An explosion occurred in the exhaust line to the scrubber during a cleaning procedure with concentrated nitric acid in a chemical reactor. Previously the reactor was treated with methanol under reflux and afterwards rinsed with drinking water. The detonation destroyed completely a section of a PN 16 condensate pipe after a flame arrestor. Glass piping, several fittings and the flame arrestor were also severely damaged. Also windows were damaged in the production room. The aim of this paper is to outline the incident investigation involving: the determination of potential explosive substances or explosive atmosphere in the pipe; a systematic analysis of ignition sources; a definition of possible scenarios of the explosion causes and sequence; a plausibility assessment of the scenarios regarding the observed damage; an evaluation of the probability of the scenarios; and a proposal of safety concepts to avoid future incidents. The investigation involved the use of Differential Scanning Calorimetric measurements (DSC) and analytical procedures from different samples remaining in different parts of the equipment after the explosion. The most probable cause of the incident was the formation of methyl nitrate in the condensing line of the flame arrestor. The detonation was initiated by the heat produced by the reaction between condensed nitric acid and methanol. Lessons learnt were used to modify the process to improve its safety.
