JP3841794B2 - Inspection method for corrosion and thinning by the two-probe method - Google Patents

Description

  It relates to the inspection method of metal corrosion and thinning by ultrasonic inspection method, especially corrosion depth, thinning depth and residual thickness of piping, equipment and bottom plate in chemical plant, petroleum plant, nuclear power plant etc. It is related to the inspection.

  In the inspection of corrosion and thinning by the ultrasonic inspection method, there is a method of measuring the plate thickness using a vertical probe. It is still effectively used as a method for precisely measuring the thinning position and thinning depth. However, the method using a vertical probe cannot be inspected unless there is a space for placing the probe on the surface of the inspection site, and there are obstacles such as H-shaped steel at regular intervals like piping in chemical plants. It was not possible to adapt to a certain place or a place where a patch plate or support material was welded. In addition, the bottom plate where the backing plate and the support material are welded to the bottom plate of the oil tank could not be inspected.

  In the inspection of chemical plant piping, there is also a method to inspect using the difference in ultrasonic wave propagation distance when the surface of the inspection site cannot be touched directly when riding on H-shaped steel called rack . However, in the method of inspecting only with the surface wave, for example, when the surface is greatly rough, the surface wave is greatly attenuated and measurement is often difficult, and the difference in the propagation distance of the surface wave is used. The method has a drawback that only the surface can be inspected, and an error occurs in the estimated depth due to the shape of the corrosion state.

According to the present invention, the inspection can be performed at a higher speed than the method using the vertical probe, and the thinning of the front and back surfaces is affected by the shape as compared with the method using the difference in surface wave propagation distance. It is now possible to measure the exact thickness of the thinning without having to.
Patent Publication No. 2002-5905

  Piping in chemical plants or the like rides on H-shaped steel called racks at certain intervals. For the part in contact with this H-shaped steel, optical means such as visual inspection cannot be used even if corrosion occurs in the pipe, and plate thickness measurement using a vertical probe cannot be performed. There was no proper way of inspection. Therefore, a method for rationally inspecting such a portion that cannot be visually observed and measured is provided. Structures such as oil tanks must be regularly inspected for bottom plates. In a method by plate thickness measurement using a general vertical probe, there is an obstacle such as a support welded portion, and a method for rationally inspecting a portion that cannot be inspected below is provided.

In the present invention, two or more probes are arranged so as to face each other at a position sandwiching the inspection site of the inspection object. Ultrasonic waves are incident on the material to be inspected from one probe. The incident ultrasonic wave becomes a longitudinal wave that propagates in the vicinity of the surface of the material to be inspected and travels toward another probe. Longitudinal waves traveling near the surface always generate transverse waves by mode conversion of ultrasonic waves during propagation. When the angle of the transverse wave generated by the mode conversion is θ, it is expressed by equation (1).

Here, if the transverse wave velocity in steel is 3230 (m / s) and the longitudinal wave velocity is 5920 (m / s), the transverse wave angle is about 33 degrees. Therefore, the longitudinal wave that travels in the vicinity of the surface always changes its mode when propagating on the surface of the material to be inspected, and proceeds while generating a transverse wave at an angle of about 33 degrees. There is a phenomenon in which a transverse wave falls at an angle. The transverse wave that has reached the back surface undergoes mode conversion again on the back surface, and a longitudinal wave is generated. This longitudinal wave propagates as a longitudinal wave that propagates in the vicinity of the front surface of the back surface and a longitudinal wave that crosses the material to be inspected. Longitudinal waves propagating on the back surface are further mode-converted during propagation to generate transverse waves at an angle of about 33 degrees, resulting in a phenomenon in which the transverse waves continuously traverse from the back surface to the surface of the material being inspected. Yes.

When this phenomenon is used, if the material to be inspected is corroded or thinned, the propagation distance of the transverse wave is shortened. Although the propagation distance of the longitudinal wave changes slightly, it is actually a minute change that does not affect the measurement result. The present invention is an inspection method in which the difference in propagation distance is measured using the properties of the ultrasonic waves, and the corrosion depth and the thinning depth are measured. Evaluation can be facilitated by using two-dimensional visual representation at least once or several times in the ultrasonic propagation path.

  The evaluation method by the attenuation amount of ultrasonic propagation and the evaluation method by increasing the detour time due to the unevenness of the surface wave propagating on the surface (Patent Document 1) are greatly affected by the attenuation of the transmission amount due to the surface condition. There is little influence of the situation, and the work efficiency is improved because it is a simple process in the field work.

  Corrosion inspection of pipes on racks is subject to inspection on both the outer surface and the inner surface, so the method of the present invention is the same regardless of whether the surface on which the probe is placed or the opposite surface. This is effective because

  If the arrival time of the ultrasonic wave is displayed on the vertical axis and the probe coordinates are displayed on the horizontal axis as the measurement results, the corrosion depth and corrosion length for each coordinate can be evaluated. The same applies to the flat plate. If the moving direction of the probe is changed, two-dimensional corrosion coordinates can be obtained.

The principle of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the path of the ultrasonic wave that first reaches the receiving probe when the ultrasonic wave is incident on the inspection object. The distance between the probes is W, the thickness of the material to be inspected is T, the acoustic velocity of the longitudinal wave is S ₁ , and the acoustic velocity of the transverse wave is S ₂ . The ultrasonic propagation time t ₀ of the path in FIG.
It becomes.

Next, Fig. 2 shows the path of ultrasonic waves that reach from the first bottom surface. This is a path through which the longitudinal wave reflected from the bottom surface is received by the receiving probe after the longitudinal wave is incident on the inspection object from the probe. The ultrasonic propagation time t ₁ of the path in FIG.
It becomes.

Fig. 3 shows the path of ultrasonic waves that arrive from the bottom with a single delay. When a longitudinal wave is incident on the inspection object from the transmitting probe, the transverse wave generated by the mode conversion crosses the inspection object, and when the transverse wave reaches the bottom surface, the longitudinal wave generated by the mode conversion is further received by the receiving probe. This is the path to reach the tentacle. The ultrasonic wave propagation time t _{2 in} FIG.

It becomes. If the value of L increases at this time, the propagation time increases. However, considering the path shown in FIG. 4, the transverse wave crosses at a position symmetrical to FIG. This is the midpoint position of the acoustic probe. Since the plate thickness T is sufficiently smaller than the inter-probe distance W, the measurement error is sufficiently within the allowable range if the first ultrasonic waveform is measured. FIG. 5 shows changes in the ultrasonic path when a thinned portion exists in this path. The distance of the transverse wave and the longitudinal wave that crosses the plate thickness is shortened depending on the thickness of the propagation distance. Accordingly, the corrosion depth and the thinning depth can be measured by measuring the difference between the propagation time when corrosion and thinning are present and the propagation time when there is no corrosion and thinning.

  Fig. 6 shows the path of an ultrasonic wave that arrives from the bottom surface with a delay of two times. The two-time delayed path is a path in which the transverse wave crosses twice in the thickness direction. As shown in FIG. 7, two paths are generated in the transverse distance of the transverse wave due to corrosion. In general, when the area of the thinned portion to be measured is large, measurement can be performed with the one-time delay path shown in FIG. 5, so measurement is performed with the path of FIG. Measurement accuracy can be ensured by measuring with a delay of 2 times.

  Similarly, FIG. 8 and FIG. 9 show the paths of ultrasonic waves that arrive from the opposite face with a delay of three times. In this case as well, the measurement can be performed without missing a smaller area of corrosion and thinning by sequentially performing the same as described above.

  The steel plate with a thickness of 12 mm shown in Fig. 10 has a semi-elliptical population thinning A with a diameter of 25 mm and a depth of 1.1 mm, a semi-elliptical thinning B with a diameter of 38 mm and a depth of 1.7 mm, and a semi-elliptical with a diameter of 56 mm and a depth of 3.4 mm. The result of making and inspecting the type artificial thinning C is shown. As can be seen from FIG. 10, the waveform of population thinning C can be read sufficiently in the one-lag route, but the other two population thinnings are difficult to read. If you look at the results of the two-time delay path, you can read population thinning B and C, but population thinning A is still difficult to see. You can see all of the thinning A, B, and C on the route that is delayed three times.

  Measured for population thinning C with a one-time delay path, the propagation time of the bottom echo is 46.6μS, and the shortened propagation time due to population thinning C is 45.7μS, so the difference in arrival time is 0.9μS Out. On the other hand, the propagation time is calculated assuming that the plate thickness T is 12mm in the equation (4), and the propagation time is calculated in the equation (4) with the plate thickness T of the reduced thickness of 3.4mm being 12mm-3.4mm = 8.6mm When the difference in propagation time is obtained, it becomes 0.9 μS, which is the same.

The propagation time t ₃ of the path delayed twice is expressed by the formula (5).

When artificial thinning B is measured with a delay of 2 times, the propagation time of the bottom echo is 50.0μS, and the shortened propagation time due to population thinning B is 49.5μS, so the difference in propagation time is 0.5 μS. On the other hand, when the propagation time difference is calculated using the equation (5) in the path of FIG. 7, it is 0.44 μS, which is almost the same. When artificial thinning A is measured, the shortened propagation time due to population thinning A is 49.7 μS, so the difference in propagation time is 0.3 μS. Similarly, when the propagation time difference is calculated by equation (5), it is 0.28 μS, which is almost the same. Thus, since it corresponds with the calculated value, the thickness reduction can be measured from the difference in propagation time.

  When performing a corrosion inspection for maintenance, an ultrasonic inspection that can measure the width and depth of thinning even when the thinned portion is covered with other members in a shorter inspection time than the conventional method. Is the method.

Illustration of the first ultrasound path Illustration of the path of ultrasonic waves from the first bottom Illustration of the path of the ultrasonic wave delayed once Explanatory diagram of ultrasonic wave path 2 delayed once Explanatory drawing when there is a thinning part in the path of ultrasonic waves delayed once Illustration of the path of ultrasonic waves delayed twice Explanatory drawing when there is a thinned part in the path of ultrasonic waves delayed twice Illustration of the path of the ultrasonic wave delayed 3 times Explanatory drawing when there is a thinning part in the path of ultrasonic waves delayed three times Figure of measurement results

Claims (2)

Transverse waves generated continuously by mode conversion from longitudinal waves traveling near the surface incident on the inspection material from the ultrasonic probe placed across the inspection site across the inspection material and transverse waves crossing the inspection material When a wave reaches the back surface of the material to be inspected, a longitudinal wave is generated by mode conversion, and a transverse wave is continuously generated from the longitudinal wave propagating in the vicinity of the back surface by the mode conversion and crosses the material to be inspected. Longitudinal waves are generated by mode conversion when the surface of the inspection material is reached, and longitudinal waves and transverse waves have the property of repeating mode conversion such that longitudinal waves are also generated by mode conversion. Use the ultrasonic wave path that the transverse wave traverses the material to be inspected at least once among the paths that the transverse wave propagates by repeating the mode conversion, and in the direction that intersects the two facing probes. The depth and reduction of corrosion , characterized by moving to express the difference in the propagation time of longitudinal and transverse waves caused by the presence of corroded and thinned parts and the intensity of ultrasonic waves, and evaluating and measuring from the images Inspection method for corrosion and thinning by the two-probe method to evaluate the depth of the meat. In the invention of claim 1, the echo reflected by the bottom surface measured every time the transverse wave generated by the mode conversion of the longitudinal wave transmitted in the vicinity of the surface crosses the material to be inspected in the plate thickness direction, and the corroded portion and the thinned portion Measure the difference in propagation time from the echo when the propagation distance is shortened by, and calculate the corrosion depth and thinning depth from the measured propagation time difference. Inspection method.