Pressurized Storage Tank
Summary
Storage tanks are used to hold crude oil and gas condensate and operate at or near atmospheric pressure. Emissions from storage tanks, predominantly flashing emissions, may be vented to the atmosphere to maintain atmospheric pressure in the storage tank. Replacing an atmospheric pressure storage tank with a pressurized storage tank and using a pressurized tank transport vehicle to receive the oil/condensate can significantly reduce emissions from methane and other volatile organic compounds (VOCs).
Description
Storage tanks are used to hold crude oil and gas condensate saturated with light hydrocarbons, including methane and other VOCs, natural gas liquids (NGLs), hazardous air pollutants (HAPs), and some inert gases. These storage tanks typically operate at or near atmospheric pressure. Upon being introduced into a storage tank, crude oil and condensate experience a pressure drop, which cause gases dissolved in the liquid to vaporize or 'flash out' of the liquid state and collect in the vapor space between the liquid and the roof of the storage tank. These vapors are known as flashing losses and account for most emissions from storage tanks. In the absence of a control device, such as a vapor recovery unit or flare, flashing emissions are vented directly to the atmosphere. Installing pressurized storage tanks to handle the produced liquids, as opposed to atmospheric storage tanks, can significantly reduce methane emissions from this process.
Pressurized storage tanks require the use of pressurized transport of the liquids (e.g., tank trucks). A blanket gas system or vapor recycle line between the storage vessel and transport tank would be required to maintain tank pressure. Inert gases such as nitrogen can be used for tank blanketing. To prevent the gas blanket from seeping out, the tank manway cover should be fume-tight and sealed. Additionally, the pressure tank should be equipped with a pressure relief valve(s) calibrated for the specific operating conditions of the tank.
Applicability
This technology is applicable where liquids transport via tank trucks is feasible.
Methane Emissions Reductions
Methane emission reductions can be determined by taking the difference in emissions from the source before and after the specific mitigation action was applied. For replacing an atmospheric pressure storage tank with a pressurized condensate storage vessel and using a pressurized tank transport vehicle, this means calculating emissions from atmospheric pressure storage tanks and subtracting zero (because using pressurized storage and transport vehicles has essentially no emissions. While using actual measurements may provide a more accurate representation of emissions/reductions from individual equipment at a given time, emissions from storage tanks can be reasonably calculated using an emission factor as follows.
ER = B × EF
Where:
ER = Emissions reduction estimate (kg CH4/year)
B = Barrels of condensate sent to tank (bbl/year)
EF = Emission factor (kg CH4/bbl)
Assumptions:
- Use the most current “large tank w/o control” or “small tank w/o flares” emission factors. Emission factors are generally developed to be representative of long-term averages for all applicable emission sources. EPA updates the emission factors from the Natural Gas Systems section of the Inventory of U.S. Greenhouse Gas Emissions and Sinks (“Greenhouse Gas Inventory”, or “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).
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.
Other Benefits
In addition to reducing emissions of methane, using pressurized condensate storage tanks in lieu of atmospheric pressure storage tanks may:
- Reduce air pollution: Reduces flashing losses from hydrocarbon storage tanks including VOCs, hazardous air pollutants, and other toxic contaminants.
Lessons Learned
References
Scott, M., & Stake, J. (2013, August). Growing condensates require optimized designs for gathering, processing. The American Oil & Gas Reporter.https://www.aogr.com/magazine/editors-choice/growing-condensates-require-optimized-designs-for-gathering-processing
Please Note: This platform reflects experiences and lessons learned from voluntary program partners. Some of these emission sources and technologies are now regulated at the federal, state, and/or local level in the United States and in other countries. The end user is solely responsible for complying with any and all applicable federal, state, and local requirements. For information on U.S. regulations for the oil and gas industry, refer to eCFR. EPA makes no expressed or implied warranties as to the performance of any technology and does not certify that a technology will always operate as advertised. Mention of names of specific companies or commercial products and services does not imply endorsement.