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AP-42 Chapter 7.1 – Organic Liquid Storage Tanks: A New Methodology for Tank Cleaning

As aboveground storage tank operators are aware, AP-42 Section 7.1 (Organic Liquid Storage Tanks) has been updated multiple times in the last year, most recently in March 2020. The updates made wholesale changes to ordinary tank emissions (working/breathing) but also added two sections specifically for tank cleaning. These additions align AP-42 with the parallel API document, API Technical Report 2568 – Evaporative Loss from the Cleaning of Storage Tanks. This document was published in November 2007 and has included these tank cleaning emissions for over 13 years.

So what’s included? There are two additions: Vapor Space Purge and Continued Forced Ventilation emissions. These processes coincide with Vapor Space Purge and Sludge Removal emissions from the API document.

During forced ventilation, additional tank cleaning calculations are required for the following:

  • Vapor Space Purge: A vapor space purge will occur each time ventilation commences after a period of standing idle. When blowers are started up, either at the top of the tank or at a shell manhole, the first air change is deemed to expel those more concentrated vapors that remain from the prior standing idle period. This first air change is characterized as a vapor space purge. Emissions associated with subsequent air changes are accounted for under continued forced ventilation (below).

  • Continued Forced Ventilation: Forced ventilation refers to the removal of vapors from a tank by means of blowers after the initial vapor space purge. The forced ventilation will then expel these vapors from the tank. This typically occurs when sludge is being manually removed from the tank bottom.

The input variables for completing these calculations are not intuitive, to say the least. For example, these variables include the molecular weight of the calibration gas for tank entry and the percent of sludge that evaporates. These variables can be extremely difficult to obtain unless the tank cleaning event is well documented.

CFR has been performing these estimates for many years and we expect state agencies to begin requiring these emissions to be accounted for in reporting.

Contact CFR for a consultation on how to better track and estimate VOC and HAP emissions from aboveground storage tanks cleaning events.

CFR has been working with tank operators for over 30 years and has a long track record of providing support for tank operators in the petroleum and chemical industry. Contact CFR to learn more about we can assist with your storage tank cleaning emissions program.


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