Importance of Magnetizing Field on Magnetic Flux Leakage Signal of Defects

• Published in: IEEE transactions on magnetics 2018-06, Vol.54 (6), p.1-6
• Main Authors: Pham, Hong Quang, Trinh, Quang Trung, Doan, Duy Tuan, Tran, Quang Hung
• Format: Article
• Language: English
• Subjects: Cartography; Defects; Dependence; Gas pipelines; Leakage; Magnetic field measurement; Magnetic flux; Magnetic flux leakage; Magnetic flux leakage (MFL); Magnetism; Magnetization; Magnetometers; Natural gas; Nondestructive testing; Permanent magnets; Petroleum pipelines; pipeline inspection; Pipelines; Quadratic equations; Saturation magnetization; Steel plates
• ISSN: 0018-9464; 1941-0069
• DOI: 10.1109/TMAG.2018.2809671

The measurement of magnetic flux leakage (MFL) is widely used as a non-destructive technique for inspecting oil and gas pipelines. In this method, many factors can affect the MFL signal, but the magnetization of the pipeline is a fundamental issue that bears consideration. We investigated the dependence of the MFL signal on the magnetizing state of pipeline samples with respect to both near-side and far-side defects by varying the number of permanent magnets in each pole of the U-core system, varying the distance between the two poles, varying the gap between the U-core system and the surface of the steel plates engineered from a gas pipeline, and varying the depth of the rectangular defects. By systematically controlling the magnetization of the samples, we observed novel behaviors of the MFL signals as a function of defect depth. For samples having above-saturated magnetization, the dependence of the MFL signal can be expressed as a quadratic function for all defect depths. However, for samples having below-saturated magnetization, the MFL signal increases linearly at low-defect depths, but exhibits a quadratic response at high-defect depths. These findings have relevance for both fundamental research and practical applications, and they provide a novel and precise cartography method for accurately quantifying the depth of defects in MFL measurement.