Some distillation columns in the chemical industry are operated in batch mode with a fairly short operating cycle. At the end of each cycle the columns are cooled and recharged. During the cooling cycle, air will be drawn into the column by the action of a vacuum relief valve. Consequently, for a finite portion of the operating cycle a flammable mixture will exist in the column.

Here we evaluate the risk posed by such an operation to see if a mitigation measure is justified. We develop a fault tree and estimate the frequency of ignition by all possible ignition sources. By comparing the risk reduction attainable by installing a lightning protection system with that attainable by using an inert blanketing system the lightning protection system is found to be the preferred solution. It provides about the same risk reduction at a lower overall cost.