Influence Of Excitement Parameters In Pulsed Eddy Current Testing For Ferromagnetic Pipe Structures And Its Mechanism

There are tens of thousands of oil wells, thousands of kilometers of oil and gaspipelines and submarine pipelines at home and abroad which mostly use ferromagneticpipeline. With the increase of these pipes service time, the increasing trend offerromagnetic pipeline damage directly affects the growth of the national economyyear by year. Pulsed eddy current testing technique is a fast and convenient detectionmethod which is used for on-line detection for the ferromagnetic pipeline, especiallyfor oil casing in oil fields. This method can test not only the single-layer ferromagneticpipelines, but the multilayer ones effectively.This paper studies the major excitement parameters which affect the pulsed eddycurrent testing of ferromagnetic pipe structures on the basis of pulsed eddy currenttheory. Laying out of the probe, number of turns of detecting coil, magnitude of theexciting current and the excitation frequency are studied on their influence on thepulsed eddy current testing of ferromagnetic pipe structures by combination ofexperiment and ANSYS finite element simulation.Experiments and simulation analysis of probe placement of different combinedspecimens are carried out by changing longitudinal, transverse probe and transverseprobe orientation and some conclusions are made. A probe has better detectionsensitivity of crack defects when probe axis parallels to the crack. Transverse probe issuperior to longitudinal probe inner pipe corrosion but inferior to longitudinal probefor detection of outer pipe corrosion. Transverse probe has higher detection sensitivityonly when it parallel to the crack defects.The increase of receiving coil turns can increase the induction voltage of the coil,but cannot improve the detection sensitivity much when the voltage has been greaterthan the background noise.The influence of different exciting current on detection is studied. Differentexcitement currents (0.5A、1A、2A、4A) are applied to carry out experiments usingcombinations of J55and20#steel. Experimental results show that detectionsensitivity of smaller excitation current is better bigger excitation current when receiving data is not affected by noise. The relevant simulations show that the greaterexcitation current results in greater magnetic field intensity H, and the magneticpermeability|ì increases accordingly, which cause the decrease of skin depth, andaffects the detection sensitivity of deep defects.Excitation frequency mainly influences the detection depth of pulsed eddy current.It has been found from the experiments and the simulated analysis performed at2Hz,4Hz,8Hz,16Hz excitation frequency that the detection depth can be effectivelyincreased by reducing the pulse frequency. The major influence zone of eddy currentfield continuously goes outward as the receiving time extends. In essence, the skindepth of low frequency is large. Theoretical analysis, experimental results andsimulation results are consistent.In this paper, major excitement parameters which affect the pulsed eddy currenttesting of ferromagnetic pipe structures are studied in detail, which provides somereference for the development of better commercial pulsed eddy current instrument.