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Air Emissions Factors and Quantification

Proposed Revisions to AP-42 Chapter 7, Section 7.1 - Organic Liquid Storage Tanks

Background and Scope

Section 7.1 presents emissions estimating methodologies for storage tanks of various types and operating conditions. The methodologies are intended for storage tanks that are properly maintained and in normal working condition. The methodologies do not address conditions of deteriorated or otherwise damaged materials of construction, nor do they address operating conditions that differ significantly from the scenarios described herein.

Currently Section 7.1 includes emissions estimating methodologies for routine emissions from fixed roof tanks and floating roof tanks. These equations were developed to estimate average annual losses for storage tanks. The proposed revisions include emissions estimating methodologies for the following types of events and situations:

  • Landing a floating roof
  • Tank cleaning
  • Tanks containing unstable liquids, such as tanks which have air or other gases injected into the liquid (sparging), tanks storing liquids at or above their boiling point (boiling), or tanks storing liquids which contain gases that have the potential to flash out of solution (flashing)
  • Variable vapor space tanks
  • Pressure tanks designed as closed systems without emissions to the atmosphere
  • Time periods shorter than one year
  • Internal floating roof tanks with closed vent systems

Additionally, the proposed revisions include the following guidance:

  • Case-specific liquid surface temperature determination
  • Adapting equations for heating cycles in fixed roof tanks
  • Applying Raoult’s Law to calculate the contribution of individual chemical species to the total emissions
  • Worked examples (Section 7.1.5)

Finally, equations in Section 7.1.6 that have been used historically to obtain approximate values have been replaced with more accurate equations.

Proposed Revisions to Chapter 7, Section 7.1 - Organic Liquid Storage Tanks

The proposed revisions to Chapter 7, Section 7.1 of AP-42 can be found here:

Public comments for Chapter 7, Section 7.1 (February 2019)

AP-42 Chapter 7, Section 7.1 proposed revisions: Organic Liquid Storage Tanks 

AP-42 Chapter 7, Section 7.1 proposed revisions: Organic Liquid Storage Tanks (redline strikeout version)

AP-42 Chapter 7, Section 7.1 proposed revisions summary

Public Comment

We seek your comment on the proposed revisions to Chapter 7, Section 7.1 of AP-42.  A 60-day extension request was received and granted by EPA.  Please submit your written comments by Nov. 26, 2018.  Comments should be e-mailed to

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