Proposed Changes to EPA AP-42 Tank Calculation Methods
September 14, 2018

The USEPA posted proposed revisions to AP-42, Chapter 7, Section 7.1 - Organic Liquid Storage Tanks. The document referred to as “AP-42” is named AP-42, Compilation of Air Pollutant Emissions Factors, Volume 1: Stationary Point and Area Sources. AP-42 contains emission factors and emission estimation methods and equations used to calculate emissions across many emission source types (storage tanks, engines, heaters, loading losses, etc.) and industries. This includes emission sources used in the oil and gas (O&G) industry.

Man on  Tank.JPG

Chapter 7, Section 7.1 includes equations, factors and methods that can be used to estimate hydrocarbon (including VOCs) emissions from fixed roof and floating roof storage tanks used by O&G facilities.

Go to Website LINK to review the associated documents for the proposed changes. The website downloads include a proposed revisions pdf, redline version pdf and a summary of changes document.

 The proposed revisions include changes to improve clarity, consistency and correctness. The changes affect methods for fixed roof and floating roof storage tanks.  Some general example changes include:

  • Scope of document
  • Updated equations and constants used – but no major changes proposed from those currently used.
  • Revised definitions and terms used
  • Flashing emissions guidance
  • Expanded Paint Solar Absorbance factor used to include new, average, aged roof/shell paint condition.
  • Recommended test methods for laboratory analyses
  • Calculation methods for pressurized and insulated tanks
  • Section for storage tank cleaning emissions
  • Added floating roof storage tank landing losses factors and equations
  • Updated table for physical parameters of certain organic liquids and refined products

The AP-42 Chapter 7 Section 7-1 Revisions Summary Document available for download includes some specific comparisons of emissions calculated using the existing Section 7-1 and the proposed methods.

Flashing Emissions

Section of the proposed revisions gives recommended methods to estimate flashing losses. These apply to fixed roof tanks storing crude oil or condensate.

Flashing emissions occur when natural gas is liberated (comes out of solution) from pressurized crude oil/condensate that undergoes a pressure drop in the tank. Depending on the pressure drop, flashing emissions can be much larger than the standing (breathing) and working losses combined.

Suggested method to estimate flashing emissions include:

  1. Direct measurement which is commonly used to measure storage tank vent gas flow rate and sample/chemically analyze vent gas. The proposed language states that direct measurement would be the preferred method when a reliable measurement method is used. At this time there is no EPA method published for direct measurement of storage tank vent gas.
    NOTE: HY-BON/EDI’s IQR measurements are accepted by State air quality regulators.
  2. Laboratory GOR which involves collecting a representative pressurized oil sample to determine the gas-to-oil ratio (GOR) and chemically analyze the liberated flash gas for the mole percent of chemical components. The result can give a VOC and hazardous air pollutant (HAP) standard cubic feet per barrel of oil throughput (SCF/bbl) factor to use in calculations.
  3. Computer simulation modeling such as E&P TANK and process simulators using equation of state can be used to calculate flashing losses. These require a representative (e.g., site specific) pressurized oil analysis and operating parameters to enhance accuracy. The results can yield the SCF/bbl factor plus speciate for VOCs and HAPs.
  4. Vasquez-Beggs equations can be used to calculate the GOR to obtain a SCF/bbl factor. For valid results, the input data used must be within specified ranges (e.g., not suitable for API gravity greater than 40 degrees). Check with your State air quality regulators for accepted input parameter ranges.

TANKS missions Estimation Software, Version 4.09D

The revisions also mention the TANKS computer program. Many operators use the TANKS computer program to calculate standing (breathing) and working loss emissions from crude oil and condensate storage tanks.

TANKS is a Windows-based software program can be used to estimate volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions from fixed roof and floating roof storage tanks. TANKS is based on the emission estimation procedures from Chapter 7 of EPA's Compilation Of Air Pollutant Emission Factors (AP-42).

The TANKS software program does not calculate flashing emissions. A separate direct measurement or calculation method must be used for flashing emissions.

You can download the software from the following link: TANKS SOFTWARE VER. 4.09D released October 2006.

Most State air permitting regulators accept TANKS 4.09 for estimating storage tank standing (breathing) and working losses from fixed roof storage tanks. The same is true for floating roof storage tanks emission calculations.

NOTE: The EPA states on its website that the TANKS model was developed using a software that is now outdated and may not reliably function with certain operating systems. EPA does not provide assistance to users of TANKs 4.09d. The model will remain on the website to be used at your discretion. The EPA recommends the use of the equations/algorithms specified in AP-42 Chapter 7 for estimating VOC emissions from storage tanks. The equations specified in AP-42 Chapter 7 can be employed with many current spreadsheet/software programs.

The impact of the proposed/final changes to Chapter 7 will have on the TANKS program emission calculation outputs remains to be determined.

Public Comments

You can submit written comments to the EPA by Nov. 26, 2018, by email to k^d-N!T.s2vo*R[Y'|]#[QEHw8m6wh$3Uu0dArs


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