JP2007033329A - Electromagnetic ultrasonic inspection method and electromagnetic ultrasonic transducer used therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic ultrasonic inspection method which can precisely inspect a corrosion of the inner surface of a heat-transfer finned pipe, carry out an inspection for a short time, eliminate the need for a coupling medium such as water or the like and carry out a preparation without requiring a long time nor a large cost. <P>SOLUTION: A pipe body 11 of the embedded heat-transfer finned pipe 2 is vibrated and resonated by generating an axisymmetric SH wave 3 from an EMAT 1 by utilizing an electromagnetic force, while moving the EMAT 1 axially in the embedded heat-transfer finned pipe 2 of an air-cooled type heat exchanger, and then a resonance frequency is detected by the EMAT 1. In the case the detected frequency differs from the resonance frequency representing a state that the wall thickness of the pipe body 11 is normal, such a judgement is made that a corrosion section occurs in the inner surface of the pipe body 11. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electromagnetic ultrasonic flaw detection method for inspecting corrosion inside an embedded finned heat transfer tube of an air-cooled heat exchanger and an electromagnetic ultrasonic transducer used therefor.

  The inner surface of the finned heat transfer tube of the air-cooled heat exchanger is corroded, and if this corrosion progresses, the fluid passing through the tube may leak out of the tube. Yes.

Conventional methods for inspecting the inner surface of a finned heat transfer tube include the eddy current flaw detection method (for example, see Patent Document 1) or a vibrator that performs flaw detection using an alternating magnetic flux generated from a coil in a probe. There is an ultrasonic flaw detection method (for example, see Patent Document 2) in which flaw detection is performed using ultrasonic waves generated by oscillating the sound.
JP 2002-296241 A JP 2001-50936 A

  However, in the eddy current flaw detection method, if the pipe is equipped with fins, this fin becomes a hindrance, and corrosion of the inner surface of the pipe cannot be inspected with high accuracy. Could not be applied. On the other hand, in the ultrasonic flaw detection method, since it takes time to inspect one heat transfer tube, the inspection speed is slow, and 100% inspection cannot be performed within a short time. Moreover, since the inside of the heat transfer tube must be filled with water to perform the inspection, post-treatment such as dehydration is required, and water may not be used depending on the type of fluid passing through the heat transfer tube. Moreover, in order to increase the inspection accuracy, it is necessary to remove the scale on the inner surface of the heat transfer tube, and preprocessing such as this removal work also takes time and cost.

  The present invention has been made in view of such circumstances, and can inspect corrosion of the inner surface of the heat transfer tube with embedded fins with high accuracy, and does not take much time for the inspection, and is a contact medium such as water. It is an object of the present invention to provide an electromagnetic ultrasonic flaw detection method that does not require time and cost for pretreatment and an electromagnetic ultrasonic transducer used therefor.

  In order to achieve the above object, the present invention provides an electromagnetic force generated from an electromagnetic ultrasonic transducer while moving the electromagnetic ultrasonic transducer along the axial direction in an embedded finned heat transfer tube of an air-cooled heat exchanger. An axisymmetric SH wave is generated by using this to vibrate the tube body of the embedded finned heat transfer tube to cause resonance, and this resonance frequency is detected by an electromagnetic ultrasonic transducer. An electromagnetic ultrasonic flaw detection method that determines that a corroded portion has occurred on the inner surface of the tube main body when the main body thickness is different from the resonance frequency when the wall thickness is normal is a cylindrical shape. Alternatively, it is composed of a permanent magnet unit formed in a cylindrical shape and a transmission coil and a reception coil wound around the outer periphery of the permanent magnet unit, and the permanent magnet unit is arranged on the large-diameter side in the radial direction. A permanent magnet unit comprising a plurality of permanent magnets having one pole and the other pole on the smaller diameter side thereof arranged so that the polarities are alternately reversed in the circumferential direction of the permanent magnet unit. A second gist is an electromagnetic ultrasonic transducer in which a plurality of slits intersecting each other are cut out at both end faces of each other.

  That is, the present inventors have conducted extensive research on a flaw detection method capable of inspecting corrosion of the finned heat transfer tube inner surface of the air-cooled heat exchanger with high accuracy. In the case of a heat pipe, an electromagnetic ultrasonic transducer (hereinafter referred to as “EMAT”) is used, and electromagnetic force is utilized from the EMAT while moving the EMAT along the axial direction inside the heat transfer pipe with embedded fins. The resonance frequency can be detected with high accuracy by generating an axially symmetric SH wave, causing the tube main body of the embedded finned heat transfer tube to vibrate and causing resonance, and detecting the resonance frequency with the EMAT. Determine the resonance frequency when the detected resonance frequency is compared with the resonance frequency when the thickness of the tube body is normal, and the resonance frequency when the detected resonance frequency is normal If different, if it be determined that the corroded portions on the inner surface of the pipe main body has occurred, it found that it is possible to check the corrosion of the heat transfer tube surface with high precision, thereby achieving the present invention. In addition, when the EMAT is used, flaw detection can be performed in a shorter time than the conventional ultrasonic flaw detection method, and the cost can be reduced and 100% inspection can be performed in the process. In addition, since the EMAT does not require a contact medium such as water, no post-treatment (dehydration, etc.) in the tube body is required, and no matter what kind of fluid passes through the heat transfer tube, flaw detection is possible. It can be performed. Furthermore, since the EMAT is non-contact flaw detection, corrosion inspection is possible even when a scale or the like is attached to the inner surface of the tube main body, so that the pretreatment time can be shortened and the pretreatment cost can be reduced. In the present invention, the finned heat transfer tube is limited to the embedded finned heat transfer tube, and is integrally formed with the fin on the outer peripheral portion (the whole or a part thereof) of the tube body of the finned heat transfer tube. In a finned heat transfer tube having a formed outer layer (for example, an L-shaped wrapping finned heat transfer tube), the resonance frequency is generated in the portion of the tube body where the outer layer is formed due to the influence of the outer layer. It is because it does not.

  On the other hand, the EMAT of the present invention comprises a permanent magnet unit formed in a cylindrical shape or a columnar shape, and a transmission coil and a reception coil wound around the outer periphery of the permanent magnet unit. A plurality of permanent magnets having one pole on the large diameter side in the radial direction and the other pole on the small diameter side in the radial direction are arranged so that the polarities are alternately reversed in the circumferential direction of the permanent magnet unit. It consists of things. Therefore, when a high-frequency current is passed through the transmission coil, an axially symmetric SH wave is generated in the tube body of the embedded finned heat transfer tube to vibrate the tube body, and resonance is caused in the tube body. This resonance frequency can be detected by the EMAT receiving coil, and can be used as the EMAT of the electromagnetic ultrasonic flaw detection method of the present invention. In addition, since a plurality of slits intersecting each other are formed in both end faces (both end faces in the axial direction) of the permanent magnet unit, the S / N ratio (signal to noise ratio) is improved by the slits. , Noise is reduced. In this case, it is more preferable to form annular slits extending in the circumferential direction at both ends (both ends in the axial direction) of the outer peripheral surface of the permanent magnet unit, since the S / N ratio is further improved. The slits preferably have a concave groove shape (flat bottom shape). When the slits are formed on the both end surfaces, in addition to the slits, one or two slits are formed in each direction. There should be more than books. In the present invention, “crossing each other” means that they may cross each other in an inclined state or may be orthogonal to each other, but the S / N ratio is higher when they are orthogonal to each other. improves.

  In the electromagnetic ultrasonic flaw detection method of the present invention, when the tube body of the embedded finned heat transfer tube is made of carbon steel (ferromagnetic material) and the fin is made of a non-ferrous material such as aluminum, The electromagnetic ultrasonic flaw detection method of the present invention can be performed on the embedded finned heat transfer tube used.

  Also, in the electromagnetic ultrasonic flaw detection method of the present invention, when the resonance order of the axisymmetric SH wave is the primary mode, the lowest mode is used, so the inner surface of the embedded fin tube is inspected for corrosion. It can be inspected with high accuracy.

  In the electromagnetic ultrasonic flaw detection method of the present invention, the EMAT includes a permanent magnet unit formed in a cylindrical shape or a columnar shape, and a transmission coil and a reception coil wound around the outer peripheral portion of the permanent magnet unit. The permanent magnet unit has a plurality of permanent magnets having one pole on the large diameter side in the radial direction and the other pole on the small diameter side in the radial direction, alternately in the circumferential direction of the permanent magnet unit. If the electrodes are arranged so that the polarities are reversed, as described above, they can be used as EMAT in the electromagnetic ultrasonic flaw detection method of the present invention, and the corrosion of the inner surface of the embedded finned heat transfer tube can be inspected with high accuracy.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an electromagnetic ultrasonic flaw detector used in an embodiment of the electromagnetic ultrasonic flaw detection method of the present invention. In the figure, reference numeral 1 denotes a substantially cylindrical EMAT, which is inserted into an embedded finned heat transfer tube (embedded fin tube) 2 and moves along the axial direction of the EMAT 1 by the electromagnetic force. Using the generated axisymmetric SH wave 3 (propagating along the pipe surface in the circumferential direction), the inner surface of the embedded finned heat transfer tube 2 is flaw-detected. Reference numeral 5 denotes a detection device including an amplifier, a burst wave oscillator, an A / D converter, and the like. Reference numeral 6 denotes a computer for analyzing an input signal. Reference numeral 7 denotes the EMAT 1 inserted into the embedded finned heat transfer tube 2. A pulse motor type delivery device for moving in the axial direction 8 is an output device.

  The embedded finned heat transfer tube 2 is used in an air-cooled heat exchanger. The tube main body 11 is made of carbon steel formed in a cylindrical shape and protrudes in a spiral shape on the outer periphery of the tube main body 11. Heat transfer fins 12 made of non-ferrous metal such as aluminum (the inner end portion of the heat transfer fins 12 is embedded in a spiral groove 11a formed on the outer peripheral surface of the tube body 11) (See FIG. 2).

  As shown in FIG. 3, the EMAT 1 includes a magnet unit 13 formed in a substantially cylindrical shape and a pair of spiral coils 14 (transmission spirals) wound around an annular recess 13a of the magnet unit 13 in the circumferential direction. Coil 14a and receiving spiral coil 14b). The magnet unit 13 has a plurality of permanent magnets 15a and 15b having one pole on the large diameter side in the radial direction and the other pole on the small diameter side in the radial direction so that the polarities are alternately reversed in the circumferential direction. Are arranged in a ring shape, and are composed of such a ring-shaped alternating magnetic field forming magnet row. Each of the permanent magnets 15a and 15b is formed in a rod shape, and the shape thereof is the same except for the polarity. When the magnet unit 13 is assembled, the magnet unit 13 is evenly arranged in the circumferential direction. The structure is divided into two. Adjacent permanent magnets 15a and 15b are bonded and fixed with an adhesive layer 15c made of an epoxy resin (see FIG. 4; not shown in FIG. 3). The magnetic field lines of the permanent magnet 15a (15b) also act to reduce the influence (escape) on the permanent magnet 15b (15a) adjacent thereto.

  The magnet unit 13 will be described in more detail. Permanent magnets 15a having S poles on the radially large diameter side and permanent magnets 15b having N poles on the radially large diameter side are alternately arranged in the circumferential direction. It consists of a ring-shaped alternating magnetic field forming magnet array formed by adhering and fixing adjacent permanent magnets 15a and 15b with an adhesive layer 15c, and forms an alternating magnetic field whose polarity is alternately reversed in the circumferential direction (FIG. 3). Further, as shown in FIG. 5, an annular recess 13 a is formed in an annular shape along the circumferential direction in the central outer peripheral portion of the magnet row (that is, the central outer peripheral portion of the magnet unit 3). In other words, each of the permanent magnets 15a and 15b has a recess 16 formed at the center outer peripheral portion so as to cross the permanent magnet 15a and 15b. A recess 13a is formed. When the EMAT 1 is inserted into the inside of the tube body 11, the annular recess 13a is configured so that both spiral coils 14 (see FIG. 7) described later (see FIG. 7) are wound and accommodated in the annular recess 13a. 11 to prevent the inner surface of the coil 11 from rubbing and being damaged. The depth is set according to the diameter of the strands of the spiral coils 14 and the width is matched to the winding width of the spiral coils 14. Is set.

  Further, as shown in FIG. 6, a plurality of end faces (that is, both end faces in the axial direction of the magnet unit 13) of the magnet row cross each end face in a penetrating manner (in this embodiment, in this embodiment). The number of the concave grooves 17a, which is five, but may be one or any number, is notched in parallel at a predetermined interval (which may or may not be equal). In addition, a plurality of (in this embodiment, five, but one or any number) concave groove-shaped slits 17b orthogonal to each of the slits 17a may be provided at predetermined intervals (even at regular intervals, etc. The slits 17a and 17b (not shown in FIGS. 3 and 4) improve the S / N ratio (signal-to-noise ratio). This has the effect of reducing noise. The width of each of the slits 17a and 17b is set to 0.2 to 0.4 mm, and the depth is set to 1.0 to 1.5 mm.

  In the annular recess 13a, the spiral coil 14 (see FIG. 5) is generated so that an eddy current for generating an electromagnetic force substantially parallel to the inner surface of the tube body 11 is generated in the embedded finned heat transfer tube 2. 3) is wound. As described above, the spiral coil 14 includes the transmission spiral coil 14a and the reception spiral coil 14b. The coils 14a and 14b are arranged in the axial direction of the magnet row (that is, the axial direction of the magnet unit 13). They are arranged alternately (see FIG. 7). Further, one end of each of the coils 14a and 14b is connected to an amplifier provided in the detection device 5 (see FIG. 1), and the other end is connected to the ground. In FIG. 5, reference numeral 18 denotes a coil placement slit formed in the outer peripheral portion of the magnet unit 13 along the axial direction thereof. The wire ends (four) of both the spiral coils 14 are smoothly taken out and connected. These take-out lines are not covered on the spiral coils 14 wound around the annular recess 13a.

  When a high-frequency current is passed through the spiral coil 14, a transverse wave polarized along the inner surface having nodes distributed uniformly in the circumferential direction at predetermined positions along the inner surface of the tube body 11. That is, the axially symmetric SH wave 3 is excited. The axially symmetric SH wave 3 is formed along the inner surface of the tube main body 11, the forming direction is the circumferential direction of the inner surface of the tube main body 11, and the amplitude direction is parallel to the inner surface of the tube main body 11. And the direction along the axial direction of the tube body 11. The excited axisymmetric SH wave 3 excites a current in the spiral coil for reception 14b, and the sound pressure can be known by detecting the voltage of this current.

  The computer 6 sends an operation signal for operating the EMAT 1 to a burst wave oscillator disposed in the detection device 5, and amplifies the burst wave generated by the burst wave oscillator with an amplifier. The axially symmetric SH wave 3 is excited on the inner surface of the tube body 11 of the embedded finned heat transfer tube 2 by being supplied to the spiral coil 14a. Different frequency signals are sequentially generated from the burst wave oscillator at predetermined time intervals. The excited axisymmetric SH wave 3 is transmitted to the inner surface of the tube body 11 in the circumferential direction to generate resonance. The axially symmetric SH wave 3 in a state where resonance is caused by the frequency is detected by the receiving spiral coil 14b, and this detection signal is sent to the signal analysis unit of the computer 6 through the amplifier and the A / D converter.

  The signal analysis unit determines the amplitude of the detection signal input from the A / D converter, that is, the sound pressure level of the axisymmetric SH wave 3, and obtains it as a function of frequency through the excitation / reception / sound pressure determination operation. A means for detecting a frequency at which the sound pressure is maximized from the obtained sound pressure distribution as a resonance frequency, and determining the presence or absence of corrosion thinning or the amount of remaining thickness according to the detected resonance frequency. The frequency of the drive signal is sent to the signal analysis unit, and the received signal is sent from the A / D converter. Therefore, the frequency at which the sound pressure is maximized is determined and detected as the resonance frequency.

  The signal analysis unit includes an LUT (look-up table) in which the relationship between the resonance frequency and the plate thickness is tabulated, and the plate thickness is determined by referring to the LUT from the resonance frequency determined earlier. . Furthermore, since the resonance frequency in the pipe body 11 in which corrosion thinning has occurred on the inner surface is different from the resonance frequency in the normal pipe body 11 obtained as described later, this (in the normal pipe body 11) ) When a resonance frequency different from the resonance frequency is detected, it can be determined and detected from the LUT that corrosion has occurred.

  On the other hand, the pipe body 11 is artificially provided with a plurality of thinned portions having different depths, the resonance frequency is obtained for each thinned portion using the EMAT1, and a large number of experimental data are collected to reduce the thickness. The relationship between the remaining thickness of the meat part (or the depth of the hole) and the resonance frequency is tabulated or numerically expressed (in this embodiment, numerically expressed). Thereby, it becomes possible to evaluate the defect size (in this embodiment, the remaining thickness of the thinned portion). Further, the resonance frequency corresponding to the normal tube body 11 is obtained by using the EMAT 1 and the experimental data is also collected. From these experimental data, for example, there is a certain relationship between the remaining thickness (or the depth of the hole) and the resonance frequency, and the smaller the remaining thickness (that is, the depth of the hole in the reduced thickness portion). It can be seen that the deeper the resonance frequency is. Therefore, the defect size can be obtained from the experimental data, or it can be determined that there is some problem on the inner surface of the tube main body 11 by changing the resonance frequency.

  The delivery device 7 is controlled by the computer 6, and when it is determined that corrosion has occurred as a result of flaw detection scanning, the delivery amount of the delivery device 7 is reduced to reduce the flaw detection pitch. The inspection accuracy can be improved by carrying out precision flaw detection. The EMAT 1 detection position (distance from the tube end of the tube body 11) is automatically taken into the computer 6 from a scanning distance detection device (not shown) built in the delivery device 7 for data analysis. It is reflected.

  The output device 8 has display means (not shown). On the display screen, the detection position (mm) of EMAT1 is set on the horizontal axis, and the resonance frequency (MHz) at the detection position is set on the vertical axis. The graph (see FIG. 8) is displayed, and corrosion is generated on the inner surface of the tube body 11 by observing the resonance frequency and amplitude (that is, fluctuation of the resonance frequency) displayed on the display screen. It is possible to know the resonance signal and the position thereof (distance from the tube end of the tube body 11). In addition, when a relationship table between the resonance frequency and the wall thickness of the tube body 11 can be projected on the horizontal side of the graph, or from the above experimental data, a table of the relationship between the resonance frequency and the wall thickness of the tube body 11. In the case of preparing the above, it becomes possible to obtain the thickness (defect size) of the corroded portion of the tube main body 11 from the resonance signal by looking at it.

  Using the electromagnetic ultrasonic flaw detector, for example, corrosion of the inner surface of the tube body 11 can be inspected as follows. That is, first, the delivery device 7 is operated, the EMAT 1 is inserted into the pipe body 11, and the entire length of the pipe body 11 is moved along the axial direction. During this movement, flaw detection is performed on the inner surface of the tube main body 11 using the axially symmetric SH wave 3 generated by the electromagnetic force from the EMAT 1 and the flaw detection data is input to the computer 6. Further, the moving distance of EMAT 1 (distance from the tube end of the tube main body 11) during the movement is detected by a scanning distance detection device, and this detection data is input to the computer 6. And based on each of these data, the presence or absence of corrosion can be confirmed from the graph projected on the display screen of the output device 8, and its size can be known. Further, if flaw detection data is printed as an XY chart on the recording paper by the output device 8, a flaw detection record for the entire length of the tube main body 11 is created. For example, in the graph shown in FIG. 8, when normal, the thickness of the tube body 11 is 3.0 mm, but at the position where the resonance signal is generated, the thickness of the tube body 11 is 1.4 mm. You can see that

  As described above, in this embodiment, the internal corrosion of the tube main body 11 of the embedded finned heat transfer tube 2 can be inspected with high accuracy. In addition, when the EMAT 1 is used, flaw detection can be performed in a shorter time than the conventional ultrasonic flaw detection method, and the cost can be reduced and 100% inspection can be performed in the process. Moreover, since the EMAT 1 does not require a contact medium such as water, no post-treatment (dehydration or the like) in the tube body 11 is required, and any type of fluid through which the embedded finned heat transfer tube 2 passes. Even so, flaw detection can be performed. Moreover, since the EMAT 1 is a non-contact flaw detection, corrosion inspection is possible even when a scale or the like is attached to the inner surface of the tube main body 11, and the pretreatment time can be shortened and the pretreatment cost can be reduced.

  In the above embodiment, when collecting a large amount of data, for example, as shown in FIG. 9, a plurality of thinned portions 25 to 29 having different depths are artificially formed on the inner surface of the tube main body 11 (test piece). The resonance frequencies f1 to f5 (MHz) are measured using the EMAT1 for each of the thinned portions 25 to 29. When the corrosion amount and the remaining thickness (remaining thickness) of each of the thinned portions 25 to 29 were set to the numerical values shown in Table 1, measurement results as shown in FIG. 10 were obtained. From these data, there is a certain relationship between the amount of corrosion or the remaining thickness and the resonance frequency, and the smaller the remaining thickness (that is, the larger the amount of corrosion and the deeper the depth of the hole in the thinned portion), the higher the resonance frequency. Can be seen to rise. Therefore, the defect size can be obtained from each of the above data, or it can be determined that there is some problem on the inner surface of the tube main body 11 by changing the resonance frequency. For example, in FIG. 9, when a corroded portion is generated at a location indicated by a black circle 30, if the measured resonance frequency at this corroded portion is determined to be fx, the remaining thickness can be determined (in this example, the remaining thickness is 2. 4 mm, the amount of corrosion is 0.6 mm), and the position is also known.

  FIG. 11 shows a modification of the EMAT1. In this example, not only slits 17 a and 17 b (see FIG. 6) are formed on both end surfaces of the permanent magnet unit 13 in the axial direction, but also on the outer peripheral surfaces of both ends of the permanent magnet unit 13 in the axial direction. Also, a plurality of annular concave grooves 21 (in this embodiment, two, but one or any number) extending in the circumferential direction may be formed by notching. The other parts are the same as EMAT1 used in the above embodiment, and the same reference numerals are given to the same parts. Even when the EMAT 1 of this example is used, the same operations and effects as the above-described embodiment are obtained. In addition, since a plurality of annular slits 21 are formed in the outer peripheral surfaces of both end portions of the permanent magnet unit 13 in the axial direction, the S / N ratio is further improved and noise is reduced.

[Examples 1 and 2 and Comparative Example 1]
An embedded finned heat transfer tube 2 having a corroded portion on the inner surface and an L-shaped wound finned heat transfer tube (not shown) were prepared. Further, an EMAT 1 having the same structure as the EMAT 1 (EMAT 1 ′) except that the slits 17a and 17b are not formed on both end faces in the axial direction of the magnet array and the magnet array shown in the above embodiment was prepared. Then, the corrosion frequency of the embedded finned heat transfer tube 2 and the resonance frequency in the vicinity thereof are measured using EMAT 1 (Example 1), and the corrosion frequency of the embedded finned heat transfer tube 2 and the resonance frequency in the vicinity thereof are measured. Measurement was performed using EMAT 1 '(Example 2), and the resonance frequency of the corroded portion of the L-shaped wound-type finned heat transfer tube and the vicinity thereof was measured using EMAT 1' (Comparative Example 1). The measurement results are shown in FIG. 12 (Example 1), FIG. 13 (Example 2) and FIG. 14 (Comparative Example 1). According to these measurement results, in Comparative Example 1, the resonance frequency could not be measured in the place where the corrosion portion was generated, and there was a lot of noise. In Example 2, although the resonance frequency could be measured, the noise In Example 1, it can be seen that the resonance frequency can be measured and the noise is very small.

  In the above-described embodiment, the EMAT 1 is formed in a substantially cylindrical shape, but is not limited to this, and may be formed in a columnar shape. In the above embodiment, the embedded finned heat transfer tube 2 is composed of a carbon steel tube body 11 and a heat transfer fin 12 made of non-ferrous metal such as aluminum. A built-in finned heat transfer tube 2 can be used. Moreover, in the said embodiment, although the said embedded finned heat exchanger tube 2 is a straight tube, it may be bent and formed in various shapes, such as U shape.

  Moreover, in the said embodiment, although the said magnet unit 13 is divided into 12 (it is comprised by 12 permanent magnets 15a and 15b), it is not limited to this, It may be divided into plurality. What is necessary is just to divide | segment into 10, 12, or 14 in order to test | inspect the internal corrosion of the heat sink tube 2 with an embedded fin. However, if it is divided into 16 or more, it is not suitable for inspecting the internal corrosion of the embedded finned heat transfer tube 2. Further, the resonance order (primary to N-order mode) used in this inner surface corrosion inspection is optimal in the primary mode, but is preferably used as long as it is up to the tertiary mode.

It is explanatory drawing which shows the electromagnetic ultrasonic testing apparatus used for one Embodiment of the electromagnetic ultrasonic testing method of this invention. It is structure explanatory drawing of an embedded finned heat exchanger tube. It is a perspective view which shows EMAT. It is explanatory drawing of said EMAT. It is a side view of a magnet unit. It is an end view of the magnet unit. It is explanatory drawing of a spiral coil. It is explanatory drawing of the graph projected on a display screen. It is explanatory drawing of a test piece. It is explanatory drawing of the graph projected on a display screen. It is a side view which shows the modification of the said EMAT. It is a figure which shows a measurement result. It is a figure which shows a measurement result. It is a figure which shows a measurement result.

Explanation of symbols

1 EMAT
2 Heat transfer tube with embedded fin 3 Axisymmetric SH wave 11 Tube body

Claims (5)

  While moving the electromagnetic ultrasonic transducer along the axial direction inside the heat transfer tube with embedded fins in the air-cooled heat exchanger, the electromagnetic ultrasonic wave is generated from this electromagnetic ultrasonic transducer using electromagnetic force and embedded. The tube body of the finned heat transfer tube is vibrated to cause resonance, and this resonance frequency is detected by an electromagnetic ultrasonic transducer.The detection result is the resonance frequency when the wall thickness of the tube body is normal. An electromagnetic ultrasonic flaw detection method characterized in that it is determined that a corroded portion is generated on the inner surface of the tube main body when different.   The electromagnetic ultrasonic flaw detection method according to claim 1, wherein the tube body of the embedded finned heat transfer tube is made of carbon steel, and the fin is made of a non-ferrous material such as aluminum.   The electromagnetic ultrasonic flaw detection method according to claim 1 or 2, wherein the resonance order of the axisymmetric SH wave is a primary mode.   The electromagnetic ultrasonic transducer includes a permanent magnet unit formed in a cylindrical shape or a columnar shape, and a transmission coil and a reception coil wound around an outer peripheral portion of the permanent magnet unit. A plurality of permanent magnets having one pole on the large diameter side in the radial direction and the other pole on the small diameter side in the radial direction are arranged so that the polarities are alternately reversed in the circumferential direction of the permanent magnet unit. The electromagnetic ultrasonic flaw detection method according to any one of claims 1 to 3. A permanent magnet unit formed in a cylindrical shape or a columnar shape, and a transmission coil and a reception coil wound around the outer periphery of the permanent magnet unit. The permanent magnet unit is arranged on the large-diameter side in the radial direction. A permanent magnet unit comprising a plurality of permanent magnets having one pole and the other pole on the smaller diameter side thereof arranged in such a manner that the polarities are alternately reversed in the circumferential direction of the permanent magnet unit. The electromagnetic ultrasonic transducer is characterized in that a plurality of slits intersecting each other are formed in the both end surfaces of the cutout.

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