GB2412430A - Hazardous gas detector - Google Patents
Hazardous gas detector Download PDFInfo
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- GB2412430A GB2412430A GB0406559A GB0406559A GB2412430A GB 2412430 A GB2412430 A GB 2412430A GB 0406559 A GB0406559 A GB 0406559A GB 0406559 A GB0406559 A GB 0406559A GB 2412430 A GB2412430 A GB 2412430A
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- gas
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- methane
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- 231100001261 hazardous Toxicity 0.000 title abstract description 18
- 239000007789 gas Substances 0.000 claims abstract description 140
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims abstract description 82
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 claims abstract description 44
- 238000010521 absorption reaction Methods 0.000 claims abstract description 44
- 230000003287 optical effect Effects 0.000 claims abstract description 42
- 230000005855 radiation Effects 0.000 claims abstract description 14
- ATUOYWHBWRKTHZ-UHFFFAOYSA-N Propane Chemical compound CCC ATUOYWHBWRKTHZ-UHFFFAOYSA-N 0.000 claims description 24
- 238000005259 measurement Methods 0.000 claims description 16
- 238000001514 detection method Methods 0.000 claims description 14
- 239000001294 propane Substances 0.000 claims description 12
- VGGSQFUCUMXWEO-UHFFFAOYSA-N Ethene Chemical compound C=C VGGSQFUCUMXWEO-UHFFFAOYSA-N 0.000 claims description 11
- 239000005977 Ethylene Substances 0.000 claims description 11
- OTMSDBZUPAUEDD-UHFFFAOYSA-N Ethane Chemical compound CC OTMSDBZUPAUEDD-UHFFFAOYSA-N 0.000 claims description 9
- 238000000862 absorption spectrum Methods 0.000 description 16
- 238000000034 method Methods 0.000 description 10
- 239000001273 butane Substances 0.000 description 9
- IJDNQMDRQITEOD-UHFFFAOYSA-N n-butane Chemical compound CCCC IJDNQMDRQITEOD-UHFFFAOYSA-N 0.000 description 9
- OFBQJSOFQDEBGM-UHFFFAOYSA-N n-pentane Natural products CCCCC OFBQJSOFQDEBGM-UHFFFAOYSA-N 0.000 description 9
- QQONPFPTGQHPMA-UHFFFAOYSA-N propylene Natural products CC=C QQONPFPTGQHPMA-UHFFFAOYSA-N 0.000 description 9
- 125000004805 propylene group Chemical group [H]C([H])([H])C([H])([*:1])C([H])([H])[*:2] 0.000 description 9
- 238000012545 processing Methods 0.000 description 8
- 239000002341 toxic gas Substances 0.000 description 6
- 239000003915 liquefied petroleum gas Substances 0.000 description 5
- 239000003345 natural gas Substances 0.000 description 4
- 238000004458 analytical method Methods 0.000 description 3
- 238000013459 approach Methods 0.000 description 3
- 239000000470 constituent Substances 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 235000013405 beer Nutrition 0.000 description 2
- 230000005540 biological transmission Effects 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 230000035945 sensitivity Effects 0.000 description 2
- 238000001228 spectrum Methods 0.000 description 2
- 230000001360 synchronised effect Effects 0.000 description 2
- 238000002835 absorbance Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 230000003197 catalytic effect Effects 0.000 description 1
- 238000012790 confirmation Methods 0.000 description 1
- 230000000593 degrading effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- -1 ethylene, propylene Chemical group 0.000 description 1
- 238000000605 extraction Methods 0.000 description 1
- 238000007689 inspection Methods 0.000 description 1
- 238000009434 installation Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 238000011896 sensitive detection Methods 0.000 description 1
- 230000011664 signaling Effects 0.000 description 1
- 238000004611 spectroscopical analysis Methods 0.000 description 1
- 231100000331 toxic Toxicity 0.000 description 1
- 230000002588 toxic effect Effects 0.000 description 1
- 238000000411 transmission spectrum Methods 0.000 description 1
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
- G01N2021/3513—Open path with an instrumental source
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
- G01N2021/396—Type of laser source
- G01N2021/399—Diode laser
Abstract
A hazardous gas detector includes a laser diode 1, driven by a circuit 11 operating the laser at wavelengths close to optical absorption lines of methane at either 1684nm or 1687.3nm and optical absorption lines or features of other targeted flammable gases at 1684.3nm, 1686.4nm and 1687nm; whilst sequentially scanning the lasers wavelength back and forth across the absorption lines or features. Radiation from the laser diode is transmitted through a monitored space and illuminates an optical detector 7. The electrical signal from the detector is processed to identify any of the targeted flammable gases present in the monitored space and calculate the quantity of each, this information being output by the detector. An estimate of the amount of hydrogen sulphide present in the monitored space is produced using a coefficient relating the amount of methane to the amount of hydrogen sulphide for the solution gas of a particular field.
Description
HAZARDOUS GAS DETECTOR
This invention is an apparatus for the detection or measurement of hazardous gases.
During the extraction, transportation and processing of oil and gas, the petrochemical industry needs to protect its employees and facilities from dangerous releases of flammable or toxic gases. The main flammable gas hazard encountered by the petrochemical industry is associated with the natural gas which is found at virtually all of its fields and facilities. In addition to natural gas, the petrochemical industry also uses or produces a number of other flammable gases including liquid petroleum gas (LPG), ethylene and propylene.
The main toxic gas hazard encountered by the petrochemical industry is associated with hydrogen sulphide, a highly toxic, corrosive gas that is present in the oil or gas of sour fields and facilities processing the output from sour fields.
Equipment for the detection of leaking flammable or toxic gases at petrochemical facilities has been developed using a number of technologies, including catalytic, electrochemical, ultrasonic and infrared. However, despite the variety of gas detectors available and considerable efforts upon the part of their developers and the petrochemical industry to perfect them and their use, a high proportion of flammable or toxic gas leaks at petrochemical facilities go undetected or are detected too late.
The object of this invention is to provide the petrochemical industry with a hazardous gas detector which will detect the majority of hazardous gas leaks and do this much earlier than existing gas detection equipment.
An ideal hazardous gas detector for the petrochemical industry would be a single gas detector capable of detecting any flammable or toxic gas that is likely to be found at its facilities with sufficient sensitivity to provide a warning before a dangerous condition is reached. In order to approach this ideal, such a detector would need to be able to detect methane, ethane, propane and butane (the main constituents of natural gas and LPG), ethylene and propylene (gases widely produced and used by the down-stream petrochemical industry) and hydrogen sulphide (found in sour oil or gas). Furthermore, such a detector would need to be able to reliably detect these gases at parts-per-million levels, in order to ensure that leaks were detected early.
The claimed invention approximates an ideal hazardous gas detector for the petrochemical industry.
The invention is described by way of examples and with reference to the accompanying drawings in which: Figure 1 shows a simple LDS based gas detector.
Figure 2 shows the variation in laser diode output power with applied drive current for a laser diode used in a simple LDS based gas detector.
Figure 3 shows the variation in output wavelength with applied drive current for a laser diode used in a simple LDS based gas detector.
Figure 4 shows the ideal transmission spectra for a single target gas absorption line to be scanned by a simple LDS based gas detector.
Figure 5 shows the drive current applied to a laser diode in a simple LDS based gas detector.
Figure 6 shows the signal from the optical detector when there is a substantial quantity of target gas present in the monitored space of a simple LDS based gas detector.
Figure 7 shows the optical absorption spectra for 1000ppm.m of methane between 1600nm and 1775nm.
Figure 8 shows the optical absorption spectra for 1000ppm.m of ethane between 1600nm and 1775nm.
Figure 9 shows the optical absorption spectra for 1000ppm.m of propane between 1600nm and 1775nm.
Figure 10 shows the optical absorption spectra for 1000ppm.m of butane between 1600nm and 1775nm.
Figure 11 shows the optical absorption spectra for 1000ppm.m of ethylene between 1600nm and 1775nm.
Figure 12 shows the optical absorption spectra for 1000ppm.m of propylene between 1600nm and 1775nm.
Figure 13 shows the optical absorption spectra for 1000ppm.m of hydrogen sulphide between 1600nm and 1775nm.
Figure 14 shows the hazardous gas detector in the first example of the claimed invention.
Figure 15 shows the optical absorption spectra for 1000ppm.m of methane between 1600nm and 1690nm.
Figure 16 shows the optical absorption spectra for 1000ppm.m of ethane between 1600nm and 1690nm.
Figure 17 shows the optical absorption spectra for 1000ppm.m of propane between 1600nm and 1690nm.
Figure 18 shows the optical absorption spectra for 1000ppm.m of ethylene between 1680nm and 1690nm.
Figure 19 shows the optical absorption spectra for a 100 metre path through the atmosphere at 100%RH, 30 C, between 1680nm and 1690nm.
Figure 20 shows the drive current applied to the laser diode in the first example of the claimed invention.
Figure 21 shows the drive current applied to the laser diode in the second example of the claimed invention.
Laser Diode Spectroscopy (LDS) is a well-known technique used for the sensitive detection of specific gas molecules. A simple, LDS based gas detector is shown in Figure 1 and comprises a laser diode 1, mounted on a temperature stabilised mount 2, driven by a laser diode drive / modulation circuit 3, the output from the laser diode being collected and collimated by optical element 4, the resulting beam being transmitted through a monitored space 5, to a receiver optical element 6, which focuses the received radiation onto a detector 7, the signal from the detector being amplified by amplifier 8 and digitised by ADC 9, then processed by signal processing system 10 to calculate the quantity of target gas in the monitored space.
The operation of a simple LDS based gas detector is illustrated by Figures 2 to 8.
Figure 2 shows the variation in output power from the laser diode as the drive current is increased, this being essentially linear when operating above the laser diode's threshold current. Figure 3 shows how the output wavelength of the laser diode varies with drive current, this effect being used to scan the laser's wavelength across the target gas' absorption line of interest. Figure 4 shows the ideal wavelength dependent transmission in and around the region of a chosen target gas absorption line resulting from introduction of a quantity of target gas into the monitored space. Figure 5 shows the current signal applied to the laser diode. This signal contains a bias component which maintains the laser diode above threshold and operating at the correct mean wavelength and a sinusoidal component, which scans the laser diode's wavelength back and forth over the region containing the absorption line. In the absence of target gas in the monitored space, the system's detector will produce a signal waveform similar to that shown in Figure 5, including both a bias component and a sinusoidal component. Figure 6 shows the output from the system's detector during a single wavelength scan cycle when a substantial quantity of target gas is present in the monitored space. The deviation from a sinusoidal waveform is produced by absorption of optical radiation whenever the wavelength of the laser diode scans through the region of the target gas' absorption line, which in Figure 7 occurs approximately half way up the positive excursion of the modulation cycle. The quantity of target gas in the monitored space can be determined by measuring the change in received intensity when the laser's wavelength corresponds to the wavelength of the optical absorption line of the target gas. The amount of absorption produced by a given quantity of gas can be determined using Beer's law: I = IO.e where I is the received intensity at the detector, lo is the intensity incident upon the sample being illuminated, is the absorption cross section of the target gas at the absorption wavelength and n is the total number of target gas molecules in the monitored space.
When considering the design of an ideal hazardous gas detector for the petrochemical industry meeting the requirements outlined on Page 1, the use of conventional LDS techniques presents the designer with a number of problems: 1. The wavelength tuning range of most laser diodes is only a few nanometres. This relatively small tuning range is insufficient to enable measurements to be made on a large number of gas species.
2. The LDS technique is only appropriate for making measurements of gases with narrow, well-resolved absorption features or lines. The gas molecules which exhibit narrow, well-resolved absorption features are small, simple molecules. Butane and propylene are not small, simple molecules and their absorption spectra do not contain narrow, well- resolved absorption features or lines.
3. Hydrogen sulphide has a Threshold Limit Value (TLV) of 1 Oppm and only exhibits weak absorption in the wavelength regions accessible with room temperature laser diodes. In the 1550nm to 1625nm region which is best suited to the detection of hydrogen sulphide, 10ppm.m of H2S will produce a maximum fractional absorbance of just 4X10-6 which is too small to be detected reliably enough for use in safety applications.
Close inspection of the absorption spectra of methane, ethane, propane, butane, ethylene, propylene and hydrogen sulphide shown in Figures 7 to 13 reveals that over the range 1600nm to 1785nm neither butane nor propylene possess absorption features suitable for conventional LDS measurements; whilst propane only possesses a single well-resolved feature, this at 1686.4nm. Also, there is no wavelength region where it is possible to make measurements of all of the flammable gases of interest and hydrogen sulphide.
The claimed invention addresses most of the problems outlined above, resulting in an LDS based hazardous gas detector which approximates the ideal for the petrochemical industry.
The LDS based hazardous gas detector of the claimed invention is shown in Figure 14. The laser diode 1 is a VCSEL, with a nominal wavelength of 1685.5nm and capable of being current tuned over the range 1684nm to 1687. 5nm, this providing access to the strong absorption lines or features of methane at 1684nm s and 1687.3nm, ethane at 1684.3nm, propane at 1686.4nm and ethylene at 1687.0nm (See Figures 14 to 18.) The output radiation from the laser diode is collected and collimated by optical element 4, the resulting beam being transmitted through a monitored space 5, to a receiver optical element 6, which focuses the received radiation onto a detector 7. The signal from the detector is amplified by amplifier 8 and digitised by ADC 9, then processed by signal processing system 12 to determine which gases, if any, are present in the monitored space and in what quantities. The determination of which flammable gases, if any, are present in the monitored space is based upon the known characteristics of the absorption lines and features of methane, ethane, propane and ethylene which the detector scans. The quantities of each gas calculated to be present in the monitored space are output separately by output interface 13.
In a preferred embodiment of the claimed invention, the drive current applied to the laser diode by drive circuit 11 is shown in Figure 20 and has the following attributes: 1. The bias component of the current sequences between levels A, B. C and D, levels chosen to operate the laser diode 1 at mean wavelengths close to the different target gas absorption lines and features at 1684.3nm, 1686.4, 1687.0nm and 1687.3nm respectively.
2. The amplitudes of the sinusoidal current components which scan the laser diode's wavelength across each target gas absorption line or feature are independently optimised for each absorption line or feature.
3. The amplitudes of both the bias and sinusoidal components are carefully chosen to avoid scanning the laser across the absorption lines of atmospheric water vapour at 1684.23nm, 1687.07nm and 1685.92nm.
The strong methane line at 1684nm is not used in this preferred embodiment because this wavelength is very close to a strong water vapour absorption line which has the potential to interfere with measurements made along open atmospheric paths (See Figure 19.).
There are two processes performed by the signal processing system 12 which need further explanation. These processes are the determination of which gases are present in the monitored space and the estimation of the amount of hydrogen sulphide present in the monitored space.
The determination of which gases are present in the monitored space is based upon analysis of the set of measurement results for the laser operating at each mean wavelength close to the strong absorption lines or features of ethane, propane, ethylene and methane respectively. In determining the gas or gases present, the analysis needs to take account of the fact that ethane and ethylene have multiple absorption features in the 1684nm to 1687.5nm wavelength range.
In a preferred embodiment of the claimed invention, the analysis of the measurement results for scans produced at bias currents A, B. C and D proceeds as follows: 1. The signals for periods corresponding to bias currents A, B. C and D are separately windowed and Fourier transformed.
2. The harmonic frequency components in each Fourier transform are normalised with respect to the amplitude of the fundamental, wavelength scanning frequency component.
3. The relative amplitude patterns of the measured harmonic components are compared with the known relative amplitude patterns for the harmonic components produced by each target gas at each bias current level.
4. The results of the comparison of the patterns at each bias current level are logically and proportionately combined to identify the gas or gases present in the monitored space.
Since the relative amplitude pattern for each target gas is known for each bias current level, all comparison results for all bias levels can make a contribution to the identification of the gas or gases present in the monitored space. The absence of harmonics in a scan at a mean wavelength at which a gas is known not to absorb can contribute to the confirmation of the presence of a particular candidate gas. By similar reasoning, the presence of harmonics in a scan at a mean wavelength at which a gas is known not to absorb can contribute to the elimination of a particular candidate gas.
Having identified the flammable gas or gases present in the monitored space, the quantity of each gas present can be calculated. This calculation uses the known absorption cross-sections of each identified gas and Beer's law to calculate the amount of gas required to produce normalised harmonic frequency components of the sizes measured.
For the claimed invention, the process of estimation of the quantity of hydrogen sulphide present in the monitored space is not based upon direct measurement of optical absorption by hydrogen sulphide. Instead, the estimate is based upon the quantity of methane measured in the monitored space and knowledge of the relative concentrations of methane and hydrogen sulphide in the solution gas of a particular oil or gas field. The justification for using this approach to estimate the amount of hydrogen sulphide present in the monitored space is as follows: 1. For sour oil or gas fields, the relative concentrations of methane and hydrogen sulphide in the solution gas of that field are known.
2. When solution gas leaks from vessels containing oil or gas from a sour field, the relative concentrations of methane and hydrogen sulphide in the leaking gas are the same as those in the solution gas inside the vessel.
3. The methane and hydrogen sulphide in solution gas are intimately mixed and do not separate or stratify when they disperse into the area surrounding a leaking vessel.
4. Any changes in the amount of hydrogen sulphide present in the solution
gas of an oil or gas field are very gradual.
The mathematics for calculating an estimate of the amount of hydrogen sulphide present in the monitored space based upon the measured quantity of methane is relatively simple. A single coefficient KH2S can be calculated, based upon the known relative concentrations of methane and hydrogen sulphide in the field's solution gas: KH2S = [hYdrOgen sulphide] / [methane] By way of example, if a hazardous gas detector of the type claimed detects 4,800ppm.m of methane when the solution gas for the field is known to contain 960,000ppm methane and 10,000ppm hydrogen sulphide, the estimated quantity of hydrogen sulphide present would be: H2S = 4,800 X KH2S = 4,800 X 10,000 / 960,000 = 50 ppm.m For a monitored space that is 10 metres long, the claimed detector would indicate an average hydrogen sulphide concentration of 5ppm, which is 50% of the TLV for hydrogen sulphide.
In order to facilitate the estimation of the amount of hydrogen sulphide present in the monitored space as described for the claimed invention, a means is provided to inform the signal processing system 12 of the calculated KH2S coefficient for a particular field's solution gas. By making it possible to update the KH2S coefficient as and when required, any gradual changes in the sourness of the solution gas of
a particular field can be accommodated.
There are a number of benefits associated with the estimation of the amount of hydrogen sulphide as described for the claimed invention: 1. It enables a single hazardous gas detector to be produced which is capable of providing warnings about both flammable and toxic gas hazards typically found at petrochemical facilities.
2. Detection of methane using LDS techniques is significantly easier than detection of hydrogen sulphide using LDS techniques. In particular, the optical absorption lines of methane accessible using room temperature laser diodes are at least an order of magnitude stronger than the accessible optical absorption lines of hydrogen sulphide.
3. For the vast majority of oil or gas fields, methane is the principle constituent of solution gas. Detecting the principle constituent of solution gas increases the probability of early detection of any leak of solution gas.
4. Unlike conventional flammable gas detectors, the claimed invention is capable of identifying the flammable gas present in the monitored space and only signalling an estimate of the amount of hydrogen sulphide present if this gas is methane. Small, background concentrations of flammable gases or vapours that are common-place at petrochemical facilities but that are not hazardous will not give rise to warnings about a hydrogen sulphide hazard.
The resulting hazardous gas detector is considerably simpler than an LDS based detector capable of directly measuring both methane and hydrogen sulphide.
This simplicity is beneficial because it will improve the reliability and robustness of the gas detector whilst simultaneously reducing its manufacturing cost.
Furthermore, if the operators of petrochemical facilities only have to install a single type of gas detector, this will reduce the installation and operation costs of their gas detection systems.
The absence of narrow, resolved absorption features in the absorption spectra of butane and propylene means that the claimed invention is not suitable for the detection of these gases. The inability to detect butane is only a minor limitation for most petrochemical gas detection applications; since butane is only a small component of natural gas and is rarely found or used on its own. (Even in LPG which can contain a significant amount of butane, there is always a larger amount of propane in the LPG which the claimed detector could detect.) However, the inability to detect propylene means that the claimed invention is not suitable for general use on downstream petrochemical facilities producing or using propylene. At such facilities, use of the claimed detector might need to be restricted to areas where propylene is not used or produced.
There are a number of ways in which the reading(s) of the claimed hazardous gas detector can be output, these including: 1. Multiple analogue or digital displays, each displaying the quantity of a particular gas.
2. A display screen with values presented for each of the gases that can be detected.
3. Multiple analogue electrical outputs, each producing a signal proportional to the quantity of a particular gas.
4. A digital electronic signal conforming to a defined protocol and containing numerical data conveying the concentration or quantity of each gas.
5. The opening or closing of relays at prescribed flammable gas or hydrogen sulphide concentrations.
The means of collection and collimation of the optical radiation from the laser diode need not be limited to the simple optical element shown in Figure 14. The optical element used for this purpose can comprise of a number of separate optical elements combined to perform the required function of laser diode radiation collection, collimation and transmission through the monitored space.
Furthermore, these optical elements need not be limited to the free-space optical elements shown. The radiation from the laser diode can be coupled into fibre- optic cable and carried to one or more optical elements which will collimate and transmit the radiation through the monitored space.
When considering the laser diode to be used in systems such as those described for the claimed invention, some of the requirements placed upon this component are difficult to meet with the DEB laser diodes conventionally used in LDS systems. In particular, requirements for relatively large wavelength scanning ranges are not readily met by DFBs. However, these requirements can be met by VCSEL laser diodes, which tune over a significantly larger wavelength range with drive current. Long wavelength VCSEL laser diodes are virtually ideal for use in the claimed invention.
The reliability of identification or discrimination of gases such as propane and ethylene can be further enhanced by applying a slope to the bias current when scanning over the wavelength regions which contain their strongest absorption features. The resulting laser diode current waveform would look similar to that shown in Figure 21. By applying a slope to the bias current, the mean wavelength of the laser diode is gradually swept through the region of the absorption feature of interest. The pattern of harmonic components produced during a sweep will be different when absorption features with different shapes are scanned. These differences can be used to discriminate propane from ethylene despite the presence of absorption features at common wavelengths.
The application of a slope to the bias current could be extended to encompass the entire wavelength range 1684nm to 1687.3nm, with the net effect of enabling an absorption spectrum for the monitored space to be determined for the range 1684nm to 1687.3nm. This spectrum would include measurements of absorption by atmospheric water vapour at 1684.23nm, 1687.07nm and 1685. 92nm. The disadvantage of capturing a spectrum for the entire wavelength range 1684nm to 1687.3nm is that the only regions of significant interest are in and around the gas absorption lines and features at 1684.3nm, 1686.4, 1687. 0nm and 1687.3nm. By spending time measuring regions of no or limited interest, the time spent measuring the regions of primary interest is reduced, with the net effect of degrading overall system signal to noise ratio. It is for this reason that the laser current drive waveforms in the preferred embodiments of the claimed invention cause the laser to scan discrete, non-continuous wavelength regions (See Figures 20 and 21.).
Whilst the hazardous gas detector with a VCSEL laser capable of scanning the range 1684nm to 1687.5nm is a preferred embodiment of the claimed invention, capable of detecting most flammable gases present at petrochemical facilities, it is not the only embodiment of the claimed invention that could make use of the technique described for estimation of the quantity of hydrogen sulphide present in the monitored space. Any LDS based gas detector capable of detecting methane with high sensitivity and specificity could be used as the basis of a combined methane and hydrogen sulphide detector employing the estimation technique described for the claimed invention. Such an LDS based gas detector need not use a VCSEL laser and could operate at any wavelength where methane exhibits sufficient optical absorption to enable sensitive measurements to be made.
In the examples of the claimed invention described, the laser diode radiation is collected and transmitted directly through the monitored space, subsequently illuminating a receiver detector. However, the claimed invention can also be beneficially employed in an apparatus where a sample of gas from an area to be monitored is drawn into a sample measurement chamber in order to be illuminated and measured using the approaches described.
The example of the claimed invention illustrated in Figure 14 shows the output from the optical detector being amplified, digitised and subsequently processed by a digital signal processing system. This means of processing the signal from the detector is shown because it is particularly suitable for the measurement of multiple frequency components in a common signal. However, it is possible to realise the claimed invention in an apparatus where the magnitudes and phases of the frequency components are determined by amplifying the detector signal and synchronously detecting the various frequency components using multiple synchronous detectors operating in parallel at different frequencies upon the same signal. The outputs from the synchronous detectors would subsequently be digitised and used for the calculation of the quantity of target gas present in the monitored space as described earlier.
Claims (9)
1. An apparatus for the detection of methane, ethane, propane or ethylene in a monitored space, the apparatus including a laser diode driven by a current comprising two components, a bias component and a scanning component, the bias component varying in a manner determined to operate the laser diode at wavelengths suitable for scanning either of methane's absorption lines at 1684nm and 1687.3nm; and one or more of the other gases' absorption lines or features at 1684.3nm, 1686.4nm and 1687.0nm, the scanning component repetitively scanning the laser diode's wavelength over the chosen absorption lines or features, the optical radiation from the laser diode being collected and transmitted through the monitored space and subsequently illuminating an optical detector, the electrical signal from this optical detector being processed to determine the gas or gases present in the monitored space and the amounts of each gas present, this information being output by the detector.
2. An apparatus as claimed in Claim 1 which having determined the amount of methane gas present in the monitored space, estimates the amount of hydrogen sulphide present in the monitored space; using a coefficient relating the amount of methane to the amount of hydrogen sulphide for the solution gas of a particular field or facility, this estimate being output by the detector.
3. An apparatus for the detection of methane gas and estimation of the amount of hydrogen sulphide gas present in a monitored space, the apparatus including a laser diode driven by a current comprising two components, a bias component and a scanning component, the bias component varying in a manner determined to operate the laser diode at one or more wavelengths suitable for scanning any of methane's optical absorption lines or features, the scanning component repetitively scanning the laser diode's wavelength over the chosen absorption line(s) or feature(s), the optical radiation from the laser diode being collected and transmitted through the monitored space and subsequently illuminating an optical detector, the electrical signal from this optical detector being processed to determine the amount of methane gas present in the monitored space; and using a coefficient relating the amount of methane to the amount of hydrogen sulphide for the solution gas of a particular field or facility, estimation of the amount of hydrogen sulphide present in the monitored space, information conveying the amount of methane present in the monitored space and the estimated amount of hydrogen sulphide present in the monitored space being output by the detector.
4. An apparatus for the detection of gases based upon the apparatus claimed in Claims 1, 2 and 3 where gas is drawn into a sample measurement chamber in order to be illuminated by laser diode radiation and measured as described in Claims 1, 2 and 3, the calculated or estimated gas concentrations being output by the detector.
5. An apparatus as claimed in Claims 2 and 3 where a means is provided to update the coefficient which is used by the detector to estimate the amount of hydrogen sulphide present in the monitored space.
6. An apparatus as claimed in Claims 1, 2 and 3 where the diode laser is a VCSEL.
7. An apparatus as claimed in Claims 1, 2 and 3 where the means of collecting the laser radiation and transmitting it through the monitored space includes combinations of free-space optical elements and / or fibreoptics.
8. An apparatus as claimed in Claims 1, 2, 3 and 4 where the means of output for the concentrations or quantities of gases calculated or estimated present in the monitored space or sample measurement chamber includes analogue electrical signals proportional to the concentration or quantity of each gas, a digital electronic signal conforming to a defined protocol and containing numerical information conveying the concentration or quantity of each gas, a numerical representation of the concentration or quantity of each gas upon a display which is associated with or forms part of the apparatus, or the opening or closing of relays at prescribed concentrations or quantities of gas, such relays and the necessary control circuitry either being associated with or forming part of the apparatus.
9. An apparatus for the detection or measurement of gases substantially as herein described above and illustrated in the accompanying drawings.
Priority Applications (7)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0406559A GB2412430A (en) | 2004-03-24 | 2004-03-24 | Hazardous gas detector |
| EP05717944A EP1723407B1 (en) | 2004-03-09 | 2005-03-08 | Gas detection |
| DE602005003337T DE602005003337T2 (en) | 2004-03-09 | 2005-03-08 | GAS DETECTION |
| AT05717944T ATE378580T1 (en) | 2004-03-09 | 2005-03-08 | GAS PROOF |
| CA2558851A CA2558851C (en) | 2004-03-09 | 2005-03-08 | Gas detection |
| US10/592,473 US7705988B2 (en) | 2004-03-09 | 2005-03-08 | Gas detection |
| PCT/GB2005/000876 WO2005088275A1 (en) | 2004-03-09 | 2005-03-08 | Gas detection |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0406559A GB2412430A (en) | 2004-03-24 | 2004-03-24 | Hazardous gas detector |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0406559D0 GB0406559D0 (en) | 2004-04-28 |
| GB2412430A true GB2412430A (en) | 2005-09-28 |
Family
ID=32188564
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0406559A Withdrawn GB2412430A (en) | 2004-03-09 | 2004-03-24 | Hazardous gas detector |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2412430A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3081922A1 (en) * | 2015-04-14 | 2016-10-19 | Yokogawa Electric Corporation | Gas concentration analyzer and gas concentration analysis method |
| US9671333B2 (en) | 2011-11-28 | 2017-06-06 | Yokogawa Electric Corporation | Laser gas analyzer |
| CN108562551A (en) * | 2018-04-11 | 2018-09-21 | 黑龙江工程学院 | A kind of method that detector accurately measures concentration of SO 2 gas |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110554002A (en) * | 2019-09-30 | 2019-12-10 | 大连艾科科技开发有限公司 | Hydrogen sulfide concentration detection module, hydrogen sulfide concentration telemeter and application |
| CN113115508B (en) * | 2021-04-07 | 2024-01-16 | 中国人民解放军国防科技大学 | Space environment simulation system based on laser wake field acceleration |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3805074A (en) * | 1973-01-02 | 1974-04-16 | Texas Instruments Inc | Spectral scan air monitor |
| US5301014A (en) * | 1991-03-27 | 1994-04-05 | Dragerwerk Aktiengesellschaft | Method and arrangement for spectroscopically measuring the concentration of a component of a gas sample |
| US5543621A (en) * | 1995-05-15 | 1996-08-06 | San Jose State University Foundation | Laser diode spectrometer for analyzing the ratio of isotopic species in a substance |
| EP1103804A1 (en) * | 1999-11-24 | 2001-05-30 | Siemens Aktiengesellschaft | Method for detection of natural gas |
-
2004
- 2004-03-24 GB GB0406559A patent/GB2412430A/en not_active Withdrawn
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3805074A (en) * | 1973-01-02 | 1974-04-16 | Texas Instruments Inc | Spectral scan air monitor |
| US5301014A (en) * | 1991-03-27 | 1994-04-05 | Dragerwerk Aktiengesellschaft | Method and arrangement for spectroscopically measuring the concentration of a component of a gas sample |
| US5543621A (en) * | 1995-05-15 | 1996-08-06 | San Jose State University Foundation | Laser diode spectrometer for analyzing the ratio of isotopic species in a substance |
| EP1103804A1 (en) * | 1999-11-24 | 2001-05-30 | Siemens Aktiengesellschaft | Method for detection of natural gas |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9671333B2 (en) | 2011-11-28 | 2017-06-06 | Yokogawa Electric Corporation | Laser gas analyzer |
| EP3081922A1 (en) * | 2015-04-14 | 2016-10-19 | Yokogawa Electric Corporation | Gas concentration analyzer and gas concentration analysis method |
| CN108562551A (en) * | 2018-04-11 | 2018-09-21 | 黑龙江工程学院 | A kind of method that detector accurately measures concentration of SO 2 gas |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0406559D0 (en) | 2004-04-28 |
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| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |