JP2000249685A - Method for investigating damage position of anticorrosion coat of buried metallic pipe and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus by a magnetic field analysis which can investigate a damage position of anticorrosion coats of buried metallic pipes in a simple constitution with a high sensitivity. SOLUTION: An a.c. signal current is sent from a transmitter 5 to a buried metallic pipe 1, thereby generating a magnetic field by a current i1 flowing in and out to an anticorrosion coat damage position 2 of the buried metallic pipe 1. At the same time, two coils inclined by 45 deg. to a ground surface within a vertical plane along an axis of the metallic pipe and having respective axes set orthogonal to each other are scanned along the pipe axis, thereby continuously detecting an intensity of the magnetic field. A sum of detection signals is synthesized and the damage position is investigated from a maximum value of an amplitude output of the synthesized signal.

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 detecting changes in the magnetic field intensity generated by the above method using two coils on the ground surface in a non-contact manner, and to an apparatus for detecting the position of a 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 a city gas, is subjected to a double anticorrosion treatment of coating with polyethylene or the like and electrolytic protection in order to prevent soil corrosion and electrolytic 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 concentrates on this damaged part and the anticorrosion current does 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, Japanese Patent Application Laid-Open Nos.
There are two types of alternating current methods, such as those disclosed in JP-A-191049, which are generally superior to the magnetic field method in detecting micro-damage of the coating and coating method. Since it is a method of detecting the 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. 6, coils Lx, which are installed on a tube axis (x-axis), a right angle (y-axis), and a vertical (z-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 the contact position with another metal tube because the change point of the magnetic field 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, the leakage current i ₁ is therefore generally very small, there is a problem that analysis of it and the data identification is often inaccurate for the coating-covering damage position 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. Since components in both directions of the axis and the z-axis can be provided, the number of magnetic field detecting coils can be reduced by half compared to the related art. In principle, a single magnetic field detection coil can be used in time, but the signal detected by one coil inclined at approximately 45 ° to the ground surface and the signal detected by another coil orthogonal to this coil (Hereinafter, referred to as “sum dynamic synthesis”), a maximum value is obtained at the center of the paint-covering defect, and a large output signal is obtained. Therefore, the synchronization signal is transmitted as in the synchronous detection method. The magnetic field intensity can be detected with high sensitivity without providing a circuit for performing the operation. Furthermore, if the output detected by the above method and summed dynamically is square-detected, twice the sensitivity to the change in magnetic field angle can be obtained as compared with the conventional method. The present invention has been accomplished based on such findings.

Therefore, the method for detecting the position of corrosion protection coating on a buried metal pipe according to the present invention provides an anticorrosion coating on the outer surface and allows an AC signal current to flow through 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 current flowing in and out, and two coils arranged at 45 ° to the ground surface in a vertical plane along the tube axis of the metal tube and arranged so that their axes are orthogonal to each other are connected to the tube shaft. , The output of the two coils is synthesized dynamically, and the change in the magnetic field intensity is measured from the synthesized output signal to prevent corrosion of the buried metal tube. The point is to search for the location of the coating damage.

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; 45 ° inclined to the ground surface and perpendicular to each other in a vertical plane along the pipe axis for detecting the magnetic field strength generated by the current flowing from the vessel to the corrosion protection coating damaged position of the buried metal pipe Two coils, a circuit that removes noise from an output signal detected by scanning the coil along the tube axis of the metal tube, and sum-dynamically combine the two output signals from which noise has been removed. The gist comprises a circuit and a display device for displaying the position of the corrosion protection coating on the buried metal tube from the combined 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,
Is 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 tube 1. The other pole of the transmitter is connected to a counter electrode 6 (for example, a magnesium anode for cathodic protection) buried underground. 3a and 3b
Is a coil for detecting the intensity of the magnetic field on the ground surface due to the current flowing from the damaged portion of the coating when the AC signal current is passed through the buried metal tube 1. When starting to wind the coil, coil 3
a and 3b are respectively φ ₁ = 45 ° with respect to the ground surface,
It is inclined at an angle of φ ₂ = 45 °, has a substantially V-shape or C-shape (φ ₁ = 135 °, φ ₂ = 135 °), and both coils are arranged so as to be orthogonal to each other on an extension of the coil axis. Have been. In addition, it is preferable that both coils are arranged in a C-shape in which the direction of the coil matches the magnetic field on the ground surface due to the current flowing in and out of the damaged portion of the coating, and the output signal is likely to increase.

In the method of the present invention, an AC signal current is passed from the transmitter 5 to the metal tube 1 and the coils 3a and 3b are connected to the metal tube 1.
Is scanned in the direction of the arrow. The on undamaged metal pipe 1 coated-covering 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 generated magnetic field changes in the direction along the tube axis, and the rate of change increases, directly above the damaged part. It becomes local maximum. Accordingly, the magnetic field strength is continuously detected by scanning the coils 3a and 3b, the sum of the respective detection signals is synthesized, and the square value of the output dynamically synthesized is obtained. The scan position of the coil is the damage position of the anticorrosion coating of the buried metal tube 1.

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 FIG. 3, reference numerals 7a and 7b denote lead wires of the coils 3a and 3b, 8a and 8b denote amplifiers, 9a and 9b denote bandpass filters, and 10 denotes a summing circuit. FIG.
Reference numeral 11 denotes an amplitude detection circuit, an example of which is shown in FIG. 1
Reference numeral 2 denotes a display such as a recorder for displaying an output signal of the amplitude detection circuit 11. 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
The coils 3a and 3b having an inclination of φ ₂ = 45 ° or φ ₁ = φ ₂ = 135 ° are scanned in a magnetic field H having an inclination θ generated by flowing an AC signal current through a buried metal tube. The induced signals (detection signals) are supplied to amplifiers 8a and 8b.
After the noise is amplified by the bandpass filters 9a and 9b, the sum of the respective signals Va and Vb is synthesized by the summing circuit 10 after the noise other than the signal frequency is removed by the bandpass filters 9a and 9b.
The amplitude output η of the signal detected in step 1 is obtained.
Will be displayed.

[0016] Although here is H ≒ 0 becomes eta ≒ 0 since Since on buried metal pipes for damage of the coating-covering only the tube current I ₀ no magnetic field component in the xz plane, the coating-covering damaged portion magnetic field is xz plane with a current density of leakage current i ₁ to flow and from the damaged portion of the coating-covering increases with decreasing distance H
Is generated with an inclination θ, η gradually increases, and immediately above the damaged portion, the z component of the magnetic field disappears, so that θ
When it becomes ≒ 0, η shows the maximum value. Therefore, the position on the tube axis (x-axis) where η is maximum on the screen of the display device 12 indicates the damaged portion, and the damaged portion of the coating of the buried metal tube can be easily searched. Can be.

[0017]

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 simulated painted-coated damaged portion is made, and an AC signal current of 625 Hz having a signal frequency less affected by a high frequency of a commercial power supply is passed through the steel pipe, and the coils 3a and 3b of the apparatus shown in FIG. Then, scanning was performed along the buried pipe 1 to search for a damaged portion. FIG. 5 shows the results of the search. The waveform of the amplitude output as a result of the experiment showed a maximum value at about 23 m, which was in good agreement with the simulated paint coating damage position.

[0018]

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 showing a schematic configuration of an exploration device of the apparatus for locating an anticorrosion coating of a buried metal pipe according to the present invention.

FIG. 3 is a diagram showing an example of a circuit configuration of a chording circuit of the searcher of the apparatus for locating a corrosion prevention coating on a buried metal pipe according to the present invention.

FIG. 4 is a diagram showing an example of a circuit configuration of an amplitude detection circuit of the searcher of the apparatus for locating the position of the anticorrosion coating of a buried metal pipe according to the present invention.

FIG. 5 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. 6 is a schematic view showing an example of a conventional method for searching for a position where a corrosion protection coating is damaged on a buried metal pipe.

FIG. 7 is a schematic view showing an example of a conventional method for searching for a position where a corrosion protection coating is damaged on a buried metal pipe.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 buried metal tube 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 summation circuit 11 amplitude detection circuit 12 display 13 operational amplifier 14 diode 15 amplifier I ₀ tube current i ₁ leakage current H field Va, the inclination phi ₁ of Vb signal η amplitude output θ magnetic _field, phi ₂ coils inclination Lx, Ly, Lz coil

Claims (3)

[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 arranged so that their axes are orthogonal to each other, along the tube axis. A buried metal pipe characterized by detecting the position of the corrosion protection coating on the buried metal pipe by detecting and synthesizing the outputs of the two coils dynamically and measuring a change in magnetic field strength from the combined output signal. Method for locating the damage of the anticorrosion coating. 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 flowing from the transmitter to a damaged position of the anticorrosion coating of the buried metal tube. Two coils arranged at a 45 ° angle to the ground surface and perpendicular to each other in a vertical plane along the tube axis for detecting the intensity of the magnetic field generated by the electric current generated by the metal tube; A circuit for removing noise from an output signal detected by scanning along an axis, a circuit for dynamically combining two output signals from which noise has been removed, and an anticorrosion coating for a buried metal tube from the combined output signal And a display device for displaying a damage position. 3. The display device according to claim 2, wherein the display device comprises a circuit for detecting a sum-dynamically synthesized output signal, and a device for displaying an amplitude output of the detected signal.
Anticorrosion coating damage location exploration apparatus according to 4.