Abstract
In order to improve the sensitivity of online magnetic flux leakage (MFL) testing for steel pipe, a sensing method based on the magnetic guiding effect is proposed and investigated in this paper. Compared to the conventional contact sensing method using a non-ferromagnetic support, the proposed method creatively utilizes a ferromagnetic one to guide more magnetic flux to leak out. Based on Hopkinson’s law, the principle of the magnetic guiding effect of the ferromagnetic support is theoretically illustrated. Then, numerical simulations are conducted to investigate the MFL changes influenced by the ferromagnetic support. Finally, the probe based on the proposed method is designed and developed, and online MFL experiments are performed to validate the feasibility of the proposed method. Online tests show that the proposed sensing method can greatly improve the MFL sensitivity.
Highlights
As an important pressure component, steel pipes are widely used for oil and gas storage and transportation
Due to the lift-off effect, the leakage field rapidly decreases with increasing radial distance, in order to realize high-sensitivity sensing, magnetic sensors are placed as closely as possible to the objects to be tested; (3) Leaked magnetic flux enhancement: A concentrating device made of ferromagnetic material is applied to affect the distribution of the magnetic flux leakage and concentrate more leaked magnetic flux into the sensor location, which can improve the sensitivity
In online magnetic flux leakage testing (MFL) testing for steel pipes, the DC magnetization method is usually used to produce a strong and uniform magnetizing field, which is suitable for both surface and sub-surface defect inspection at high speed
Summary
As an important pressure component, steel pipes are widely used for oil and gas storage and transportation. Due to the lift-off effect, the leakage field rapidly decreases with increasing radial distance, in order to realize high-sensitivity sensing, magnetic sensors are placed as closely as possible to the objects to be tested; (3) Leaked magnetic flux enhancement: A concentrating device made of ferromagnetic material is applied to affect the distribution of the magnetic flux leakage and concentrate more leaked magnetic flux into the sensor location, which can improve the sensitivity. In online MFL testing for steel pipes, the DC magnetization method is usually used to produce a strong and uniform magnetizing field, which is suitable for both surface and sub-surface defect inspection at high speed. It can be seen that even for the contact method, there is still an inevitable lift-off distance from the pipe surface to the sensor location because of the thickness of the non-ferromagnetic support, restricting the sensitivity improvement, which is the main challenge of the conventional method.
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