Worst-case Scenario for Flammable Mixtures

For the risk management program, where the concentration of the regulated flammable substance in the mixture is one percent or more by weight of the mixture, the entire weight of the mixture must be applied toward the 10,000 pound threshold quantity for the flammable substance unless the owner or operator can demonstrate that the mixture itself does not have an NFPA flammability hazard rating of 4 (40 CFR §68.115(b)(2)). When calculating the worst-case release scenario of a flammable mixture in the off-site consequence analysis (OCA) section of a risk management plan (RMP) pursuant to §68.25, should the model include the entire quantity of the mixture or just the quantity of the regulated substance present in the mixture?

The owner or operator should conduct the analysis using the total quantity of all regulated flammable substance or substances in the mixture. Non-flammable components should not be included. However, if additional (non-regulated) flammable substances are present in the mixture, they should be included in the quantity used in the analysis.

Additional guidance for preparing an OCA for flammable mixtures is available in section 5.2 of the Risk Management Program Guidance for Offsite Consequence Analysis (EPA550-B-99-009).