The underground storage tank usually has small volume and typically cylinder shape. A magnetic flux leakage (MFL) inspection device was developed for underground storage tank. In order to be suitable for curvature plates, the magnetization structure was designed, which contains three substructure and can cause the non-uniform magnetic field appeared on the cross section. The impact by the non-uniform magnetic field and uneven placement sensors were analyzed. The MFL testing requires effective signal progressing methods. A signal circuit without high-pass filter was used in hardware signal progressing. A zero-phase filter was adopted in the digital signal processing. The defect was characterized by peak-peak value of defect signal. The channel independent calibration method was carried out. An experimental underground tank with a diameter of 2.4m was established. The experimental results for cracks with different depths, lengths and widths were presented, and the results proved that the presented signal progressing was effective.

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