On November 20, 2019, US Environmental Protection Agency (EPA) finalized revisions to its guidance for estimating emissions from storage tanks as published in Chapter 7.1 of the Compilation of Air Pollutant Emissions Factors (AP-42), Volume 1. The EPA TANKS 4.09D emissions estimation software, which already had known errors and was no longer supported by EPA, has been rendered further out of date as a result of the new release.

Changes from the proposed revisions of AP-42 Chapter 7.1 to the final version include:

  • Meteorological data were updated from 30-year average values for the period of 1961 through 1990 to 20-year average values for the period of 1991 through 2010
  • Methods for estimating flashing emissions that were included in the proposed revisions were removed in the final version
  • The final version points the user to AP-42 Section 5.2 for estimating emissions from underground gasoline storage tanks

There were no other changes in the factors, equations, or values for variables from the July 2018 proposal to the November 2019 final version. Revisions from the proposal that are now final include adjustments to the temperature equations, as well as new guidance for the following:

  • Estimating emissions from tank cleaning and degassing events
  • Calculating net throughput for working loss from cumulative changes in the liquid level, rather than from pumping volume
  • Estimating emissions from tanks storing hot stocks, accounting for whether the tank is uninsulated, partially insulated, or fully insulated
  • Estimating emissions from low-pressure tanks
  • Reference to ASTM D6377 for determining the true vapor pressure of crude oils, and cautions with respect to the use of ASTM D2879 to determine the true vapor pressure of stocks
  • Retirement of historical approximations where more accurate equations are now available. Retired equations include:

the old equation for fixed-roof tank working loss:

OldEquationforFixed-RoofTank

and

the old equation for range of the vapor pressure:

OldEquationforRange

Impact of Temperature Equations Changes

Fixed-Roof Tanks. For fixed-roof tanks, the revisions to the temperature equations generally result in a slightly lower estimate of emissions when assuming the paint condition is “new” (which corresponds to the previous designation of “good”), but a slightly higher estimate when assigning the solar absorptance value for the “average” condition.

For example, consider a white tank that is 60 ft in diameter and 48 ft tall, storing diesel in Port Arthur, TX, with a throughput of 600,000 bbl/yr. Estimated emissions from this tank decrease by 2-4% when applying the changes if the “new” paint condition is assigned, but increase by 1-2% if the “average” paint condition is assigned. (The range in the percent change is due to other variables, such as whether the bulk liquid is assumed to be in equilibrium with ambient conditions or is assigned a warmer-than-ambient value).

External Floating-Roof Tanks. For external floating-roof tanks, the effect of the revisions depends on the assumptions regarding the liquid bulk temperature relative to ambient conditions. The trend appears to be a slightly higher emissions estimate when assuming the liquid bulk temperature to be in equilibrium with ambient conditions, but a slightly lower estimate when assigning a measured liquid bulk temperature that is warmer than ambient.

For example, consider a white external floating-roof tank that is 150 ft in diameter and 48 ft tall, storing RVP 7 crude oil in Port Arthur, TX, with a throughput of 3,000,000 bbl/yr. Estimated emissions from this tank increase by 1-3% if calculating the liquid bulk temperature from ambient temperature, but decrease by 3-5% if assigning an elevated value of 80oF as the liquid bulk temperature. (The range in the percent change is due to other variables, such as whether the paint condition is deemed to be “new” or “average”).

Summary

The revisions recently finalized by EPA to its AP-42 Chapter 7.1 guidance for estimating emissions from storage tanks will have varying degrees of impact on emissions estimates. Changes to the guidance for estimating emissions from routine tank operations pertain primarily to refinements to the equations for estimating temperatures in a storage tank. The changes to these equations will result in higher estimated emissions in some cases, but lower in others. Although the estimated emissions will typically be within ten percent of the previously estimated values, the difference may be greater in certain scenarios.

Guidance has also been finalized for scenarios that had not previously been addressed in AP-42 Chapter 7.1, such as estimating emissions from the cleaning of storage tanks and adjustment to the equations for estimating emissions from routine operations in the case of partially or fully insulated tanks.

Changes to emission factors always raise questions as to when the new factors must be used for both permits and emissions inventories. Because these programs are administered at the state level, implementation may vary depending on facility location.

Trinity's experts will be digesting the final version of Chapter 7.1 and releasing an updated version of BREEZE TankESP in the near future and hosting a webinar to discuss the landscape of tank emission calculations moving forward.

For more information, please contact Trinity at (800) 229-6655 or complete the Contact Us form on our website.