculated. Thirdly, the paper makes analysis of the probe model with ANSYS, the simulation analysis of the burial depth, size of defects; pipe wall thickness, magnetization intensity and lift-off were made. After the design of the magnetic flux leakage testing probe, including the coil sensor, excitation coil and core excitation, the parameters of the excitation coil were calculated, and finally the probe was produced. Finally, the steel rotating scanning device and test sample tube were produced; the experimental magnetic flux leakage testing device was achieved. Then experimental testing on magnetic flux leakage testing device probe were made, factors affect magnetic flux leakage testing were experimentally studied. Finally, results of the characteristics of speed, lift-off and frequency of magnetic flux leakage were reached.This study shows that the prerequisite of MFL is how to generate leakage field at pipe defects, the core of magnetic flux leakage testing is the probe, and its role is to encourage and receive magnetic field signals, respectively, to achieve these two functions are the sensors and the excitation device. The pipe needs to ensure full magnetization, but not too much, otherwise it will affect both sensitivity and signal to noise ratio (SNR). Whether with AC or DC magnetization, if the steel tube is over magnetized, the signal to noise ratio will decrease. The magnetic field leakage is strongest when magnetization direction is perpendicular to the long axis of the defect. When the magnetization direction is parallel with the defect axis, there is almost no leakage magnetic field signal. For axial injury, it should be vertical scanning coil sensor, and for the circumferential injury, it should be horizontal coil sensor. The key of MFL is to achieve the relative motion of sensor and the tube. For the detection method of the probe scanning/steel pipe rotating in situ, although the rotation and the scanning speed is limited, but it can achieve vertical and horizontal detection.The finite element simulation study with ANSYS shows that the surface defects generates strongest leakage magnetic field, for the defects with same shape and size, the relationship of the buried depth and the magnetic field leakage amplitude were approximately linearly. For defects with the same Depth and in the same magnetic conditions, the influence of width to the peak amplitude is not great, only a slight increase. In the case of certain defects in width, within a certain range, the peak to peak of vertical component of leakage magnetic field is roughly linear with the depths of defect. The thickness of Pipe wall influences the distribution of magnetic field lines is because of the inclusion capacity of the wall, the thinner wall, the fewer magnetic field lines, the easier leakage. The leakage of magnetic field lines can be found when the current density reaches a certain value, and it rises with the increase of current density, but it decreased after reaching a certain level. The axial component of Leakage magnetic field rapidly decreases when the sensor lift-off increases, the radial leakage magnetic field also rapidly decreases. When the lift-off value is high, leakage magnetic field changes slowly. As the value and the relationship between magnetic flux leakage signals is nonlinear, the smaller the lift-off value, the faster the decrease. That is, the smaller lift-off and the greater fluctuation impact on the detection sensitivity.Tests show that AC probe and DC probe are different in the signal characteristics, magnetic intensity, and sensitivity and so on. The signal to noise ratio of AC probe is higher than the DC probe, magnetic it requires lower intensity, and the sensitivity is higher to the same lift-off. The results of magnetization characteristics correspond to the magnetization theory of ferromagnetic materials; the speed characteristic of magnetic flux leakage is consistent with the theory of electromagnetic induction. Lift-off characteristic is consistent with the ANSYS simulation analysis, with the increase of sensor lift-off value the signal decreases rapidly. The fluctuation is more visible when the lift-off value is small. The lift-off with high fluctuation should be avoided. For the magnetic leakage detection device in this paper, the appropriate lift-off of DC probe is 1-3mm. the appropriate lift-off of AC magnetic flux leakage probe is of the range of 1-10mm. the suitable frequency range of AC magnetic flux leakage is 1 kHz-10 kHz. With the appropriate lift-off, testing speed and magnetization, the magnetic leakage detection device with rotating tube in situ/probe line scanning developed in this paper can achieve longitudinal and transverse detection.