For oilfield storage tanks, NSPS OOOO and OOOOa have similar requirements regarding applicability, emission limitations, recordkeeping and reporting requirements. Some exceptions for storage tanks are different applicability dates for the two rules and NSPS OOOOa has additional requirements for closed vent systems (piping) used to control NSPS OOOOa affected facility storage tanks.

These rules affect storage tanks that handle crude oil, intermediate hydrocarbon liquids, condensate or produced water.

Below is a summary of key parts of the rules. Unless specified, the requirements apply to NSPS OOOO and OOOOa. Consult the written rules for details.



  • NSPS OOOO affects storage tanks constructed, modified or reconstructed after August 23, 2011, and on or before September 18, 2015.
  • NSPS OOOOa affects storage tanks constructed, modified or reconstructed after September 18, 2015.
  • These rules do not apply to a storage tank that was constructed on or prior to August 23, 2011, and not reconstructed or modified after August 23, 2011.
  • A storage vessel (tank) that is in crude oil, intermediate hydrocarbon liquids, condensate or produced water service and has the potential for VOC emissions equal to or greater than 6 tons per year (tpy) is defined as an “affected facility” if constructed, reconstructed or modified after the rule dates.
  • If a storage tank is an affected facility, then the operators must comply with rule requirements for that storage tank.
  • An affected storage tank that reduces its potential to emit (PTE) emissions to less than 4 tpy for 12 consecutive months may remove it emission controls. This can occur when the oil throughput is reduced. For such a tank, it does not require emission controls but is still defined as an affected facility.
  • The rule exempts process vessels (such as vapor recovery towers), surge control vessels, knockout vessels. Also exempt are temporary tanks onsite for less than 180 days and pressure vessels designed to operate at more than 15 psig with no emissions.
  • Storage vessels with a capacity greater than 100,000 gallons used to recycle water that has been passed through two stage separation are exempt.


Emission Limitations

  • NSPS OOOO/OOOOa affected facility storage tanks that have emissions greater than or equal to 6 tpy VOCs must reduce VOC emissions by 95% or greater.
  • There are no emission limitations on methane (CH4) for storage tanks.


Emission Control Options

Typical emission controls include:

  • Vapor recovery units (VRUs)
  • Enclosed combustors (VCUs)
  • Open-tipped (pipe) flares.

The most desirable control method is a VRU since this recovers the natural gas product and sends the gas to the sales line or back to the process for facility use. Gas recovery lowers emissions and increases profits to the facility.

Guidance on controlling emissions from storage tanks at oil and gas production facilities can be found in the September 2015 EPA Compliance Alert: https://goo.gl/F12VdS


Emission Estimation

  • Storage tanks VOC emissions result from flashing, breathing (standing) and working emissions. See Texas Commission on Environmental Quality document link: https://goo.gl/J3MehD
  • Flash emissions can be determined by direct measurement of vent gas, pressurized gas-oil-ratio (GOR) sampling/analysis and processor simulation models.
  • Breathing and working emissions can use methods given in EPA’s AP-42, Compilation of Air Pollutant Emission Factors, Chapter 7: Liquid Storage Tanks. The AP-42 equations do not calculate flash emissions. Note that the calculation methods for storage tanks in AP-42 were revised in 2020.


Pollutants Covered

Storage vessels included in NSPS OOOO and OOOOa only have VOC limitations.


Ways to Avoid NSPS OOOO and OOOOa

  • Install a vapor recovery unit (VRU) and capture the vent gas. VRUs are considered process equipment by EPA and the gas recovered by the VRU does not count toward the VOC potential to emit (PTE) emissions for the storage tanks. VRUs should meet the EPA minimum definition of a VRU (proper compressor selection, etc), and should be specifically designed for high BTU, “wet” gas streams.
  • Obtain an air permit for the facility that is “practically enforceable” from the air permitting agency such that the permit limits VOCs to less than 6 tons per year per storage tank. Such tanks are exempt from NSPS OOOO and OOOOa. The air permit will require that each storage tanks emit less than 6 tpy VOC. The storage tank emissions used to determine applicability would be based on after control emissions.
  • Use centralized batteries to process the crude oil/condensate which allow more efficient VRU systems. Centralized batteries receiving oil from several to many wells flowing can have a more constant flow of oil to storage tank which improves efficiency of a VRU system. VR Towers can also be used to effectively eliminate oxygen ingress.
  • Ensure storage tank hatches and piping to the control device (flare/combustor) or VRU operate in a leak-free manner.
  • Use tankless facility designs that eliminate the use of storage tanks See link to Cimarron Tankless Facilities Design.


NSPS OOOOa only – Closed Vent Systems (Piping) Design Certification

The rule requires certification by a qualified professional engineer that the closed vent system is properly designed to ensure that all emissions from the unit being controlled are sent to the control device and allow for proper control.

This applies only to NSPS OOOOa required closed vent systems for emission control devices used to control vent gas from affected storage tanks, centrifugal compressor wet seal fluid degassing systems, reciprocating compressors rod packing and pneumatic pumps.


Initial Notification

No initial notification to EPA or State regulatory agency for storage tanks with applicable requirements under NSPS OOOO and OOOOa.



Consult the rules for the recordkeeping requirements for affected source storage tanks.



The annual reports for each are due before the dates listed below for affected facilities subject to NSPS OOOO or OOOOa.

  • OOOO: January 15 each year.
    • The initial annual report was due January 15, 2014, for the period of October 15, 2012 – October 14, 2013. The NSPS OOOO report due January 15, 2020 covers the period of October 15, 2018 – October 14, 2019.
  • OOOOa annual reports due October 30 each year.
    • The initial annual report was due October 30, 2019, for the period of August 2, 2018 – August 1, 2019.


Depending on the State regulatory agency, Part 70/Title V permitted facilities may include the NSPS OOOO/OOOOa required report in the Part 70/Title V annual reports which is typically due by March 31 each year covering the previous calendar year.

Review the rule for details on reporting requirements. Report data required includes constructed, modified or reconstructed location, VOC emission rate, records of deviations from standards, affected tanks removed or put back into service, and statement that have met requirement of the rule.

Contact your EPA regional office and State environmental agency to obtain the correct contact information and method (e.g., CDX, email, hardcopy) required for submitting your NSPS OOOO and OOOOa reports.


NSPS OOOOa Guidance Document

The EPA has published a guidance document available online entitled: Small Entity Compliance Guide for Oil and Natural Gas Sector: Emission Standards for New, Reconstructed, and Modified Sources 40 CFR Part 60 Subpart OOOOa. Go to Document LINK to download the guidance document.


Stay in Compliance

Cimarron can assist your company with NSPS OOOO and OOOOa compliance. Our thorough understanding of oil and gas processes and expertise with storage tank emission controls (VRUs, VRTs, VCUs) and leak monitoring makes us a leader in providing certainty to your compliance needs.

Our IQR (Identify, Quantify and Rectify) services include fugitive leak monitoring (LDAR) that meets NSPS OOOOa requirements. Our IQR services can also assist with emission controls selection for your facility.

Check out Cimarron Energy’s Tankless Facilities at: https://www.staging2.cimarron.com/tankless-facilities as a way to produce without storage tanks.

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