CN113790860B - Methane leakage detection method and device for underground limited space gas equipment - Google Patents
Methane leakage detection method and device for underground limited space gas equipment Download PDFInfo
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- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 title claims abstract description 298
- 238000001514 detection method Methods 0.000 title claims abstract description 71
- 238000000034 method Methods 0.000 claims abstract description 31
- 239000007789 gas Substances 0.000 claims abstract description 22
- 239000003345 natural gas Substances 0.000 claims abstract description 17
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- 238000001228 spectrum Methods 0.000 claims description 32
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- 238000007084 catalytic combustion reaction Methods 0.000 description 1
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- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
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Abstract
The invention provides a methane leakage detection method and device for underground limited space gas equipment. The method comprises the following steps: reading a second harmonic signal of a laser frequency modulation signal demodulated from the laser echo signal by the laser detection device; calculating a methane concentration based on the second harmonic signal; if the methane concentration exceeds a set threshold, it is determined that natural gas leakage has occurred. The invention calculates the methane concentration based on the second harmonic signal of the laser frequency modulation signal, and because the peak value of the even harmonic signal is just positioned at the center of the methane absorption spectrum line, compared with the prior art, the invention directly calculates the methane concentration based on the laser echo signal, thereby improving the calculation precision of the methane concentration and the precision of methane leakage detection.
Description
Technical Field
The invention belongs to the technical field of natural gas leakage monitoring, and particularly relates to a methane leakage detection method and device for underground limited space gas equipment.
Background
Natural gas is used as an important energy source in cities and is widely applied to power supply, heat supply, cooking and other aspects of urban residents. The gas pipeline is spread over the underground of the whole city as the main artery of the city, becomes a resource and an energy source for national economy development and people life guarantee, and has an important role of a city life line. The urban gas safety operation relates to the development of cities and society, and the task of gas supply safety management is becoming heavy. Town gas pipes face risks from many aspects, mainly including corrosion, construction damage, artificial damage and other external influences, welded junction cracking and other quality problems, geological disasters and the like. Because natural gas is combustible gas, natural gas leaks and has the risks of burning and explosion, and timely discovery and rapid disposal after natural gas equipment leaks have great significance for guaranteeing the life and property safety of people.
The main component of natural gas is methane, and natural gas leakage detection is mainly realized based on monitoring of methane concentration in air. At present, natural gas leakage detection is mainly carried out in a traditional manual regular inspection mode, and detection means mainly comprise leakage point sound distinguishing by human ears, soapy water spraying, a portable methane detector and the like. The existing portable methane on-site detection equipment is mainly a catalytic combustion type sensor, is a consumption type methane detection instrument, has short service life and slow response, needs frequent calibration, has poor anti-interference capability on the humidity, temperature and pressure change of the peripheral environment and interference gas, is limited by regional barrier conditions in a direct contact type measurement mode, is mainly used for carrying out data statistics in a manual recording mode, and is easy to generate the phenomena of inaccurate counting, even missed detection and missing report.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a methane leakage detection method and device for underground limited space gas equipment.
In order to achieve the above object, the present invention adopts the following technical scheme.
In a first aspect, the invention provides a methane leakage detection method for underground limited space gas equipment, comprising the following steps:
reading a second harmonic signal of a laser frequency modulation signal demodulated from the laser echo signal by the laser detection device;
calculating a methane concentration based on the second harmonic signal;
if the methane concentration exceeds a set threshold, it is determined that natural gas leakage has occurred.
Further, the laser detection device includes: the laser device comprises a controller, a transceiver signal processor connected with the controller, a laser driver connected with the transceiver signal processor, an echo preamplifier, a laser transmitter connected with the laser driver, and a laser receiver connected with the echo preamplifier; the receiving and transmitting signal processor demodulates the second harmonic signal from the laser echo signal and sends the second harmonic signal to the controller.
Still further, the method of demodulating the second harmonic signal from the laser echo signal includes:
demodulating a frequency modulation signal from the laser echo signal;
equally dividing the frequency modulated signal within one sampling period into a plurality of small segment signals;
performing fast Fourier transform on each small segment of signal to obtain a frequency spectrum signal of each small segment of signal;
and connecting amplitude values of the second harmonic of each small-section signal together to obtain a frequency spectrum signal of the second harmonic in one sampling period.
Further, the method of calculating the methane concentration based on the second harmonic signal includes:
measuring the methane concentration y at a unit distance from the laser detection device using a standard methane concentration measuring instrument i And the peak mean value x of the second harmonic i I=1, 2, …, N is the number of measurements;
the following quadratic fit model is established:
y＝ax 2 +bx+c
wherein x is the peak mean value of the second harmonic, y is the methane concentration, and a, b and c are the second term coefficient, the first term coefficient and the constant term respectively;
by means of (x) i ，y i ) Training the quadratic fit model to determine a, b and c;
substituting the peak value average value of the second harmonic at the detection point which is located at a unit distance from the laser detection device into a trained quadratic fit model to obtain the methane concentration at the detection point.
Further, if the spectrum signal of the second harmonic meets the following conditions, calculating a value of methane concentration according to the quadratic fit model; otherwise, the value of methane concentration is 0; the conditions are as follows:
0.42<W L /W<0.58
0.42<W R /W<0.58
0.75<H L /H R <1.33
in which W is L 、W R The frequency difference between the maximum point of the second harmonic spectrum signal and the minimum point adjacent to the left and the minimum point adjacent to the right is respectively, and w=w L +W R ，H L 、H R The difference between the maximum value point of the second harmonic spectrum signal and the minimum value point adjacent to the left and the minimum value point adjacent to the right.
In a second aspect, the present invention provides a methane leak detection apparatus for an underground confined space gas facility, comprising:
the second harmonic acquisition module is used for reading a second harmonic signal of the laser frequency modulation signal demodulated from the laser echo signal by the laser detection device;
a methane concentration calculation module for calculating a methane concentration based on the second harmonic signal;
and the methane leakage judging module is used for judging that the natural gas leakage occurs if the methane concentration exceeds a set threshold value.
Further, the laser detection device includes: the laser device comprises a controller, a transceiver signal processor connected with the controller, a laser driver connected with the transceiver signal processor, an echo preamplifier, a laser transmitter connected with the laser driver, and a laser receiver connected with the echo preamplifier; the receiving and transmitting signal processor demodulates the second harmonic signal from the laser echo signal and sends the second harmonic signal to the controller.
Still further, the method of demodulating the second harmonic signal from the laser echo signal includes:
demodulating a frequency modulation signal from the laser echo signal;
equally dividing the frequency modulated signal within one sampling period into a plurality of small segment signals;
performing fast Fourier transform on each small segment of signal to obtain a frequency spectrum signal of each small segment of signal;
and connecting amplitude values of the second harmonic of each small-section signal together to obtain a frequency spectrum signal of the second harmonic in one sampling period.
Further, the method of calculating the methane concentration based on the second harmonic signal includes:
measuring the methane concentration y at a unit distance from the laser detection device using a standard methane concentration measuring instrument i And second harmonicPeak mean value x of (2) i I=1, 2, …, N is the number of measurements;
the following quadratic fit model is established:
y＝ax 2 +bx+c
wherein x is the peak mean value of the second harmonic, y is the methane concentration, and a, b and c are the second term coefficient, the first term coefficient and the constant term respectively;
by means of (x) i ，y i ) Training the quadratic fit model to determine a, b and c;
substituting the peak value average value of the second harmonic at the detection point which is located at a unit distance from the laser detection device into a trained quadratic fit model to obtain the methane concentration at the detection point.
Further, if the spectrum signal of the second harmonic meets the following conditions, calculating a value of methane concentration according to the quadratic fit model; otherwise, the value of methane concentration is 0; the conditions are as follows:
0.42<W L /W<0.58
0.42<W R /W<0.58
0.75<H L /H R <1.33
in which W is L 、W R The frequency difference between the maximum point of the second harmonic spectrum signal and the minimum point adjacent to the left and the minimum point adjacent to the right is respectively, and w=w L +W R ，H L 、H R The difference between the maximum value point of the second harmonic spectrum signal and the minimum value point adjacent to the left and the minimum value point adjacent to the right.
Compared with the prior art, the invention has the following beneficial effects.
According to the invention, the second harmonic signal demodulated from the laser echo signal by the laser detection device is read, the methane concentration is calculated based on the second harmonic signal, and if the methane concentration exceeds the set threshold value, the natural gas leakage is judged to occur, so that the automatic detection of the natural gas pipeline leakage is realized. The invention calculates the methane concentration based on the second harmonic signal of the laser frequency modulation signal, and because the peak value of the even harmonic signal is just positioned at the center of the methane absorption spectrum line, compared with the prior art, the invention directly calculates the methane concentration based on the laser echo signal, thereby improving the calculation precision of the methane concentration and the precision of methane leakage detection.
Drawings
FIG. 1 is a flow chart of a method for detecting methane leakage in an underground limited space gas facility according to an embodiment of the invention.
Fig. 2 is a block diagram showing the construction of a laser detection device.
Fig. 3 is a waveform diagram of a second harmonic spectrum signal.
FIG. 4 is a block diagram of a methane leakage detection device for an underground limited space gas facility according to an embodiment of the present invention.
Detailed Description
The present invention will be further described with reference to the drawings and the detailed description below, in order to make the objects, technical solutions and advantages of the present invention more apparent. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Fig. 1 is a schematic flow chart of a methane leakage detection method of an underground limited space gas device according to an embodiment of the invention, which comprises the following steps:
step 101, reading a second harmonic signal of a laser frequency modulation signal demodulated from a laser echo signal by a laser detection device;
step 102, calculating methane concentration based on the second harmonic signal;
step 103, if the methane concentration exceeds a set threshold, determining that natural gas leakage has occurred.
In this embodiment, step 101 is mainly used for obtaining the second harmonic signal. The second harmonic signal is simply referred to, and is actually the second harmonic signal of the laser frequency modulation signal demodulated from the laser echo signal by the laser detection device. The laser detection device is used for generating and transmitting laser signals to the detection area and receiving reflected laser echo signals. When methane leaks from the detection area, methane gas absorbs the laser emission signal, specifically, modulates the frequency of the laser echo signal, that is, the intensity of the laser echo signal has a strong correlation with the concentration of methane. Thus, the concentration of methane can be calculated by performing signal data processing on the laser echo signal. The prior art generally calculates the methane concentration directly based on the laser echo signal, and the method is simple and feasible. However, long-term practice shows that the peak value of the even harmonic signal is exactly located at the center of the absorption line, and the peak value of the odd harmonic component has an offset relative to the center of the absorption line, that is, the absorption effect of methane on the even harmonic signal is better than that of the odd harmonic. The higher the harmonic frequency is, the smaller the amplitude of each subharmonic component is, and the second harmonic is the strongest harmonic signal, so the embodiment calculates the methane concentration by extracting the second harmonic signal from the laser echo signal, and compared with the prior art, the embodiment directly calculates the methane concentration based on the laser echo signal, and can improve the calculation precision of the methane concentration, thereby improving the detection precision of methane leakage. The method for generating the second harmonic signal is more, and a phase-locked loop circuit is generally used, and the specific generation method is not limited in this embodiment, and a second harmonic signal generation method different from the phase-locked loop circuit will be provided later.
In this embodiment, step 102 is mainly used to calculate the methane concentration. As described above, the correlation of the intensity of the second harmonic signal with the methane concentration is superior to the correlation of the intensity of the laser echo signal with the methane concentration, and therefore, the present embodiment calculates the methane concentration based on the intensity of the second harmonic signal. The methane concentration is positively correlated with the intensity of the second harmonic signal, i.e., the stronger the second harmonic signal, the greater the methane concentration. And a relation model of the methane concentration and the second harmonic signal intensity can be established according to experimental data, and the methane concentration is calculated according to the obtained second harmonic signal intensity by using the relation model. Modeling methods are numerous, for example, a logistic regression model of methane concentration and second harmonic signal strength can be built; an artificial neural network can be adopted, the second harmonic signal intensity is taken as input, the methane concentration is taken as output, and experimental data are utilized for training to establish a relation model of the two. The present embodiment is not limited to a specific calculation method, and a specific embodiment will be given later.
In this embodiment, step 103 is mainly used to determine whether leakage occurs according to the calculated methane concentration. Typically by setting a threshold, if the methane concentration exceeds the set threshold, then a leak is considered to have occurred; otherwise no leakage occurs. The magnitude of the threshold has great influence on the judging result, and the excessive threshold can judge real leakage as no leakage, so that the leakage rate is increased; if the threshold value is too small, the false alarm rate may be increased by judging that the leakage is not originally leakage, so that the threshold value is selected in consideration, and the magnitude of the threshold value is generally determined through repeated experiments.
As an alternative embodiment, the laser detection device includes: the laser device comprises a controller, a transceiver signal processor connected with the controller, a laser driver connected with the transceiver signal processor, an echo preamplifier, a laser transmitter connected with the laser driver, and a laser receiver connected with the echo preamplifier; the receiving and transmitting signal processor demodulates the second harmonic signal from the laser echo signal and sends the second harmonic signal to the controller.
The embodiment provides a technical scheme of the laser detection device. The laser detection device mainly comprises a controller, a transceiver signal processor, a laser driver, an echo preamplifier, a laser transmitter and a laser receiver. The connection relation of each part is shown in figure 2, and the transceiver signal processor is respectively connected with the input end of the controller and the laser driver and the output end of the echo preamplifier; the output end of the laser driver is connected with the input end of the laser emitter; the input end of the echo preamplifier is connected with the output end of the laser receiver. The receiving and transmitting signal processor receives the control signal of the controller on one hand; and the other side outputs a driving signal to the laser driver, and the driving signal is amplified by the laser driver and then drives the laser emitter to generate a laser emission signal. The laser receiver can adopt an InGaAs photodiode to convert a received laser signal into an electric signal and then output the electric signal to the echo preamplifier; the electric signals are amplified by the echo preamplifier and then output to the transceiver signal processor; the receiving and transmitting signal processor demodulates the second harmonic signal of the frequency modulation signal from the laser echo signal and outputs the second harmonic signal to the controller.
As an alternative embodiment, the method for demodulating the second harmonic signal from the laser echo signal includes:
demodulating a frequency modulation signal from the laser echo signal;
equally dividing the frequency modulated signal within one sampling period into a plurality of small segment signals;
performing fast Fourier transform on each small segment of signal to obtain a frequency spectrum signal of each small segment of signal;
and connecting amplitude values of the second harmonic of each small-section signal together to obtain a frequency spectrum signal of the second harmonic in one sampling period.
The embodiment provides a technical scheme for demodulating a second harmonic signal from a laser echo signal. The second harmonic demodulation methods mainly include two methods: one is a quadrature phase locking method; one is the fourier transform method. In the prior art, a quadrature phase locking method is adopted to generate a second harmonic signal, and the second harmonic signal is completed by a phase locking amplifier hardware module, so that the hardware complexity and the hardware cost of the system are increased, and more random noise is introduced. The second method of the improved type is adopted in the embodiment, namely a segmented fast fourier transform method: dividing the frequency modulation signal of one sampling period into a plurality of equal segments; and then, respectively performing fast Fourier transform FFT on the signals of each small segment to obtain a frequency spectrum (amplitude frequency) signal of each small segment, and connecting amplitude values of second harmonic waves of the signals of each small segment together to obtain a frequency spectrum signal of the second harmonic waves in one sampling period. Since the FFT can be implemented by software, the hardware cost is low and the reliability is high. The number of the segments of each small segment is reduced in a segmented way, so that the total number of segments of the absorption signal of a single sampling period is increased, the interference of noise can be reduced to the greatest extent, and the accuracy of second harmonic is improved. In addition, since the time complexity of the FFT is O (n×logn), the time complexity of the FFT performed after dividing into a plurality of small segments is reduced to O (M (n/m×log (n/M))) =o (n×log (n/M)), and M is the number of the divided small segments, so that the calculation amount can be reduced, the calculation speed can be increased, and the real-time calculation of the methane concentration can be advantageously realized.
As an alternative embodiment, the method for calculating the methane concentration based on the second harmonic signal comprises:
measuring the methane concentration y at a unit distance from the laser detection device using a standard methane concentration measuring instrument i And the peak mean value x of the second harmonic i I=1, 2, …, N is the number of measurements;
the following quadratic fit model is established:
y＝ax 2 +bx+c
wherein x is the peak mean value of the second harmonic, y is the methane concentration, and a, b and c are the second term coefficient, the first term coefficient and the constant term respectively;
by means of (x) i ，y i ) Training the quadratic fit model to determine a, b and c;
substituting the peak value average value of the second harmonic at the detection point which is located at a unit distance from the laser detection device into a trained quadratic fit model to obtain the methane concentration at the detection point.
This example gives a technical solution for calculating methane concentration. In the embodiment, the methane concentration is calculated by establishing a quadratic fit model of the second harmonic peak mean value and the methane concentration. As previously mentioned, the methane concentration is positively correlated with the second harmonic intensity, i.e., the stronger the second harmonic signal, the higher the methane concentration. In order to improve the fitting accuracy, the embodiment adopts a quadratic fitting model, namely, the methane concentration is expressed as a quadratic function y of a second harmonic peak value mean value x, and the quadratic function is concretely expressed as above. Firstly, using standard methane concentration measuring instrument to experimentally measure methane concentration y at unit distance from laser detection device i And the peak mean value x of the second harmonic i The method comprises the steps of carrying out a first treatment on the surface of the Then by (x) i ，y i ) The composed training data set trains the quadratic fit model, thereby determining model parameters a, b and c. The trained quadratic fit model is provided, and the quadratic fit model can be used for obtaining the peak value average value of the actually measured second harmonicTo obtain methane concentration.
As an alternative embodiment, if the spectral signal of the second harmonic meets the following condition, calculating a value of methane concentration according to the quadratic fit model; otherwise, the value of methane concentration is 0; the conditions are as follows:
0.42<W L /W<0.58
0.42<W R /W<0.58
0.75<H L /H R <1.33
in which W is L 、W R The frequency difference between the maximum point of the second harmonic spectrum signal and the minimum point adjacent to the left and the minimum point adjacent to the right is respectively, and w=w L +W R ，H L 、H R The difference between the maximum value point of the second harmonic spectrum signal and the minimum value point adjacent to the left and the minimum value point adjacent to the right.
The embodiment provides a technical scheme for improving the accuracy of calculating the methane concentration based on the second harmonic intensity. The embodiment is actually a modification of the previous embodiment, namely, firstly, the waveform of the second harmonic spectrum signal is checked to check whether the waveform parameters meet the requirements, and if the waveform parameters meet the requirements, the methane concentration is calculated according to the fitting model; if not, the methane concentration is set to 0. This process can avoid misjudging background noise or other disturbances as second harmonic signals, resulting in errors in the calculation of methane concentration. The second harmonic spectrum signal and parameters are shown in fig. 3. The second harmonic spectrum signal satisfies the requirements of the 3 inequalities, the first 2 inequalities being equivalent to 0.42/0.58<W L /W R <0.58/0.42, which is a requirement for the frequency domain symmetry of the waveform (i.e., the left and right widths are as equal as possible); the last 1 inequality is the requirement for amplitude symmetry (i.e. the left and right peaks are as equal as possible).
Fig. 4 is a block diagram of a methane leakage detection device for an underground limited space gas facility according to an embodiment of the present invention, the device includes:
the second harmonic acquisition module 11 is used for reading a second harmonic signal of the laser frequency modulation signal demodulated from the laser echo signal by the laser detection device;
a methane concentration calculation module 12 for calculating a methane concentration based on the second harmonic signal;
a methane leak determination module 13 for determining that a natural gas leak has occurred if the methane concentration exceeds a set threshold.
The device of this embodiment may be used to implement the technical solution of the method embodiment shown in fig. 1, and its implementation principle and technical effects are similar, and are not described here again. As well as the latter embodiments, will not be explained again.
As an alternative embodiment, the laser detection device includes: the laser device comprises a controller, a transceiver signal processor connected with the controller, a laser driver connected with the transceiver signal processor, an echo preamplifier, a laser transmitter connected with the laser driver, and a laser receiver connected with the echo preamplifier; the receiving and transmitting signal processor demodulates the second harmonic signal from the laser echo signal and sends the second harmonic signal to the controller.
As an alternative embodiment, the method for demodulating the second harmonic signal from the laser echo signal includes:
demodulating a frequency modulation signal from the laser echo signal;
equally dividing the frequency modulated signal within one sampling period into a plurality of small segment signals;
performing fast Fourier transform on each small segment of signal to obtain a frequency spectrum signal of each small segment of signal;
and connecting amplitude values of the second harmonic of each small-section signal together to obtain a frequency spectrum signal of the second harmonic in one sampling period.
As an alternative embodiment, the method for calculating the methane concentration based on the second harmonic signal comprises:
measuring the methane concentration y at a unit distance from the laser detection device using a standard methane concentration measuring instrument i And the peak mean value x of the second harmonic i I=1, 2, …, N is the number of measurements;
the following quadratic fit model is established:
y＝ax 2 +bx+c
wherein x is the peak mean value of the second harmonic, y is the methane concentration, and a, b and c are the second term coefficient, the first term coefficient and the constant term respectively;
by means of (x) i ，y i ) Training the quadratic fit model to determine a, b and c;
substituting the peak value average value of the second harmonic at the detection point which is located at a unit distance from the laser detection device into a trained quadratic fit model to obtain the methane concentration at the detection point.
As an alternative embodiment, if the spectral signal of the second harmonic meets the following condition, calculating a value of methane concentration according to the quadratic fit model; otherwise, the value of methane concentration is 0; the conditions are as follows:
0.42<W L /W<0.58
0.42<W R /W<0.58
0.75<H L /H R <1.33
in which W is L 、W R The frequency difference between the maximum point of the second harmonic spectrum signal and the minimum point adjacent to the left and the minimum point adjacent to the right is respectively, and w=w L +W R ，H L 、H R The difference between the maximum value point of the second harmonic spectrum signal and the minimum value point adjacent to the left and the minimum value point adjacent to the right.
The foregoing is merely illustrative of the present invention, and the present invention is not limited thereto, and any changes or substitutions easily contemplated by those skilled in the art within the scope of the present invention should be included in the present invention. Therefore, the protection scope of the invention is subject to the protection scope of the claims.
Claims (6)
1. The methane leakage detection method for the underground limited space gas equipment is characterized by comprising the following steps of:
reading a second harmonic signal of a laser frequency modulation signal demodulated from the laser echo signal by the laser detection device;
calculating a methane concentration based on the second harmonic signal;
if the methane concentration exceeds a set threshold, determining that natural gas leakage has occurred;
the method for demodulating the second harmonic signal from the laser echo signal comprises the following steps:
demodulating a frequency modulation signal from the laser echo signal;
equally dividing the frequency modulated signal within one sampling period into a plurality of small segment signals;
performing fast Fourier transform on each small-section signal to obtain a frequency spectrum signal of each small-section signal;
connecting amplitude values of second harmonic of each small section of signal together to obtain a spectrum signal of the second harmonic in a sampling period;
the method for calculating the methane concentration based on the second harmonic signal comprises the following steps:
measuring the methane concentration y at a unit distance from the laser detection device using a standard methane concentration measuring instrument i And the peak mean value x of the second harmonic i I=1, 2, …, N is the number of measurements;
the following quadratic fit model is established:
y＝ax 2 +bx+c
wherein x is the peak mean value of the second harmonic, y is the methane concentration, and a, b and c are the second term coefficient, the first term coefficient and the constant term respectively;
by means of (x) i ，y i ) Training the quadratic fit model to determine a, b and c;
substituting the peak value average value of the second harmonic at the detection point which is located at a unit distance from the laser detection device into a trained quadratic fit model to obtain the methane concentration at the detection point.
2. The method for detecting methane leakage in an underground limited space gas facility according to claim 1, wherein the laser detection means comprises: the laser device comprises a controller, a transceiver signal processor connected with the controller, a laser driver connected with the transceiver signal processor, an echo preamplifier, a laser transmitter connected with the laser driver, and a laser receiver connected with the echo preamplifier; the receiving and transmitting signal processor demodulates the second harmonic signal from the laser echo signal and sends the second harmonic signal to the controller.
3. The method for methane leak detection in an underground limited space gas facility of claim 2, wherein if the spectral signal of the second harmonic satisfies the following condition, calculating a value of methane concentration from the quadratic fit model; otherwise, the value of methane concentration is 0; the conditions are as follows:
0.42<W L /W<0.58
0.42<W R /W<0.58
0.75<H L /H R <1.33
in which W is L 、W R The frequency difference between the maximum point of the second harmonic spectrum signal and the minimum point adjacent to the left and the minimum point adjacent to the right is respectively, and w=w L +W R ，H L 、H R The difference between the maximum value point of the second harmonic spectrum signal and the minimum value point adjacent to the left and the minimum value point adjacent to the right.
4. A methane leak detection apparatus for an underground confined space gas facility, comprising:
the second harmonic acquisition module is used for reading a second harmonic signal of the laser frequency modulation signal demodulated from the laser echo signal by the laser detection device;
a methane concentration calculation module for calculating a methane concentration based on the second harmonic signal;
a methane leak determination module for determining that a natural gas leak has occurred if the methane concentration exceeds a set threshold;
the method for demodulating the second harmonic signal from the laser echo signal comprises the following steps:
demodulating a frequency modulation signal from the laser echo signal;
equally dividing the frequency modulated signal within one sampling period into a plurality of small segment signals;
performing fast Fourier transform on each small-section signal to obtain a frequency spectrum signal of each small-section signal;
connecting amplitude values of second harmonic of each small section of signal together to obtain a spectrum signal of the second harmonic in a sampling period;
the method for calculating the methane concentration based on the second harmonic signal comprises the following steps:
measuring the methane concentration y at a unit distance from the laser detection device using a standard methane concentration measuring instrument i And the peak mean value x of the second harmonic i I=1, 2, …, N is the number of measurements;
the following quadratic fit model is established:
y＝ax 2 +bx+c
wherein x is the peak mean value of the second harmonic, y is the methane concentration, and a, b and c are the second term coefficient, the first term coefficient and the constant term respectively;
by means of (x) i ，y i ) Training the quadratic fit model to determine a, b and c;
substituting the peak value average value of the second harmonic at the detection point which is located at a unit distance from the laser detection device into a trained quadratic fit model to obtain the methane concentration at the detection point.
5. The methane leak detection apparatus for an underground confined space gas facility of claim 4, wherein the laser detection apparatus comprises: the laser device comprises a controller, a transceiver signal processor connected with the controller, a laser driver connected with the transceiver signal processor, an echo preamplifier, a laser transmitter connected with the laser driver, and a laser receiver connected with the echo preamplifier; the receiving and transmitting signal processor demodulates the second harmonic signal from the laser echo signal and sends the second harmonic signal to the controller.
6. The apparatus for methane leak detection in an underground limited space gas facility of claim 5, wherein if the second harmonic spectral signal satisfies the following condition, a value of methane concentration is calculated from the quadratic fit model; otherwise, the value of methane concentration is 0; the conditions are as follows:
0.42<W L /W<0.58
0.42<W R /W<0.58
0.75<H L /H R <1.33
in which W is L 、W R The frequency difference between the maximum point of the second harmonic spectrum signal and the minimum point adjacent to the left and the minimum point adjacent to the right is respectively, and w=w L +W R ，H L 、H R The difference between the maximum value point of the second harmonic spectrum signal and the minimum value point adjacent to the left and the minimum value point adjacent to the right.
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