Pursuant to the risk management program regulations under 40 CFR §68.115(b)(2)(iii), regulated substances in naturally occurring hydrocarbon mixtures prior to entry into a natural gas processing plant or a petroleum refining process unit do not need to be considered when determining whether more than a threshold quantity is present at a stationary source. Why did EPA only exempt regulated substances in hydrocarbon mixtures prior to entry into a natural gas processing plant or a petroleum refining process unit, but not after entry?

EPA specifically exempted from threshold determinations certain naturally occurring hydrocarbon mixtures prior to processing in order to reflect EPA’s intent to exempt mixtures that do not have a National Fire Protection Association (NFPA) rating of 4 and simplify the task of applying the judgmental criteria of NFPA-4 for these pervasive mixtures. These mixtures may have a flash point and boiling point corresponding to NFPA-4 criteria; however, they contain non-volatile components and have a low potential for vapor cloud explosions. Moreover, EPA believes that exploration and production facilities do not have the turbulent or confined conditions that lead to vapor cloud explosions.

The potential for a vapor cloud explosion increases as these mixtures undergo processing in a petroleum refining process or a natural gas processing plant. Processes at these facilities are more complex, these facilities may contain increased site congestion from equipment and buildings, flammable substance may be stored in large quantities, and there may be many ignition sources. Mixtures from these processes may also meet NFPA-4 criteria and would need to be considered for threshold determinations. Thus, EPA did not exempt naturally occurring hydrocarbon mixtures subsequent to entry into a natural gas processing plant or a petroleum refining process unit.

Note that only naturally occurring hydrocarbon mixtures meeting the rule’s definition for crude oil, condensate, produced water, or field gas are eligible for the naturally occurring hydrocarbon mixtures exemption at §68.115(b)(2)(iii).