Measuring and testing – With fluid pressure – Leakage
Reissue Patent
1998-10-08
2001-10-09
Larkin, Daniel S. (Department: 2856)
Measuring and testing
With fluid pressure
Leakage
Reissue Patent
active
RE037403
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the measurement of fugitive air pollution emissions from a wide range of processes utilized in the natural gas, petroleum, petrochemical, and chemical industries.
BACKGROUND
Fugitive emissions are air pollution emissions that are not released from stacks designed as release points. Instead, fugitive emissions escape from industrial processes by means such as evaporation from wastewater treatment areas or leaks at process components. Leaks may occur at process components such as pumps, compressor seals, flanges, valves, pipe thread connections, and open ended lines on valves that are shut off. Rising concerns over hazardous air pollutants and greenhouse gases have led to the need for improved quantification of fugitive emissions to the environment.
Fugitive emissions are very difficult to quantify without expensive and time consuming measurement programs. Consequently, the oil, gas, and chemical industries typically use techniques to estimate the emission rate. These techniques are easy to apply but have several large uncertainties inherent in there use, as described below.
All emission measurement methods rely on determining the concentration of the compound(s) being emitted from the source and an estimation of the amount of dilution that takes place between the source and the point of concentration measurement. For example, when using an enclosure technique, the leaking component is wrapped with a nonpermeable material and a clean purge gas (such as nitrogen) sweeps through the enclosure at a measured flow rate. The known flow rate of purge gas provides a known dilution rate of the compound(s) leaking from the component. In the case of methane (CH
4
), the emission rate using an enclosure measurement can be calculated from the purge flow rate through the enclosure and the concentration of methane in the outlet stream as follows:
Q
CH4
=F
purge
×C
CH4
×10
−6
where:
Q
CH4
=emission rate of methane from the enclosed component (ml/min),
F
purge
=the purge flow rate of the clean air or nitrogen (ml/min), and
C
CH4
=the measured concentration of methane in the exit flow (ppm).
The enclosure measurement technique is relatively accurate, but requires extensive time and effort to set up. Processes and components of concern must be carefully wrapped with the non-permeable material so that no unwanted gas or air enters the enclosure. The time and expense associated with this technique makes it prohibitive for routine monitoring of gas leaks.
Another technique to determine leak rate uses correlations which have been developed to relate the concentration measured near a leaking component to its actual leak rate. These relationships have been developed by correlating leak rates measured at components using the enclosure technique to the maximum concentrations measured at either 1 cm or 1 mm from the components using a portable volatile organic compound (VOC) analyzer. Correlations have been reported from work sponsored by the EPA (CMA, 1989) and the American Petroleum Institute (API) (Webb and Martino, 1992). These correlations essentially provide an empirical expression of the average dilution of the leaking material as it travels from the leak to the detector of the VOC. A plot of leak rate versus VOC screening value has been completed by others (CMA, 1989), and the scatter in measurements was demonstrated to be three orders of magnitude.
A theoretical ideal correlation between emissions and the resulting portable VOC analyzer screening concentration can be determined for a given VOC. This correlation depends on the volumetric sampling flow rate which is drawn into the instrument. If a methane leak is entirely captured by the instrument during screening, the concentration measured by the instrument will be:
C
VOC
=
Q
LEAK
Q
VOC
×
10
where:
C
VOC
=concentration read by VOC (ppm),
Q
LEAK
=volumetric leak rate of methane from the component (ml/min), and
Q
VOC
=volumetric sampling flow rate of air drawn into the VOC (ml/min).
For instance, one commonly used instrument for screening components for fugitive emissions draws a nominal sample flow of 1000 ml/min. If a leak rate of 10 ml/min is entirely captured during screening, the resulting concentration will be 10,000 ppm, or 1%. Similarly, a leak rate of 1 ml/min would result in a VOC concentration of 1000 ppm, or 0.01%.
In practice, the actual ideal concentration is rarely achieved because the leak is not completely captured. The amount of the leak which is captured using this technique can vary significantly and is affected by the following: the sampling flow rate; the distance of the sampling probe from the leak; the ambient wind speed or air movement; and the characteristics of the leak such as its velocity upon leaving the component and the area over which the leak occurs. Larger sampling distances and increased ambient wind speeds reduce the influence of the sample flow on the air movement around the leak and give the plume from the leak more opportunity to diffuse away from the probe and avoid capture. For larger leaks, the plume may have enough momentum to overcome the flow field generated by the sampling flow. For leaks which escape from several points around a component, the area covered by the sampling probe flow rate may not be large enough to capture the plume.
For the foregoing reasons, there is a need for less time consuming, yet precise methodology for measuring fugitive air emissions. An instrument for performing this new methodology should allow for rapid, accurate and reliable quantification of emission concentrations from processes and components.
SUMMARY
The present invention is directed to an instrument for measuring air emissions which allows for rapid, accurate and reliable measurement of the emission rate of air pollutants from processes and components. An instrument for measuring fugitive air emissions having features of the present invention comprises an air mover capable of high flow rates for inducing a vacuum and an air flow measurement device which is connected to the air mover for measuring the rate of air flow. A first sample hose is connected to the air mover to draw air at a high flow rate to enhance capture of the air pollutants and can be positioned near a leaking component. A second sample hose, which can be connected directly to a gas analyzer, such as a portable VOC analyzer, draws air at a lower flow rate and is positioned on the opposite side of the leak being measured by said first sample hose. A means for connecting said instrument to a gas analyzer, such as a VOC analyzer, allowing for the concentration of the air pollutant to be determined if air is sampled from the first sample hose or for determination of background levels of contaminants if air is sampled from the second sample hose.
Alternatively, samples can be collected from the first and second sample hoses and analyzed at a later time. It is preferable that the flow rate in the first sample hose is adjustable, such as by attaching a gas regulator to the air mover, so that the vacuum created by the first sample hose can be adjusted for measuring leak rates under varying conditions. For example, breezy conditions at some sites may require a higher flow rate, ie. greater vacuum, to completely capture the leak, compared to a similar measurement indoors. Applying a high flow rate is an advantage of the present invention since the application of a high flow rate allows for a more complete capture of the gas emissions from leaking components.
A three-way valve can be used to allow for sample collection or analysis of air from the first sample hose or the second sample hose. Most preferably, a solenoid can be used for rapid and automatic switching between measurement of contaminants in the first and second sample hoses. Continuous application of the vacuum created by the first sample hose while measuring contaminants in the second sample hose allows for background measurements to be made, which can be high if surro
Gas Research Institute
Larkin Daniel S.
Pauley Petersen Kinne & Fejer
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