Vent Line and Fugitive Emissions Study at a US Gasoline Dispensing Facility

Audio version
Posted / Last update: 24-05-2011
Publication: PetrolPlaza Technology Corner
Issued: December 2009
Author: Ted Tiberi, ARID Technologies, Inc.
  • Figure 1: AC-250 and ARIDAS Gear Mounted on Vent Line

  • Figure 2: Storage Tank Pressure Profile; Case 1: High Back Pressure

  • Figure 3: Case 1: High Back Pressure; CARB Correlation with Pressure Intervals

  • Figure 4: Storage Tank Pressure Profile; Case 2: Low Back Pressure

  • Figure 5: Case 2: Low Back Pressure; CARB Correlation with Pressure Intervals

  • Figure 6: Emissions Reductions and Savings Summary

  • Figure 7: ARID Evaporative Loss Model

  • Figure 8: ARID Evaporative Loss Model (continued)

  • Figure 9: American Meter AC-250

  • Figure 10: ARIDAS Equipment (shown without Flowmeter)

  • Figure 11: continued - ARIDAS Equipment

  • Figure 12: continued - ARIDAS Equipment


A study was undertaken at a US Gasoline Dispensing Facility located in Federal Way, Washington. The primary purpose of this study was to quantify gasoline storage tank evaporative emissions. These emissions are comprised of both vent emissions escaping through a pressure/vacuum (p/v) valve and fugitive emissions, which may be emitted anywhere within the storage tank hardware, fuel dispensers, nozzles and vapor piping system. Two secondary goals of this study were to compare the total measured evaporative losses with the hydrocarbon losses estimated by ARID’s proprietary Evaporative Loss Model (ELM), and to assess the impact of elevated storage tank pressures on fugitive emissions for a site passing the standard leak decay test.


ARID supplied an American Meter AC-250 dry gas flow meter equipped with a pulse counter for recording direct measurements of vent line emissions (Figure 9 for technical specifications and other details on the meter). ARID also supplied our sensors and remote data acquisition gear (ARIDAS – ARID Data Acquisition System. Figure 10 to 12). This equipment includes an ambient temperature sensor, an atmospheric pressure sensor, and a tank pressure sensor. In addition, a modem is included which allows remote data acquisition for monitoring data in real-time and for downloading batches of data at various time intervals.

The AC-250 dry gas meter and ARIDAS sensors were mounted on the vapor vent line as seen in Figure 1. The modem and power supply for the ARIDAS equipment were mounted inside the kiosk at the site.

Total Evaporative losses are equal to the measured vent emissions plus the fugitive emissions. The fugitive emissions, in-turn are a function of the average storage tank pressure. Therefore, ARID applied a CARB correlation for estimating the fugitive component of the total emissions based on the average pressure data collected by our equipment. (For a station passing a standard 2 inch pressure decay test, there are still allowed leakages).
To check the accuracy of the fugitive correlation, we wanted to make a more direct measurement of the fugitive emissions. One straightforward means to accomplish this is to simply reduce the back pressure on the storage tank system. Since the storage tank pressure will be reduced, the flow through various fugitive leak sources will in-turn be reduced, and the fugitive emissions will then be preferentially directed through the meter and be readily measured.
If one assumes that the Total Evaporative Loss rate is relatively constant (with variables such as temperature, RVP, A/L ratio, ORVR penetration, and throughput being held approximately the same), the measured vent emissions will increase and the fugitive emissions will decrease. By reducing the back pressure on the storage tank system, we did not add any incremental emissions to the environment; we simply re-directed a larger proportion of the “fugitive losses” through our meter for direct measurement.

This is pioneering work, and by making direct measurements, we have very accurately quantified the total evaporative losses at this site. Previous attempts at such emissions studies have relied upon sophisticated air dilution schemes to indirectly process a portion of vent emissions through a complicated sampling train of sensors and flow meters, with questionable results.

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