JP3167654B2 - Method and apparatus for locating corrosion protection coating on buried metal pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a method and an apparatus for searching for a position of a corrosion protection coating on a buried metal pipe by a magnetic field method, and more particularly to a coating damage of a coated metal pipe buried underground. An AC signal current flows between the metal tube and a counter electrode buried in the ground at a position and a contact position between the metal tube and another metal structure, and a current flowing into and out of the corrosion prevention coating damaged position of the metal tube. The present invention relates to a method for contactlessly detecting a change in magnetic field intensity generated by the above-mentioned method using at least two coils on the ground surface, and an apparatus for detecting the position of corrosion prevention coating on a buried metal pipe.

[0002]

2. Description of the Related Art In general, the outer surface of a metal pipe laid underground, for example, a steel pipe buried in city gas, is subjected to a double anticorrosion treatment of a coating of polyethylene or the like and a cathodic protection in order to prevent soil corrosion and electric corrosion. Have been. However, if the coating is damaged by the construction of a third party after laying these coated metal pipes, the anticorrosion current from the cathodic protection device will concentrate on this damaged part and the anticorrosion current will not spread to the surrounding area. The anticorrosion function decreases.
In particular, when this coating-covered damaged part comes into contact with another metal buried object, the anticorrosion current flows into the metal buried object that is not originally the target of anticorrosion. There is a risk of soil corrosion and electric corrosion of the pipe.

[0003] In the case where the coating material is damaged and the effect of the cathodic protection is concerned as described above, it is necessary to search for the damaged position by some method, and to take measures such as excavation and repair. Conventionally, as a method of searching for the location of this coating covering damage,
There are known a potential method in which the position is determined from a change in the potential of the ground surface through a current flowing through the damaged portion, and a magnetic field method in which the position is determined from a change in the strength of the magnetic field at the damaged portion.

[0004]

SUMMARY OF THE INVENTION The potential method includes a DC method,
For example, JP-A-61-210935 and JP-A-6-210935
There are two types of alternating current methods as disclosed in Japanese Patent Application Laid-Open No. 3-194949, which are generally superior to the magnetic field method in detecting micro-damage of a coating and covering method. Since it is a method of detecting a potential distribution, there is a drawback that workability is poor such that it is necessary to increase the sensitivity by spraying water on a pavement road surface having a high electric resistance such as asphalt.

On the other hand, as a magnetic field method, for example,
Japanese Patent No. 300991 discloses, as shown in FIG. 4, coils Lx, which are installed on a tube axis (x-axis), a right angle (y-axis), and a vertical (x-axis) at positions shifted laterally from immediately above a buried metal tube. Ly,
A method is disclosed in which Lz is moved in the direction of the tube axis to detect a change in the magnetic field intensity generated from the metal tube 1, and a portion where the magnetic field intensity changes suddenly is detected as a contact position with another metal tube 4. The method of detecting a change in magnetic field strength from a change in I ₀ requires three coils, which complicates the circuit, and in general, it is difficult to identify a contact position with another metal tube because the magnetic field change point is not clear. There is a problem.

For example, in Japanese Patent Application Laid-Open No. 63-78063, a transmitter 5 is connected to a metal tube 1 and a counter electrode as shown in FIG. xz coil axis magnetic field intensity generated by the leakage current i ₁ to enter
A method is disclosed in which a coil 3 installed in parallel with a plane is detected by moving along a tube axis, and a local maximum value of the detection signal is detected as a paint-coating damage position. However, since the leakage current i ₁ is generally very small, there are problems that the specification of the damaged position of the coating material is often inaccurate and that the analysis of the data is not easy.

In the conventional magnetic field method, since the strength of the magnetic field is detected by reading the amplitude of the received signal of the coil, the detection sensitivity is low.
There is also a synchronous detection method that makes a part of a transmission signal highly sensitive as a reference signal as disclosed in Japanese Patent No. 10935,
There is a drawback in that a circuit for transmitting a synchronization signal is required, which complicates the circuit configuration.

The present invention solves the problems of the above-described conventional method for detecting the position of the corrosion protection coating on a buried metal pipe, and makes it possible to simply determine the position of the corrosion protection coating on the buried metal pipe and the contact position with another buried metal body. The purpose of the present invention is to easily and accurately search.

[0009]

As a result of the present inventor's research to achieve the above-mentioned object, the conventional measurement of the magnetic field in the xz plane has x
A total of two coils are used, one in each of the two directions, the axial direction and the z-axis direction.
By inclining, one magnetic field detecting coil can have components in both the x-axis and the z-axis, so that the number of magnetic field detecting coils can be reduced by half compared to the conventional one. In principle, Although one magnetic field detection coil can be used in time, a signal detected by one coil inclined at approximately 45 ° with respect to the ground surface is correlated with a signal detected by another coil orthogonal to this coil. As a result, the magnetic field intensity can be detected with high sensitivity without providing a circuit for transmitting the synchronization signal as in the synchronous detection method. Furthermore, if the sum of squares of the correlation output detected by the above method is used, the angle of the magnetic field can be obtained. The present invention has been accomplished based on the finding that the sensitivity to the change is twice as high as that of the conventional method.

Therefore, the method for detecting the position of the corrosion protection coating on a buried metal pipe according to the present invention provides an anticorrosion coating on the outer surface of the buried metal pipe, and applies an AC signal current to the metal pipe buried in the ground so that the corrosion protection coating damage position of the buried metal pipe is detected. A magnetic field is generated by the inflowing and outgoing electric current, and the magnetic field is set to 45
Another coil is provided which is tilted and arranged so that its axes are orthogonal to each other, and the two coils are scanned along the tube axis to continuously detect the magnetic field strength, respectively.
The gist of the present invention is to find the position of corrosion protection coating damage on a buried metal pipe by calculating the correlation between the outputs of the two coils and measuring the change in magnetic field strength from the correlation output signal.

Further, the present invention provides an apparatus for locating the position of an anticorrosion coating on a buried metal tube, comprising: a transmitter for passing an AC signal current between a metal tube having an anticorrosion coating on the outer surface buried underground and a counter electrode; 2 is disposed perpendicular to the ground surface and perpendicular to each other in a vertical plane along the pipe axis for detecting the magnetic field intensity generated by the current flowing into and out of the corrosion protection coating damaged position of the buried metal pipe from the vessel. Two coils, a circuit for removing noise from an output signal detected by scanning the coil along the tube axis of the metal tube, and a circuit for phase shifting the output signal from which noise has been removed. The gist of the present invention includes a circuit for performing a correlation operation on a signal and a display device for displaying a damaged position of the anticorrosion coating of the buried metal tube from the correlation output signal.

[0012]

Next, embodiments of the present invention will be specifically described with reference to the drawings. FIG. 1 is a schematic view showing a method for searching for a position where a corrosion protection coating is damaged on a buried metal pipe according to the present invention.
In FIG. 1, reference numeral 1 denotes a buried metal pipe which is an object of cathodic protection,
Reference numeral 4 denotes another metal structure (for example, a water pipe) in contact with the buried metal pipe, and the buried metal pipe 1 and the metal structure 4 are in an electrically connected state at the damaged portion 2 of the coating. Reference numeral 5 denotes a transmitter for passing an AC signal current through the buried metal pipe 1, and the other pole of the transmitter is connected to a counter electrode 6 buried underground (for example, a magnesium anode for cathodic protection). Reference numerals 3a and 3b denote coils for detecting the magnetic field strength on the ground surface due to the current flowing in and out of the damaged portion of the coating when the AC signal current flows through the buried metal tube 1. Coil 3a and 3b Are inclined at angles of φ ₁ = 45 ° and φ ₂ = 45 °, respectively, and are substantially V-shaped or C-shaped (φ ₁ = 135 °).
°, φ ₂ = 135 °), and both coils are arranged so as to be orthogonal to each other on the extension of the coil axis.

In the method of the present invention, the metal tube 1 is transmitted from the transmitter 5.
And the coils 3a and 3b scan the vicinity of the conduit of the metal tube 1 in the direction of the arrow. On the metal pipe 1 is not damaged the coating-covering is the magnetic field generated from only tube current I ₀ is substantially constant in the direction along the tube axis. As the density of the current flowing into and out of the damaged part of the coating increases as it approaches the damaged part, the intensity of the generated magnetic field changes in the direction along the tube axis, and the rate of change increases to just above the damaged part. It becomes local maximum. Accordingly, when the magnetic field strength is continuously detected by scanning the coils 3a and 3b, the correlation between the respective detection signals is obtained, and the value of the sum of squares of each correlation output signal is obtained.
The scanning position of the coil having the maximum value is the buried metal tube 1
This is the position of the anticorrosion coating damage.

FIG. 2 shows a schematic configuration of an apparatus for locating a corrosion prevention coating on a buried metal pipe for performing the above method. In the figure, 7a and 7b are lead wires of the coils 3a and 3b, 8a and 8b are amplifiers, 9a and 9b are band-pass filters, 10a and 10b are phase shift circuits, and 11a and 11b are multiplication circuits for obtaining correlation. , 12a and 12b are low-pass filters. 13a and 13b are correlation outputs E ₁ ,
A squaring circuit for squaring E ₂ , an addition circuit 14 for obtaining the sum of squares, and a display 15 such as a recorder for displaying an output signal of the addition circuit 14. In addition, 12a, 12b
13a and 13b can provide the same effect as the squaring circuit even with an envelope detector or a full-wave rectifier circuit. In FIG. 2, illustration of a buried metal tube, a transmitter, and the like is omitted.

In this device, φ ₁ along the xz plane
= Φ ₂ = coil 3a having a slope of π / 4 (45 °)
And 3b are scanned in a magnetic field H having an inclination θ generated by flowing an AC signal current through the buried metal tube, and the induction signals (detection signals) obtained are amplified by the amplifiers 8a and 8b and then band-passed. After noises other than the signal frequency are removed by the filters 9a and 9b, the phases are shifted by 45 ° by the phase shifters 10a and 10b, respectively, and the multiplications 11a and 11b are performed.
b, the output of the coils 3a and 3b is phase-shifted by the circuit, and the correlation operation is performed between the signals. The low-pass filters 12a and 12b remove the AC component from the correlation output signal of the multiplication circuit, thereby reducing the DC component. Is extracted. The DC-correlated outputs E1 and E2 obtained in this way are squared by the squaring circuit 13a.
13b, and is added by the adder 14, and the square sum output η
Is obtained and displayed on the display device 15.

[0016] Here, the magnitude of the output of the coils 3a and 3b is proportional to _{e sa = Hcos (θ + π} / 4) e sb = Hcos (θ-π / 4). Therefore, the outputs E ₁ and E ₂ after correlation are E ₁ = A · H ² cos (θ + π / 4) cos (θ−π /
4) cosπ / 4 E ₂ = A · H ² cos (θ + π / 4) cos (θ−π /
4) cos (−π / 4) (A is the gain of the amplifiers 8a and 8b), and the sum of squares output η of E ₁ and E ₂ is η = (E ₁ ) ² + (E ₂ ) ² = k · H ⁴ (cos 2θ) ² ... (1) Here, k is a proportional constant.

On a buried metal tube having no damage to the coating, H 電流 0 and η = 0 since only the tube current I ₀ is obtained.
As approaching the damaged portion of the coating, the current i ₁ flowing into and out of the damaged portion of the coating increases, the current density increases, and the magnetic field H is generated with a gradient θ in the xz plane, so that η gradually increases. Immediately above, since the z component of the magnetic field disappears, θ = 0 °, and η shows a maximum value.

Therefore, the position on the tube axis (x-axis) 1 where the sum of squares output η is maximum on the screen of the display device 15 indicates the damaged portion, and the coating of the buried metal tube is easily performed. Can detect the damaged part.

In the present invention, since the sum of squares of the correlation output of the detection signal by the coil is calculated for the purpose of searching for a damage position, the sum of squares output .eta. Changes by cos 2θ, and the sensitivity to the change of the magnetic field angle θ is twice that of the conventional method, so that the change of the magnetic field intensity appears clearly, so that the search is easy. The low-pass filters 12a and 12b can be used without taking the sum of squares.
Even if the outputs E ₁ and E ₂ are directly displayed on a two-channel display or a recorder, the damaged portion of the coating of the buried metal tube can be detected.

[0020]

EXAMPLE 100 cm ² (10 cm × 10 cm) at 23 m from the feeding point of a 100 mm diameter polyethylene-lined steel pipe buried horizontally at a depth of 1.5 m from the ground surface
A 725 Hz AC signal current having a signal frequency less affected by hum is passed through the steel pipe, and the coils 3a and 3b of the apparatus shown in FIG. 2 are connected to the buried pipe as shown in FIG. The damaged area was searched by scanning along. FIG. 3 shows the results of the search. The waveform of the sum-of-squares output η of the experimental result shows a local maximum value at about 23 m, which is in good agreement with the equation (1).

[0021]

As described above, according to the present invention, the number of coils is smaller than that of the conventional device, and the position of the corrosion protection coating on the buried metal tube can be detected with high sensitivity by using a simple circuit configuration without using the synchronous detection method. The damage can be accurately grasped by conducting a diagnosis of the buried metal pipe periodically or when the reduction of the cathodic protection effect is discovered, so that the location of the excavation when repairing the coating can be determined. It is possible to significantly reduce wasteful costs due to misidentification and to prevent accidents such as gas leaks from steel pipes buried in city gas.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a method of searching for a position of a corrosion protection coating on a buried metal pipe according to the present invention.

FIG. 2 is a block diagram illustrating a schematic configuration of a detector of the corrosion prevention coating damage position detecting device for a buried metal pipe according to the present invention.

FIG. 3 is a graph showing an output waveform when an anticorrosion coating damage position of a buried metal pipe is detected based on a test result of the present invention.

FIG. 4 is a schematic view showing an example of a conventional method for detecting the position of a corrosion protection coating on a buried metal pipe.

FIG. 5 is a schematic view showing an example of a conventional method for detecting the position of a corrosion protection coating on a buried metal pipe.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 buried metal pipe 2 damaged part of coating 3, 3 a, 3 b coil 4 other metal structure 5 transmitter 6 counter electrode 7 a, 7 b lead wire 8 a, 8 b amplifier 9 a, 9 b bandpass filter 10 a, 10 b phase shift circuit 11 a , 11b Multiplication circuit 12a, 12b Low-pass filter 13a, 13b Square circuit 14 Addition circuit 15 Display device I ₀ Tube current i ₁ Leakage current E ₁ , E ₂ Correlation output H Magnetic field η Output θ Magnetic field gradient φ ₁ , φ ₂ Lx, Ly, Lz coil

Claims (3)

(57) [Claims] An anti-corrosion coating is applied to an outer surface of the metal pipe, and an AC signal current is applied to a metal pipe buried in the ground to generate a magnetic field by a current flowing into and out of the buried metal pipe at a position where the anti-corrosion coating is damaged. The magnetic field strength is continuously increased by scanning two coils, which are arranged at 45 ° with respect to the ground surface in a vertical plane along the tube axis and whose axes are orthogonal to each other, along the tube axis. Detecting and correlating the output of the two coils, and measuring the change in the magnetic field intensity from the correlation output signal to detect the position of the corrosion protection coating on the buried metal pipe; How to locate coating damage. 2. A transmitter for passing an AC signal current between a counter electrode and a metal tube buried in the ground and having an outer surface provided with an anticorrosion coating, and flows from the transmitter to a position where the anticorrosion coating of the buried metal tube is damaged. Two coils arranged in a vertical plane along a tube axis for detecting a magnetic field intensity generated by a current flowing at an angle of 45 ° with respect to the ground surface and orthogonal to each other; A circuit that removes noise from the output signal detected by scanning along the axis, a circuit that shifts the phase of the output signal from which noise has been removed, a circuit that performs a correlation operation on the phase-shifted signal, and A display device for displaying the position of the anticorrosion coating of the buried metal pipe. 3. The anticorrosion coating damage detecting device according to claim 2, wherein the display device comprises a circuit for calculating a sum of squares of the correlation output signal, and a device for displaying the sum of squares.