Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2004-03-18
2004-11-23
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S338100
Reexamination Certificate
active
06822236
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to detecting gases, and more particularly to methods of and apparatus for detecting trace amounts of target gases, such as natural gas, remotely along long-range paths in the free atmosphere, wherein the exemplary natural gas detection is enabled by simultaneously remotely detecting methane and ethane along the same long-range path in the free atmosphere.
2. Description of the Related Art
In the field of gas detection, attempts are made to detect one specific, or “target”, gas even though local conditions render such detection difficult. Such difficulty may be based, for example, on the fact that the target gas may only be present in “trace amounts”, such as one or a few parts per billion (PPB). Moreover, the target gas may be mixed with, or present with, water vapor and/or undesired (non-target) background, or “competitive”, gases that may be in the same atmosphere as contains the target gas, for example. The background gases are referred to as “competitive” gases because there are overlaps in absorption spectra of the trace gases and the background gases, e.g., in the infrared absorption spectra of such gases.
Many industries require facilities to detect target gases, such that there is a general need for accurate, fast and cost-competitive detection of target gases. However, the natural gas pipeline distribution system is the largest chemical distribution system in the United States. As a result, although equipment for detecting target gases other than natural gas has wide application in the United States, for example, the natural gas pipeline industry has the greatest need for accurate, fast, and cost-competitive chemical leak detection equipment and methods. This need relates in part to regulations that require gas utilities to perform periodic surveys for natural gas leaks.
Initially, in target gas detection for the natural gas industry, there is a need to distinguish between natural gas as a target gas, and other combustible gases. The main constituents of natural gas are methane and ethane, with methane being the primary component. However, methane is produced by many natural biological sources, including animal and plant. Thus, if an elevated methane level is sensed, it does not necessarily mean that there is a natural gas leak. In contrast, there are no substantial natural ethane emission sources. However, ethane generally does not exceed twenty-percent of natural gas. As a result, ethane is both more difficult to detect, but is a better indicator of natural gas than methane. Thus, to have an optimal natural gas detector, there is a need for the detector to simultaneously detect both methane and ethane to assure that the detected gas is from a natural gas leak and not from a natural emission source.
This need to simultaneously and independently detect both ethane and methane is not met by current gas detection equipment. For example, flame ionization detectors (FID) cannot distinguish natural gas from such competitive gases. As a result, when currently available FID equipment is used in an attempt to detect natural gas, the FID equipment provides “false natural gas alarms” based on the detection of leaking propane tanks, leaking gasoline cans, so-called “sewer gas”, and all other combustible gases. A natural gas pipeline utility using the FID equipment must respond to each false natural gas alarm although there is in fact no natural gas leak. Another limitation of the FID equipment is that during the detection process, it is generally necessary to place the equipment very close to the ground and within a “cloud” of the target gas that is to be detected. As a result, the FID detection process is relatively slow, and FID equipment cannot be used at a place remote from the locale of the gas leak, for example.
Also, Fourier Transform Infrared (FT-IR) spectro-radiometers use an interferometer to determine the spectral content of light passing through the free atmosphere. However, the output of such FT-IR instruments is based on a combination of all of the gases that are optically “active” (e.g., infrared absorptive) within the spectral region of the instrument. Thus, their temporal response is generally poor. Moreover, FT-IR systems are expensive, have very limited detection range through the free atmosphere, and cannot detect very low concentrations of target gases.
Further, in contrast to the FID and FT-IR techniques, tunable diode laser absorption Spectroscopy (TDLAS), laser absorption spectroscopy (LAS), and differential absorption laser-based radar (DIAL) all use laser emission sources that are narrow band. For example, the DIAL devices typically monitor only one or two very narrow spectral absorption lines. Laser-based techniques are more costly to manufacture, maintain and use compared to broadband techniques such as gas correlation radiometry (GCR). However, gas correlation radiometry (GCR) is generally a passive technique that relies on solar illumination or scattering, or on thermal emission background. Thus, GCR instruments do not have an active source of energy that is directed through the free atmosphere to the instrument. Further, while GCR instruments may be provided with filters that improve a signal to noise ratio by generally limiting the overall bandwidth of light admitted to a detector of the GCR instrument, such filters do not provide an optimized bandwidth around an optimized central bandpass wavelength. As a result, the sensitivity of such GCR instruments may be as low as 10 to 100 parts per million (PPM).
Moreover, the FT-IR, TDLAS, LAS, DIAL and GCR technologies provide separate background gas and target gas channels that are interrogated sequentially. That is, light transmitted along a path through the free atmosphere and then through the background channel may be detected by a detector first. After such detection, the light transmitted through the same path through the free atmosphere and then through the target gas channel is detected by the same detector. The resulting temporal, or sequential, spacing of the alternating detection of the background channel and the target gas channel may vary from 0.1 second to several minutes. That is, it generally takes more than 0.1 seconds for these systems to provide a complete data set consisting of a target gas absorption measurement and an atmospheric background measurement, and during that time period, there may be changes in the atmospheric conditions along the path of the light. Thus, the light that is transmitted along the path and through the target gas channel may have been subjected to different atmospheric conditions along the light path (e.g., atmospheric turbulence and variability) than the light transmitted through the background channel. As a result, the accuracy of these instruments is subject to a sensitivity limitation when used in a dynamic atmosphere. Atmospheric turbulence and variability generally limit the ultimate sensitivity of these instruments in that the same value of instrument output provided at different times may not be based on the same amount of the target gas. Further, attempts to avoid such atmospheric-induced inaccuracies, e.g., attempts to distinguish between signals generated based on a target gas and on the varying atmospheric conditions, have generally been limited to situations in which light is transmitted only a few feet through a detection path that may contain the target gas to be detected. For example, it may be practical to provide known modulation imposed on light transmitted along a detection path that is only a few feet long from transmitter to detector. Given the few feet between the transmitter and the detector, a conductor may easily input the characteristics of the know modulation to the detector so that demodulation will be accurate. However, problems are faced in accurately demodulating the modulates light when the detection path must, for practical purposes, be hundreds or thousands of feet long
In addition to the accuracy limitation due to limited sensitiv
Nelson Loren D.
O'Brien Martin J.
Hannaher Constantine
Martine & Penilla LLP
Moran Timothy
Ophir Corporation
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