JP2004333225A - Tank corrosion diagnostic method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tank corrosion diagnostic method for retaining reliability without affecting the diagnosis of corrosion even if sludge precipitates. <P>SOLUTION: In the corrosion diagnostic method in a tank for storing liquid W by coating the inner surface of the tank 1 made of steel consisting of a cylindrical drum section 1A and a disc-like bottom section 1B before setting it longitudinally, vibration meters 2A, 2B, 2C are provided on the outer surface of the bottom section 1B, Lamb waves 7 that are generated in rust 4 and are propagated to the bottom surface 1B are detected to diagnose corrosion. Even if sludge S precipitates from the liquid W and is deposited at the bottom section 1B, corrosion and leakage can be reliably detected for diagnosis. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of diagnosing corrosion of a tank storing a liquid, and more particularly to a method of diagnosing corrosion of a tank suitable for diagnosing corrosion of a tank storing a liquid in which sludge or powdery solid matter easily precipitates.
[0002]
[Prior art]
In thermal power plants, nuclear power plants, chemical plants, etc., for example, to store liquids such as water, various tanks are required, but in the case of such a tank, usually, a vertical and substantially cylindrical shape Steel containers are often used.
[0003]
Incidentally, in the case of such a steel tank, rust prevention is indispensable. Therefore, it is customary to apply a rust prevention coating such as an epoxy resin on the inner surface of the container.
[0004]
However, even if such a rust-preventive coating is applied, it is necessary to check the soundness of the tank over a long period of use. Methods are applied, and a typical one is a so-called ultrasonic diagnostic method.
[0005]
Therefore, the prior art of this ultrasonic diagnostic method will be described with reference to FIG. 7. The tank 1 shown here is composed of a substantially cylindrical body 1A and a disk-shaped bottom 1B bulging outward in a dome shape. Made of steel containers.
[0006]
Then, on the inner surface of the tank 1, that is, on the inner surfaces of the body portion 1A and the bottom portion 1B, a rust preventive coating of epoxy resin is applied in a thickness of 0.5 mm to 1.0 mm, although not shown in this figure. As shown in the figure, the support member 10 is installed vertically with the bottom 1B facing down.
[0007]
Here, at the time of the corrosion diagnosis, first, the vibrometer 2 is attached to the outer periphery of the body 1A of the tank 1 as shown in the figure. At this time, a vibration detector of a type that detects acceleration by a piezoelectric element (piezoelectric ceramic element) is usually used as the vibrometer 2, and for example, an attractive force of a permanent magnet is used for attachment.
[0008]
This prior art detects an elastic wave called acoustic emission (for example, see Patent Document 1) that appears when rust is generated in the tank 1, and diagnoses corrosion due to rust. Is transmitted to the oscillograph 13 and the personal computer 14 via a cable 12.
[0009]
The data is processed by the personal computer 14 to diagnose the occurrence of rust and the degree of corrosion due to the rust. At this time, the waveform of the detection signal is displayed on the oscillograph 13 and the processed diagnostic result is displayed on the personal computer 14. Is displayed on the monitor.
[0010]
Although not particularly related to the generation of rust, a technique for detecting a defect in an object using a Lamb wave is also known (see Patent Document 2). Here, the Lamb wave is an elastic wave propagating through the object. At this time, the slowest propagation speed is a so-called _A0 mode Lamb wave propagated as bending vibration of the plate-like object.
[0011]
[Patent Document 1]
JP-A-3-243846
[Patent Document 2]
JP-A-2002-195987
[Problems to be solved by the invention]
The prior art described above does not consider sludge settling in the tank, and has a problem that the longer the service period of the tank, the more difficult it is to make a corrosion diagnosis.
[0014]
Here, sludge usually consists of a mixture of a solid powdery sediment and a viscous liquid sediment, and as the tank is used for a long period of time, the sedimentation increases, and eventually a considerable thickness is formed from the bottom of the tank. Reach you.
[0015]
At this time, in the related art, since the elastic wave propagating through the liquid in the tank is detected, if the sludge precipitates, the elastic wave propagates through the sludge depending on the place where the rust occurs in the tank. To reach the vibrometer provided in the tank body.
[0016]
However, in the portion of the liquid where the sludge has settled, the elastic wave propagating there is greatly attenuated. Therefore, as a result, in the prior art, even if rust is generated, there is a possibility that the rust cannot be detected due to sludge precipitation, and therefore, the corrosion diagnosis becomes difficult. is there.
[0017]
Here, the probability of occurrence of corrosion or leakage expected in such a tank increases as the tank is used for a long period of time, so that the necessity of a corrosion diagnosis increases. It is useless when it matters, and reliability cannot be maintained.
[0018]
SUMMARY OF THE INVENTION An object of the present invention is to provide a method for diagnosing corrosion of a tank, in which even if sludge precipitates, the corrosion diagnosis is not affected and reliability can be maintained.
[0019]
[Means for Solving the Problems]
The object is to provide a method for diagnosing corrosion of a tank in which an inner surface of a steel container having a substantially cylindrical body and a substantially disk-shaped bottom is coated with a rust-preventive coating and then stored vertically to store a liquid. Provided on the outer surface of the bottom of the steel container is a vibration detecting means that sets the thickness direction of the member constituting the bottom to an acceleration detection direction, and the corrosion generated in the steel container is removed from a portion where the corrosion occurs. The diagnosis is achieved based on the Lamb wave transmitted to the vibration detecting means via the bottom.
[0020]
In this case, the Lamb wave is also be a Lamb wave of A ₀ mode can achieve the above object, further, the vibration detecting means is disposed at least three in a position away from each other at the outer surface of said bottom The above object can also be achieved by specifying the position of the corroded portion generated in the steel container on the basis of a difference in detection time of Lamb waves detected by the at least three vibration detecting means.
[0021]
According to the above means, the Lamb wave generated from the corroded portion of the steel tank is detected by the vibration detecting means (piezoelectric ceramic vibrometer) installed at the bottom of the tank, and thereby the corrosion and the leakage are inspected.
[0022]
An epoxy resin, for example, having a thickness of 0.5 to 1.0 mm is applied to the inner surface of the steel tank as a rust-preventive coat. At this time, corrosion occurs and grows in a gap between the epoxy resin coat and the steel plate. Due to the growth of the rust, micro-vibration is generated by peeling of the resin coat of several tens of μm or self-destruction of the rust itself, and a Lamb wave is generated and propagated.
[0023]
The frequency of the Lamb wave generated by the growth of rust at this time is from several tens of kHz to several hundreds of kHz, and the frequency band varies depending on the material of the tank steel plate and the rust-preventive coat and the state of rust. Further, when the corrosion penetrates at the corroded portion and the liquid inside leaks, a small but Lamb wave is generated due to the fluid force vibration accompanying the liquid outflow from the penetrated portion.
[0024]
These Lamb waves propagate through the bottom plate of the tank. When sludge accumulates at the bottom from liquid retained in the tank, the sound wave (longitudinal wave) propagating in the liquid is greatly attenuated to minus 20 dB or more at 1 m, and therefore, is detected when the tank has a diameter of several meters. Becomes impossible.
[0025]
On the other hand, the Lamb wave propagates through the bottom member (also referred to as a mirror plate), so that the attenuation is small and detection is possible. In other words, the Lamb wave is less susceptible to attenuation due to sediment in the tank, and can be measured because the attenuation per unit length is small.
[0026]
Vibration mode of a Lamb wave in this case mainly the A ₀ mode propagates in-plane vibration, therefore, it become possible to efficiently detected by the vibration detecting means installed in the end plate of the tank bottom.
[0027]
Further, by providing three or more vibration detecting means on the outer peripheral portion of the tank bottom plate, it is possible to identify a position where a Lamb wave is generated from a time difference from a portion where corrosion progresses to each vibration detecting means. Further, when Lamb waves are frequently generated from the corroded part, the corroded part and the leaked part can be specified by integrating and displaying the Lamb wave arrival number at each position.
[0028]
The effect of the above means is that, in a steel tank storing a liquid in which sludge settles, a ram wave generated from a corrosion progressing part and a leaking part propagates through a tank bottom end plate and is detected by a vibration detecting means installed at the tank bottom. By doing so, it is possible to detect and specify the location of corrosion and leakage of the steel tank.
[0029]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a tank corrosion diagnosis method according to the present invention will be described in detail with reference to the illustrated embodiments.
[0030]
FIG. 1 shows an embodiment of the present invention. In the drawing, a tank 1 is a tank for which corrosion is to be diagnosed. The tank 1 has a substantially cylindrical body 1A and a dome-shaped bulging disk ( It is made of a steel container having a bottom (substantially disk shape) 1B. This point is the same as in the case of the prior art in FIG.
[0031]
Although not shown, an inner surface of the tank 1, that is, the inner surface of the body 1A and the bottom 1B, is coated with a rust-preventive coating of epoxy resin with a thickness of 0.5 mm to 1.0 mm. , With the bottom 1B facing down, and vertically installed by the support member 10. This point is the same as in the case of the conventional technique described with reference to FIG.
[0032]
Here, in the embodiment of FIG. 1, reference numerals 2A, 2B and 2C denote vibrometers, which are basically the same as the vibrometer 2 in the prior art of FIG. 7 differs from the prior art shown in FIG. 7 in that the mounting position is the bottom 1B of the tank 1. Next, they are also constituted by a vibrometer in which the thickness direction of the bottom 1B is used as the acceleration detection direction. Technology is different.
[0033]
Thus, the Lamb wave (described later) propagating to the bottom portion 1B can be predominantly detected by the vibrometers 2A, 2B, and 2C, which is a feature of this embodiment.
[0034]
These three vibrometers 2A, 2B, and 2C are individually connected to an oscillograph 13 and a personal computer 14 by three cables 12, and a detection signal is taken into the personal computer 14, and data processing is performed to generate rust and corrode. At this time, the waveform of the detection signal is displayed on the oscillograph 13 and the diagnosis result is displayed on the monitor of the personal computer 14. This point is not much different from the prior art.
[0035]
Next, the operation of this embodiment will be described. Here, first, at the time of the corrosion diagnosis, three vibrometers 2A, 2B, and 2C are attached to the bottom 1B. At this time, the vibrometers 2A, 2B, and 2C are located near the periphery of the bottom 1B. Are arranged at equal intervals on a circle at an equal distance from the center.
[0036]
At this time, as these vibrometers 2A, 2B, and 2C, those of a type that detects acceleration by a piezoelectric element (piezoelectric ceramic element) are usually used, and for attachment, for example, an attractive force of a permanent magnet is used. . This point is not much different from the prior art.
[0037]
In this embodiment, the ram wave generated when rust is generated in the tank 1 is detected by the vibrometers 2A, 2B, and 2C to diagnose corrosion due to rust. Will be described with reference to FIG.
[0038]
As in the tank 1 shown in FIG. 1, a steel tank for storing liquid is coated with an epoxy resin or the like having a thickness of about 0.5 to 1.0 mm for rust prevention as described above. Has a rust-proof coating. Here, FIG. 2 shows the rust-proof coating 3 applied to the bottom 1B of the tank 1 at this time.
[0039]
However, even when the rust preventive coating 3 is provided in this manner, rust 4 may be generated between the inner surface of the bottom 1B and the rust preventive coating 3 as shown in FIG. At this time, once the rust is generated, the generated rust 4 gradually grows, and finally, there is a possibility that the bottom 1B may even be penetrated by the rust.
[0040]
It is known that when the thickness of the rust 4 approaches millimeters in the rust growth process at this time, the grown rust 4 expands at a certain growth rate. Delamination 5 appears, causing a partial fracture phenomenon.
[0041]
When the breaking phenomenon occurs due to the peeling 5 of the rust preventive coating 3 in this manner, a minute local out-of-plane vibration 6 is generated on the bottom 1B of the tank 1 and the Lamb wave 7 is generated by the out-of-plane vibration 6. It is.
[0042]
At this time, as described above, as the rust 4 grows, the rust-preventive coating 3 peels off by about several tens of μm, or the rust 4 itself self-destructs to generate micro vibrations. As a result, Lamb waves and the like are generated. It is also considered to be done.
[0043]
The frequency of the Lamb wave 7 generated by the growth of the rust 4 at this time is from several tens of kHz to several hundreds of kHz, and the frequency band varies depending on the steel plate and the material of the rust-preventive film constituting the tank 1. Also depends on
[0044]
At this time, when the corrosion penetrates at a portion corroded by the rust 4 and the liquid inside leaks, a small but Lamb wave is generated due to the flow force vibration accompanying the liquid flowing out from the penetrated portion. Is also as described above.
[0045]
Here, the state of generation of the Lamb wave 7 depends on the degree of progress of corrosion, and the probability of its detection depends on the detection sensitivity of the vibrometer, but occurs several times an hour and is detected. It is known empirically.
[0046]
The Lamb wave 7 thus generated propagates in the plane direction on the bottom 1B of the tank 1 as shown in the figure, and reaches the vibrometers 2A, 2B, and 2C in the peripheral portion. At this time, as described above, these vibrometers 2A, 2B, and 2C are configured by vibrometers that use the thickness direction of the bottom 1B as the acceleration detection direction.
[0047]
As a result, the Lamb wave 7 generated by the rust 4 is efficiently detected by the vibrometers 2A, 2B, and 2C, detected as acceleration, and a detection signal is supplied to the oscillograph 13 and the personal computer 14.
[0048]
Also, at this time, an elastic wave having a frequency higher than the audible frequency, that is, an ultrasonic wave 8 is also generated. It hardly propagates 1B.
[0049]
Here, FIG. 3 shows a situation where the liquid W such as water is stored in the tank 1 and the sludge S has settled, and FIG. 4 shows this situation in an enlarged manner. Hereinafter, the propagation state of the wave caused by the rust 4 generated on the bottom 1B of the tank 1 will be described with reference to FIGS.
[0050]
First, since the Lamb wave 7 propagates as bending vibration of the bottom 1B, even if the sludge S accumulates in the tank, it propagates almost without being affected, and the attenuation is only about several dB per 1 m of the propagation distance.
[0051]
Therefore, in the case of this embodiment, the Lamb wave 7 is reliably detected by the vibrometers 2A, 2B and 2C while maintaining a good S / N (noise-to-signal ratio).
[0052]
On the other hand, ultrasonic waves are hardly attenuated when propagating in liquid (water). However, when the sludge S is deposited in the liquid W, the mixture of the powdery solid and the viscous liquid, which are components of the sludge S, receives a large attenuation of several tens of dB at a propagation distance of 1 m. Therefore, as described in the related art, detection becomes almost impossible.
[0053]
For this reason, the ultrasonic waves 8 are attenuated in the liquid W on which the sludge S is deposited, and are below the detection limit by the vibrometers 2A, 2B, and 2C installed at the bottom 1B of the tank 1, and therefore, are not detected in this embodiment. The vibration caused by the Lamb wave 7 is only.
[0054]
Here, FIG. 3 shows a state in which vibrations of three channels are measured by the vibrometers 2A, 2B, and 2C. Corrosion due to rust 4 is determined based on a time difference between signals of the respective vibrometers 2A, 2B, and 2C. It is installed so that the position where the error occurs can be specified.
[0055]
At this time, the installation of the vibrometers 2A, 2B, and 2C is reliable and simple as described above, since the tank 1 is made of steel. Good.
[0056]
The electric signals detected by the vibrometers 2A, 2B, and 2C are transmitted through individual cables 12, displayed in a waveform on an oscillograph 13, and subjected to data processing by a personal computer 14 to specify the generation position.
[0057]
Here, FIG. 5 shows, as described above, the three vibrometers 2A, 2B, and 2C arranged at equal intervals on the circumference at an equal distance from the center near the periphery of the bottom 1B of the tank 1. 1 shows an example of the obtained electrical output signal.
[0058]
In FIG. 5, the horizontal axis represents time in units of microseconds, and the vertical axis represents voltage (comparison value V). 1 is the output of the vibrometer 2A, and ch. 2 is the output of the vibrometer 2B, ch. Reference numeral 3 denotes an output of the vibration meter 2C.
[0059]
In this case, the time when the Lamb wave 7 arrives at each of the vibrometers 2A, 2B, and 2C is proportional to the distance of the generation position of the Lamb wave 7 from the center of the bottom 1A, and the time difference (shift) is on the order of microseconds. ) Occurs.
[0060]
Therefore, from the time difference and the propagation speed of the Lamb wave 7 (approximately 3000 m / sec), the position at which corrosion by the rust 4 has occurred can be calculated on the order of 10 mm, and the position of the rust 4 can be specified.
[0061]
Since the propagation speed V of the Lamb wave 7 at this time is determined by the material, shape, dimensions, and the like of the tank 1, it can be calculated and obtained in advance, but strictly, it is desirable to actually measure and calibrate. .
[0062]
Next, FIG. 6 shows the number of occurrences of the signal at the generation position P of each Lamb wave on the tank bottom (end plate) calculated from the arrival time difference of the Lamb wave detection signal by the above-described three-channel vibrometer. It is an example of a dimensional graph.
[0063]
At this time, in a place where corrosion due to rust is progressing, there is accumulation of the number of hits several times or more even in a measurement period of about one hour. On the other hand, thermal expansion and accidental vibration are at most once or twice at most. Therefore, from the three-dimensional graph display shown in FIG. 6, the corrosion of the bottom 1B of the tank 1 can be detected at an early stage. It can be seen that according to embodiments of the invention, appropriate leakage prevention measures are possible.
[0064]
By the way, in the above embodiment, the case where the tank 1 is constituted by the cylindrical body 1A and the disk-shaped bottom 1B bulging outward in a dome shape has been described. It is needless to say that a tank having a vertical body made of a steel container having a body portion and a substantially disk-shaped bottom portion can be embodied regardless of a fine shape.
[0065]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, since corrosion can be reliably detected even if sludge precipitates, accurate corrosion diagnosis is always obtained and the soundness of the tank can be easily determined.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing one embodiment of a tank corrosion diagnosis method according to the present invention. FIG. 2 is a cross-sectional view for explaining a principle of Lamb wave generation due to rust.
FIG. 3 is a sectional view of a tank to which one embodiment of the present invention is applied.
FIG. 4 is an enlarged sectional view of a tank to which one embodiment of the present invention is applied.
FIG. 5 is a waveform chart showing an example of a measurement result according to an embodiment of the present invention.
FIG. 6 is a graph illustrating an example of a measurement result according to an embodiment of the present invention, which is displayed in three dimensions.
FIG. 7 is an explanatory view showing an example of a conventional tank corrosion diagnosis method.
[Explanation of symbols]
1 tank 1A body 1B bottom (end plate)
2A, 2B, 2C Vibration meter (vibration detector)
3 Rust prevention coating 4 Rust 5 Peeling (peeled part appeared in rust prevention coating 3)
6 Micro vibration generator 7 Lamb wave ( _A0 mode Lamb wave)
8 Ultrasonic wave 10 Support member 12 Cable 13 Oscillograph 14 Personal computer S Sludge W Liquid (water)

Claims (3)

In a corrosion diagnosis method for a tank in which a rust-proof coating is applied to the inner surface of a steel container consisting of a substantially cylindrical body and a substantially disk-shaped bottom, and a liquid is stored vertically,
On the outer surface of the bottom of the steel container, provided is a vibration detecting unit that sets a thickness direction of a member constituting the bottom to an acceleration detection direction,
A method for diagnosing corrosion of a tank, wherein the corrosion of the steel container is diagnosed based on a Lamb wave propagated from the site where the corrosion occurred to the vibration detecting means via the bottom portion. In the method for diagnosing tank corrosion according to claim 1,
A tank corrosion diagnostic method, wherein the Lamb wave is an _A0 mode Lamb wave. In the tank corrosion diagnosis method according to claim 1 or 2,
At least three vibration detecting means are installed at positions separated from each other on the outer surface of the bottom portion,
A tank corrosion leak diagnosis, wherein a position of a corrosion site generated in the steel container is specified based on a difference between detection times of Lamb waves detected by the at least three vibration detection means. Method.

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