Multi-gas leakage synchronous monitoring system based on the distributed optical fiber sensing technology

Abstract

In the process of long-distance transportation of different gas, the remote pipeline plays an irreplaceable role in energy transmission. When the pipeline is laid in remote areas for a long distance, it is easy to be influenced by geological disasters and complex working conditions, which may lead to corrosion and leakage. therefore, it is necessary to conduct pipeline gas real-time safety monitoring. An optical fiber gas leakage synchronous monitoring system was proposed and demonstrated based on distributed optical sensing technology for simultaneous multi-gas measurements. In this study, we discuss that the principle of multi-gas leakage synchronous monitoring system is investigated and then validated by the theoretical simulation experiments. Furthermore, gas concentration and leakage location discrimination tests are also conducted in laboratory. The experimental results show that the output intensity values increased obviously along with the gas concentration changes, and the response time of the sensor system is about 40 seconds, and it’s concluded that the multi-gas leakage synchronous monitoring system based on distributed optical fiber sensing technology exhibited good sensing and location discrimination performance.

Keywords

Multi-gas measurement dielectric film filter (DFF)optical fiber sensing

1. Introduction

In aerospace, a wide variety of gas sensors in various types of aerospace vehicles are indispensable for the key gas monitoring components, which are mainly used for monitoring the change of harmful gas components in manned spaceships, fuel combustion detection during the work of spacecraft engines, gas or liquid leakage judgment at key parts of spacecraft, and planetary atmospheric environment detection during deep space exploration. So the gas sensors provide very important data references for the quality of aerospace products, safety, cosmic environmental exploration and the health of astronaut [1]. Real-time monitoring of all kinds of gas components during spacecraft in orbit will directly determine the performance of aerospace vehicles. If the gas components can not be measured in real time and accurately, it will cause different degrees of damage for the spacecraft, shorten the life of the spacecraft in serious cases, and even lead to catastrophic accidents. In the face of the extreme complexity and harsh space application environment of advanced spacecraft during orbit operation, it is difficult for traditional probe gas sensing technology to complete real-time and accurate distributed measurement [2], Owing to the particularity of working environment such as aerospace vehicle, the technical requirements of gas sensor applied to aerospace vehicle are more strict, such as smaller volume, lower power consumption, lighter quality, higher sensitivity and more stable reliability [3]. In recent decades, with the continuous development of optical technology, the optical fiber which used for sensor provide the advantages of anti electromagnetic interference, insulation, remote-sensing and multiplexing, so the optical fiber sensor has been widely used in many detection fields [4]. Compared with traditional mechanical or electronic sensors, optical fiber sensors are more in line with the development needs of modern sensing technology. At the same time, optical fiber gas sensors in aerospace, energy gas long-distance transportation, coal resources deep mining and other complex and harsh application environment, real-time measurement and monitoring of various gas states make it more prominent unique advantages, providing an essential guarantee for aerospace and industrial production safety. The optical fiber gas monitoring system in complex and harsh environment not only needs to be able to detect the gas type and concentration, but also to locate the leakage source of the long distance gas quickly and accurately.

In view of the fact that the excellent properties of optical fiber, and make it more and more important in the field of gas sensors, similar to pressure, temperature, displacement and other optical fiber detecting sensors, there are many kinds of principles to use optical fiber to detect leakage gases, mainly using the characteristics of fluorescence, scattering, absorption and refractive index change of matter to detect the leakage gas concentration. With the appearance of distributed feedback semiconductor laser, the sensing accuracy of spectral absorption fiber gas has been greatly improved, and more gases can be detected by spectral absorption fiber gas sensor. In order to improve the measurement accuracy, two types of optical fiber gas sensors based on differential absorption [5, 6] and harmonic detection [7, 8] are derived. The performance of the optical fiber gas sensor based on spectral absorption and harmonic detection largely depends on the stability of the central wavelength of the narrow-band light source. At the same time, the poor reusability of the narrow-band light source leads to the disadvantages of the system, such as the redundant structure, the inability to measure a variety of gases synchronously and the high cost. At the same time, another type of optical fiber gas sensor has reported in some research papers, In 1965, the Hirschfeld proposed the concept of evanescent wave, and with the in-depth research and 80 years of development, researchers in various countries have improved and established a relatively complete theory and method of optical fiber gas sensor based on evanescent wave. Ke et al. reported the optical fiber H ${}_{2}$ S gas sensor based on evanescent field, the disadvantages of light source drift and zero drift were solved by using double optical fiber optical path, and The results show that the gas detection sensitivity of gas sensor reached to 0.169 $\mu$ w/ppm [10]. Butt et al. proposed a carbon monoxide (CO) gas sensor based on evanescent field absorption, and the gas sensor possessed the excellent characteristics of exponential attenuation in the low refractive index region for the waveguide, which provides a good solution for the optical fiber sensor of various gases [11]. Nima Bavili and other scholars have proposed a hydrogen sensor which were composed of nano-palladium film and hydrogen adsorbable polymer cladding. The minimum detection time of the hydrogen sensor system is less than 100s, and the minimum detection concentration range is less than 70 ppm in 1% hydrogen detection atmosphere [12]. Swain et al. have presented a new principle to enhance the initial evanescent field to increase the sensitivity of gas detection [13]. AhmetCicek demonstrated an ultrasonic gas sensor based on evanescent coupling of spoof surface acoustic waves between two surface phononic crystals containing trapezoidal grooves on rigid slabs, and the gas sensing can be achieved by measuring the output acoustic intensity at constant frequency, which exhibits a steep decrease with carbon dioxide (CO ${}_{2}$ ) volume fraction up to 2000 ppm [14]. It can be seen that in addition to using various optimized sensing fibers for evanescent wave optical fiber sensors, it is very important to explore sensitive films suitable for the measured gases with high sensitivity and robustness, and the manufacturing process is demanding.

From the development of optical fiber gas sensors, the optical fiber light source in the optical fiber gas sensing system has experienced the process from wide spectrum to narrow band, and the gas measurement range is from single point to multi-point, and then to distributed sensing detection. As a DFB light source with stable power, narrow line width and easy modulation, the sensitivity of various optical fiber gas sensors has been greatly improved. At the same time, the use of two or more DFB lasers to detect a variety of gases is also common, but most of its research mainly focuses on a variety of forms of reuse mode and single point specific gas detection sensitivity and accuracy, and there are still many shortcomings, (a) Limited to the choice of narrow-band light source, the same optical fiber gas detection system gas detection gas types are limited; if a variety of gas synchronous measurement, it is bound to choose a number of narrow-band light source, which results high cost, complex structure and other shortcomings, (b) At present, the optical fiber gas sensor is usually composed of several discrete optical components. The detection fiber is twice as high as the detection path, the attenuation of optical power signal and the low integration of the system, which leads to the shortcomings of low precision and small range in long distance multi-point detection, which can not be effectively applied to the long range and long distance gas synchronous fiber detection system.

So a novel distributed optical fiber gas sensing technology is studied, which has the advantages of multiple gas synchronous monitoring, high spatial resolution and so on. It can accurately locate and measure the gas information in any specific range and long distance measurement range. The article is divided into four sections: the sensing mechanism of the multi-gas leakage synchronous monitoring gas sensor based on the distributed optical fiber sensing technology is introduced, and the sensing principle and the influence factors of sensing system sensitivity are analyzed in Section 2. Section 3 describes the fabrication of the multi-gas leakage synchronous monitoring gas sensor based on distributed optical fiber sensing technology, and the actual test experiments for various gases were arranged, the experimental results are analyzed, and the sensitivity of the gas sensing system is discussed. At last, the fourth part mainly puts forward suggestions and opinions on the improvement of the optical sensor system.

2. Theory

The working mechanism of optical fiber gas sensor based on spectral absorption is the selective absorption of light by various gas molecules in near infrared band and Bill Lambert’s theorem. Many gaseous substances show strong absorption in specific regions of electromagnetic spectrum, such as ultraviolet/visible, near infrared or mid infrared regions, and the specific gas exhibited the specific absorption line or absorption band, which provided a solid physical basis for accurate detection of the gases, as shown in the Table 1, the absorption spectra in different spectra regions originated differently for different gas.

Table 1
The central wavelength of the gas absorption peak in the infrared band

Gas	O ${}_{2}$	CO ${}_{2}$	CO	H ${}_{2}$ O	NO ${}_{2}$	CH ${}_{4}$	C ${}_{2}$ H ${}_{2}$	NH ${}_{3}$	H ${}_{2}$ S
Absorption peak central wavelength ( $\mu$ m)	0.761	1.573	1.567	1.365	0.8	1.665	1.53	1.544	1.578

If the wavelength of the illuminant covers the absorption line of the gas, when the light emitted by the light source passes through the atmosphere with the gas, due to the beer Lambert principle and its absorption effect, resulting in the attenuation of the received light intensity, so the received light intensity could be calculated as in Eq. (1).

$\displaystyle I=I_{0}\exp[-\alpha(\lambda)\textit{LC}]$ (1)

Where the parameter $I$ is the received light intensity, the parameter $I_{0}$ is the input light intensity in optical fiber path the parameter $\alpha$ is the coefficient of gas absorption under certain pressure, the parameter $L$ is the length of optical fiber path, and the parameter $C$ is the concentration for the detective gas So we can obtain the concentration of gas in Eq. (2).

$\displaystyle C=-\frac{1}{\alpha(\lambda)L}\ln\frac{I}{I_{0}}$ (2)

Figure 1.

The experiment setup of multi-gas leakage synchronous monitoring optical fiber sensing system.

The experimental configuration of the multi-gas leakage synchronous monitoring which was based on distributed optical sensing mechanism is shown in Fig. 1. The beam of broadband light was generated from a broadband tunable laser light source, and the broadband laser light enters the optical path of the detection fiber. Take advantage of the dielectric film filter 0(DFF ${}_{0}$ ), dielectric film filter 1(DFF ${}_{1}$ ), dielectric film filter N (DFFn) in the optical fiber path, the different central wavelengths ( $\lambda_{0}$ , $\lambda_{1}$ , $\lambda_{2}$ , $\ldots$ $\lambda_{N}$ ) narrow band light were selected, and the absorption spectrum wavelength corresponding to the same or different detection gases in the gas detection zone can be detected and sorted, then they are reflected back to analyze, and the reflected light in different optical fiber gas sensing regions with different central wavelengths which containing information of the type and concentration field of the gas were detected respectively, and then enter the fiber coupler and array wave-guide gratings (AWG), so intensity of light were received by photo-detector arrays and optical fiber spectrometer (Anritsu MS9740A) after demodulation, And finally, the type and the concentration of the gas were analyzed and calculated furtherly. So according to the method of adjusting and scanning the reflected light spectrum information in the optical fiber optical path, the concentration and the location region of the gas can be realized, and then the high spatial resolution accurate measurement of the gas information in different detection zones can be realized, the final electric signal output intensity is obtained.

3. Experiment and discussion

For the sake of demonstration of optical fiber gas sensor in real gas measurements, the a series of gas detection experiments were first carried out by using 1-m-long single mode fiber (SMF) as sensing section to detect gas respectively in gas sensing zone one and zone two of the Fig. 1, the gas sensing zone one was filled with the methane (CH ${}_{4}$ ) gas which concentration range is 0 $\sim$ 5% around optical fiber sensing path, and the gas sensing zone two was filled with the ethylene (C ${}_{2}$ H ${}_{2}$ ) gas which concentration range is 0 $\sim$ 2% around optical fiber sensing path. As shown in Table 1, the gas absorption lines are different for the methane and acetylene, so the configuration in Fig. 1 enabled us to detect different gas concentration in different sensing zones synchronously.

Figure 2.

The methane and ethylene gas in two separate zones.

Figure 3.

Initial intensity of reflected light.

The performance of multi-gas leakage synchronous monitoring optical fiber sensing system was tested with the experiment set-up in Fig. 2. A succession of detecting experiments were carried out for two gases in two different sensing zones, owing to the three dielectric film filters along in the optical fiber path, so the initial intensity of reflected light is depicted in Fig. 3, the output signal intensity in the graph represents there is no gas leakage in zone one and zone two.

In the multi-gas leakage optical fiber monitoring system, the first problem to be solved is not only to identify the type of gas, but also to determine the location of gas leakage. In view of the specific absorption spectrum for a specific gas, when the gas detection area leaks, the optical output intensity of the optical fiber will decrease with the extension of the transmission distance. In Fig. 4, we discuss the experiment results for different gas leakage situations, as shown on the differences between situations (a), (b), (c) reflected light in Fig. 4, according to the changes of output intensity of $I_{\lambda_{0}}$ , $I_{\lambda_{1}}$ and $I_{\lambda_{2}}$ , so the concentration of gas and the location of gas leakages could be calculated occur in both ethylene in region one and methane in region two accurately.

Figure 4.

The output intensity values vs. different gas leakage in two separate detective zones.

Figure 4 shows that the multi-gas leakage synchronous optical fiber monitoring system exhibits different intensity output value for different gas leakage in different leakage zones. Compared with initial intensity of reflected light in Fig. 3, the red intensity output value in Fig. 4 represent the signal output intensity decline and gas leakage in specific regions. In Fig. 4b, the signal output intensity $I_{\lambda_{2}}$ declined only, and the test results indicated methane leakage in zone two, and according to the signal output changes it could be calculated the concentrations of methane leakage. By contrast, the signal output intensity $I_{\lambda_{1}}$ and $I_{\lambda_{2}}$ has dropped in Fig. 4a and c, the experimental results showed that the methane (CH ${}_{4}$ ) leaks in zone two, or the methane (CH ${}_{4}$ ) and ethylene (C ${}_{2}$ H ${}_{2}$ ) leaks in zone one and zone two together. When the leakage of ethylene (C ${}_{2}$ H ${}_{2}$ ) gas occured in zone one, the light intensity entering zone two decreased to some extent, at the same time the the signal output intensity $I_{\lambda_{2}}$ is partly weakened correspondly in Fig. 4a. To the contrary, in Fig. 4c, the leakage of ethylene (C ${}_{2}$ H ${}_{2}$ ) and methane (CH ${}_{4}$ ) gas occured in zone one and zone two respectively, owing to the absorption of ethylene in the optical fiber of zone one, so the signal output intensity $I_{\lambda_{1}}$ has dropped similar to the Fig. 4a, meanwhile, due to the absorption of methane gas in the optical fiber of zone two, so the signal output intensity $I_{\lambda_{2}}$ has dropped even worse compared with Fig. 4a and b. and the and the response time of the sensor system is about 40 seconds. Overall, according to the light intensity of the specific absorption spectrum which reflected from each gas detection zone, the concentration of gas could be detected, and the location of gas leakage can be accurately located.

4. Conclusion

In the paper, it demonstrated an multi-gas leakage synchronous monitoring based on the distributed optical fiber sensing technology, the fundamental sensing principle and characteristic are explained. the optical fiber sensing system is not only sensitive to multiple gas leakage concentration simultaneously, but also it could obtain the gas leak location accurately. a series of gas concentration measurement detection experiments were implemented, the experiment results indicated that the optical fiber testing system exhibited good gas concentration sensing and gas leakage location identification performance.

Although an optical fiber multi-gas leakage monitoring system has been built, the gas detection system still needs to be improved, and more effective design methods need to be put forward to enhance the stability and sensitivity enhancement in the optical fiber sensing system in the follow-up work.

Footnotes

Acknowledgments

The authors would like to acknowledge the supports from the Henan Province Natural Science Foundation (Grant No. 212300410422), Henan Province Project of Postgraduate education reform and quality improvement (Grant No. YJS2021AL042), Open Fund Project of Key Laboratory of Anhui Province for Mine Intelligent Equipment and Technology in Anhui University of Science and Technology “Research and application of laser cladding remanufacturing key technology for hydraulic column used in coal mine” (Grant No. KSZN202002003). Research team development project of Zhongyuan University of Technology (Grant No. K2021TD002)

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Multi-gas leakage synchronous monitoring system based on the distributed optical fiber sensing technology

Abstract

Keywords

1. Introduction

2. Theory

Table 1 The central wavelength of the gas absorption peak in the infrared band

Footnotes

Acknowledgments

References

Table 1
The central wavelength of the gas absorption peak in the infrared band