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USEPA Finalizes Changes to AP-42 Chapter 7.1 – Organic Liquid Storage Tanks

Our love affair with aboveground storage tank air emissions continues. The U.S. EPA finalized revisions to Chapter 7.1 of AP-42 – Organic Liquid Storage Tanks on November 20, 2019. As a result of these revisions, the EPA TANKS 4.09D emissions estimation software, which already had known errors and was no longer supported by USEPA, is now further out dated.

Revisions that are now final include adjustments to the temperature equations, as well as new guidance for the following:

  • Meteorological data were updated from 30-year average values for the period of 1961 through 1990 to the 20-year average values for the period of 1991 through 2010

  • 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 heated liquid and 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 liquid true vapor pressure and cautions with respect to the use of ASTM D2879

  • Retirement of historical approximations where more accurate equations are now available

Retired equations include the old equation for fixed-roof tank working loss and the old equation for range of the vapor pressure.

The impact of these temperature equation changes affect both fixed roof and external floating 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 assuming the paint condition is “average.”

For external floating-roof tanks, the effect of the revisions depends on the liquid bulk temperature assumptions relative to ambient conditions. Emissions are slightly higher when assuming the liquid bulk temperature is in equilibrium with ambient conditions, but slightly lower when assigning a liquid bulk temperature that is warmer than ambient.

In summary, changes to the methodologies for estimating emissions from above ground storage tanks are primarily refinements to the equations for estimating temperatures in a storage tank. The changes to these equations will sometimes result in higher estimated emissions and sometimes in lower emissions.

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

CFR Environmental has a long track record of providing support related to air permitting, compliance, and reporting projects for the petroleum and chemical industry, including expertise in AP-42 compliance. To guide its clients in the petroleum and chemical industries, CFR Environmental has created a software program, CFR-E, using Chapter 7.1 of AP-42 that accurately calculates air emissions from the storage of bulk organic liquids.

The CFR-E software contains hundreds of chemicals and petroleum products, meterological data for any major city in North America, and can calculate VOC emissions for all major types of storage tanks. Our air expertise also includes Renewable Operating Permits (ROP), Permits to Install (PTI) and Potential to Emit (PTE) projects for compliance with state and federal regulations.

Contact us to learn more about our air consulting services.

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