It is a common practice to insulate storage tanks containing reactive chemicals to protect against fire exposure. While this mitigation technique is appropriate for vessels handling non-reactive chemicals, reactive chemicals storage represents a special challenge and must be examined on a case-by-case basis. For certain classes of reactive chemicals, given a sufficiently long hold time, the insulation will always lead to a runaway reaction.
If insulation is to be used, special handling is required in order to insure that after the fire is extinguished, the vessel contents do not reach a temperature that causes a runaway within 48 hours. The 48 hours time limit is selected arbitrarily and should be long enough for most installations to empty the tank contents, inject and circulate additional inhibitor into the tank, cool the tank contents, and/or use the vessel contents in the process.
For vessels containing reactive liquids or non-reactive liquids that are known to be foamers or where two-phase flow is possible due to the disengagement characteristics of the vessel/relief system use the total surface area of the vessel as wetted surface area when estimating heat input into the vessel. Existing guidelines from API and NFPA-30 ignore the impact of two-phase flow on wetted area selection and can lead to non-conservative designs. Assuming a constant heat flux input, a vessel that is 30% full, for example, will result in a higher reaction rate than a vessel that is 90% full. This effect has to be established using advanced simulation techniques such as those embodied in SuperChems™ and SuperChems™ for DIERS.
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