JPH0652255B2 - In-pipe magnetic flaw detection method - Google Patents

Description

The present invention relates to a pipe magnetic flaw detection method for detecting pitting corrosion that occurs on the inner and outer surfaces of a steel pipe.

[Prior Art] Conventionally, as a method of detecting flaws on the inner and outer surfaces of a steel pipe, there is a magnetic flaw detection method in which a magnetic sensor detects a leakage magnetic flux generated from a pit portion by magnetizing the steel pipe.
This magnetic flaw detection method, as shown in FIG.
A magnet 2 for magnetizing the inspected steel pipe 1 and a magnetic sensor 3 for detecting the magnetic flux φ generated from the pitting corrosion of the inspected steel pipe magnetized by the magnet 2 are integrally formed therein. The detection head 4 is inserted, and the magnetic head 4 is inserted into the magnet 2
The inspected steel pipe 1 is run in the direction of arrow A in the figure while being magnetized from the inner surface. Here, a plurality of magnetic sensors 3 are provided at regular intervals in the circumferential direction when flaw detection is performed on the entire circumference of the inspected steel pipe 1. Then, the magnetic head 4 is connected to the tube 1 to be inspected.
A magnetic flux φ is generated from the pitting corrosion when passing through the pitting portion 5 which is generated in the magnetic field. The magnetic flux φ is detected by the magnetic sensor 3 provided in the magnetic head 4, converted into an electric signal, and measured (not shown). Output to the department. Then, the electric signal is measured by the measuring unit to indirectly detect the pitting corrosion generated in the steel pipe 1 to be inspected.

[Problems to be Solved by the Invention] However, the conventional magnetic flaw detection methods have the following problems. That is, the inner surface of the steel pipe 1 to be inspected and the magnet 2
If there is a change in the relative distance between the and, the magnetizing force of the magnet 2 changes. Therefore, even if the pitting corrosion has the same size, variations in the magnetizing force cause variations in detection sensitivity. Therefore,
With the conventional method, it was difficult to accurately determine the size of pitting corrosion. Further, the output voltage level from the magnetic sensor 3 is lowered due to the change in the relative distance, and thus pitting corrosion may not be detected.

Therefore, the present invention is capable of compensating for the relative distance variation between the inner surface of the steel pipe to be inspected and the magnet, eliminating the variation in the detection sensitivity to pitting corrosion of the same size, and improving the flaw detection accuracy. The purpose is to provide.

[Means and Action for Solving Problems] The present invention is based on the following principle. That is, artificial pitting having a hole diameter of 5φ and a wall thickness of 70% is formed in the inspected steel pipe, and the output voltage V _B of the magnetic sensor due to the leakage magnetic flux from the sound portion and the output voltage of the magnetic sensor due to the magnetic flux generated from the pitting portion. When the correlation with Vc is obtained, it has a very good linear relationship as shown in FIG.

Therefore, in the first invention of the present application, after the leakage magnetic flux of the pipe to be inspected is detected by each magnetic sensor and converted into an electric signal, the output voltage for the leakage magnetic flux in the sound portion of the pipe to be inspected is obtained by the moving distance averaging method. , Compensation voltage is created for each magnetic sensor by comparing and dividing this output voltage with a preset reference voltage, and the output signal from each magnetic sensor is multiplied by the corresponding compensation voltage. The variation in detection sensitivity to pitting corrosion is automatically compensated.

Then, it is advantageous to input the output signal from each magnetic sensor to the limiter circuit to limit the level and then obtain the output voltage with respect to the leakage magnetic flux in the sound portion of the pipe to be inspected by the moving distance averaging method. .

In addition, the second invention of the present application provides an initial bias compensation voltage for each magnetic sensor output obtained by subtracting a preset bias compensation reference voltage from the output voltage of each magnetic sensor before magnetizing the tube to be inspected. Stored, then magnetize the pipe to be inspected and detect the leakage magnetic flux generated from the pipe to be inspected by each magnetic sensor, convert it into an electric signal, and output voltage of the corresponding magnetic sensor by the stored initial bias compensation voltage. After compensating the initial bias voltage of the output signal from each magnetic sensor by comparing and subtracting the subtraction output with the reference voltage for bias compensation, the output voltage for the leakage magnetic flux in the sound part of the pipe to be inspected is calculated by the moving distance averaging method. Then, a compensating voltage is created for each magnetic sensor by comparing and dividing this output voltage with a preset reference voltage. The output signal initial bias voltage is compensated from the gas sensor is obtained by the corresponding compensation voltage to automatically compensate for variations in the detection sensitivity to pitting by multiplying respectively.

[Embodiment] An embodiment of the magnetic flaw detection method of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the basic structure of the measuring section in the first embodiment of the method of the present invention, and the measuring system is similar to the conventional example shown in FIG. 3n integrally formed with the magnetic sensors 3a, 3b ... 3n
The steel pipe 1 to be inspected is magnetized from the inner surface by the magnet 2, and the resulting magnetic flux is detected by the magnetic sensors 3a to 3n arranged at regular intervals along the inner circumference of the steel pipe to be converted into an electric signal. It is supposed to be converted.

In FIG. 1, reference numeral 11 denotes a multiplexer, which extracts the electric signals S3a to S3n output from the magnetic sensors 3a to 3n in a time division manner according to a control signal from the microcomputer 13. The extracted time division signal Si (where i = 3a to 3n) is amplified by the signal amplifier 12 to a predetermined level and then given to the microcomputer 13. The microcomputer 13 uses the time division signal S amplified by the signal amplifier 12.
A low-pass filter (hereinafter abbreviated as LPF) 14 that removes the magnetic sensor output signal from the pitting portion and extracts only the magnetic sensor output signal from the detection portion by inputting i and performing frequency analysis, and an arbitrary reference a reference voltage generator 15 for generating a voltage _{E R,} compares the output voltage _{E B} of the magnetic sensor output signal in the normal area extracted by the this reference voltage _{E R LPF14, E R / E} B becomes compensation a divider 16 which calculates and outputs a voltage E _K, a multiplier 17 for compensating the output voltage E _I by multiplying the compensation voltage E _K to the output voltage E _I of the time division signal Si, the multiplexer 11 And a pulse generator 18 for generating a start pulse SP and a clock pulse CK as control signals. The cutoff frequency c of the LPF 14
Is set so as to satisfy c << s as compared with the signal frequency s of the magnetic sensor output signal for pitting corrosion.

Now, as shown in FIG. 2, when the start pulse SP and the clock pulse CK from the pulse generator 18 are given to the multiplexer 11, the multiplexer 11 outputs the output signals S3a to S3n from the magnetic sensors 3a to 3n in a time division manner. The signal amplifier 12 is extracted and is used as the time-division signal Si.
It is introduced into the microcomputer 13 via the and stored. Then, by the frequency analysis by the LPF 14,
Of the time-division signal Si, the magnetic sensor output signal for the pitting portion of the inspected conduit 1 is removed, and only the magnetic sensor output signal for the sound portion is sequentially taken out, and the output voltage E _B of the magnetic sensor output signal for this sound portion is obtained. Are provided to the divider 16. Then, the divider 16 sequentially compares the output voltage E _B with the reference voltage E _R from the reference voltage generator 15,
The compensation voltage E _K = E _R / E _B is calculated, and this compensation voltage E _K is given to the multiplier 17. Then, the multiplier 17 outputs the output signals S3a ...
S3n are sequentially given in a time division manner, and each signal S3a to S3n is given.
Multiplies the corresponding output voltage E _I and the compensation voltage E _K performed for each, compensation to the output voltage E _I by compensation voltage E _K is performed. That is, the compensation voltage E _K is a ratio for changing the magnetic sensor output voltage due to the leakage magnetic flux of the sound portion to a predetermined level, and as shown in FIG. 12, the output voltage due to the leakage magnetic flux from the sound portion is a hole. Since it has a linear relationship with the output voltage due to the magnetic flux generated from the eclipse, the compensation voltage E _K
By multiplying the output voltage by the magnetic flux generated from the pitting corrosion, the variation in the detection sensitivity caused by the variation in the relative distance between the steel pipe 1 to be inspected and the magnet 2 is automatically compensated for the pitting corrosion.

FIG. 3 shows that each of the above-mentioned components is configured by hardware,
A pitting corrosion (hole diameter 10 mmφ, wall thickness 70) when an electromagnet is used as the magnet 2 for magnetizing the steel pipe 1 to be inspected and the magnetic sensor 3 is used as one channel and the magnetizing current I (A) of the electromagnet is intentionally changed It is a figure which shows the relative detection sensitivity characteristic before compensation (broken line in a figure) and after compensation (solid line in a figure) with respect to (%). Here, the change in the magnetizing current I corresponds to the change in the magnetizing force due to the change in the relative distance between the inspected steel pipe 1 and the magnet 2. As is clear from the figure, by compensating the output voltages from the magnetic sensors 3a to 3n by the method of this embodiment, variations in the detection sensitivity due to changes in the magnetizing force are eliminated for pitting corrosion of the same size. Therefore, highly reliable flaw detection can be performed, and the size of pitting corrosion can be detected with high accuracy.

By the way, when a steel pipe that transports a gas such as a gas is used as the inspected steel pipe 1, the inspection head 4 often travels with the gas. In this case, the compressibility of the gas makes it difficult for the inspection head 4 to run at a constant speed, which causes variations in the sampling pitch in the longitudinal direction of the steel pipe in the above-described time-division method. May be missing. Further, since the signal frequency of the output signal for the pitting corrosion of the magnetic sensor 3 changes according to the traveling speed of the detection head 4, the cutoff frequency c of the LPF 14 is changed.
Needs to be changed based on the traveling speed of the detection head 4.

Therefore, a second embodiment of the distance synchronous sampling method as a magnetic flaw detection method that does not depend on the traveling speed of the detection head 3 will be described next with reference to FIGS. 4 and 5.

FIG. 4 is a block diagram showing the basic structure of the second embodiment of the present invention. The same parts as those in FIG. 1 are designated by the same reference numerals and detailed description thereof will be omitted. In FIG. 4, reference numeral 21 is an encoder for measuring the travel distance of the detection head 4, and the travel distance pulse of the encoder output indicated by DP in FIG. 5 is sent to the multiplexer 11 and the pulse generator 18 of the microcomputer 13. Given. The multiplexer 11 is activated by the input of the traveling distance pulse DP, and in response to the input of the traveling distance pulse DP, CK in FIG.
The output signals of the magnetic sensors 3a to 3n are sequentially extracted in response to the rising edge of the clock signal CK from the pulse generator 18 that generates the clock signal shown in FIG. As a result, a signal indicated by Si in FIG. 5 is output from the multiplexer 11, the signal Si is amplified to a predetermined level by the signal amplifier 12, and then given to the microcomputer 13 and stored therein.

The microcomputer 13 includes the reference voltage generator 1
In addition to 5, the divider 16, the multiplier 17, and the pulse generator 18, a moving distance averaging unit 22 and a reference distance setting unit 23 are provided. The moving distance averaging unit 22 uses the multiplexer 1
The output voltage of each magnetic sensor 3a to 3n extracted in 1 is set to the reference distance (for example, 2
cm, 5 cm, or 7 cm), and the averaged voltage E _B ′ is given to the divider 16. Then, as in the case of the first embodiment,
The divider 16 compares it with the reference voltage E _R to calculate the compensation voltage E _K = E _R / E _B ′, and the multiplier 17 calculates the compensation voltage for each output voltage E _I of each of the magnetic sensors 3a to 3n. Voltage E _K
Is multiplied by. As a result, by obtaining the average moving distance of the output voltages of the magnetic sensors 3a to 3n, this moving distance average value can be regarded as the output voltage of the magnetic sensor due to the leakage magnetic flux of the sound portion, and thus the first embodiment is performed. Similar to the example, it is possible to automatically compensate for variations in detection sensitivity to pitting corrosion.

On the other hand, in the first and second embodiments, in order to detect the difference in detection sensitivity, the magnetic sensors 3a to
The output voltage of 3n is extracted by the frequency analysis method by the LPF 14 or the moving distance averaging method. However,
When a through hole or the like is formed as pitting corrosion on the steel pipe 1 to be inspected, the magnetic sensor output signal due to the magnetic flux generated by this through hole becomes a signal with a large amplitude. It is difficult to remove completely by the averaging method. Therefore, when the first and second embodiments are applied to the steel pipe 1 to be inspected having the through holes and the like, a decrease in the compensation accuracy of the detection sensitivity cannot be avoided.

Therefore, in order to prevent the deterioration of the compensation accuracy as described above, the LPF 14 or the moving distance averaging unit 22 is passed through a limiter circuit that clips and removes the output signal from the signal amplifier 12 above a predetermined level and below a predetermined level. Should be given to.

FIG. 6 is a block diagram showing a third embodiment for preventing the deterioration of the compensation accuracy, in which 31 is a limiter circuit, the output terminal of which is the same as that of the LPF 14 of the first embodiment. It is connected. This limiter circuit 31 has a series resistance R
And a pair of diodes D1 and D2 which are connected to the output side of the series resistor R with opposite polarities,
The other terminals of the pair of diodes D1 and D2 are connected to the output terminal of the LPF 14. The output voltage E _{B of the} LPF 14 is used as the reference voltage of the limiter circuit 31, and the voltage level E _I of the magnetic sensor output signal Si and the output voltage E _{B of the} LPF 14 are compared to obtain E _B ± E _D (E _D is Diode D
If the value of E _I is larger or smaller than the offset voltage of 1, D2), then E _I is a diode D1 or D2.
Since it is clipped by, the magnetic sensor output signal Si is removed. That is, when the amplitude waveform of the signal at the point F in FIG. 6 has a large amplitude value T as shown by F in FIG. 7, it passes through the limiter circuit 31 and becomes a small amplitude as G in FIG. By further passing through the LPF 14, it is smoothed as indicated by H in FIG. Therefore,
The limiter circuit 31 does not automatically apply a large-amplitude signal due to pitting corrosion such as through holes to the LPF 14, and highly accurate sensitivity compensation can be realized.

By the way, as shown in FIG. 8 (only 3a to 3d are shown), the output voltage characteristics of the magnetic sensors 3a to 3n with respect to the leakage magnetic flux in the sound part of the steel pipe change corresponding to the magnetizing current I (A) supplied to the electromagnet. As a result, the detection sensitivity to the magnetizing current I varies depending on the magnetic sensors 3a to 3n. The initial bias voltage E of each magnetic sensor 3a to 3n when the magnetizing current I = O (A)
i also varies. Therefore, if the automatic gain compensation is performed by the above-described embodiments under the condition that the initial bias voltage exists, the variation in the detection sensitivity may increase. Therefore, by performing the automatic gain compensation after compensating the initial bias voltage of each of the magnetic sensors 3a to 3n, it is possible to prevent the measurement accuracy from deteriorating due to the initial bias voltage.

FIG. 9 is a basic configuration diagram showing a fourth embodiment of the present invention in which automatic gain compensation is performed after initial bias voltage compensation. 41 and 42 are subtractors, 43 is a reference power source for bias voltage compensation, Reference numeral 44 denotes an initial bias compensation voltage memory, and since the other components are the same as those in FIG. 1, the same reference numerals are given and description thereof is omitted. In FIG. 9, first, the output signals of the magnetic sensors 3a to 3n before magnetizing the steel pipe 1 to be inspected are time-divisionally extracted by the multiplexer 11, and the voltage level is amplified to the desired value Ei by the signal amplifier 12. After that, it is supplied to the subtractor 41, and compared with the reference voltage Ec from the bias voltage compensating power source 43 and subtracted. Then, the difference output E _D (= Ei−Ec) from the subtractor 41 is given to the subtractor 42 and stored in the initial bias compensation voltage memory 44 as the initial bias compensation voltage E _P. The output voltage of each of the magnetic sensors 3a to 3n is compensated to zero volt by comparing and subtracting the output voltage E _D from the magnetic field sensor with the initial bias compensation voltage E _P stored in the memory 44. Under this condition, when the inspected steel pipe 1 is magnetized with the magnetizing current I, the output voltage Ei of each of the magnetic sensors 3a to 3n.
Is compensated for the initial bias voltage by the compensating voltage E _P in the subtractor 42, and is applied to the LPF 14 according to the output characteristics shown in FIG. 10 (only 3a to 3d are shown).

Therefore, by applying the output voltage of the subtractor 42 to the microcomputer 13 to automatically compensate the detection sensitivity,
It is possible to completely eliminate variations in detection sensitivity among the magnetic sensors 3a to 3n.

In addition, although the case where the present invention is applied to the frequency analysis method is shown in the fourth embodiment, it goes without saying that the same effect can be obtained even when applied to the moving distance averaging method. Further, in each of the above-described embodiments, the constituent device may be configured by hardware or software.

[Effects of the Invention] As described in detail above, according to the first invention of the present application, a magnet for magnetizing the tube to be inspected and a magnetic flux generated from the magnetized tube to be inspected are detected inside the tube to be inspected. In the in-tube magnetic flaw detection method of detecting the pitting corrosion generated in the pipe to be inspected by inserting the detection head consisting of a plurality of magnetic sensors arranged in the pipe circumferential direction, the magnetic flux leaking flux of the pipe to be inspected is detected by each magnetic sensor. After each is detected and converted into an electric signal, the output voltage for the leakage magnetic flux in the sound part of the pipe to be inspected is obtained by the moving distance averaging method, and this output voltage is compared and divided with a preset reference voltage. Compensation voltage is created for each magnetic sensor, and the output signal from each magnetic sensor is multiplied by the corresponding compensation voltage to automatically compensate for variations in detection sensitivity to pitting corrosion. Therefore, it is possible to compensate for the relative distance variation between the inner surface of the inspected person and the magnet without depending on the traveling speed of the detection head, and as a result, it is possible to eliminate the variation in the detection sensitivity to pitting corrosion of the same size, and the accuracy of flaw detection It is possible to provide a method for in-tube magnetic flaw detection that is particularly effective when a steel pipe that transports a gas such as a gas is used as the pipe to be inspected.

According to the second invention of the present application, the initial bias compensation voltage for each magnetic sensor output obtained by subtracting the preset bias compensation reference voltage from the output voltage of each magnetic sensor before magnetizing the tube to be inspected. Then, magnetize the tube to be inspected and detect the leakage magnetic flux generated from the tube to be inspected by each magnetic sensor, convert it into an electric signal, and output the corresponding magnetic sensor by the stored initial bias compensation voltage. After compensating the initial bias voltage of the output signal from each magnetic sensor by comparing and subtracting the subtraction output of the voltage and the reference voltage for bias compensation, the output voltage for the leakage magnetic flux in the sound part of the pipe to be inspected by the moving distance averaging method. Compensation voltage is created for each magnetic sensor by comparing and dividing this output voltage with a preset reference voltage. Since the output signals in which the initial bias voltages from the respective magnetic sensors have been compensated are multiplied by the corresponding compensation voltages, variations in the detection sensitivity to pitting corrosion are automatically compensated. Therefore, in addition to the first invention, It is possible to prevent the measurement accuracy from deteriorating due to the initial bias voltage of each magnetic sensor, and it is possible to provide an in-tube magnetic flaw detection method capable of further improving flaw detection accuracy.

[Brief description of drawings]

1 to 3 are diagrams showing a first embodiment of the present invention. FIG. 1 is a block diagram showing a basic configuration, FIG. 2 is a signal waveform diagram for explaining the operation, and FIG. The figures are characteristic diagrams for explaining the effect, and FIGS. 4 and 5 are the second figures of the present invention.
FIG. 4 is a block diagram showing a basic configuration, FIG. 5 is a signal waveform diagram for explaining the operation,
FIGS. 6 and 7 are diagrams showing a third embodiment of the present invention, FIG. 6 is a block diagram showing a basic configuration, FIG. 7 is a signal waveform diagram for explaining the operation, and FIG. Figures through 10
FIG. 8 is a diagram showing a fourth embodiment of the present invention, FIG. 8 is a characteristic diagram for explaining a problem, FIG. 9 is a block diagram showing a basic configuration, and FIG. 10 is for explaining effects. FIG. 11 is a schematic diagram showing a conventional magnetic flaw detection method, FIG.
FIG. 2 is a characteristic diagram for explaining the principle of the present invention. DESCRIPTION OF SYMBOLS 1 ... Inspected steel pipe, 2 ... Magnet, 3 (3a-3n) ... Magnetic sensor, 4 ... Detection head, 5 ... Pitting part, 11 ... Multiplexer, 12 ... Signal amplifier, 13 ... Microcomputer, 14 ... LPF, 15 ... reference voltage generating unit, 16 ... divider, 17 ... multiplier, 18 ... pulse generator, 21 ... encoder, 22 ... moving distance averaging unit, 23 ... reference distance generating unit,
31 ... Limiter circuit, 41, 42 ... Subtractor, 43 ... Bias voltage compensation reference power source, 44 ... Initial bias compensation voltage memory.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Seigo Ando, Marunouchi, 1-2, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor, Yasuji Hosouhara, 4--12, Tsukoshi, Warabi-shi, Saitama Prefecture (72) Inventor Michio Ozawa 5-1, Hirano-cho, Higashi-ku, Osaka City, Osaka Prefecture Osaka Gas Co., Ltd. (72) Inventor, Kaichi Yasui 705 Miyukiyama, Tenpaku-ku, Nagoya, Aichi (56) References -125560 (JP, A) JP-A-49-46993 (JP, A) JP-A-59-3347 (JP, A)

Claims (3)

[Claims] 1. A detection comprising, inside a tube to be inspected, a magnet for magnetizing the tube to be inspected and a plurality of magnetic sensors arranged in a circumferential direction of the tube for detecting magnetic flux generated from the magnetized tube to be inspected. In a pipe magnetic flaw detection method in which a head is inserted to detect pitting corrosion generated in the pipe to be inspected, the magnetic flux leaked from the pipe to be inspected is detected by each of the magnetic sensors and converted into an electric signal, and then the moving distance average is calculated. Obtain an output voltage for the leakage magnetic flux in the sound part of the tube to be inspected by the method, create a compensation voltage for each of the magnetic sensors by comparing and dividing this output voltage and a preset reference voltage, A method of magnetic flaw detection in a pipe characterized in that the variation of the detection sensitivity to the pitting corrosion is automatically compensated by multiplying the output signal from each of the magnetic sensors by the corresponding compensation voltage. . 2. The output signal from each magnetic sensor is input to a limiter circuit to limit the level, and then the output voltage with respect to the magnetic flux leakage in the sound part of the pipe to be inspected is obtained by the moving distance averaging method. The in-tube magnetic flaw detection method according to claim (1). 3. A detection comprising a magnet for magnetizing the tube to be inspected and a plurality of magnetic sensors arranged in the tube circumferential direction for detecting magnetic flux generated from the magnetized tube to be inspected, inside the tube to be inspected. In a pipe magnetic flaw detection method in which a head is inserted to detect pitting corrosion generated in the pipe to be inspected, a bias compensating reference voltage preset from the output voltage of each magnetic sensor before magnetizing the pipe to be inspected The initial bias compensation voltage for each magnetic sensor output obtained by subtracting is stored, then the tube under test is magnetized and the leakage magnetic flux generated from the tube under test is detected by each of the magnetic sensors and converted into an electrical signal. Then, by comparing and subtracting the subtraction output of the corresponding magnetic sensor output voltage and the bias compensation reference voltage with the stored initial bias compensation voltage, the output signal from each magnetic sensor is After compensating for the initial bias voltage, the moving voltage averaging method is used to obtain the output voltage for the leakage magnetic flux in the sound part of the pipe to be inspected, and the output voltage and the preset reference voltage are compared and divided to obtain the compensation voltage. Is created for each of the magnetic sensors, and the output signals of which the initial bias voltage from each of the magnetic sensors has been compensated is multiplied by the corresponding compensation voltage, thereby automatically compensating for variations in the detection sensitivity to pitting corrosion. A method of magnetic flaw detection in a tube, which comprises: