JPS61153539A - Detecting method of leak in boiler heat conduction pipe - Google Patents

Abstract

PURPOSE:To improve decision precision by deciding leakage when either of thee increase pattern of the cumulative generation frequency of a burst type AE and the signal level of the specific frequency band of an AE original signal exceeds a set value and the plant control signal of a boiler decreases below a set value. CONSTITUTION:The burst type AE is detected 19 in the AE original signal obtained from the output of an AE sensor 5 fitted to a heat conduction pipe 2. When the increase pattern of the cumulative generation frequency of this burst type AE increases above the set increase pattern, a comparing part 24 outputs the 1st detection signal. Further, a comparing part 29 outputs the 2nd detection signal when the signal level of the specific frequency band of the AE original signal increase the set value. Then, a decision on a leak is made only when the plant control signal from the boiler decreases below the set value after at least either of the 1st and the 2nd signals is outputted. Consequently, the precision of the leak decision making is improved.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for detecting leakage in boiler heat exchanger tubes, and particularly to a method for early detection of fluid leakage in heat exchanger tubes by acoustic emission (hereinafter referred to as AE). The present invention relates to a preferred leak detection method.

[Background of the invention]

The boiler water wall header, as shown in Figure 2, is a very complicated place where the water wall pipe, cage side wall, cage floor pipe, etc. intersect, so the temperature of each pipe is particularly high when starting and stopping the boiler. Due to the difference, transient excessive thermal stress is likely to occur, and cracks often occur at the welded toes of the pipes that connect these pipes, and can propagate to the heat transfer tubes, resulting in leakage accidents.

especially.

As DDS (daily start/shutdown) operation has recently become widespread, these pipe joints are subjected to much more severe thermal cycles than in the past. Therefore, in addition to examining the structure, it is important to immediately detect any cracks that occur and cause leakage, and to prevent the cracks from expanding by changing or stopping the boiler operating conditions.

Conventional methods have been used to detect this leakage situation.

The following methods have been adopted or considered. Namely.

(b) Boiler maintenance personnel will visually inspect it.

(b) Detect audible sounds associated with leakage using a microphone.

(c) AE sound associated with leakage (high frequency sound above audible frequency)
leakage is detected based on a change in the pattern of increase in the cumulative number of sudden-onset AEs.

(d) Observe the frequency spectrum of sudden-onset AE, and detect leakage by detecting an increase in signal level in a specific frequency band.

AE is a phenomenon in which elastic waves are generated by the strain energy that is suddenly released when a material is deformed by an external force and dislocations and cracks inside the material move until it breaks. Double signal frequency is several MH from the audible frequency
It has a z component, but usually covers a range of 20 kHz to 2 MHz. Among these methods, method (a) is
The drawback is that leak detection is delayed and dangerous. In addition, method (b) is difficult to distinguish from audible sounds generated from the boiler itself, such as combustion noise, or audible sounds generated from peripheral equipment, and there are almost no practical examples of method (c) and (d). A
The method according to E will be explained in detail in the first part.

First, the method (c), that is, the conventional method of detecting AE sound (high frequency sound higher than an audible sound frequency) using an AE sensor, and its problems will be described below. When a leak occurs in a heat transfer tube, the steam pressure inside the tube is high.

High-frequency sound above the audible frequency is excited and propagates through the heat exchanger tube. Therefore, the AE sensor can be attached directly to the heat exchanger tube, or the AE sensor can be attached via a waveguide.
By attaching an E-sensor to detect high-frequency sound as an electrical signal.

Leakage can be detected. At this time, the AE multiplied signal characteristics extracted by the AE sensor include the level (amplitude) and frequency distribution, but if the presence or absence of leakage is determined only based on the magnitude of the level, the AE signal level will change due to changes in the steam flow in the heat transfer tube, etc. Because it changes, it has drawbacks such as making incorrect decisions.

On the other hand, when focusing on the frequency spectrum of the AE signal,
The AE multiplied signal frequency spectrum when there is no leakage shows a gentle pattern as shown at 15 in FIG. 5, but when leakage occurs, as shown at 16 in the same figure,
Only the signal level of a specific frequency band (near 0.4 MHz in the figure) increases.

Therefore, leakage can be detected by detecting changes in these spectra. This is what I mentioned above (
This is method 2). However, this method does not depend on the steam pressure, steam flow rate, steam temperature, etc. inside the heat transfer tube.

The frequency band in which the signal level increases due to the occurrence of leakage differs depending on the pipe diameter, etc., and it has the disadvantage that it is very difficult to set one frequency band (center frequency and band Ii).

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a leakage detection method that can reliably detect leakage from a boiler water wall header by a method using AE.

[Summary of the invention]

The present invention detects sudden type AE from the AE original signal obtained from the output of an AE sensor attached to the heat exchanger tube when detecting leakage in boiler heat exchanger tubes using AE, and detects a pattern of increase in the cumulative number of sudden type AE occurrences. A signal (first signal) to be output when the increase is greater than the set increase pattern and turn;
A signal (second signal) is output when the signal level of a specific frequency band of the E original signal is higher than the set value, and a signal (second signal) is output when the plant control signal from the boiler is lower than the set value. 3 signal), and a leak is determined only when at least one of the first signal and the second signal, which is the AE multiplied signal, and the third signal are output. When it is determined that this is the case, an alarm signal is generated.

Figure 2 shows the water wall header of the boiler. In the figure, when the boiler is started, a temperature difference of 30 to 40°C transiently occurs between the cage side tube 1 and the ice wall tube 2, so thermal stress acts, and the vicinity of the welded end of the filler bar 6 becomes stressed. This is most severe, and cracks are likely to occur at the location indicated by reference numeral 4, causing leakage and blowouts.For this reason, generally, the ice wall tube 2 and the cage side tube 1 are fixed by a reinforcing plate 3, and the location indicated by reference numeral 4 is the most severe. Although the boiler has a structure that reduces the stress acting on its surroundings, as the boiler approaches the end of its service life, leakage becomes more likely to occur due to a decrease in strength due to material deterioration.

The present invention attempts to detect such leaks and blowouts at an early stage.

The following five basic matters of the present invention will be explained in detail.

First, the installation of the AE sensor will be described. As shown in FIG. 2, the AE sensor 5 is attached to the reinforcing plate 3 and the ice wall tube 2. FIG. 3 shows the method for attaching the AE sensor to the reinforcing plate 3 and the water wall pipe 2, as well as the basic parts of the AE measurement system. The AE sensor is installed as shown in FIG. 3, since both the reinforcing plate 3 and the ice wall tube 2 are at high temperatures.

This is done via a waveguide 7 for heat insulation.

The wave guide 7 is made of stainless steel, and has a conical shape at the part where it is attached to the reinforcing plate 3 and the ice wall tube 2. Welding is not suitable for installing the waveguide 7 due to the risk of damage to the boiler heat exchanger tubes.
Bonding is performed using ceramic bond 8. The AE sensor 5 is attached to the end of the waveguide 7 with a room temperature curing adhesive. The AE sensor 5 is attached to the water wall tube 2 by processing the bottom of the conical portion of the waveguide 7 to match the curvature of the ice wall tube 2, and fixing it with a ceramic bond 8 in the same way as when attaching it to the reinforcing plate 3. .

Next, the AE measurement system shown in FIG. 3 will be explained.

The electrical signal captured by the AE sensor 5 is input to a preamplifier 9, where it is amplified by 60 dB, and its output is input to the AE measuring device 10. The AE measuring device 10 amplifies the input signal by about 40 dB and also
In order to obtain the number of E occurrences, the amplified signal is processed with an appropriate threshold value to obtain a sudden type AE multiplied signal, and one pulse signal is generated for each sudden type AE multiplied signal. This pulse signal is input to the pen recorder 11 at the next stage, and the cumulative number of AE occurrences with respect to the elapsed time is recorded. On the other hand, the AE multiplied signal frequency spectrum is observed by inputting the original signal amplified by the AE measuring device 10 (the AE multiplied signal which has not been processed by the threshold value) to the frequency analyzer 12.

Figure 4 shows an example of the cumulative number of AEs actually measured using the method described above. In Figure 6, the horizontal axis is the elapsed time;
The vertical axis indicates the cumulative number of AE occurrences. In a state where no leakage occurs, the AE exhibits a linear increasing pattern indicated by 13 due to the steam flow or environmental noise generated around the boiler body.

On the other hand, when a leak occurs, the AE shows a pattern of rapid increase from the start of the leak, as shown by 14.
This allows the occurrence of leakage to be recognized.

Moreover, FIG. 5 shows an example of the frequency spectrum of AE actually measured by the method described above.

When the boiler heat exchanger tubes are normal, they show a spectrum as shown by code 15, and there is no frequency with a particularly prominent peak.On the other hand, when leakage begins to occur, a specific frequency like code 16 appears. A characteristic spectrum with an outstanding peak at around 0.4 MHz in the figure is observed. Therefore, by evaluating this frequency spectrum together with the pattern in which the cumulative number of AEs rapidly increases, which is indicated by reference numeral 14 in FIG. 4, it is possible to confirm the occurrence of leakage.

Note that the AE sensor used has a frequency of 100
A sound pressure detection sensor that detects only sounds of kHz or higher is suitable.

Furthermore, by using two or more AE sensors in the present invention to provide a positioning function and setting a leak detection monitoring range, leaks can be detected without being affected by environmental noise generated from the boiler. .

[Embodiments of the invention]

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

FIG. 1 is a configuration diagram showing an example of the configuration of a device used in an embodiment of the leakage detection method according to the present invention. In FIG.
E sensor, 9 is AE multiplied signal 60d from AE sensor 5
The signal from the preamplifier 9 is input to the AE measuring device H.

The signal input to the AE measurement device
The signal is amplified by about 40 dB to create the original AE signal.

This AE [signal is processed by an appropriate threshold value in the sudden AE signal extraction section 19, and the 6-pulse signal generation section 20 from which the sudden AE multiplied signal is extracted receives the input signal from the sudden AE signal extraction section 19. If there is, one pulse signal is generated with one sudden type AE multiplied signal, this pulse signal is accumulated in the first calculation section 21, and the output signal of the cumulative number of pulses is sent to the first comparison section 24. is input. Reference numeral 23 denotes a level setting unit that determines a setting level for the cumulative number of AE occurrences by adding a little amount to the reference level 22 based on the increasing pattern of the cumulative number of AE occurrences as shown by reference numeral 13 in FIG. 4. . In the first comparison section 24, the first calculation section 21
The cumulative number of pulses outputted from the level setting section 23 is compared with the set level outputted from the level setting section 23, and when the cumulative number of pulses exceeds the set level, a binarized first detection signal is output.

On the other hand, the above-mentioned AE original signal also has a specific frequency band, for example 0.4MH! - It is input to the band pass filter 25 having a band of ±50 kHz, and only the AE multiplied signal having only this frequency band component is passed and sent to the second calculation unit 26. In the second calculation unit 26, the specific frequency band A
The signal level multiplied by E is calculated, and the result is sent to the second comparator 29. In the second comparison section 29.

The signal level from the second calculation unit 26 is compared with the set level determined by the level setting unit 28 based on the signal reference level 27, and when the signal level exceeds the set level,
A binarized second detection signal is output.

Further, 30 is a flow rate converter that converts the flow rate of the boiler into an electric signal, and a flow rate abnormality decrease signal generator 31 to which the output signal is inputted is a flow rate converter that converts the flow rate of the boiler into an electric signal.
outputs a detection signal. Also.

32 is a pressure converter that converts the boiler pressure into an electrical signal, and an abnormal pressure drop signal generator 33 that inputs the output signal.
outputs a binarized fourth detection signal. Note that the third and fourth detection signals are normal plant control signals, and may be used in the present device.

Above mentioned 1. Second. The third and fourth detection signals are O
It is input for a warning signal generation section consisting of R circuits 34 and 35 and an AND circuit 36. The warning signal generating section 37
At least one of the detection signal and the second detection signal is inputted, and the third detection signal is a plant control signal. When the fourth detection signal is input, a warning signal is sent to the alarm unit 38 (for example, a speaker).

Generates a warning sound.

In the embodiments described above, one AE sensor detects leakage at the boiler water wall header, but one-dimensional positioning can be performed using two AE sensors, or four AE sensors. If two-dimensional positioning is performed using AE, normal defect positioning using AE becomes possible, and a monitoring range can also be set.

Leakage detection can be performed without being affected by boiler environmental noise that occurs outside the monitoring range.

[Effects of the invention] In the conventional technology, boiler safety personnel inspect leaks and
The situation was extremely dangerous as the eruption had to be discovered. On the other hand, according to the present invention, these dangers are eliminated, and if the system is connected in such a way that the warning signal obtained along with the leak alarm can be input to the boiler control system,
Since a boiler trip is immediately applied when a leak occurs,
Safety during boiler operation is further improved.

Furthermore, if leakage is determined based only on the leakage sound signal (level, frequency band) as in the conventional technology, there is a risk of erroneously determining leakage because the frequency range is inherently noisy. The accuracy of leakage determination can be improved by incorporating a plant control signal indicating a decrease in steam flow rate into the determination material.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing the configuration of a device used in an embodiment of the leak detection method according to the present invention, FIG. 2 is an explanatory diagram showing an example of a boiler water wall header to which the leak detection device according to the present invention is applied,
Fig. 3 is an explanatory diagram showing the method of attaching the AE sensor to the armpit and the basic parts of the AE measurement system, and Fig. 4 is a chart showing an example of the occurrence pattern of the cumulative number of AE occurrences with respect to elapsed time. FIG. 5 is a chart showing an example of the AE frequency spectrum. Explanation of symbols 5...AE sensor 7...Wave guide 9
...Preamplifier ■, ...AE measuring device 19
...Sudden type AE extraction part

Claims (4)

[Claims] (1) Check leakage from boiler heat transfer tubes using AE (acoustic)
This is a leakage detection method that detects leakage using the
A first signal that detects sudden type AE from the E original signal and outputs when the increasing pattern of the cumulative number of occurrences of the sudden type AE is greater than a set increasing pattern, and a specific frequency of the AE original signal. A second signal that is output when the signal level of the band is higher than the set value, and a third signal that is output when the plant control signal from the boiler is lower than the set value, A method for detecting a leak in a boiler heat exchanger tube, characterized in that a leak is determined only when at least one of the signal and the second signal and the third signal are output. (2) A boiler heat exchanger tube leakage detection method according to claim 1, characterized in that an alarm signal is generated when a leak is determined. (3) In the boiler heat exchanger tube leakage detection method according to claim 1 or 2, a boiler characterized in that a sound pressure detection sensor that detects only sounds with a frequency of 100 kHz or more is used as the AE sensor. Method for detecting leakage in heat transfer tubes. (4) Any one of claims 1 to 3
In the boiler heat exchanger tube leakage detection method described in
A method for detecting leakage in boiler heat exchanger tubes, characterized by using two or more sensors to provide a positioning function and setting a leakage detection monitoring range.