JPH03285161A - Remote eddy current flaw detection method - Google Patents

Abstract

PURPOSE:To easily perform flaw detection work and to miniaturize an apparatus by exciting a transmission coil and measuring the boundary distance between a direct magnetic field affected region and an indirect magnetic field dominating region and adjusting the distance in a pipe axis direction between transmission and receiving coils so that the receiving coil is present in the latter region to allow both coils to integrally run. CONSTITUTION:A transmission coil T and a receiving coil R are integrally moved in the direction shown by an arrow 11. The magnetic flux 12 from the coil T pierces pipe 10 to propagate through an external space 13 along the outer surface of the pipe 10 and again pierces the pipe 10 to reach the receiving coil R. The electromagnetic wave speed of the magnetic flux 12 is lower in the thick wall part of the pipe 10 to a large extent and phase difference can be detected and, since the amplitude of a receiving signal also changes in a corroded reduce thickness part 14, the detection of said part 14 can be carried out. Since the distance between the coils T, R is variable, by setting the distance L to 2.5 times or more the diameter D of pipe to select the same so that the coil R is present in an indirect magnetic field, a detection signal reduced in noise is obtained and the reduced thickness part 14 can be certainly detected with high sensitivity. Since the distance L can be made as short as possible, an apparatus 35 is miniaturized and, even when the pipe 10 has a bent part, the apparatus is allowed to run smoothly to carry out work.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a remote eddy current flaw detection method used to inspect metal pipes such as underground steel pipes for corrosion thinning.

The conventional remote eddy current flaw detection method, also known as the remote field eddy current flaw detection method, involves arranging a transmitting coil and a receiving coil in a metal tube with an interval in the tube axis direction. In order to prevent the receiving coil from being directly influenced by the magnetic field from the transmitting coil, it is necessary to place it at a distance of 2 to 4 times the normal tube diameter (i.e. the outer diameter of the tube). When alternating current is applied to the transmitting coil, the magnetic flux from the transmitting coil passes through the tube, passes through the external space, travels along the outer surface of the tube, penetrates the tube again, and is received by the receiving coil.The electromagnetic waves of this magnetic flux are transmitted through the tube. The speed when passing through the thick part is much smaller than the speed when passing through the air, which is the external space, and therefore the propagation time, that is, the difference between the transmitted signal of the transmitting coil and the received signal of the receiving coil The phase difference changes in proportion to the wall thickness of the tube, and from this,
Detects corrosion thinning of pipes based on phase difference.

Problems to be Solved by the Invention In such a remote eddy current flaw detection method, the type and diameter of the tube to be inspected have a great influence on the eddy current generation, which is the basis of the isolated eddy current phenomenon, and on the magnetic field distribution inside and outside the tube. Therefore, the optimum flaw detection conditions (i.e., (a) transmitting coil frequency, (b) excitation power, and (
c) Determining the axial direction of the transmitting coil and the receiving coil (i.e., the distance in the tube axis direction) requires a huge amount of hands-on experiment work. Furthermore, even if these flaw detection conditions are determined in advance in a laboratory or the like, the optimum flaw detection conditions may change due to variations in the material of the pipe to be actually inspected in the field, and the inner walls of the pipe caused by corrosion may change. It is not possible to obtain a clear detection signal with little noise.

An object of the present invention is to provide a remote eddy current flaw detection method that allows remote eddy current flaw detection to be performed under optimal flaw detection conditions at a site where a pipe to be inspected is installed. In addition, in this type of remote eddy current flaw detection method, as mentioned above, the distance between the transmitting coil and the receiving coil must be 2 to 4 times the pipe diameter, which increases the size of the inspection equipment. There's a problem. As the configuration of the inspection device increases in this way, it becomes difficult to pass the pipe through bends,
Or it becomes difficult to insert it into the tube.

Another object of the present invention is to provide a remote eddy current flaw detection method in which the configuration of the inspection device can be made as compact as possible.

Means for Solving the Problems The present invention provides a remote eddy current flaw detection method in which a transmitter coil and a receiver coil are placed in a pipe with a gap in the tube axis direction for flaw detection. Prepare a flaw detection device with a variable distance, excite the transmitting coil with a predetermined frequency and predetermined excitation power, and make the distance variable so that the boundary between the area of direct magnetic field influence and the area dominated by indirect magnetic field. Based on the measurement results, the distance between the transmitting coil and the receiving coil is set so that the receiving coil is in the area dominated by the indirect magnetic field, and the transmitting coil and the receiving coil are integrally connected. This is a remote eddy current flaw detection method characterized by running the pipe along the axis of the pipe.

Further, the present invention provides a remote eddy current flaw detection method in which a transmitting coil and a receiving coil are placed in a pipe at intervals in the tube axis direction, and the distance between the transmitting coil and the receiving coil in the tube axis direction is variable. A flaw detection device is prepared, the transmitting coil is excited by selecting each of a plurality of combinations of frequency and excitation power, and the distance is varied for each combination to differentiate the region affected by the direct magnetic field and the region dominated by the indirect magnetic field. The distance serving as the boundary is measured, and based on the measurement result, the combination of frequency and excitation power is determined so that the receiving coil, which is placed at a predetermined distance from the transmitting coil, is in the area dominated by the indirect magnetic field. This is a remote eddy current flaw detection method characterized by the selection of

According to the present invention, a flaw detection device is prepared in which the distance between the transmitting coil and the receiving coil in the tube axis direction, that is, the distance in each axis direction is variable, and the transmitting coil is normally set at a frequency assumed empirically. and excitation power, and in this state, the distance is made variable to measure the distance that is the boundary between the area affected by the direct magnetic field and the area controlled by the indirect magnetic field, and based on the measurement result, the area controlled by the indirect magnetic field is determined. The distance between the transmitting coil and the receiving coil is set so that the receiving coil exists at A plurality of types are changed, and the distance is made variable for each combination, and the distance serving as the boundary between the region affected by the direct magnetic field and the region dominated by the indirect magnetic field is measured. In this way, a reception signal with less noise is obtained from the receiving coil. Remote eddy current testing is performed using the frequency, excitation power, and distance. In this way, remote eddy current testing is performed for the combination of frequency and excitation power. Set the optimum distance.

Further, according to the present invention, the transmitting coil is excited by selecting each of a plurality of combinations of frequency and excitation power, and the distance is varied for each combination to differentiate the area of influence of the direct magnetic field and the area of control of the indirect magnetic field. The distance serving as the boundary is measured, and based on the measurement result, a combination of frequency and excitation power is selected so that the receiving coil, which is placed a predetermined distance from the transmitting coil, is in the area dominated by the indirect magnetic field. In this way, the distance between the transmitting coil and the receiving coil in the tube axis direction is set in advance, and the combination of frequency and excitation power to perform optimal remote eddy current flaw detection is selected. Since the arrangement of the receiving coil can be selected, it is possible to place the transmitting coil and the receiving coil close to each other.

Embodiment FIG. 1 is a sectional view for explaining the principle of an embodiment of the present invention. The present invention is implemented to inspect the state of corrosion and thinning of a metal pipe 10 such as a steel pipe buried underground. By moving the transmitting coil T and the receiving coil R integrally in the direction of the arrow 11, corrosion thinning of the tube 10 can be inspected.

In the flaw detection device 35, the magnetic flux 12 from the transmitting coil T passes through the tube 10, passes through the external space 13 in the air, is transmitted along the outer surface of the tube, and passes through the tube 10, which is the object to be detected, again to the receiving coil. Reach R. The speed of the electromagnetic wave, which is this magnetic flux 12, is much smaller in the thick walled part of the tube 10 than in the external space 13, so the propagation time, that is, the transmitted signal from the transmitting coil T and the received signal from the receiving coil R, The phase difference can be detected in accordance with the wall thickness of the pipe 10, and thus the corroded thinned portion 14 can be detected. Due to the thinned portion 14, the received signal amplitude of the receiving coil R also changes.

FIG. 2 is a graph showing the relationship between the phase difference between the transmitted signal of the transmitting coil T and the received signal of the receiving coil R and the wall thickness of the tube 10. From this, it is understood that the phase difference and the wall thickness of the tube 10 are proportional.

FIG. 3 is an electrical circuit diagram showing the overall configuration of an embodiment of the present invention. The transmitting coil T and the receiving coil R are connected by a connecting rod 15 made of a non-magnetic material, and the traction wire 1
20-80 Hz from a zero frequency generator 18 which is driven by a traction drive 17 via a
The oscillation output, preferably 30 to 40 Hz, is amplified by a power amplifier 19 and excited by a transmitting coil T.

The reception signal from the reception coil R is amplified by the signal amplifier 20 and output to the line 22 via the filter 21. The zero phase difference detection circuit 23 outputs the signal from the transmission coil T to the transmission signal via the signal amplifier 20 and the filter 21. The recorder 24 detects the phase difference with the received signal from the receiving coil R.
to record on the recording paper. The amplitude of the received signal from filter 21 is detected by amplitude detection circuit 25, and this amplitude is also recorded on recording paper by recorder 24.

FIG. 4 is a graph showing the experimental results of the inventor of the present invention, in which the phase difference between the transmitted and received signals inside the tube was measured by changing the distance in the tube axis direction between the transmitting coil and the receiving coil for each of the five flaw detection conditions. The pipe 10 is an SGP steel pipe, and its outer diameter is 318.
mmφ, the tube wall thickness is 6.9 mm, and the oscillation frequency of the frequency generator 18 is 40 Hz. The transmitting coil T has a wire diameter of 1 Ommφ and the number of turns of 750 turns, and the receiving coil R has a wire diameter of 0.1 mmφ and the number of turns of 3000 turns. For line p1, the excitation power of the transmitting coil T is 1.2W.
(=0.2AX6V), and line p2 is 0.002
W (=0.0IAx0.2V). For reference, line p3 shows each case when the excitation power is 30W (=IAX
30V). From these graphs, it can be seen that the area directly influenced by the magnetic field is 1.5D or less for line p1, 0.6D or less for line p2, and 2.5D or less for line p3.

In lines p1 to p3, the distance between reference marks S1 to S3 is
This value is the boundary between the area of influence of the direct magnetic field 12a of the transmitting coil T and the area of influence of the indirect magnetic field that is farther away, and is a so-called transition point.

In remote eddy current testing, a distance larger than the transition points 51 to S3 is set, and in order to shorten this distance as much as possible, a distance slightly larger than the transition points 51 to S3 is actually set. is preferably defined as an excitation power of 0.0
When selecting 02W and 1,2W, the distance is 0.0.
6D or more, ], 5D or more can be selected, and when the excitation power is 30W, the distance is 2.
It is possible to select 5D or higher.

FIG. 5 shows the experimental results of the present inventor in which the amplitude of the received signal inside the tube was measured by changing the distance in the tube axis direction between the transmitting coil and the receiving coil for each flaw detection condition, line 9.
1 to q3 are the lines p1 mentioned in relation to FIG. 4 above.
- It corresponds to the conditions of p3. In FIG. 5, the distance positions r1 to r3, which are the boundaries between the direct magnetic field influence area and the indirect magnetic field domination area, are so-called transition points.

From such experimental results, according to the present invention, the excitation power of the transmitting coil T is selected to be 0.002W and 1.2W, and the distance is selected to be 0.6D or more and 1.5D or more, respectively, to reduce the thickness of the thinned part. 1
It is understood that it is possible to detect the thinning condition of 4. Furthermore, when the excitation power is selected to be 30 W, it can be seen that remote eddy current flaw detection can be performed by setting the distance to approximately 2.5 D or more corresponding to the transition point r3.

According to the present invention, in the flaw detection device 35, the transmitting coil T
The distance between the receiving coil R and the receiving coil R in the tube axis direction is variable. Such a flaw detection device 35 is first prepared. Next, the transmitting coil T is excited at, for example, 40 Hz and 30 W to obtain the line p3 in FIG. 4 while changing the distance. From this, it can be seen that the distance of the transition point S3 is approximately 2.5D. Based on this measurement result, the distance between the transmitting coil T and receiving coil R is selected to be approximately 2.5D or more, and the distance between the transmitting coil T and receiving coil R is selected to be approximately 2.5D or more.
exists in the region dominated by the indirect magnetic field, and in this state, the transmitting coil T and receiving coil R are run along the tube axis direction. In this way, a detection signal with less noise is obtained from the receiving coil R, and the corroded and thinned portion 14 can be reliably detected by the highly sensitive and clear detection signal.

In the laboratory, the optimal flaw detection conditions for the tube 10 to be inspected installed at the site and similar tubes, that is, the frequency,
By roughly assuming the excitation power and distance, and slightly changing the flaw detection conditions in the laboratory, it is easy to find the optimum flaw detection conditions for the pipes to be actually inspected at the site. It becomes possible to find it. Furthermore, in the present invention, it is possible to find the transition points 51 to s3 and thereby arrange the receiving coil in the region dominated by the indirect magnetic field of the transmitting coil, so that the distance can be made as short as possible. Therefore, the present flaw detection device 35 can be downsized, and even if the tube 10 has a bent portion, it can run smoothly and perform flaw detection work.

Further, as another embodiment of the present invention, for the pipe 10 to be inspected, the distance between the transmitting coil T and the receiving coil R is 111! Let L be, for example, a distance 15 times the pipe diameter (i.e. L = 1.5D)
), the flaw detection bag has a variable distance between the transmitting coil T and receiving coil R in the tube axis direction for on-site experiments! 35, then select and fix, for example, 40 Hz as the normally assumed excitation frequency, vary the excitation power, and as shown in No. 4 [2I, transition point S
Find the excitation power such that 1 to s3 is a distance 15 times the pipe diameter. The value of such excitation power is roughly determined in advance in the laboratory, and adjusted by actual measurements on site. It is 1.2W. In this way, the receiving coil R is placed a predetermined distance L (=i5D) from the transmitting coil T, and the frequency and excitation are adjusted as described above so that the receiving coil is in the area dominated by the indirect magnetic field. A combination with electric power is selected. As a result, a highly sensitive and clear detection signal corresponding to the thinning M14 can be obtained from the receiving coil R.

Note that it is also possible to perform remote eddy current flaw detection using the amplitude of the receiving coil R in the same manner.

A specific configuration of the flaw detection device 35 in which the distance between the transmitting coil T and the receiving coil R is variable is shown in FIG.

FIG. 7 is a simplified exploded perspective view of the flaw detection device 35.

8 is a front view of the support means 37, FIG. 9 is a sectional view of the support means 37, and FIG. 10 is a sectional view of another support means 48. The transmitting coil T is a support means 37
The tube 10 is wound around the tube 10 so that it can be moved in the axial direction of the tube 10. This support means 37 is a plate-shaped body 3 around which the transmitting coil T is wound.
8, a pair of end plates 40, 41 fixed to both ends in the axial direction by bolts 39, support members 42, 43 that support these end plates 41 on the left and right at the bottom thereof, and these support members 42. Wheels attached to 43 44.45
and has. The connecting rod 15 is inserted through the plate-like body 38 and the end plates 40° 41, and the fixing piece 46 is fixed using bolts 76° 77.
Fixed by

Another support means 48 is also fixed to the connecting rod 15 .

11 is a perspective view of the receiving coil R, FIG. 12 is a plan view of the receiving coil R viewed from the outside in the radial direction of the tube 10, and FIG. 13 is a side view of the receiving coil R. and
FIG. 14 is a front view of the receiving coil R. A pair of end plates 5
Mounting members 53 are installed at intervals in the circumferential direction between the end plates 51 and 52, and a plate-like member 5 is installed between the end plates 51 and 52.
4 is placed. The mounting members 53 are arranged at equal intervals in the circumferential direction of the end plates 51 and 52. The end plates 51, 52, plate-shaped body 54, etc. have the same structure as the support means 37 for the transmitting coil T described above, and are attached to a support body 56 on which wheels 55 are provided.

The receiving coil R is wound around the core 57. A boss 64 is fixed to the end plate 51 by a bolt 75, and a tightening bolt 66 extends in the radial direction from the boss 64. By tightening the support rod 15 using this bolt 66, the support means 48 can be fixed to the support rod 15. Also, by loosening this bolt 66, the support means 48 is displaced and adjusted in the axial direction of the connecting rod 15,
The distance between the transmitting coil T and the receiving coil R can be made variable.

Effects of the Invention As described above, according to the present invention, the optimum flaw detection conditions, that is, the transmitting coil is set so that a received signal with low noise can be obtained from the receiving coil for the pipe to be inspected both in the laboratory and in the field. The frequency and excitation power as well as the distance in the tube axis direction between the transmitting coil and the receiving coil can be easily determined. Therefore, remote eddy current flaw detection can be easily performed both in the laboratory and in the field.

Furthermore, since the distance between the transmitting coil and the receiving coil can be made as short as possible, the configuration of the inspection device can be made smaller, and it becomes easier to pass through bends in the tube and insert it into the tube.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view for explaining the principle of an embodiment of the present invention, FIG. 2 is a graph showing the relationship between the phase difference of transmitted and received signals and the wall thickness of the tube 10, and FIG. A diagram showing the electrical configuration of the embodiment, FIG. 4 is a graph showing the relationship between the phase difference of the transmitting and receiving signals inside the tube and distance, and FIG. 5 is the experimental result of the inventor. A graph showing the relationship between the amplitude of the received signal inside the pipe and the distance, FIG. 6 is a sectional view showing a flaw detection device 35 according to an embodiment of the present invention, and FIG. 7 is a simplified exploded perspective view of the flaw detection device 35. 8 shows the support means 37.
FIG. 9 is a sectional view of the supporting means 37, FIG.
11 is a perspective view of the receiving coil R, FIG. 12 is a plan view of the receiving coil R,
FIG. 13 is a side view of the receiving coil R, and FIG. 14 is a front view of the receiving coil R. 10...Pipe, 11 Running direction, 12...Magnetic flux, 18
Frequency generator, 19... Power amplifier, 20... Signal amplifier, 21... Filter, 23... Phase difference detection circuit, 24... Recorder, 25... Amplitude detection circuit, T...
・Transmission coil, R...reception coil

Claims (2)

[Claims] (1) A flaw detection device in which the distance between the transmitting coil and the receiving coil in the tube axis direction is variable in the remote eddy current flaw detection method in which flaw detection is performed by arranging a transmitting coil and a receiving coil in the tube with an interval in the tube axis direction. prepare a transmitter coil, excite the transmitting coil with a predetermined frequency and predetermined excitation power, make the distance variable, and measure the distance that is the boundary between the area affected by the direct magnetic field and the area dominated by the indirect magnetic field; Based on the measurement results, the distance between the transmitting coil and the receiving coil is set so that the receiving coil is located in the area dominated by the indirect magnetic field, and the transmitting coil and the receiving coil are made to travel together along the tube axis. A remote eddy current flaw detection method characterized by: (2) A flaw detection device in which the distance between the transmitting coil and the receiving coil in the tube axis direction is variable in the remote eddy current flaw detection method in which a transmitting coil and a receiving coil are placed in a pipe with a space between them in the tube axis direction. , the transmitter coil is excited by selecting each of a plurality of combinations of frequency and excitation power, and the distance is varied for each combination, so that the boundary between the area of influence of the direct magnetic field and the area of influence of the indirect magnetic field is and, based on the measurement result, select the combination of frequency and excitation power such that the receiving coil, which is disposed a predetermined distance from the transmitting coil, is in a region dominated by the indirect magnetic field. A remote eddy current flaw detection method characterized by: