What Is Magnetic Flux Leakage?

Magnetic leakage refers to the part of the magnetic field outside the magnetic screen device in the magnetic screen device. Magnetic flux leakage is the magnetic field energy leaked into the air (space) by a magnetic source through a specific magnetic circuit.

Magnetic leakage is

There is a certain gap between the coils, and a small gap is formed between the gaps

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Magnetic leakage detection principle

When the ferromagnetic plate is magnetized by a magnetization device, an induced magnetic field can be generated in the plate. If there are volume defects such as corrosion and mechanical damage on the plate, the magnetic field lines will leak to the outside of the plate, which will form a leakage magnetic field on its surface. The principle is as follows: Shown in Figure 1.

The magnetic flux leakage detection of the tank bottom plate is carried out by scanning using a device that integrates a magnetizing device (permanent magnet or electromagnet) and an array magnetic field probe. The magnetizing device magnetizes the detection part of the tank bottom plate to make it saturated or near magnetic. The saturation level, the magnetic field sensor converts the new type of magnetic flux leakage caused by defects on the bottom plate into electrical signals, and then through amplification, filtering and signal processing, for the thickness of the tank bottom wall thinning defects, the equivalent depth of the defect can be given, Crack-type defects can be analyzed by the waveform of the magnetic flux leakage signal.

Testing equipment and test plates

1. Detection equipment

The testing instrument adopts the floor leakage detection system of Floormap3Di produced by British Silverwing Company. The system uses air-gap magnetoresistive sensor technology, adding an additional set of sensors to detect changes in magnetic field strength caused by top defects, and combined with traditional MFL magnetic leakage marks to identify upper and lower defects. Floormap3Di system has 64 detection channels, 256 The sensor reduces the sensitivity to the defect direction while enhancing the lateral resolution of smaller defects.

The system detects a scanning width of 300mm, a maximum detection thickness of 12.5mm, a maximum penetration of 6mm coating, and a detection sensitivity of 20% of the board thickness. The physical system is shown in Figure 2.

2.Test plate

The test plate is produced by referring to JB / T 10765-2007 "Non-destructive testing method for magnetic leakage detection of atmospheric metal storage tanks". The calibration test plate is used for functional testing of each independent channel of the testing instrument. The comparison test board is used to evaluate the depth equivalent of the detected defect.

Detection process

1. Detection object

Above-ground storage tanks are composed of bottom plates (lap welding), tank walls, tank roofs (with fixed roofs and floating roofs), and pipes around the tanks. The material is generally ordinary carbon steel. The thickness of the bottom plate of the storage tank is generally 8-12mm. Because the oil contains moisture or chemical components such as H ₂ S will cause corrosion to the bottom plate, an anticorrosive layer will be added during the manufacturing process. Affected by various factors, the damage of the anticorrosion layer will cause different degrees of corrosion on the upper surface of the bottom plate, and the corrosion of the lower surface will be more serious. Corrosion defects are also a major defect of the tank floor. The storage tanks have different volumes, and the diameter of the bottom plate ranges from a dozen meters to tens of meters.

2. Detection process

The percentage of magnetic flux leakage detection on the bottom plate of this tank inspection is 100%, and the testing basis is JB / T 10765-2007 "Non-destructive testing method for magnetic flux leakage testing of atmospheric metal storage tanks".

(1) Before the test, first carry out the tank cleaning work, including emptying, replacing and cleaning the storage tank, so as to meet the requirements for entering the storage tank for testing.

(2) The instrument is preheated after starting up, and the probe lift-off value is adjusted according to the thickness of the base plate to ensure the accuracy of the test results.

(3) Calibrate the instrument. Use the calibration test board to test the function of each independent channel of the instrument. By scanning and comparing the artificial defects of the test plate, adjusting the instrument's set value and the height of the sensor, the instrument signal indication is consistent with the percentage of the desired plate thickness artificial defect depth.

(4) Determine the reference of the storage tank and number the bottom plate (draw the floor plan) so that the scanning mode can be easily determined during the scanning.

(5) Select the scanning mode. Scanning can be performed in manual and automatic modes, and the scanning speed is kept as uniform as possible. ^[2]