JP6235839B2 - Method for inspecting micro crack on inner surface of tube and apparatus for inspecting micro crack on inner surface of tube - Google Patents

Description

  The present invention relates to a method for inspecting a micro crack on the inner surface of a pipe and an apparatus for inspecting a micro crack on the inner surface of a pipe that can inspect the micro crack generated on the inner surface of the pipe.

A tube defect inspection method for inspecting defects generated on the outer surface or inner surface of a tube by a guide wave is widely known.
For example, it is known that a probe is provided at one end of a tube, a guide wave is transmitted from the probe, and a reflected wave reflected by a defect is received by the probe. According to this method, a corrosion-reduced defect of 20% or more of the wall thickness of the pipe (defect of 1 mm or more in depth for a 5 mm-thick pipe) or a crack defect (crack of 50% or more of the wall thickness of the pipe). ) (A defect having a depth of 2.5 mm or more in a 5 mm thick tube) can be detected (for example, see Patent Document 1).

JP 2005-10055 A

However, the defect inspection method described in Patent Document 1 cannot detect a microcrack having a depth of 300 μm.
Also known is an apparatus that transmits ultrasonic waves from the outer periphery of the tube toward the center of the tube and receives ultrasonic waves reflected by the inner peripheral surface of the tube. In this method, the ultrasonic wave reflected by the fine crack is mixed with the ultrasonic wave reflected by the inner peripheral surface of the tube, and only the ultrasonic wave reflected by the fine crack cannot be extracted. As a result, a microcrack having a depth of 300 μm cannot be detected.
Therefore, a part of the tube to be inspected is cut out and cut in the axial direction of the tube, thereby confirming whether or not a micro crack has occurred in the tube.
In view of the above circumstances, the present invention provides a method for inspecting a micro crack on the inner surface of a pipe and a micro crack inspecting apparatus for the inner surface of the pipe capable of detecting a micro crack generated on the inner surface of the pipe without cutting the pipe to be inspected. For the purpose.

The present invention transmits a Lamb wave from a transmitting probe installed on the outer peripheral surface of a tube to be inspected, and transmits the Lamb wave from a receiving probe installed at a predetermined interval from the transmitting probe on the outer peripheral surface of the tube. A transmitting / receiving step for receiving, a peak identifying step for identifying the peak of the A0 mode wave and the peak of the S0 mode wave from the Lamb wave received by the reception probe, and the wave of the S0 mode from the peak of the A0 mode wave A peak detection step for detecting whether or not there is a third peak between the peaks, and a determination for determining that there is a microcrack on the inner surface of the tube when the third peak is detected in the peak detection step And a process.
According to the present invention, when the third peak is detected between the peak of the A0 mode wave and the peak of the S0 mode wave, it is determined that there is a microcrack on the inner surface of the tube. Thereby, even if it does not cut | disconnect the pipe | tube to test | inspect, it can be determined whether there exists a micro crack in the inner surface of a pipe | tube.

The present invention relates to a transmission probe installed on the outer peripheral surface of a tube to be inspected, a reception probe installed on the outer peripheral surface of the tube at a predetermined interval from the transmission probe, and directed from the transmission probe to the reception probe. Transmitting means for transmitting a Lamb wave, receiving means for receiving the Lamb wave transmitted from the transmitting means from the reception probe, A0 mode wave peak and S0 mode wave from the Lamb wave received by the receiving means. Peak specifying means for specifying the peak of the first and peak detecting means for detecting whether or not there is a third peak between the peak of the wave of the A0 mode specified by the peak specifying means and the peak of the wave of the S0 mode; And determining means for determining that there is a microcrack on the inner surface of the tube when the peak detecting means detects the third peak.
According to the present invention, when the third peak is detected between the peak of the A0 mode wave and the peak of the S0 mode wave, it is determined that there is a microcrack on the inner surface of the tube. Thereby, even if it does not cut | disconnect the pipe | tube to test | inspect, it can be determined whether there exists a micro crack in the inner surface of a pipe | tube.

In one aspect of the present invention, the peak specifying means includes an A0 peak specifying means for specifying an A0 mode wave peak from the Lamb wave received by the receiving means, and an A0 mode wave specified by the A0 peak specifying means. It is preferable to include an envelope creating unit that creates an envelope of the Lamb wave based on a peak, and an S0 peak identifying unit that identifies an S0 mode wave peak from the envelope created by the envelope creating unit. .
In this way, the peak of the A0 mode wave and the peak of the S0 mode wave can be easily detected.

In the aspect of the invention, it is preferable that the predetermined interval is an interval at which the determination unit can specify the peak of the wave in the A0 mode and the peak of the wave in the S0 mode.
In this way, the peak of the A0 mode wave and the peak of the S0 mode wave can be reliably identified.

In one aspect of the invention, it is preferable that the predetermined interval is determined by the thickness of the tube and the frequency of the Lamb wave.
In this way, the predetermined interval can be determined by the thickness of the tube and the frequency of the Lamb wave.

  As described above, according to the present invention, when the third peak is detected between the peak of the A0 mode wave and the peak of the S0 mode wave, it is determined that there is a microcrack on the inner surface of the tube. Even if the tube to be inspected is not cut, it can be determined whether or not there is a fine crack on the inner surface of the tube.

It is a schematic diagram which shows the outline | summary of the micro crack inspection apparatus of the pipe inner surface which is embodiment of this invention. It is a block diagram which shows the control structure of the micro crack inspection apparatus of the pipe inner surface shown in FIG. It is a figure which shows the dispersion | distribution curve for determining the frequency of the Lamb wave transmitted from a transmission probe. It is a figure which shows the distance from the transmission probe suitable for a measurement to the receiving probe when the frequency and tube thickness of the Lamb wave transmitted from a transmission probe change. It is a flowchart which shows the control procedure of the micro crack inspection apparatus shown in FIG. It is a figure which shows the waveform which the receiving means received, Comprising: The waveform of the pipe | tube without a micro crack is shown. It is a figure which shows the waveform which the receiving means received, Comprising: The waveform of the pipe | tube with a micro crack is shown. It is a figure which shows the cross section of a sample tube. It is a figure which shows the waveform which the receiving means received in the case of test | inspection of a sample tube. It is a figure which shows the state which removed the scale from the sample tube. It is a figure which shows transition of the peak of an A mode wave, and a 3rd peak in the case of test | inspection of a sample tube.

  Exemplary embodiments of a micro crack inspection method and a micro crack inspection apparatus for a pipe inner surface according to the present invention will be described below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments.

FIG. 1 is a conceptual diagram showing a micro crack inspection apparatus for a pipe inner surface according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a control configuration of the micro crack inspection apparatus for a pipe inner face shown in FIG. is there.
As shown in FIG. 1, the micro crack inspection apparatus for the inner surface of the pipe is configured to include an apparatus body 2, a transmission probe 3, and a reception probe 4. As shown in FIG. 2, the apparatus main body 2 includes an input unit 21, a setting unit 22, a transmission unit 23, a reception unit 24, a display unit 25, a determination unit 26, and a control unit 27.

  The input means 21 is for inputting various set values to the apparatus main body 2 and giving various instructions, and has various keys such as a numeric key 211 and a start key 212. The setting means 22 is for setting the frequency of the Lamb wave used for inspection of fine cracks, and sets the frequency input from the numeric keypad 211 or the like as the frequency of Lamb wave used for inspection. The Lamb wave frequency to be set has a sufficient speed difference between the A0 mode wave and the S0 mode wave dispersed from the Lamb wave, and the receiving unit 24 can identify the A0 mode wave and the S0 mode wave. The transmission unit 23 is for generating and transmitting a Lamb wave having the frequency set in the setting unit 22 and includes a connection terminal 231 to which the transmission probe 3 is connected. The receiving unit 24 is for receiving the Lamb wave transmitted from the transmitting unit 23 and includes a connection terminal 241 to which the receiving probe 4 is connected. The display means 25 is for displaying the wave received by the receiving means 24 on the display 251, and the display 251 displays a horizontal axis for displaying the time axis and a vertical axis for displaying the amplitude. The judging means 26 is for judging whether or not there is a micro crack on the inner surface of the tube T, and comprises an A0 peak identifying means 261, an envelope creating means 262, an S0 peak identifying means 263, and a peak detecting means 264. Yes.

  The A0 peak specifying means 261 is for specifying the peak PA (see FIG. 6A) of the A0 mode wave from the Lamb wave received by the receiving means 24. The peak of the Lamb wave received by the receiving means 24, That is, the largest amplitude is the peak PA of the A0 mode wave. The envelope creating means 262 is for creating the envelope E (see FIG. 6B) of the Lamb wave received by the receiving means 24. The A0 mode wave peak PA specified by the A0 peak specifying means 261 is used. A Lamb wave envelope E is created with reference to. The S0 peak specifying means 263 is for specifying the peak of the S0 mode wave from the Lamb wave received by the receiving means 24. The S0 peak specifying means 263 has a predetermined timing (the wave of the S0 mode wave) in the envelope E created by the envelope creating means 262. The amplitude that has increased again at the timing at which arrival is expected) is assumed to be the S0 mode wave peak PS (see FIG. 6B). The peak detecting means 264 is for detecting whether or not there is a third peak PC (see FIG. 7B) between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave. is there. Then, as shown in FIG. 7B, the determination unit 26 determines that there is a microcrack on the inner surface of the tube T when the peak detection unit 264 detects the third peak PC, while FIG. ), When the peak detector 264 does not detect the third peak, it is determined that there is no microcrack on the inner surface of the tube T.

  The control means 27 is for comprehensively controlling the input means 21, setting means 22, transmission means 23, reception means 24, display means 25, and determination means 26 described above. Specifically, the transmission unit 23 transmits the Lamb wave, and the reception unit 24 receives the Lamb wave transmitted from the transmission unit 23. Further, the Lamb wave received by the receiving unit 24 is displayed on the display unit 25. Further, the determination unit 26 detects whether or not there is a fine crack.

  The transmission probe 3 is for transmitting the Lamb wave generated by the transmission means 23 and is connected to a connection terminal 231 provided in the transmission means 23, and as shown in FIG. Installed on one side of the area to be inspected. Specifically, it is installed so that the transmission direction of the Lamb wave is parallel to the axis O passing through the center of the tube T.

  The receiving probe 4 is for receiving the Lamb wave in the receiving means 24 and is connected to a connection terminal 241 provided in the receiving means 24 as shown in FIG. Specifically, as shown in FIG. 1, a predetermined direction from the transmission probe 3 is set so that the reception direction coincides with the transmission direction of the transmission probe 3 on the outer peripheral surface of the tube T to be inspected and the other side of the inspection region. Install with a gap of. The predetermined interval (the interval between the transmission probe 3 and the reception probe 4) is specified by the A0 mode wave peak PA (see FIGS. 6 and 7) and the S0 mode wave peak PS (see FIGS. 6 and 7). Use an interval that is possible.

  In the case of inspecting a micro crack on the inner surface of the tube T using the micro crack inspection apparatus described above, first, a person who inspects the tube T (hereinafter referred to as “inspector”) uses an ultrasonic thickness gauge (not shown). ) To measure the thickness of the tube T to be inspected. Thereby, the thickness of the tube T to be inspected is specified. Next, the inspector determines the frequency of the Lamb wave transmitted by the transmission means 23 and the distance between the transmission probe 3 and the reception probe 4 (interprobe distance). The frequency of the Lamb wave transmitted by the transmission means 23 is determined based on the dispersion curve shown in FIG. The dispersion curve shows the relationship between the frequency of the Lamb wave and the velocity of the A0 mode and S0 mode waves dispersed from the Lamb wave, and the frequency of the Lamb wave that causes a difference in velocity between the A0 mode and the S0 mode. Is determined as the frequency of the Lamb wave transmitted by the transmission means 23. The interprobe distance is determined based on the arrival time difference between the A0 mode and S0 mode waves shown in FIG. Specifically, the arrival time difference between the waves in the A0 mode and the S0 mode is determined to be a predetermined time.

  Here, as shown in FIG. 4, the basic condition is that the tube T to be inspected has a thickness of 5 mm, the Lamb wave frequency is 1.0 MHz, and the distance between probes is 300 mm. The frequency of the Lamb wave and the interprobe distance are determined so that the arrival time difference between the S0 mode wave and the S0 mode wave is the same as the basic condition.

  The frequency of the Lamb wave is not limited to that determined by the basic conditions. As shown in FIG. 3, when the thickness of the tube T to be inspected is 5 mm, the Lamb wave frequency is 0.8 to 1.2 MHz, Preferably, it can be set arbitrarily in the range of 0.9 to 1.1 MHz. Further, when the wall thickness of the tube T to be inspected is 6.5 mm, the frequency of the Lamb wave is 0.7 to 1.2 MHz, preferably 0.7 to 1.0 as shown in FIG. Can be set. Furthermore, as shown in FIG. 3, when the thickness of the tube T to be inspected is 4.5 mm, the frequency of the Lamb wave is 0.9 to 1.2 mm, preferably 1.0 to 1.0 as shown in FIG. It can be set arbitrarily within the range of 1.2 MHz. Similarly, the interprobe distance is not limited to that determined by the basic conditions, and can be arbitrarily set as long as the arrival time difference between the waves in the A0 mode and the S0 mode can be obtained more than a certain value.

  Next, the inspector installs the transmission probe 3 and the reception probe 4 on the outer peripheral surface of the tube T to be inspected. Specifically, the transmission probe 3 is installed on one side of the region to be inspected so that the transmission direction of the Lamb wave is parallel to the axis O passing through the center of the tube T, and the other side of the region to inspect the reception probe 4 In addition, the probe is installed such that the reception direction coincides with the transmission direction of the transmission probe 3 and the interprobe distance determined from the transmission probe 3 is opened.

  Next, as shown in FIG. 5, the inspector sets the frequency of the Lamb wave determined by using the numeric keypad 211 or the like in the apparatus main body (setting unit 22) (step S1: Yes). Next, the inspector instructs the apparatus main body (control means 27) to start the inspection using the start key 212 or the like (step S2: Yes). Then, the apparatus main body (control unit 27) gives an instruction to start inspection to the transmission unit 23 and the reception unit 24. The transmission means 23 given the inspection start instruction from the control means 27 generates a set Lamb wave with a predetermined frequency and transmits a Lamb wave with a predetermined length (Step S3). As a result, a Lamb wave having a predetermined frequency is transmitted from the transmission probe 3 toward the reception probe 4. The transmitted Lamb wave of a predetermined frequency is dispersed when propagating through the region (tube T) to be inspected. On the other hand, the receiving means 24 given the inspection start instruction from the control means 27 starts to receive the Lamb wave. Thereby, the receiving means 24 receives the Lamb wave of a predetermined length from the receiving probe 4 (step S4). The Lamb waves received by the receiving means 24 are dispersed when propagating through the tube T to be inspected, and are composed of A0 mode waves and S0 mode waves. The wave in the A0 mode has a larger amplitude than the wave in the S0 mode and reaches the receiving means 24 earlier than the wave in the S0 mode. The wave in the S0 mode has a smaller amplitude than the wave in the A0 mode and the wave in the A0 mode. The reception means 24 is reached later.

  When the receiving unit 24 receives a Lamb wave having a predetermined length, the control unit 27 gives an instruction to display the waveform to the display unit 25 and gives an instruction to determine whether there is a microcrack to the determination unit 26. The display unit 25 given the waveform display instruction from the control unit 27 displays the Lamb wave of the predetermined length received by the receiving unit 24 (see FIGS. 6A and 7A) on the display 251. (Step S5).

  On the other hand, the determination unit 26 that has been instructed by the control unit 27 to determine the presence or absence of a microcrack first gives a peak specification instruction to the A0 peak specification unit 261. The A0 peak specifying unit 261 given the peak specifying instruction specifies the peak PA of the A0 mode wave from the Lamb wave received by the receiving unit 24 (step S6). As described above, the A0 mode wave has the largest amplitude among the waves received by the receiving means 24 as the peak PA of the A0 mode wave.

  When the wave peak PA in the A0 mode is specified, the determination unit 26 gives an envelope generation instruction to the envelope generation unit 262. The envelope creation means 262 given the instruction to create the envelope curve has the Lamb wave envelope E (see FIGS. 6B and 7B) based on the A0 mode wave peak PA identified by the A0 peak identification means 261. b) (see step S7).

  When the envelope E is created, the determination unit 26 gives a peak specifying instruction to the S0 peak specifying unit 263. The S0 peak specifying unit 263 given the peak specifying instruction specifies the peak PS of the wave in the S0 mode (step S8). Note that, as described above, the S0 mode wave has an amplitude that has increased again at a predetermined timing (a timing at which the arrival of the S0 mode wave is expected) in the envelope E created by the envelope creating means 262. Let it be the peak PS of the wave.

  When the peak PS of the S0 mode wave is specified, the determination unit 26 gives a peak detection instruction to the peak detection unit 264. The peak detection means 264 to which the peak detection instruction is given determines whether or not the third peak PC (see FIG. 7B) exists between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave. Is detected (step S9).

  As shown in FIG. 7, when the peak detection unit 264 detects the third peak PC, the determination unit 26 determines that there is a fine crack on the inner surface of the tube T (step S10: Yes). On the other hand, as shown in FIG. 6, if the peak detection means 264 does not detect the third peak PC, the determination means 26 determines that there is no fine crack on the inner surface of the tube T (step S10: NO).

  The micro crack inspection method described above is verified with a sample tube. The sample tube was used for the piping of a gas boiler. The outer diameter is 28.6 mm, the wall thickness is 5.3 mm, and scale accumulation and microcracks are observed on the inner surface of the sample tube. The sample tube is halved in the axial direction for verification.

  In the verification, the frequency of the Lamb wave is 1.0 MHz and the distance between the probes is 300 mm. In the state where the sample tube is halved, accumulation of scales (porous scale and dense scale) and fine cracks C are recognized as shown in FIG. The depth of the fine crack C is 272 μm.

  First, as shown in FIG. 8, in a state where scale accumulation and fine cracks are observed, the transmission unit 23 transmits the Lamb wave and the reception unit 24 receives the Lamb wave. In the Lamb wave received by the receiving means in this state, as shown in FIG. 9A, a third peak PC is recognized between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave. .

  Next, as shown in FIG. 10, the scale is removed with a toothbrush or the like, and in a state where the fine crack C is recognized, the transmission unit 23 transmits the Lamb wave and the reception unit 24 receives the Lamb wave. In the Lamb wave received by the receiving means 24 in this state, the third peak PC is recognized between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave as shown in FIG. 9B. It is done.

  Next, with the fine cracks removed by a grinder or the like, the transmission unit 23 transmits the Lamb wave and the reception unit 24 receives the Lamb wave. In the Lamb wave received by the receiving means 24 in this state, as shown in FIG. 9C, a third peak PC is recognized between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave. I can't.

  From the above, it is verified that the third peak PC is recognized between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave when there is a fine crack on the inner surface of the tube.

  As shown in FIG. 11, the amplitude of the peak PA of the A0 mode wave is found to be about 15% attenuated by the scale and about 30% attenuated by the fine cracks. As a result, the attenuation of the amplitude of the peak PA of the wave in the A0 mode can be used as an auxiliary index when inspecting the accumulation of scale and the presence or absence of fine cracks.

  Further, as shown in FIG. 11, the amplitude of the third peak PC is recognized to be attenuated by about 5% due to the scale. Thereby, the attenuation of the amplitude of the third peak PC can be used as an auxiliary index when examining the accumulation of the scale.

  The microcrack inspection apparatus according to the embodiment of the present invention described above identifies the A0 mode wave peak PA and the S0 mode wave peak PS from the Lamb wave received by the receiving means 24, and the A0 mode wave peak. When the third peak PC is detected between the PA and the S0 mode wave peak PS, it is determined that there is a microcrack on the inner surface of the tube T. Therefore, even if the tube T to be inspected is not cut, the tube T Can determine whether there are microcracks on the inner surface

  Further, the peak PS of the S0 mode wave is identified from the envelope E of the Lamb wave created based on the peak PA of the A0 mode wave, and the interval from the peak PA of the A0 mode wave to the peak PS of the S0 mode wave Therefore, it is possible to easily detect whether or not there is a third peak PC between the peak PA of the A0 mode wave and the peak PS of the S0 mode wave. .

  Further, since the interval between the transmission probe 3 and the reception probe 4 is an interval that can specify the peak PA of the A0 mode wave and the peak PS of the S0 mode wave, the peak PA of the A0 mode wave and the S0 mode wave Can be reliably identified.

  Further, since the interval between the transmission probe 3 and the reception probe 4 is determined by the thickness of the tube T to be inspected and the frequency of the Lamb wave, the predetermined interval can be determined by the thickness of the tube T and the frequency of the Lamb wave. .

  Further, in the micro crack inspection method according to the embodiment of the present invention, the peak PA of the A0 mode and the peak PS of the wave of the S0 mode are identified from the Lamb wave received by the reception probe 4, and the wave of the A0 mode is identified. When the third peak PC is detected between the peak PA and the S0 mode wave peak PS, it is determined that there is a microcrack on the inner surface of the tube T. Therefore, the tube T can be cut without cutting the tube T to be inspected. It can be determined whether or not there are fine cracks on the inner surface of the substrate.

  As described above, the present invention can detect the presence or absence of micro cracks on the inner surface of the pipe without destroying the pipe, so the presence or absence of micro cracks on the inner surface of the pipes of various pipes constituting a plant such as a gas plant. Suitable for inspection.

2 device main body 21 input means 211 numeric keypad 212 start key 22 setting means 23 transmission means 231 connection terminal 24 reception means 241 connection terminal 25 display means 251 display device 26 determination means 261 A0 peak identification means (peak identification means)
262 Envelope creation means 263 S0 peak identification means (peak identification means)
H.264 Peak detection means 27 Control means 3 Transmission probe 4 Reception probe T Tube O Axis line C Fine crack PA A0 mode wave peak E Envelope PS S0 mode wave peak PC Third peak

Claims (4)

A transmission / reception step of transmitting a Lamb wave from a transmission probe installed on the outer peripheral surface of the tube to be inspected and receiving the Lamb wave from a reception probe installed at a predetermined interval from the transmission probe on the outer peripheral surface of the tube When,
A peak identifying step of identifying the peak of the A0 mode wave and the peak of the S0 mode wave from the Lamb wave received by the reception probe;
A peak detecting step of detecting whether or not there is a third peak between the peak of the A0 mode wave and the peak of the S0 mode wave;
A determination step of determining that there is a microcrack on the inner surface of the tube when the third peak is detected in the peak detection step;
I have a,
The peak detection step includes
An A0 peak identification step for identifying the peak of the A0 mode wave from the received Lamb wave;
An envelope creating step of creating an envelope of the Lamb wave based on the peak of the wave of the A0 mode identified in the A0 peak identifying step;
An S0 peak identification step for identifying a wave peak of the S0 mode from the envelope created in the envelope creation step;
A method for inspecting microcracks characterized by comprising: A transmission probe installed on the outer peripheral surface of the pipe to be inspected;
A receiving probe installed at a predetermined distance from the transmitting probe on the outer peripheral surface of the tube;
Transmitting means for transmitting a Lamb wave from the transmitting probe toward the receiving probe;
Receiving means for receiving the Lamb wave transmitted from the transmitting means from the receiving probe;
Peak identifying means for identifying the peak of the A0 mode wave and the peak of the S0 mode wave from the Lamb wave received by the receiving means;
Peak detecting means for detecting whether or not there is a third peak between the peak of the wave of the A0 mode specified by the peak specifying means and the peak of the wave of the S0 mode;
Determining means for determining that there is a microcrack on the inner surface of the tube when the peak detecting means detects the third peak;
Equipped with a,
The peak specifying means includes
A0 peak specifying means for specifying the peak of the A0 mode wave from the Lamb wave received by the receiving means;
An envelope creating means for creating an envelope of the Lamb wave based on the peak of the wave of the A0 mode identified by the A0 peak identifying means;
S0 peak identifying means for identifying the S0 mode wave peak from the envelope created by the envelope creating means;
An apparatus for inspecting micro cracks on the inner surface of a pipe, characterized by comprising: 3. The micro crack inspection apparatus for an inner surface of a pipe according to claim 2 , wherein the predetermined interval is an interval at which the determination unit can identify the peak of the wave of the A0 mode and the peak of the wave of the S0 mode. . The micro crack inspection apparatus for a pipe inner surface according to claim 2 or 3 , wherein the predetermined interval is determined by a thickness of the pipe and a frequency of the Lamb wave.