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Magnetic flux leakage



Definition

Magnetic flux leakage is the magnetic field energy leaked into the air (space) by a magnetic source through a specific magnetic circuit. The magnetic field of a magnet is closed inside and does not show magnetism to the outside. When a magnetic pole is formed outside, a magnetic field is generated. The magnetic field is radiated to the outside in an open loop. To be precise, a magnetic field is a form of magnetic leakage.

Magnetic leakage of electromagnetic coil

There is a certain gap between the coils, and a small electromagnet is formed between the gaps. It has its own magnetic induction line, and it is closed, so it will produce The magnetic induction intensity which is different from the theory, in fact, this is the non-ideal situation of the electromagnet, which can generally be determined by experimental methods.

This is related to the density, number of turns, material, voltage, and current of the coil.

Methods to reduce magnetic flux leakage

1. Magnetic core

1) Using wound cores, ring magnetic cores and other structures without seams to reduce magnetic flux Hinder.

2) Reduce the working magnetic flux density Bm value of the transformer and increase the beam magnetic capacity of the iron core.

3) Use high-grade or high-permeability iron core materials to increase the beam magnetic capacity of the iron core.

2. In terms of coil structure, in theory, the most ideal way is to wind the primary and secondary coils, which can maximize the coupling between primary and secondary and reduce leakage inductance.

However, in practice, in order to solve the problem of withstand voltage between primary and secondary, it is difficult to achieve this method. Make the secondary, and finally wind around the remaining half of the primary, so that the entire secondary is fully enclosed in the primary, the coupling effect is better, and the leakage inductance is small.

3. The outer casing of the transformer is used. The iron casing can confine the leakage magnetic field inside to prevent outward diffusion, but the casing will generate heat, so the distance between the casing and the transformer is required.

4. Sandwich winding method

When winding the coil, the primary coil can be divided into two parts, and the secondary coil can be sandwiched in the middle. In this way, the coupling between the primary and secondary coils is enhanced, and the magnetic flux leakage will be reduced. First of all, in order to dissipate heat, the iron core of the transformer is not completely surrounded by the coil. Because the coil has current flowing through it, it must generate a magnetic field, which is not converted into electrical energy by the secondary coil. This part of the unconverted magnetic field is magnetic leakage. Moreover, we know that in order to prevent the generation of eddy currents, we make the core of the transformer into slices, and superimpose multiple slices. Since the magnetic lines of induction are not completely closed, there will be magnetic leakage between the slices. Heat dissipation and eddy current are the causes of magnetic leakage.

Corrosion detection

Magnetic flux leakage detection principle

When the ferromagnetic sheet is magnetized with a magnetization device, an induced magnetic field can be generated in the sheet. If there is corrosion on the sheet For volume defects such as mechanical damage, the magnetic lines of force will leak to the outside of the sheet, thereby forming a leakage magnetic field on the surface. The principle is shown in Figure 1.

The magnetic flux leakage detection on the bottom of the storage tank is carried out by scanning through a device integrated with a magnetization device (permanent magnet or electromagnet) and an array magnetic field probe. The magnetization device magnetizes the detected part of the bottom of the storage tank To make it reach the level of saturation or near magnetic saturation, the magnetic field sensor converts the new model of magnetic leakage generated by the defect on the bottom plate into an electrical signal, and then through amplification, filtering and signal processing, it can give the The equivalent of the defect depth can be analyzed by the waveform of the magnetic flux leakage signal for the defect of the crack nature.

Testing instrument and test board

1. Testing instrument

The testing instrument adopts the floormap3Di magnetic flux leakage detection system of the storage tank bottom plate produced by British Silverwing Company. . The system uses air-gap magnetoresistive sensor technology, adds a set of additional sensors to detect changes in magnetic field intensity caused by top defects, and combines with traditional MFL magnetic flux leakage signs to identify upper and lower defects. Floormap3Di system has 64 detection channels, 256 The sensor enhances the lateral resolution of detecting smaller defects while reducing the sensitivity to the defect direction.

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

2. Testing test board

The testing test board is made according to JB/T 10765-2007 "Non-destructive testing method for magnetic flux leakage testing of atmospheric metal storage tanks". The calibration test board is used to test the functions of each independent channel of the testing instrument. The comparison test panel is used to evaluate the depth equivalent of the detected defects.

Testing process

1. Test object

The above-ground storage tank consists of bottom plate (lap welding), tank wall, tank top (with fixed roof and floating roof). Divide) and the pipeline composition around the storage tank. The material is generally ordinary carbon steel. The thickness of the bottom of the storage tank is generally 8-12mm. Because the oil contains water or chemical components such as H2S, it will corrode the bottom, and an anti-corrosion layer will be added to the bottom during the manufacturing process. Affected by various factors, the damage of the anti-corrosion layer will cause different degrees of corrosion on the upper surface of the bottom plate, and the corrosion on the lower surface will be more serious. Corrosion defects are also the main defects of the bottom of the tank. The volume of the storage tank is different, and the diameter of the bottom plate ranges from a dozen meters to tens of meters.

2. Testing process

The rate of magnetic flux leakage testing for the bottom plate of the can-opening inspection is 100%, and the testing basis is JB/T 10765-2007 "Non-destructive testing of atmospheric metal storage tank leakage "Magnetic Inspection Method", the inspection steps are:

(1) Before the inspection, clean the tank first, including tank emptying, replacement and cleaning, in order to meet the requirements of entering the storage tank for inspection.

(2) The instrument is turned on for preheating, and the probe lift-off value is adjusted according to the thickness of the bottom plate to ensure the accuracy of the test results.

(3) Calibrate the instrument. Use the calibration test board to test the functions of each independent channel of the instrument. By scanning and comparing the artificial defects of the test board, adjusting the setting value of the instrument and the height of the sensor, so that the signal indication of the instrument is consistent with the percentage of the required thickness of the artificial defect depth.

(4) Determine the base of the storage tank and number the bottom plate (draw the floor map) to facilitate the determination of the scanning mode during scanning.

(5) Select the scan mode. Scanning can be carried out in manual and automatic modes, and the scanning speed should be kept as uniform as possible.

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