Vapor Recovery Units | US EPA (2024)

Applicable Industry Segments

Additional Resources

Vapor recovery units are relatively simple systems that can capture the vapors from low-pressure tanks about 95 percent of the time (5 percent of time required for annual maintenance). Due to the relatively low cost of the technology, VRUs can provide substantial returns where there are market outlets for the recovered, BTU-rich vapors. VRUs also capture volatile organic compounds (VOCs) and hazardous air pollutants (HAPs).

VRUs are commonly used to reduce emissions from crude oil and condensate storage tanks, but where VRUs are already in place the vapor collection system can be modified to capture emissions from other low pressure vent sources found onsite including pipeline pigging operations, compressor seal and blowdown vents, and dehydrator vents.

VRUs may be installed on a single storage tank or multiple tanks such as a gathering/boosting station tank battery. Hydrocarbon vapors are drawn out of the storage tank under low-pressure (typically between 0.25 and 2 psig) and are first piped to a liquid separator (suction scrubber) to remove any liquids that condense out of the saturated vapor. The liquids are usually recycled back to the storage tank. From the scrubber, the vapors flow through a specially designed wet gas compressor capable of handling low suction pressure.VRUs are equipped with a control pilot to shut down the compressor and permit the back flow of vapors into the tank to prevent the creation of a vacuum in the top of a storage tank when liquids are withdrawn and the liquid level drops. The vapors are then metered and directed to a low-pressure sales line, a production compressor suction, or onsite fuel gas supply. Figure 1 shows a VRU attached to a crude oil/condensate storage tank. The large vapor space in a storage tank modulates pressure and flow variations from other low pressure sources such as reciprocating compressor rod packing vents or dehydrator vents. A VRU should be sized to handle double the average volume of vapors expected from the storage tanks and other connected equipment so that the compressor runs half the time.

Vapors recovered from storage tanks and other processes can contain natural gas liquids (NGLs) and other organics that have a heat content (i.e., the amount of energy released when a volume of gas is burned) higher than that of pipeline quality natural gas. The heat content of pipeline quality natural gas is typically between 950 and 1,100 Btu per standard cubic foot (scf), while the Btu content of the recovered vapors from storage tanks can exceed 2,000 Btu per scf (depending on the composition of the collected vapors). Therefore, on a volumetric basis, the recovered vapors can be more valuable than methane alone. Furthermore, gas condensate from a VRU suction scrubber has a much higher gravity than crude oil, and if sold independently, bears a higher price than crude oil.

Methane emission reductions can be determined by taking the difference in emissions from the source before and after the specific mitigation action was applied. Emissions can be estimated multiple ways including applying emission factors, direct measurement, and using simulation software. Specific methodologies vary by the source controlled by the VRU, but generally post-VRU emissions may be calculated by applying a 95 percent reduction to pre-VRU emission estimates.

Emission factors are generally developed to be representative of long-term averages for all applicable emission sources. The Natural Gas and Petroleum Systems section of the Inventory of U.S. Greenhouse Gas Emissions and Sinks (“Greenhouse Gas Inventory”, or “GHGI”) provides emission factors for storage tanks, reciprocating compressors, and centrifugal compressors. EPA updates the emission factors from the GHGI every year, so specific emission factors may change. To find the current emission factor, navigate to the GHGI website for Natural Gas and Petroleum Systems and click on the page for the most recent inventory. On that page, you will find links for Annex 3.5 (Methodology for Estimating CH₄, CO₂, and N₂O Emissions for Petroleum Systems) and Annex 3.6 (Methodology for Estimating CH₄, CO₂, and N₂O Emissions for Natural Gas Systems). Methane emission factors can be found in Table 3.5-3 (Petroleum Systems) and Table 3.6-2 (Natural Gas Systems).

Subpart W of EPA’s Greenhouse Gas Reporting Program (GHGRP) also provides calculation methodologies at 40 CFR 98.233. Subpart W calculation methodologies are available for:

• Storage tanks: the subpart W methodology mainly relies on the use of simulation software, although emission factors are provided for storage tanks at low production sites (i.e., less than 10 barrels per day)
• Reciprocating compressors: the subpart W methodology depends on the industry segment and uses direct measurement or an emission factor
• Centrifugal compressors: the subpart W methodology depends on the industry segment and uses direct measurement or an emission factor
• Glycol dehydrators: the subpart W methodology uses simulation software.

Post-VRU emissions may then be calculated as:

E_VRU = E × (1 – VRU)

Where:

E_VRU = Controlled annual total volumetric methane emissions at standard conditions in cubic feet

E = Annual total volumetric methane emissions from all emission sources that the VRU would control at standard conditions in cubic feet

VRU = 0.95 (95% control)

The calculation methodology in this emissions reduction section is based upon current information and regulations (as of August 1, 2023). EPA will periodically review and update the methodology as needed.

In addition to reducing emissions of methane, the use of VRUs may:

• Provide additional revenue: Provides opportunities for additional returns where there are market outlets for the high BTU vapor.
• Reduce air pollution: Emissions of VOCs, HAPs, and other toxic contaminants will be reduced.

Lim, Y. F., Foo, D. C. Y., & Ooi, M. B. L., (2022, February 1). Optimizing vapor recovery from storage tanks. Chemical Engineering. https://www.chemengonline.com/optimizing-vapor-recovery-from-storage-tanks/?printmode=1

U.S. Environmental Protection Agency. (2022, May). Methane Challenge Program – BMP Commitment Option Technical Document. https://www.epa.gov/system/files/documents/2022-05/MC_BMP_TechnicalDocument_2022-05.pdf

U.S. Environmental Protection Agency. (2019, March 15). Natural Gas STAR Methane Challenge Program – ONE Future Commitment Option Technical Document. https://www.epa.gov/sites/default/files/2016-08/documents/methanechallenge_one_future_supp_tech_info.pdf