Inside a plant specialised in thio-chemistry, an explosion occurred at 9:55 am on a unit undergoing a restart procedure, less than one year after major facility modifications. Following the unintentional intake of ammonia into the oven on an acid nitrosyl sulphate production unit, both the nitrous gas blower and its discharge line violently exploded during a rise in process temperature. Onsite damage was considerable: the blower and a portion of the discharge line at the oxidation capacity inlet were destroyed, pipes running through the damaged zones, projectiles of materials ejected distances up to 100 m, destruction of a cladding protection between the blower and the combustion ovens. The shockwave also shattered window panes in the control room 25 m away as well as the nearby cafeteria. No victims were reported. The production cycle was stopped. The temperature rise and operating regime, at the pumping threshold, of the blower would have triggered an ammonium nitrate and nitrite dust explosion. The nitrite self-decomposition would have been responsible for causing a more violent self-decomposition of the ammonium nitrate.
The site operator reacted by modifying the process in order to limit nitrite formation, improve process controls and operations (by installing both a high temperature alarm in the condensation liquid phases and a low temperature alarm on the ovens, plus sensor redundancy on the key process parameters of air and ammonia flow rates and screen and condensate temperatures). The operator also installed a protective wall and steel structure around the blower to limit exposure to projectiles of debris.
From an organisational standpoint, the operator revised the entire set of procedures and operating guidelines to avoid a precipitous degradation of the catalytic screens and to better test measurement and alarm instrumentation. More specifically, the screens were to be disassembled if the oven had to be stopped for more than 2 weeks, and an acid wash would be performed as a preliminarily step to installing new screens. This protocol was completed by a series of specific actions to be taken in the event of fire or degraded operations.
Lastly, a more rigorous methodology was adopted to ensure unit documentation remained up-to-date, in addition to drafting guidelines and analysing risks prior to introducing any modifications in the units.
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