Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-08-28
2003-12-02
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339100, C250S338500
Reexamination Certificate
active
06657198
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a system and method for the detection of moisture in natural gas. More specifically, the present invention relates to a technique for determining the level of water vapor present within an industrial natural gas pipeline.
Natural gas has long been used as an energy source because of its low cost and widespread availability. After natural gas is mined, it is purified through several sequential processes, and distributed via networks of underground pipelines that typically transport the gas at a pipe pressure of several hundred pounds per square inch (PSI). Natural gas is sold to the customer as an energy product, and the energy content is generally expressed in British Thermal Units (BTU). The rate that gaseous product is pumped to the customer is measured in standard million cubic feet (SMCF), which is based on the gas volume at a standard pressure and temperature (typically 1 atmosphere pressure/14.73 PSI, and 70 degrees F.).
Contaminants in natural gas, such as water, reduce the BTU capacity of the gas, thereby resulting in a less efficient energy product. Contaminants also corrode delivery pipelines over time potentially resulting in serious safety hazards while also necessitating the costly replacement of segments of the pipeline (downtime for the pipelines can cost upwards of several thousand dollars per second). Accordingly, companies engaged in the mining, purification, and distribution of natural gas continuously monitor the quality of the gas at various stages of production and distribution to prevent such occurrences. One contaminant of particular interest is water vapor (H
2
O). Excessive buildup of water vapor is a primary cause of pipeline corrosion, and it acts to dilute the natural gas thereby reducing its BTU capacity (thereby making the gas a less efficient energy source).
Distributors of natural gas typically have set maximum allowable levels of H
2
O within natural gas for various stages of natural gas production and distribution. The final product that is delivered to the customer (usually a large consumer supplier such as Southern California Gas, or Pacific Gas and Electric), is termed “mainline gas.” The typical maximum allowable level of H
2
O in mainline gas is 7 lbs of H
2
O per measured million standard cubic feet of CH
4
(MMscf); 1 lb/MMscf is approximately 21.1 parts per million by volume, ppmv). This level is termed the “tariff”. When H
2
O levels exceed tariff levels, plant operation can be suspended resulting in substantial loss of revenue and associated customer lawsuits.
Conventional techniques for measuring water vapor in natural gas rely primarily on the use of chemical sensors. These sensors operate by monitoring the capacitance or dielectric constant of a sensor element (made from compounds such as phosphorous pentoxide (P
2
O
5
) and aluminum oxide) subjected to a sample from the mainline gas. The electrical properties of the sensors change in a quantitative measurable manner as a function of the amount of water vapor present in the sample gas and such changes are translated into water concentration measurements. In such chemical sensors, a low pressure sample of pipeline gas is delivered to the sensor element via a regulation (pressure reduction) system. The gas sample measured by the pipeline is at a much lower pressure than the pipeline itself (typically 10-30 PSI, compared to 800 PSI in the pipeline). Such sensors are typically housed in sampling shelters that also house the accompanying regulation system.
As the sensing elements in chemical sensors are necessarily exposed to gas samples, contaminants in the gas stream such as glycols, amines, and oils directly contact the sensors. While chemical sensors can provide reliable measurements for short periods of time after calibration, the exposure to the contaminants (glycols and amines in particular) soil the sensor, thereby causing drifts in the calibration. This condition results in erroneous readings and can lead to eventual failure if the contaminants build up. Various filters (coalescing, adsorbents, and particle filters) have been employed to minimize the effects of glycol and amine contamination, but historically these filtration schemes are only temporary solutions. This is due in part because the filters are easily saturated with contaminants or they leak and require replacement at irregular intervals.
It should therefore be appreciated that there remains a need for a reliable and durable system and method for detection of water levels in natural gas.
SUMMARY OF THE INVENTION
The current invention utilizes absorption spectroscopy, a technique that has been utilized for decades to measure the concentration of water vapor in air, and in various laboratory environments. With such spectroscopy techniques, a light source is passed through a gas sample and detected by a detector opposite the light source. The light source can be a conventional hot filament, a glow bar, a laser, or any suitable emitter in the wavelength region of interest. By monitoring the amount of light absorbed by the sample, at specific wavelengths, the concentration of the target gas can be accurately determined.
A common problem with absorption spectroscopy is interference among constituents in the gas sample being measured. This interference occurs when the gas of interest (in this case H
2
O) absorbs light at the same, or nearly the same, wavelength as another gas present in the sample. Natural gas, which is composed of >95% CH
4
, has water vapor at typically less than 1% by volume. Conventional spectroscopic methods (i.e., non-laser based) are not suitable for measurements of H
2
O in a CH
4
background because the absorption by CH
4
, which is present in much larger quantities, completely obscures the much weaker absorption by H
2
O at all wavelengths in the visible and infrared region.
The current invention operates in a wavelength range with minimal CH
4
absorption and preferably utilizes laser light sources for absorption spectroscopy, thereby minimizing the effects of interference due to the extremely high spectral purity of the laser (narrow line width). The current system incorporates a laser as its light source such as those used in automated, unattended, field instrumentation that operate at wavelengths between 1.6 and 2.7 microns (&mgr;m). The preferred lasers are the tunable diode lasers (“TDL”) detailed in U.S. Pat. No. 5,257,256, which is hereby fully incorporated by reference. TDLs are widely utilized in optical communications, laser printers, bar code readers, CD players, and laser pointers. Alternatively, a color center laser which operates in the 1-3 &mgr;m region may be utilized, but such lasers are not always suitable for use in commercial field instrumentation due to their relatively large physical size, high power consumption, high maintenance requirements (they must be cryogenically cooled), and cost.
Laser-based measurements of water vapor in air use commercially-available TDLs operating at wavelengths near 1.38 &mgr;m, where water vapor has a strong absorption band. However, this wavelength is not suitable for measurements of H
2
O in a CH
4
background because CH
4
absorption in the 1.38 micron region is extremely strong and completely obscures absorption by H
2
O (see the spectrum of CH
4
in the 1-2 &mgr;m region
200
which is shown in FIG.
2
).
The present system measures water vapor at another absorption band, 1.88 &mgr;m, where absorption by CH
4
is much weaker (see
FIG. 3
which illustrates transmission spectra
300
(transmission=1−absorption) of CH
4
325
and H
2
O
350
over wavenumbers
5260
-
5330
(wavenumber=1 &mgr;m, times 10,000)). There are several H
2
O absorption lines that can be used to monitor H
2
O in a natural gas background, but it is a wavelength range in the CH
4
absorption spectrum, 1.877-1.901 &mgr;m, that is preferred as it has strong H
2
O absorption lines, thereby allowing water vapor to be measured in a pure CH
4
background (see
FIG. 4
which shows a spectru
Kukkonen, III Carl A.
Moran Timothy
SpectraSensors, Inc.
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