Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2000-07-24
2002-09-24
Dang, Hung Xuan (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339010
Reexamination Certificate
active
06455854
ABSTRACT:
INDUSTRIAL FIELD
The present invention relates to the infrared detection of hydrocarbon gases (which term includes vapours).
BACKGROUND ART
The use of non-dispersive infrared spectroscopy to detect gases is well established. It essentially involves transmitting infrared radiation along a path in an area being monitored; the wavelength of the infrared radiation is chosen so that it is absorbed by the gas of interest (hereafter called the “target gas”) but not substantially absorbed by other gases in the atmosphere of the area being monitored. If monitoring out-of-doors, the wavelength should ideally not be absorbed by liquid water (e.g. in the form of condensation, rain or spray). The intensity of the radiation that has passed along the path in the area being monitored is measured and the attenuation in the intensity of the radiation gives a measure of the amount of the target gas in the monitored area.
However, factors other than absorption by the target gas also attenuate the infrared radiation including obscuration of the detecting beam, atmospheric scattering of the radiation, contamination of optical surfaces, e.g. by dirt or condensation, and ageing of components. The reliability of infrared gas detectors is significantly improved by the use of a reference; such a reference is usually infrared radiation at a different wavelength, which ideally is a wavelength at which the target gas does not exhibit significant absorption. The ratio between the signal obtained at the wavelength where the target gas does absorb (the “sample wavelength”) and the signal obtained at the wavelength where the target gas does not significantly absorb (the “reference wavelength”) compensates for the attenuation caused by non-target gases since ideally the signal at the reference wavelength and the signal at the sample wavelength will both be affected by such non-target gas attenuation.
A known infrared detector is a so-called “fixed point” detector, which has a very short path length (e.g. up to 10 cm) and so only monitors a relatively small space. It can be used to detect leakages of hydrocarbons from oilrigs, pipelines, storage tanks or refineries. The provision of such detectors in open spaces away from a leakage site may result in the leakage not being detected since prevailing atmospheric conditions (e.g. wind speed, wind direction and temperature) could carry the gas away from the detectors, which would then not register the leakage. It is therefore a difficult task to position such fixed point detectors and usually a compromise is drawn in the location of detectors, based on likely leak sites and typical prevailing weather conditions; also the number of such detectors that can be provided is limited by cost. Generally fixed-point detectors are used for monitoring of specific items of equipment and apparatus that are liable to leak, for example pipeline joints and valves.
Fixed-point detectors are coupled with an alarm that indicates the detection of a target gas in the immediate neighbourhood of the detector. Because such detectors are placed near the source of any leak, any significant leaking target gas will be in relatively high concentration in and around the detector. It is therefore possible to set the alarm such that the amount of the target gas present before the alarm is triggered is relatively large, thereby avoiding the giving of false alarms. The giving of false alarms is a substantial problem since it could result in the shutting down of a facility, for example an oil rig or an oil refinery.
To overcome the above-mentioned shortcomings of fixed-point detectors, longer path-length gas detectors, so called “open-path optical gas detectors”, are used, in which radiation at sample and reference wavelengths is transmitted along an open-path which passes through the atmosphere in the space to be monitored. The length of the path can vary from one to a thousand meters, depending on the application, and so allows a much greater space to be monitored than is the case with fixed-point detectors. When used out-of-doors, the open nature of the optical path means that the beam is exposed to prevailing atmospheric weather conditions, which can seriously affect the operation of the instrument. For example, rain, snow, mist, fog, sea spray, blizzards and sand or dust storms scatter or absorb radiation at the reference and sample wavelengths. The level of absorption and scattering by such weather conditions depends on the size, shape, nature and optical properties of the droplets, drops or particles concerned. Unfortunately such attenuation is not uniform across the infrared spectrum, i.e. the attenuation at the sample wavelength and the attenuation at the reference wavelength are not identical which gives rise to errors in the measurement of the amount of target gas and can, in extreme cases, lead to the failure to trigger an alarm or the triggering of a false alarm. The matter is complicated considerably because different weather conditions exhibit different relative and absolute attenuation at the sample and reference wavelengths. For example, one sort of fog can attenuate the radiation at the reference wavelength more than the radiation at the sample wavelength whereas a different sort of fog will attenuate the radiation at the sample wavelength more than the radiation at the reference wavelength. The variability of atmospheric attenuation for different weather conditions makes it very difficult to compensate for the effects of weather upon this sort of gas detector.
In order to minimise the differential attenuation between the sample wavelength and the reference wavelength, it is preferable that the two wavelengths are as close as possible to each other. However, this is not always possible since there may not be a suitable reference wavelength, i.e. a wavelength at which the target gas is only minimally absorbed, near the sample wavelength. The situation is made even more complex because of the need to avoid cross-sensitivity to other atmospheric gases, the absorption/refraction characteristics of water droplets that may be present in the path of the infrared beam and the band shapes and tolerances of the filters used to restrict the sample and reference wavelengths. Thus there may be several hundred nanometers between the sample and reference wavelengths. This separation can result in significant differences between the absorption characteristics of the sample and reference wavelengths under different weather conditions, as set out above.
Instead of measuring the reference signal at a single wavelength, it has been proposed (see for example GB-1,402,301, GB-1,402,302, U.S. Pat. No. 4,567,366, EP-0,744,615 and GB-2,163,251) to use two reference wavelengths located on either side of a sample wavelength and take, as the reference signal, the average of the signals at the two reference wavelengths. This arrangement requires the measurement of light absorption at two different reference wavelengths, which in turn requires either the use of separate light beams to measure the absorption at each of the two reference wavelengths or the use of a mechanical arrangement to bring two filters into alignment with a single light-sensitive detector. Both solutions will work satisfactorily in a laboratory but not in the field, particularly not in the harsh environments encountered on offshore oil/gas platforms or in the Middle East, the Tropics, the Arctic, etc. The use of two reference light beams (in addition to the sample light beam) requires careful alignment (within micron tolerances) of the detectors and it is difficult enough to align the detectors for the sample beam and a single reference beam, let along aligning an additional detector for a second reference beam. Furthermore, the buffeting of the detectors in the environment of the North Sea, for example, can displace the alignment. In addition, the use of an additional reference beam makes the system expensive. The use of a mechanical arrangement (e.g. a spinning filter wheel) to bring sample and reference filters periodically into alignment
Andrus Sceales Starke & Sawall LLP
Dang Hung Xuan
Zellweger Analytics Limited
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