RU2598694C2 - Device and method of measuring concentration of gaseous substances - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to measurement equipment and can be used for remote measurement of concentration of gaseous substances, including methane, in oil and gas industries, in power engineering etc. Device for measuring concentration of gaseous substances includes a unit of laser radiation with a wavelength varying in the absorption spectral range of a detected molecule and an analytical signal detector optically connected to the unit of laser emitter through a single-mode optical fibre and an analytical single-pass cell, as well as a unit of control, data receiving and processing, the laser emitter unit includes optically series-connected a module of diode laser, a fibre-optic splitter, one end of the fibres of which through the cell is optically connected to a comparing signal detector, and the second end through an additional fiber-optic cable delivering the radiation to an object of research and the analytical cell with fibre-optic input and output, is optically connected to an analytical signal detector. Unit of control, data receiving and processing is made up of three modules, namely: a digital programmable module, a module of digital-to-analog and analog-to-digital converters (DAC and ADC) and a module of analog signals converters, herewith by electric connections of outputs of the analytical signal detector, signal comparator, as well as control signals for the module of diode laser radiation the control is carried out over the diode laser power, its frequency tuning, registration, processing and comparing the analytical signal with the signal of comparison and, finally, calculation of concentration of the object of research. Method involves generating by the diode laser with a fibre radiation output of an optical radiation with a pulse duration overlapping the spectral absorption line of the analyzed gas, splitting this radiation with the help of the fiber splitter into the comparison channel for comparison at the calculation of concentration and to provide additional frequency stabilizing of the diode laser radiation at the level of 0.0002 cm^-1 along the line of methane absorption, input of the second part of the diode laser radiation into the analytical channel consisting of the single-pass cell with fibre-optic input and output, as well as fibre-optic cables for radiation delivery to the cell and outputting the radiation to the detector of analytical signal, determination of concentration of gaseous substances from the analytic and the comparison channels absorption spectra.

EFFECT: invention provides higher sensitivity and accuracy of measurements of volume concentration of methane on remote routes with the help of optical fibre and a single-pass optical cell of small length (less than 100 mm).

5 cl, 2 dwg

Description

The proposal relates to measuring equipment and can be used for remote measurement of the concentration of gaseous substances, including methane in the oil and gas industry, in the electric power industry and so on. A particularly significant effect can be obtained from its use in the extraction and transportation of oil and gas.

A known method for the detection and measurement of methane concentration in places of occurrence of coal seams using an optical fiber and an optical spectrometer (Patent US 2014/0138528 A1, May 2014). The spectrometer is located at or near the well, exploring the coal seam, and includes a radiation source and detector, as well as a probe located in the well and optically, through an optical fiber connecting the test sample to the spectrometer. The probe consists of an open optical cell, the optical fiber connecting the radiation source is docked to the upper part, and the second optical fiber is connected to the other (lower) one to transmit the studied (characteristic) radiation from the sample to the detector. The cell can freely move along the borehole and measure the methane content in cross sections of coal seams. Various types of spectrometers (Fourier spectrometers, spectrometers operating in the visible UV, IR spectral ranges) can be used as an optical spectrometer, which allow recording methane absorption spectra at various wavelengths. Mono-mode fibers, or fiber bundles, are used to transmit and receive radiation, where the central part of the beam transmits radiation and other (peripheral) fibers transmit characteristic radiation.

The disadvantage of this method is the low accuracy of the measurements, due to the limitation of the input optical power due to non-linear effects in the fiber, which serves as a sensitive element. In addition, when registering the absorption spectra of the measured sample, there is the possibility of serious interference and interference of the absorption spectra of methane with other gases, significantly limiting the sensitivity of the device. The second disadvantage of this method is the small radius of the system, not exceeding several hundred meters.

The works of foreign authors are known [Chan, K., et al. "An Optical Fiber-Based Gas Sensor for Remote Absorption Measurement of Low-Level Methane Gas in Near-Infrared Region", J Lightwave Tech. LT-2 (1984) pp. 234-237; Mohebati, A., et al. "Remote Detection of Gases by Diode Laser Spectroscopy", J Modern Optics 35 (1998) pp. 310-324], in which for the first time using the near-IR diode lasers and fiber optics, remote methane detection was demonstrated. In the first work, a Fabry-Perot-type multimode diode laser was used for these purposes, generating at a wavelength of 1.61 μm, a single-pass optical cuvette 0.5 m long and an optical fiber 1 km long at one end. Sensitivity to methane detection was 0.07% of the lower safety level (5% by volume). In the second work, methane was detected using a Fabry-Perot diode laser at a wavelength of 1.33 μm, an optical cuvette of 1 m length with a fiber input and output, and a short fiber (not more than a few km). Sensitivity to methane detection was +/- 0.05%.

The main disadvantage of these works on the remote detection of methane by absorption spectroscopy with diode lasers and optical fibers for delivering radiation to an optical cuvette was a significant limitation of their field of application due to the technical complexity of the systems, the low radiation power of diode lasers, the low efficiency of radiation input into the fiber and, as a result, the small range of these systems.

Known absorption fiber-optic gas analyzer containing sequentially installed and optically coupled emitter, input optical fiber, multi-way cell, consisting of three spherical mirrors, output optical fiber, unit for recording and processing information. A spectral integral demultiplexer is installed between the output optical fiber and the recording unit, and on the continuation of the collective mirror sphere in the immediate vicinity of its edge, the ends of the input and output optical fibers are installed on one side, both lens mirrors are mounted with the possibility of joint rotation relative to the center of curvature of the mirror collective in the common meridional plane of all mirrors (RU 2091764, G01N 21/61, 1997).

Also known is an optical absorption gas analyzer containing an optically coupled laser source of infrared electromagnetic radiation with a wavelength from the absorption region of the analyzed gas, a multi-pass gas cell made in the form of an integrating sphere with an internal reflective coating, where the optical input and output are located asymmetrically with respect to the center of the sphere, a filter and a radiation receiver connected through an amplifier to the processing unit and the registration of the information signal (RU 2022249, G01N 21/61, 1994). The inner surface of the integrating sphere can be made ellipsoidal (WO 2004/013600, G01N). To increase the accuracy and reliability of the studies, the optical absorption gas analyzer contains a broadband optical emitter located along its radiation with a tubular gas cuvette with internal reflective walls and two photodetectors equipped with light filters in the region of absorption and transparency of the analyzed gas, respectively, connected to the differential processing unit and recording information signals (US 6469303, G01J 005/02, 2002; US 2004/0007667, G01N 21/61).

Known optical absorption gas analyzer containing a source of electromagnetic radiation with a wavelength of at least from the absorption region of the analyzed gas, located along the radiation tube gas cuvette with internal reflective walls and a photodetector connected through an amplifier to the information processing and recording unit, characterized in that the tubular gas cuvette is made spiral (Archimedes spiral) and through holes are made in its walls at an acute angle to its plane of symmetry from the electromagnetic radiation source (RU 2262684).

The main disadvantages of the above gas analyzers:

1. The listed gas analyzers are difficult to manufacture and operate;

2. The use of optical fiber is extremely limited in view of the difficulties with attaching and removing radiation to single-pass ditches; the radius of action of such systems does not exceed several kilometers;

3. high instability of the received signal due to interference effects during the passage of coherent radiation of the laser source through the cuvette, which significantly reduces the sensitivity during registration of the gas component;

4. when measuring at two fixed wavelengths, it is impossible to ensure high selectivity of the analysis, since the influence of spectral interference is high, and it is also impossible to simultaneously measure the concentration of more than one gas;

5. some of the systems do not meet the requirements for explosion safety when used in explosive atmospheres.

The objective of the present invention is to provide a simple-to-design methane detector device with high accuracy in measuring methane concentrations at remote distances in hazardous areas using single-mode optical fiber and near-infrared diode lasers as radiation sources.

The technical result of the invention is to increase the sensitivity and accuracy of measuring methane concentration at remote distances from the radiation source and receiver (more than 50 km) using a single-mode optical fiber and a diode laser with a fiber output of radiation generating at a wavelength of 1.651 μm.

The indicated technical result is achieved by the fact that in a methane detector containing a laser radiation unit with a wavelength that varies in the absorption spectral range of the detected molecule, and an analytical signal detector, optically coupled to the laser emitter unit through a single-mode optical fiber and an analytical cell with fiber input and output, as well as a control unit for receiving and processing data, a laser emitter unit contains an optically serially connected diode laser module, a fiber branch the one end of the fiber of which through the comparison cuvette is optically connected to the detector of the comparison signal, and the other end is optically connected to the analytic signal detector through the fiber-optic cable that delivers radiation to the test object and the analytical cell with fiber input and output, and the control unit data reception and processing is made in the form of three modules, namely a digital programmable module, a module of digital-to-analog and analog-to-digital converters (DAC and ADC) and an analog signal converter module In this case, by means of electrical connections of the outputs of the detector of the analytical signal, the comparison signal, as well as the control signals of the diode laser module, the radiation power of the diode laser is controlled, its frequency is tuned, registration, processing and comparison of the analytical signal with the comparison signal and ultimately calculation concentration of the investigated object.

Regarding the object of the invention — the device, the technical result is achieved in that, according to the proposal, the device for measuring the concentration of the studied object contains a laser radiation unit (1) with a diode laser module (5) and a built-in comparison channel module (7, 8), an analytical cell with fiber input and output (3), optically coupled to the diode laser module and the analytic signal detector (4) through a fiber splitter (6) and a fiber optic cable (12), and the analytic signal detector is connected to the mode For analog signal converters (11), connected in series with the DAC and ADC module (10).

In specific embodiments of the device:

- the diode laser module (5) is a diode laser with distributed feedback with the output of radiation into a single-mode fiber, generating in the near IR wavelength range (1.651 μm), with a radiation power of at least 20 mW;

- analytical cell (3) with a fiber input and output, an optical path length of 50 mm and a total loss of not more than 1 dB;

- detectors of the analytical signal (4) and the comparison signal (8) contain InGaAs p-i-n photodiodes;

- fiber optic cable (12) can be made in the form of a single-mode fiber waveguide with optical radiation losses of approximately 0.24 dB / km;

- digital programmable module (9) and the DAC and ADC module (10) can be performed on the basis of a microcontroller or on the basis of programmable logic integrated circuits (FPGA).

The graphic image (Fig. 1) shows schematically a device for measuring the concentration of gaseous substances that implements a method for measuring the concentration of gaseous substances.

The methane detector contains a laser radiation unit (1) operating at a wavelength that varies in the absorption spectral range of the detected molecule, and an analytical signal detector (4) optically coupled to the laser emitter unit via a single-mode optical fiber (12) and an analytical single-pass cell with a fiber input and output (3), as well as a control unit, receiving and processing data (2), the laser emitter unit contains optically serially connected diode laser module (5), fiber splitter (6), one end of the wave which is optically connected through the comparison cell (7) to the detector of the comparison signal (8), and the second end is connected via optical fiber cable (12), which delivers radiation to the test object and the analytical cell with fiber input and output (3), to detector of the analytical signal (4), and the control unit, receiving and processing data (2) is made in the form of three modules, namely, a digital programmable module (9), a module of digital-to-analog and analog-to-digital converters (DAC and ADC) (10) and a module analog signal converters fishing (11).

The diode laser module (5) is a diode laser with distributed feedback with the output of radiation into a single-mode fiber, generating in the near IR wavelength range (1.651 μm), with a radiation power of at least 20 mW.

An analytical cell (3) with a fiber input and output, an optical path length of 50 mm and a total loss of not more than 1 dB. The cuvette can be made in the form of a bracket with fiber-optic collimators at the ends, or in the form of a tube with built-in flanges from fiber-optic collimators.

Detectors of the analytical signal (4) and the comparison signal (8) can be made of InGaAs p-i-n photodiodes. The diameter of the active area of photodiodes is at least 100 microns.

Fiber optic cable (12) can be made in the form of a single-mode fiber waveguide with optical radiation losses of not more than 0.24 dB / km.

The digital programmable module (9) and the DAC and ADC module (10) can be performed on the basis of a microcontroller or on the basis of programmable logic integrated circuits (FPGA).

The device operates as follows.

A diode laser (5), at the command of a digital programmable module (9) and a DAC and ADC module (10), generates a sequence of short pulses at a wavelength that coincides with the absorption line of the detected gas. The current pulses of the diode laser are trapezoidal - this allows you to additionally scan the radiation in the region of the center of the absorption line, capturing the entire line contour and thereby ensuring high selectivity to the selected gas component. Using the information from the detector of the comparison signal (8), the device is tuned, namely, the diode laser emitter is brought into the specified wavelength range. This is due to a change in the temperature of the Peltier thermocouple on which the diode laser is located. Peltier thermocouple current is controlled from a digital programmable module (9) through a DAC and ADC module (10) and an analog signal converters module (11). Temperature measurement is carried out by means of a temperature sensor located on the Peltier thermocouple next to the diode laser. The electrical signal from the temperature sensor enters the digital programmable module (9) through the analog signal converters module (11) and through the DAC and ADC module (10). The Peltier thermocouple and temperature sensor are required elements of both serial and prototype diode laser modules designed for spectral measurements. At the end of the tuning process, the device enters the stabilization mode of the wavelength tuning range from the spectrum of the substance in the comparison cuvette, and it is ready to take measurements. Next, part of the radiation with the help of a fiber splitter enters the comparison cell (7) and is recorded by the comparison signal detector (8). The comparison channel performs two functions: 1 - the main function of the comparison channel when calculating the concentration, 2 - performing additional temperature stabilization of the scanning cycles of the pump current of the diode laser along the gas absorption line in the comparison cell containing the known concentration of the gas under study broadened to the atmosphere by air. The next part of the radiation of the diode laser from the fiber splitter enters the fiber optic cable (12), is brought to the desired object of study, and then goes to the input of the cell (3). Direct absorption of laser radiation occurs in the cell in the presence of a detectable gas. The output radiation from the cell using the output fiber-optic cable (12) enters the input of the analyzer of the analytical signal, amplifies, digitizes (10, 11), and then using the digital programmable module (9), the concentration of the detected gas is calculated.

Example

A distributed feedback diode laser (ROS) with a single-mode fiber output generates at a wavelength of 1.651 microns, a radiation power of at least 20 mW. There is a single-mode fiber cable of considerable length at least 25 km at one end for transferring the radiation of a diode laser to the object of study (the place of methane leakage in the pipeline). There is a single-pass open cell (dustproof) with fiber inlet and outlet, 50 mm long.

The duration of the trapezoidal pulse of the pump current is 1 ms, the repetition rate of 200 Hz. With a typical coefficient of current tuning of 0.02 cm ^-1 / mA for ROS lasers of structure A ₃ B ₅ and pump currents of not more than 100 mA, the scan frequency scan in the pulse was not less than 2.0 cm ^-1 . This adjustment is enough to register the R4 multiplet of the 2v ₃ methane band. In ROS lasers, the typical lasing line width is 5 MHz, which provides spectral resolution of the multiplet, limited only by Doppler line broadening at room temperatures. At frequencies of modulation of the pump current pulses of at least 25 kHz, the spatial resolution in a single-mode quartz fiber will be less than 10 km.

A fiber splitter with a 1/4 fission factor directs part of the radiation (25% in power) to the comparison channel to perform the comparison procedure when calculating the methane concentration and to provide additional frequency stabilization of the diode laser radiation at a level of 0.0002 cm ^-1 along the absorption line of the reference gas - methane in a comparison ditch.

The second part of the radiation (75% in power) is brought to the analytic cell using the fiber optic cable, which registers the absorption on the R4 multiplet. A second optical fiber of the same length of at least 25 km transfers the radiation emerging from the cell and carrying information about the absorbing object to the analytic signal detector. There is an amplification of the analytical signal, digitization and the calculation of concentration. The sensitivity to measuring methane concentration at these parameters in an open atmosphere and at temperatures of -40 + 50 C °, as well as at values of attenuation of laser radiation in a fiber of 0.24 dB / km, will be less than 0.02% of the volume concentration of methane. This is quite enough to measure gas leaks at concentrations substantially lower than explosive ones. The calculation is done in the "on-line" mode with an update frequency of less than 30 ms.

Technical characteristics of the described device obtained experimentally: the length of the optical fiber is at least 25 km, the optical length of the analytical cell is 50 mm, the sensitivity to measuring the volume concentration of methane is less than 0.02%, the recording time is less than 30 ms.

Using the present invention allows to increase the sensitivity and accuracy of measuring the volumetric concentration of methane on remote routes using optical fiber and a single-pass optical cuvette of small length (less than 100 mm).

Hardware and software that provide high technical characteristics of the described device, after upgrading the optical fiber and receiving parts (nodes 4, 12, 3 of Fig. 1), significantly increase the reception range, as well as the number of analytical methane detection channels (see Fig. 2, where the first detection cuvette 13 is shown, the second cuvette 14 and the Nth detection cuvette are N).

For these purposes, along the path of the optical fiber (upper part of the fiber (12), Fig. 1), analytical channels with an optical cuvette and returnable optical fiber are added in parallel through fiber splitters (with a division ratio, for example, 95/5%) with an appropriate analytic signal detector. The analog signal converters module (11) in this case is optimized for the number of analytical channels. The advantage of the above connection scheme is due to the independence (autonomy) of the analytical channels in the event of failure of one of them.

Claims (5)

1. A device for measuring the concentration of gaseous substances, including methane, containing a laser radiation unit with a wavelength that varies in the absorption spectral range of the detected molecule, and an analytical signal detector that is optically coupled to the laser emitter unit through a single-mode optical fiber and an analytical cell with fiber input and an output, as well as a control unit for receiving and processing data, a laser emitter unit contains an optically serially connected diode laser module, a fiber a coupler, one end of the fiber of which is connected optically to the detector of the comparison signal through the comparison cuvette, and the other end is optically connected to the analytic signal detector, and the control unit, through the fiber-optic cable that delivers radiation to the test object and the analytical cell with fiber input and output, data reception and processing is made in the form of three modules, namely a digital programmable module, a module of digital-to-analog and analog-to-digital converters (DAC and ADC) and an analog converters module signals, while through the electrical connections of the outputs of the detector of the analytical signal, the comparison signal, as well as the control signals of the diode laser module, the radiation power of the diode laser is controlled, its frequency is tuned, registration, processing and comparison of the analytical signal with the comparison signal and ultimately calculation of the concentration of the studied object, while the fiber cable is made in the form of a single-mode fiber waveguide with loss of optical radiation at a wavelength of 1.65 microns m approximately 0.24 dB / km. 2. The device according to claim 1, characterized in that the detectors of the analytical signal and the comparison signal contain InGaAs p-i-n photodiodes with a fiber input. 3. The device according to claim 1, characterized in that the digital programmable module and the DAC and ADC modules can be made on the basis of a microcontroller or on the basis of programmable logic integrated circuits (FPGA). 4. The device according to claim 1, characterized in that, along the path of the optical fiber, analytical channels with an optical cuvette and return optical fiber are added in parallel through the fiber couplers, as well as with the corresponding analyzer of the analytical signal, while the module of analog signal converters is optimized for the number of analytical channels . 5. A method for measuring the concentration of gaseous substances, including methane, comprising generating a diode laser with a fiber output of optical radiation with a pulse duration that overlaps the absorption spectral line of the test gas, branching this radiation with a fiber splitter into a comparison channel to perform a comparison procedure for calculating the concentration and to provide additional frequency stabilization of the diode laser radiation at a level of 0.0002 cm ^-1 along the methane absorption line, the introduction of a second radiation of the diode laser into the analytical channel, consisting of a single-pass cell with fiber input and output, as well as fiber-optic cables for delivering radiation to the cell and to the analyzer signal detector, determining the concentration of gaseous substances from the absorption spectra of the analytical channel and the comparison channel.

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