Do quantities in co-located vessels have to be aggregated for worst-case release scenarios?

Pursuant to the risk management program (RMP) regulations, a facility owner or operator must conduct an off-site consequence analysis that includes modeling of a worst-case release. If a single process is comprised of several co-located vessels, must the total quantity in all the vessels be aggregated to determine the worst-case release quantity?

A worst-case release is the release of the largest quantity of a regulated substance from a vessel or process line failure that results in the greatest distance to a specified endpoint (§68.3). For substances in vessels, a facility owner or operator must assume a release of the largest amount in a single vessel. Guidance on worst-case release scenarios is available in Section 2.2 of the Risk Management Program Guidance for Offsite Consequence Analysis (EPA550-B-99-009).