JP2733009B2 - Tube demagnetization method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for demagnetizing a tube for measuring magnetostrictive stress in a non-contact manner on the tube.

[0002]

2. Description of the Related Art Steel pipes used for gas piping systems and the like are:
It is used buried in soil for many years. Due to land subsidence or load from vehicles running on the ground,
A bending stress occurs in the steel pipe. If a state in which a large bending stress is generated continues, the steel pipe may be damaged by stress corrosion cracking or the like. For diagnosis to prevent steel pipe breakage, it is necessary to measure the bending stress acting on the steel pipe.

In general, as a method of measuring a stress, a method of pasting a strain gauge on the surface of an object to be inspected and detecting a strain to convert the stress into a stress, or a method in which a stress is generated in a steel pipe, include:
There is a method using an X-ray apparatus for detecting that the crystal lattice of the steel pipe is distorted and the angle of the X-ray diffracted by the crystal lattice changes.

However, a magnetostrictive stress measurement method is known as a method for easily measuring stress at a site where a steel pipe is laid. Magnetostrictive stress measurement uses the principle that the change in permeability of a steel pipe is proportional to the stress.
That is, the degree of magnetization of the steel pipe is taken out as a voltage value by the detection coil, and the stress is measured because this value is proportional to the stress value.

However, prior to measuring the magnetostrictive stress of the steel pipe, if the steel pipe is already magnetized due to geomagnetism or the like, an error occurs in the measurement result. Demagnetization must be performed.

FIG. 6 shows a configuration for measuring the output of the magnetostrictive sensor 2 in the circumferential direction of the steel pipe 1 using the magnetostrictive stress measuring device. A rail on which a truck on which a magnetostrictive sensor 2, a demagnetizing magnet 4 is mounted, and the like travels is attached to the steel pipe 1. The bogie is run along the rails by driving the running motor, and the magnetostrictive stress is measured in the circumferential direction of the steel pipe 1. The control device controls the entire magnetostrictive stress measurement device, such as processing of data output from the magnetostrictive sensor 2 and running control of the bogie. The display displays measurement data and the like. In addition, the rail is moved in the pipe axis direction, and measurement is performed at a plurality of positions.

As described above, demagnetization must be performed before measuring the magnetostrictive stress of the steel pipe 1. In that case, FIG.
, Measurement points P1 to Pn are provided on the circumference of the steel pipe 1 at every angle θ = 5 °. Next, the magnetostrictive stress measuring device moves the bogie so that the demagnetizing magnets 4 sequentially reach the measurement points P1 to Pn. At each of the measurement points P1 to Pn, a current is applied to the demagnetizing magnet 4 as shown in FIG. 7 to generate an alternating magnetic field that gradually decreases to zero, thereby performing demagnetization. In this way, the measurement points P1 to Pn
At first, the bogie goes around the steel pipe while performing demagnetization processing.

Next, the magnetostrictive stress measuring device moves the bogie so that the magnetostrictive sensor 2 reaches the measurement points P1 to Pn subjected to the demagnetization process in the opposite direction. 2, the magnetostrictive stress is measured.

FIG. 8 is a flowchart showing a magnetostrictive stress measuring method including a conventional method of demagnetizing a pipe in a magnetostrictive stress measuring apparatus. The process is started in step a1. In step a2, the magnetostrictive stress measuring device moves the bogie so that the demagnetizing magnet 4 reaches the measurement point P1 at which demagnetization starts, and supplies a current as shown in FIG. Then, an AC magnetic field corresponding to the current is generated to excite the steel pipe 1 and perform a demagnetization process. Next, at step a3, the carriage is moved in the circumferential direction L by an angle θ = 5 °. Next, in step a4, it is determined whether or not it is a demagnetization start point, and if not, the process returns to step a2.
The demagnetization process described above is performed. Thus, step a2
The process of ~ a4 is repeated, the carriage is moved so that the demagnetizing magnet 4 sequentially reaches the measuring points P1 to Pn, and the demagnetizing process is performed at each of the measuring points P1 to Pn. As described above, the bogie makes a round of the steel pipe 1, and the demagnetizing magnet 4 returns to the demagnetizing start point P1.

Next, the process proceeds to step a5, where the magnetostrictive stress measuring device includes the demagnetizing magnet 4 and the magnetostrictive sensor 2 shown in FIG.
Is moved in the circumferential direction L by the interval t. That is, the magnetostrictive sensor 2 reaches the measurement point P1 at the demagnetization start point, and sets this point as the measurement start point. Next, step a6
, The magnetostrictive stress is measured by the magnetostrictive sensor 2. Next, in step a7, the carriage is moved by an angle θ = 5 ° in the circumferential direction M opposite to that when performing the demagnetization processing. That is, the magnetostrictive sensor 2 is moved to the measurement point Pn.

Next, the process proceeds to step a8, where it is determined whether or not the position of the magnetostrictive sensor 2 is the measurement start point. If not, the process returns to step a6 to perform the above-described magnetostriction stress measurement processing. In this way, the processes of steps a6 to a8 are repeated, the carriage is moved so that the magnetostrictive sensor 2 sequentially reaches the measurement points Pn to P1, and the magnetostriction stress measurement process is performed at each of the measurement points Pn to P1. As described above, the cart goes around the steel pipe 1 and the magnetostrictive sensor 2 is moved to the measurement start point P1.
To reach. Next, the process proceeds to step a9, where the measurement of the magnetostrictive stress is completed, and the measured data is processed.

[0012]

As described above, in the conventional demagnetization processing for the magnetostrictive stress measurement of the steel pipe 1, current flows through the demagnetizing magnet 4 at each of the measurement points P1 to Pn.
The current is gradually reduced to zero to complete. This demagnetization process needs to be performed at a total of 72 points at a pitch of θ = 5 ° along the circumference of the tube. Therefore, the time required for the demagnetization process to sequentially perform the demagnetization process at 72 locations is
The time is about twice as long as the measurement time of the magnetostrictive stress, and the efficiency of the entire magnetostrictive stress measurement is reduced.

An object of the present invention is to provide a method for demagnetizing a tube which can improve the efficiency of a demagnetization process.

[0014]

SUMMARY OF THE INVENTION The present invention relates to a method for demagnetizing a tube for measuring magnetostrictive stress by measuring a stress acting on the tube by moving a magnetostrictive sensor in a circumferential direction of the tube. The coil is moved in the circumferential direction of the tube while exciting so as to generate an AC magnetic field of a predetermined strength or more, and at least at a position around the tube, the intensity of the AC magnetic field generated from the demagnetizing coil is gradually reduced. A method of demagnetizing a tube, characterized in that the tube is demagnetized to zero.

[0015]

According to the present invention, the demagnetizing coil is moved in the circumferential direction of the tube while being excited so as to generate an AC magnetic field having a predetermined strength or more. The intensity of the alternating magnetic field generated from is gradually reduced to zero.

Therefore, focusing on an arbitrary point on the circumference of the tube, the intensity of the AC magnetic field is gradually reduced to zero in the process of passing and leaving the demagnetizing coil, which is equivalent to demagnetizing. Is done. That is, demagnetization can be performed at all points on the pipe circumference. With this, for each location where magnetostriction measurement is performed,
It is not necessary to perform the demagnetization process, and the efficiency of the demagnetization process can be improved.

[0017]

FIG. 1 shows a configuration for applying an embodiment of the present invention to measure the output of a magnetostrictive sensor 2 in the circumferential direction of a steel pipe 1 using a demagnetizing stress measuring device. A rail 5 on which a truck 3 on which a magnetostrictive sensor 2, a demagnetizing magnet 4, and the like are mounted travels is attached to the steel pipe 1. By driving the traveling motor 6, the carriage 3 travels along the rails 5, and the magnetostrictive stress is measured in the circumferential direction of the steel pipe 1.
The control device 8 controls the entire magnetostrictive stress measuring device such as processing of data output from the magnetostrictive sensor and running control of the bogie 3, and the display 9 displays measurement data and the like.

FIG. 2 is a block diagram showing a schematic electrical configuration of the magnetostrictive stress measuring apparatus shown in FIG. Processing circuit 20 realized by microcomputer or the like
Detects the position of the bogie 3 in the circumferential direction of the steel pipe 1 in response to the output from the encoder 21 and controls the driving of the motor 6 to control the running of the bogie 3.

The processing circuit 20 controls the demagnetizing magnet 23 by applying a current to the coil of the demagnetizing magnet 4 to generate an AC magnetic field, and also controls the operation of the magnetostrictive sensor 2.

As described above, it is necessary to demagnetize the steel pipe 1 before measuring the magnetostrictive stress. In this embodiment,
As shown in FIG. 3, the demagnetizing magnet 4 is set to a measurement point Q1 which is a demagnetizing start point. A constant AC current shown in FIG. 4 is applied to the demagnetizing magnet 4 to move the steel tube 1 at a constant speed in the circumferential direction L while exciting the steel tube 1 by generating a constant AC magnetic field.

When the demagnetizing magnet 4 returns to the measurement point Q1 of the demagnetization start point after making a round of the steel pipe 1, the current of the demagnetizing coil is gradually reduced to zero. When demagnetization is performed as described above, for example, focusing on any point on the tube circumference, the intensity of the AC magnetic field is gradually reduced to zero while the demagnetizing coil passes and separates. It is demagnetized. Therefore, demagnetization can be performed at all points on the pipe circumference.

Next, as shown in FIG. 3B, the magnetostrictive sensor 2 is set at the measurement point Q1 at the demagnetization start point. This point is referred to as the measurement start point Q1 again, and the magnetostrictive stress is measured at every angle θ = 5 ° pitch along the circumference of the tube in the circumferential direction M in a direction opposite to the demagnetization, that is, at the measurement points Q1 to Qn.

FIG. 5 is a flowchart showing a method for measuring magnetostrictive stress including the method for demagnetizing a pipe according to the present invention in the magnetostrictive stress measuring apparatus shown in FIG. The process is started in step b1. In step b2, the magnetostrictive stress measuring device moves the carriage 3 so that the demagnetizing magnet 4 reaches the measurement point Q1 at the demagnetization start point, and the fixed magnet shown in FIG. An alternating current is applied, an alternating magnetic field corresponding to the current is generated, and the steel pipe 1 is excited to perform a demagnetization process. In step b3, the demagnetizing magnet 4 is moved in the circumferential direction L. In step b4,
It is determined whether or not the current demagnetization point is the demagnetization start point.
And the above-described demagnetization processing is continued. In this way,
The processes of steps b3 and b4 are repeated, and the demagnetizing magnet 4 goes around the pipe while performing the above-described demagnetizing process. In step b4, when the demagnetizing magnet 4 reaches the demagnetization starting point, the carriage 3 is stopped, and in step b5, the current flowing through the coil of the demagnetizing magnet 4 is gradually reduced to zero. Thus, the demagnetization process ends.

Next, proceeding to step b6, the magnetostrictive stress measuring device moves the bogie in the circumferential direction L by an interval t between the demagnetizing magnet 4 and the magnetostrictive sensor 2 shown in FIG. That is, the magnetostrictive sensor 2 sets the demagnetization start point as the measurement start point.

Next, at step b7, the magnetostrictive sensor 2
To measure the magnetostrictive stress. Next, in step b8, the angle θ = 5 in the circumferential direction M opposite to that when performing the demagnetization process.
The cart is moved by an amount of ° to cause the magnetostrictive sensor 2 to reach the measurement point Q2. Next, the process proceeds to step b9, where it is determined whether or not the position of the magnetostrictive sensor 2 is the measurement start point. If not, the process returns to step b7 to perform the above-described magnetostriction stress measurement processing. In this way, the processing of step b7 to step b9 is repeated, and the carriage 3 is sequentially moved so that the magnetostrictive sensor 2 reaches the measurement points Q1 to Qn.
In step n, a magnetostrictive stress measurement process is performed. As described above, the trolley 3 goes around the steel pipe 1 and the magnetostrictive sensor 2 returns to the measurement start point. Next, the process proceeds to step b10, where the measurement of the magnetostrictive stress is completed, and the measured data is processed.

As described above, by performing the magnetostrictive stress measurement, the time required for the demagnetization process as a pre-process of the magnetostrictive stress measurement substantially corresponds to the time required for the bogie 3 to make one round in the circumferential direction L of the steel pipe 1. About 20 seconds. Conventionally, it takes about 120 seconds to stop and demagnetize each point where the measurement of magnetostrictive stress is performed as described above, and it takes about 120 seconds to perform all demagnetization. As a result, the time required for demagnetization is reduced to about 1/6. Although the time required for the entire magnetostrictive stress measurement is conventionally 3 minutes, the time required for the entire magnetostrictive stress measurement according to the present embodiment is about 1 minute and 20 seconds.
About 1/2.

The demagnetization may be performed one or more times. Further, although the demagnetization and the measurement are performed in the opposite directions, the directions may be switched after the rails are moved in the tube axis direction.

[0028]

As described above, according to the present invention, the demagnetizing coil is moved in the circumferential direction of the tube while generating an AC magnetic field of a predetermined strength or more, and demagnetized at least at a position around the tube. The intensity of the AC magnetic field generated from the coil is gradually reduced to zero. Therefore, the time required for demagnetization of the tube can be significantly reduced, and a method of demagnetizing the tube for improving the efficiency of the demagnetization process can be provided.

[Brief description of the drawings]

FIG. 1 shows a configuration for measuring an output of a magnetostrictive sensor 2 in a circumferential direction of a steel pipe 1 by using a magnetostrictive stress measuring device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a schematic electrical configuration of the magnetostrictive stress measuring device shown in FIG.

FIG. 3 is a view for explaining a tube demagnetization method and a magnetostriction stress measurement method of the present invention.

FIG. 4 is a demagnetizing magnet 4 used in the demagnetizing method of the present invention.
4 is a time chart of a current flowing through the power supply.

FIG. 5 is a flowchart showing a magnetostriction stress measurement method including the tube demagnetization method of the present invention in the magnetostriction stress measurement apparatus shown in FIG.

FIG. 6 is a view for explaining a conventional method of demagnetizing a tube.

FIG. 7 is a time chart of a current flowing through a demagnetizing magnet 4 in a conventional tube demagnetizing method.

FIG. 8 is a flowchart showing a magnetostrictive stress measurement method including a conventional pipe demagnetization method in the magnetostrictive stress measurement apparatus shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel pipe 2 Magnetostrictive sensor 3 Cart 4 Magnet for demagnetization 5 Rail 6 Motor 8 Control device 9 Display device 20 Processing 21 Encoder

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kuroda Takashi Tsukasa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Yoshiaki Sakai 1-1-2, Marunouchi, Chiyoda-ku, Tokyo No. Nippon Kokan Co., Ltd. (56) References JP-A-3-186725 (JP, A) JP-A-62-254409 (JP, A) JP-A-4-93109 (JP, U)

Claims (1)

(57) [Claims] 1. A demagnetizing method for a tube for measuring a stress acting on a tube by moving a magnetostrictive sensor in a circumferential direction of the tube, wherein the demagnetizing coil has a predetermined strength or more. While energizing to generate an AC magnetic field, it is moved in the circumferential direction of the tube, and the strength of the AC magnetic field generated from the demagnetizing coil is gradually reduced to zero at least at a position around the tube. The method of demagnetizing the pipe.