Research Of The Key Technical Problems In The Internal Pipeline Detection With Magnetic Flux Leakage

The internal pipeline detection is an important means to determine pipeline deformation, corrosion, cracks, and defective degrees, the result of which provides a scientific basis for the pipeline operation, maintenance, and its safety evaluation. In this thesis, with a focus study on pipeline defect inspection and pre-judgment method and with a main research into magnetic flux leakage (MFL) detection theory and application, the defect detection capability of inspection device used in two kinds of structure is analysed and inner-outer defect identification (ID/OD) method is proposed under the condition of weak magnetic field, which has laid a technical basis for the evaluation of the pipeline defect integrity. Based on the research into the micro-mechanism structure of Ferro-magnet material magnetic domain, the method for testing stress concentration region under the weak magnetic conditions is presented; the feasibility of the method is proved by experiments and an effective way in resolving the safety assessment of pipeline is provided.In this thesis, the internal pipeline defect detection mechanism and detection model are researched. By applying magnetic dipole method in the different excitation direction, the defect magnetic field distribution is calculated. By applying the finite element method for the same specification defects under the conditions of high-saturation or near saturation to surface magnetic leakage distribution, the inner-outer defects surface magnetic field distribution in the weak magnetic field is emulationally calculated. Through the research of two kinds of device structures used in MFL detection, a solution to the traditional complex structures in the pipeline MFL detection equipment is proposed and so is it to the low capacity within the pipelines. And also by the research into the space weak magnetic fields detection, an efficient identification method to pipeline inner-outer defect distribution is offered.The signal characteristics and the identification ability for the two kinds of structural defect MFL detection device (used in the magnetic poles of mid-region and magnetic poles of post-region detection) are experimentally analyzed; With the magnetic poles mid-region detection accuracy up to inner defect minimum identification surface diameter ofφ3mm, depth of 10% wall thickness of the pit defect, and outer defect minimum identification surface diameter ofφ3mm, depth of 20% wall thickness of pit defect, a through-hole up to diameter ofφ2mm can be detected, while with the magnetic poles post-region detection accuracy up to inner defect minimum identification surface diameter ofφ4mm, depth of 20% wall thickness of the pit defect, and outer defect minimum identification surface diameter ofφ4mm, depth of 40% wall thickness of pit defect, a through-hole up to diameter ofφ3.5mm can be detected. By comparing the experimental results, under the conditions of reducing appropriate testing requirements, magnetic post-region detection can be used as a simplified means for pipe inspection device.The signal characteristics and the identification ability for the inner-outer defect identification under the conditions of weak magnetic field: detection is experimentally verified, which can efficiently detect inner surface diameter ofφ2mm, depth of 10% wall thickness of pit defect, while for the outer width of 10mm, 50% wall thickness of groove defect, there is no valid detection signal, therefore, such a method can be used as an effective way to identify inner-outer defect.According to relative theories of ferromagnetics, the internal magnetic properties of ferromagnetic in the magnetic domain model are studied in this thesis; and the micro-mechanism of ferro-magnet material magnetic domain structure is analyzed. Through the research on the movement variation of magnetic domain structure, that is, the movement of domain wall and rotation of magnetic moment, the effects on the magnetic domain caused by outer magnetic field and outer stress is analyzed and so is variation caused by such material magnetic properties. Through theoretical analysis and simulation experiments, the stress concentration area detection method under a weak magnetic field conditions is presented in this paper, so as just to solve the problem of pre-judgment in defect regions of pipeline integrity evaluation.Q235 stress model specimen is designed in the experiment. Stress region detection experiments are carried out under the conditions of non-external excitation, the weak magnetic fields and the strong magnetic fields. A standard specimens of non-stress concentration area under the geomagnetic condition and two standard specimens with different stress values are directly detected in such a magnetic method that stress concentration region signal characteristics under the condition of non-external excitation is obtained, with the result that the stress concentration region's surface magnetic induction intensity in horizontal component and vertical component reaches 1.4Gs of the signal peak-peak value; stress concentration region signal characteristics under the condition of external excitation is obtained, by changing the relative position between specimen and excitation device, with the result that the stress concentration region's surface magnetic induction intensity in horizontal component and vertical component reaches 14Gs-24Gs of the signal peak-peak value. Experiment results show that the stress concentration area is not sensitive under the conditions of strong magnetic fields and the stress concentration area under the condition of weak magnetic field can be detected and detection results have very good repeatability while under the conditions of non-excitation conditions, the stress concentration area can be measured, but the signal is greatly influenced by the external conditions, and after loading strong magnetic field, the effective signal itself disappears with unrecoverable nature. Experimental results show that the stress concentration region detection method under the conditions of weak magnetic field is feasible, the method of which can be used as an effective way to detection stress concentration region.In this paper, theoretical analysis and experimental research verify the basic theoretical correctness and feasibility of the method.