In a chemical plant producing fertilisers, an ammonia release at the level of the hot ammonium nitrate solution station intoxicated 4 employees, 2 of whom were working for a subcontractor. The installation was placed in safety mode and local fire-fighters were notified. The 4 injured personnel were all hospitalised for exams and cleared to leave 5 hours later.

The accident occurred even though the unit in question had been operating since the previous evening. Regulation of the nitric acid (HNO3) flow rate, which is usually an automated process, was switched to manual mode when encountering difficulties in stabilising the reaction medium pH. A maintenance service call had been scheduled at 9 am on the day of the accident.

During this service call, the low flow and high flow safety mechanisms were inhibited for the time it took to complete testing. After manipulating the nitric acid intake valve, the flow of HNO3 stopped suddenly, causing an excess build-up of ammonia in the reactor. The technician unsuccessfully attempted to reactivate the flow safety mechanisms, before deliberately tripping the reactor. Basic vapours were then discharged via the facility’s vents and by the degassing of non-recycled condensates discharged into the gutters crossing the unit.

A defective nitric acid valve caused this accident : a broken pin was found inside the station at the time of its disassembly. This equipment problem could not be detected before disassembly, and the control room relay did not indicate any defect. The lack of a unit-wide alarm, compounded by an inadequate risk analysis prior to maintenance work, was also cited. As for feedback from this accident, personnel training improvements were envisaged, especially aimed at temporary safety device downtime and the installation of ammonia detectors connected to an alarm.