When evaluating the worst-case release scenario for spills of liquid toxic substances, the owner or operator should assume that the maximum quantity within the vessel or pipe was released instantaneously to form a liquid pool (40 CFR §68.25(d)(1)). For regulated toxic substances that are normally gases at ambient temperature, the worst-case release is assumed to occur over a ten minute period (40 CFR §68.25(c)(1)). For regulated toxic gases that are liquefied by refrigeration, should the worst-case release be evaluated as a liquid or gas release?
The worst-case release scenario for gases handled as refrigerated liquids at ambient pressure may be evaluated as either a liquid or a gas, depending on containment. If the released substance is not contained by passive mitigation systems or if the contained liquid pool formed would have a depth of one centimeter or less, the owner or operator must assume that the substance is released as a gas in 10 minutes (40 CFR §68.25(c)(2)(i)). If the released substance is contained by passive mitigation systems and forms a liquid pool with a depth greater than one centimeter, the substance may be evaluated as a liquid and assumed to form an instantaneous pool (40 CFR §68.25(c)(2)(ii)).