Method for measuring deposition of scales at inner wall of Austenitic boiler tube based ultra-low-frequency vortex

Abstract

The invention relates to a method for measuring the deposition of scales at an inner wall of an Austenitic boiler tube based ultra-low-frequency vortex, comprising the following steps: 1) an exciting coil is passed by ultra-low-frequency sine wave current to generate a ultra-low-frequency alternating magnetic field; 2) a sensor is attached on the outer wall of an Austenitic tube; 3) the ultra-low-frequency alternating magnetic field penetrates through the wall of the Austenitic tube to enter the tube, and vortex current is generated on ferromagnetic deposited scales so as to block and weaken the original alternating magnetic field; 4) a receiving coil positioned in the radial direction or axial direction of the tube wall of the exciting coil receives the ultra-low-frequency alternating magnetic field passing the path of the deposited scales on the inner wall of the tube; 5) the receiving coil sends the received signal containing path variables into a detecting device; and 6) the detecting device obtains a relevant curve of the phase change of the electromagnetic field and the change signal of the content of the scales by phase analysis of a computer. In the invention, the adopted ultra-low-frequency alternating magnetic field can penetrate through the tube wall, so the detection can be carried out on the positions and quantity of the deposited scales of the Austenitic stainless steel tube.

Description

[0001] technical field [0002] The invention relates to the field of electromagnetic nondestructive testing, in particular to a method for measuring scale accumulation on the inner wall of an austenitic boiler tube based on ultra-low frequency eddy currents. Background technique [0003] The superheater and reheater tubes of power plant boilers are clogged and exploded due to the accumulation of oxides. As early as the 1960s and 1970s, foreign countries focused on the formation and peeling of the oxide layer on the surface of the steam flow parts. In the tubes on the heating surface of the boiler, the scale peeling and accumulation on the inner wall of the austenitic tube is especially serious, which is mainly caused by the difference in thermal expansion coefficient between the austenitic tube and the scale. In recent years, there have been more and more accidents of early scale peeling, accumulation and blockage of the inner wall of such superheaters and reheaters in Chin...

AI Technical Summary

Problems solved by technology

Disadvantages: First, it is not sensitive to a small amount of oxide scale, and it needs a certain amount of accumulation to react. When the accumulation amount is large, the linear relationship is poor; second, some materials of the boiler tube will be ferromagnetic under high temperature service. In the magnetic measurement There will be misjudgment of oxide scale accumulation

Third, austenitic boiler tubes are prone to magnetism during processing and deformation, which seriously interferes with the magnetic nondestructive testing method.

Therefore, the scale measurement of the elbow is prone to large errors

Fourth, the oxide skin on the inner wall of the tube has a great influence on the quantification

Fifth, when the sensor is moved during measurement, the magnetic adsorption will change the original state of the accumulation, and after the accumulation is magnetized, the measurement data will become significantly larger again, and the detection repeatability will be poor

Sixth, during the inspection process, the scanning speed and scanning direction have a great influence on the accuracy of the results, which are greatly affected by the level of the inspectors

Seventh, pipes with ferrite spray coating on the outer wall cannot be tested

[0007] But its disadvantages are: first, it is not sensitive to a small amount of scale, and it needs a certain amount of accumulation to react, and the linear relationship is poor when the amount of accumulation is large; second, some materials of boiler tubes will be ferromagnetic under high temperature service. There will be misjudgment of scale accumulation during magnetic measurement

Third, austenitic boiler tubes are prone to magnetism during processing and deformation, which seriously interferes with the magnetic nondestructive testing method.

Therefore, the scale measurement of the elbow is prone to large errors

Fourth, the oxide skin on the inner wall of the tube has a great influence on the quantification

Fifth, when the sensor is moved during measurement, the magnetic adsorption will change the original state of the accumulation, and after the accumulation is magnetized, the measurement data will become significantly larger again, and the detection repeatability will be poor

Sixth, during the inspection process, the scanning speed and scanning direction have a great influence on the accuracy of the results, which are greatly affected by the level of the inspectors

Seventh, pipes with ferrite spray coating on the outer wall cannot be tested

Eighth, the oxide accumulation thickness cannot be measured

[0008] The invention patent application with the publication number CN101587096A discloses a method for non-destructive testing of the scale thickness distribution in stainless steel pipes, using conventional eddy currents with a frequency of several thousand to tens of thousand Hz, combined with computer acquisition and signal data corresponding to the position of the sensor phase to determine However, due to the inherent existence of the skin effect of eddy current, under the conventional eddy current detection frequency, for thick-walled pipes, or the thickness of the oxide skin accumulation exceeds a certain range, the eddy current field will be greatly weakened, so that the thickness of the oxide skin The increase and decrease cannot be truly reflected, especially when the oxide skin is blocked and close to filling the inner wall of the pipe, the thickness is the largest, which shows the insufficiency of this method

Second, in order to establish the relative position between the computer acquisition and the acquisition point, a mechanical positioning device similar to an encoder must be placed on the sensor in the axial and circumferential directions, which will inevitably make the device very complicated and cumbersome, and it will be difficult for the bends that are most likely to be clogged by oxide scales. Difficult to locate accurately at pipe diameter change

Moreover, the arrangement of power plant boilers is dense, and there is only a narrow space between the tubes, and the detection device that is too large cannot be tested on site.

Oxide scale detection in engineering is often carried out during emergency repairs. The time limit is very short, and multiple rows of pipes need to be inspected within a limited time. The fine scanning that takes a lot of time has no practical application value in engineering applications.

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