To address Section 112(r) of the Clean Air Act amendments, the U.S. Environmental Protection Agency (EPA) promulgated the Risk Management Plan (RMP) rule, which requires all companies using specific regulated toxic and flammable substances to develop an RMP and to revise and resubmit the plan to EPA every five years.
The purpose of the RMP is to reduce chemical risk in surrounding communities, assist first responders in the event of an accidental release, and inform citizens of local chemical hazards. As part of the RMP, companies must conduct a hazard assessment, known as an Offsite Consequence Analysis (OCA), to evaluate the potential impacts that an accidental release may have on an area, as well as the worst-case and accidental release modeling scenarios.
The worst-case scenario is defined by EPA as the release of "the largest quantity of a regulated substance from a single vessel or process line failure that results in the greatest distance to an endpoint."1 This endpoint is essentially the distance at which the hazardous substance no longer poses a threat of causing serious injuries from short-term exposures. Similarly, EPA defines alternative scenarios as those "that are more likely to occur than the worst-case scenario."1
In General Guidance on Risk Management Programs for Chemical Accident Prevention (40 CFR Part 68) - CHAPTER 4: Offsite Consequence Analysis, EPA also defines the meteorological conditions for the alternative scenarios: typical meteorological conditions, average temperature, and average humidity data at the facility site (see Figure 3). The Guidance assumes 3 meters per second and D stability class for alternative scenarios, if no onsite data are available.
Many facilities will be required to update their OCA modeling this year as part of their RMP update, and this tool may prove to be a valuable, time-saving resource. No matter which program is used to conduct your OCA, it's important to review the documentation used previously to ensure that the parameters and assumptions are still applicable, and to include any changes to the modeling scenarios resulting from this review in the update.