JPS61132856A - Eddy current flaw detecting method by insertion probe coil - Google Patents

Abstract

PURPOSE:To execute an eddy current flaw detection in all parts of a tube by inserting an insertion probe coil into the tube, and turning it in the peripheral direction. CONSTITUTION:An insertion probe coil 3 is inserted into a bobbin 4 from a tube expanded part D until a flange plate 6 abuts on a tube plate 2. The probe coil 3 is in parallel to the axial direction, in the side face of the bobbin 4, and to its end face, a pair of coils 5 being parallel to the direction vertical to the axial direction are wound round and formed. Accordingly, when an eddy current flaw detection is executed in the peripheral direction by turning the probe coil centering around the tube axis direction, only a defect generated in the tube can be detected as a discontinuous part without being influenced by a discontinuous part formed in the tube axis direction by the plate 2, the tube expanded part D and a tube end 9, etc., and a flaw detection can be executed in all parts of the tube.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an improvement in an eddy current flaw detection method using an interpolated probe coil for heat exchanger tubes and the like.

(Conventional technology) Condenser cooling pipes in thermal and nuclear power plants, steam generator heat transfer pipes, various heat exchanger pipes in petrochemicals, etc.
Periodic maintenance inspections using eddy current testing are widely carried out to check for damage caused by corrosion and to confirm safety.

In this method, a self-comparison type or absolute value type interpolation probe coil, in which a pair of detection coils are formed adjacent to each other in the circumferential direction on a rod-shaped bobbin such as Bakelite, is fed into the tube using compressed air, and then manually or rolled up. This involves winding up a cord cable using a device, etc., and detecting flaws while running it at a constant speed over the entire length of the pipe. The basic principle of flaw detection is ``By bringing a coil through which an alternating current (frequency of 100 Hz to several MHz) is passed close to a metal material, an induced current (eddy current) is generated in the metal material, which occurs when a defect exists. Defects can be identified by detecting disturbances in the induced current and changes in coil impedance (or changes in induced voltage).

Heat exchanger tubes, etc. are supported in contact with a large number of tube support plates, and pseudo signals are generated at these parts. Therefore, if there is a defect in this part, the pseudo signal and the signal due to the defect are combined, causing the defect to occur. Accurate information such as signal amplitude and phase could not be obtained, making defect evaluation ambiguous.

However, this problem has been solved in recent years with the establishment of multi-frequency eddy current flaw detection. This method involves simultaneously inputting several different test frequencies into a single eddy current detection coil, separating and extracting the flaw detection signals corresponding to each test frequency, and then adding, subtracting, multiplying and dividing these results to eliminate unnecessary signals. This method extracts only the necessary defect signals.

(Problems to be Solved by the Invention) Generally, as shown in FIG. 5, the ends of tubes such as heat exchanger tubes are attached to the tube plate 2 by expanding and joining from the inner surface of the tubes 1 using a Settle tube expander. The pressure for expansion joint is 80kgf/al.
This is the most practical and economical method for attaching tubes to heat exchangers, etc. used in the following.Aluminum plastic tubes and other copper alloy tubes used in various heat exchangers such as condensers are joined using this method. Further, when the unrestricted portion is expanded, a very large residual stress is generated in that portion, so the expanded portion is usually stopped within the pipe slope. Note that the tube expansion rate is 3 to 5% in the case of aluminum plastic tubes and 5 to 10% in case of cupronickel tubes.

At the end of the tube, a coarse spurious signal is generated,
With the multi-frequency eddy current flaw detection method, it is difficult to separate the pseudo signal from the defect signal, and an area where flaw detection cannot be detected is inevitable. The reason for this is that at the tube end, there are a tube sheet signal, a tube expansion signal, and a tube end signal, and these signals are close to each other, are complex, and have extremely coarse signal levels.

5 and 6 show a test example in which the vicinity of the tube plate 2 was inspected using a self-comparison type interpolation probe coil 23 in which a pair of coils 21 and 21 were formed adjacent to each other in the circumferential direction of the rod-shaped bobbin 22. show. The tube plate 2 is made of naval brass with a thickness of 351 m, and the tube 1 has an outer diameter of 31.75 txi and a wall thickness of 1.2 m.
When the tube 1 is expanded and joined to the tube sheet 2 at a tube expansion rate of 3% using a No. 51 aluminum plastic tube, the undetectable area B starts at a position C = approximately 10 m from the back surface of the tube sheet 2, Defects occurring within this range will not be detected. In the figure, A indicates a flaw detectable range, and D indicates an enlarged tube portion formed in the tube plate 2. Furthermore, it can be seen from FIG. 6 that the flaw detection signal is extremely overscaled in the undetectable area B. In addition, A in the figure
, B correspond to FIG. 5A and B.

The undetectable area B that occurs at the end of the pipe described above is a problem, especially on the seawater inlet side of the condenser cooling pipe. That is, corrosion called inleft attack occurs in a concentrated manner at the cooling pipe inlet. This is because the corrosion-resistant coating formed inside the tube is damaged by the turbulent flow of seawater at the inlet of the cooling tube, promoting erosion. Inleft attack occurs directly under and near the tube plate, so it is impossible to detect flaws in the area B.
It is highly desirable to eliminate the formation of

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an eddy current flaw detection method using an interpolated probe coil that allows eddy current flaw detection to be performed in all parts of a pipe in a simple manner.

(Means for solving problems) The present invention takes the following measures to achieve the above object.

That is, in an eddy current flaw detection method using an interpolation probe coil, an interpolation probe coil in which a pair of coils are wound adjacently is inserted into a pipe, and the interpolation probe coil is moved to detect defects in the pipe. , the interpolation probe coil has a pair of coils wound in parallel to the axial direction on the side surface of the bobbin and parallel to the axial direction on the end surface thereof, and the probe coil is wound around the tube axis direction. Detect flaws by moving.

(Function) In the present invention, since the interpolation probe coil is formed by winding a pair of coils parallel to the axial direction on the side surface of the bobbin and parallel to the direction perpendicular to the axial direction on the end surface, By rotating the probe coil around the tube axis, eddy current flaw detection in the circumferential direction can detect discontinuities formed in the tube axis direction by the tube sheet, tube expansion section, tube end, tube support plate, etc. Only defects occurring in the tube can be detected as discontinuities without being affected, and flaw detection can be performed in all parts of the tube.

(Example) Next, embodiments of the present invention will be described with reference to the drawings.

FIGS. 3(1) and 3(2) show a self-comparison type inner layer probe coil 3 exclusively used for flaw detection at the end of a tube according to the present invention. The probe coil 3 is inserted into the end of the tube expanded and joined to the tube plate 2 as shown in FIG. There is.

The bobbin 4 is made of an insulating material such as Bakelite and is formed into a stepped rod shape so as to be engaged with the tube end, that is, the tube expansion portion, and the inner surface of the vicinity thereof while maintaining a certain clearance. The size of the clearance is usually 0.5 to 1. Ow
It is considered to be a degree. Further, the length of the bobbin 4 is determined so that the tip of the bobbin 4 is located in the flaw detectable range A in FIG. This is because the probe coil 3 is used exclusively for the tube end, and the conventional interpolated probe coil 23 is used for eddy current flaw detection in areas other than the tube end. Note that A in Figure 1.

B and C have been transcribed to facilitate reference to each area in FIG.

A pair of coils 5.5 are wound adjacent to each other on the bobbin 4, the side surfaces thereof being parallel to the axial direction, and the end surfaces thereof being parallel to the direction perpendicular to the axial direction. The pair of coils 5.5 act as so-called self-comparison type coils, and can suppress disturbances due to variations in tube expansion rate and lift-off effects when rotating the coils. An absolute value type coil was also prototyped, but it was found to be impractical for the purpose of flaw detection due to the disturbance.

A flange plate 6 made of an insulating material is provided on the tube end side of the bobbin 4 on which the coil 5.5 is wound. A recess 8 that can be engaged with the bobbin 4 is formed concentrically with the bobbin 4.

Note that 7 is a cord connected to the coil 5.5 from the back side of the flange plate 6.

The interpolation probe coil 3 constructed as described above is inserted into the tube from the expanded tube portion until the flange plate 6 comes into contact with the tube plate 2, as shown in FIG. Thereafter, the interpolation probe coil 3 is rotated at least 180 turns about the tube axis direction manually or mechanically to detect defects formed at the tube end.

The rotation speed of the interpolation probe coil 3 is usually about 6 Or
pm to 500 rpm.

As mentioned above, since the present invention performs eddy current flaw detection in the circumferential direction,
Only defects occurring in the tube can be detected as discontinuous portions without being affected by discontinuous portions formed in the tube axis direction by the tube plate, tube expansion portion, tube end, etc.

Figure 2 shows the defect 1 caused in the lower part of the tube end by this method.
The flaw detection signal (IVP-P) when 0 was detected is shown.

The interpolation probe coil 3 is detected by rotating to the left (or right) at a rotational speed of 60 rpm, with the coils 5, 5 being horizontal at the end face as a reference (0°).

FIG. 4 shows a second embodiment of the invention. In the figure, the interpolation probe coil 3'' has coils 5'' and 5'' wound around a cylindrical bobbin 4° parallel to the axial direction and the perpendicular direction thereof, and a support at one end of the bobbin 4°. A rod 12 is fixed in the tube axis direction. To perform flaw detection using the probe coil 3', the support rod 12 is used to push it into the tube and rotate it at various locations. The pushing pitch is
The length of the probe coil is slightly smaller than 3", but
The dimension of L can be appropriately determined depending on the positioning accuracy of the defect. This method is suitable because defects can be easily found even in areas where the tube support plates are in contact, without applying sophisticated multi-frequency eddy current testing.

(Effects of the Invention) As explained above, in the eddy current flaw detection method using the interpolated probe coil of the present invention, the side surface of the bobbin is parallel to the axial direction,
Moreover, flaws are detected by inserting and rotating an interpolated probe coil, which has a pair of coils wound adjacent to each other parallel to the axial direction on the end face, into the tube.
Detects only defects occurring in the pipe as discontinuous parts without being affected by discontinuities in the pipe axis direction due to tube sheets, tube expansion parts, pipe ends, tube support plates, etc. arranged in the circumferential direction of the pipe. It is possible to completely eliminate the undetectable area. Moreover,
There is no need for sophisticated signal processing as in the multi-frequency eddy current testing method, and therefore, the signal processing device used in the method of the present invention can be one that is used in normal eddy current testing methods, and is excellent in economy and workability. .

[Brief explanation of the drawing]

Fig. 1 is a vertical cross-sectional view of the tube end portion during eddy current flaw detection showing the first embodiment of the method of the present invention, Fig. 2 is a diagram showing flaw detection signals by the method of the present invention, and Fig. 3 (1) +2) A side view and a front view of an interpolation probe coil according to the method of the present invention, FIG. 4 is a cutaway longitudinal cross-sectional view showing a second embodiment of the method of the present invention, and FIG. FIG. 6, which is a longitudinal cross-sectional view of the tube end portion, is a diagram showing the eddy current flaw detection signal of FIG. 5. 1...tube, 2...tube plate, 3,3°...interpolation probe coil, 4.4''...bobbin, 5.5"...coil. Patent applicant: Co., Ltd. Kobe Steel, Ltd. Same as above New Japan Nondestructive Inspection Co., Ltd.I2 Figure flk6■

Claims (1)

[Claims] 1. In an eddy current flaw detection method using an interpolation probe coil, in which a pair of coils are wound adjacent to each other and an interpolation probe coil is inserted into a pipe, and the interpolation probe coil is moved to detect defects in the pipe. , the interpolation probe coil has a pair of coils wound in parallel to the axial direction on the side surface of the bobbin and parallel to the axial direction on the end surface thereof, and the probe coil is wound around the tube axis direction. An eddy current flaw detection method using an interpolated probe coil, which detects flaws by moving them.