An ammonia container or pressurised drum (NH3) was leaking within a frozen foods warehouse located in an industrial estate. The accident occurred at the time a refrigerating unit condenser was being replaced after being partially emptied of its NH3 contents the previous day; 1,500 kg of NH3 at -18°C were transferred into four 930-litre (450-kg) drums rented to a chemicals distributor by the refrigerationist in charge of the works. Three full drums and a fourth, half-full drum were stored outside the warehouse under night-time supervision.
The following day at 11:50 am and with these containers not having been handled in the meantime, one of drums built in 1998 and then recertified in 2003 (49-bar water pressure, 32.5-bar static pressure, -20 °C < T < +50 °C temperature) abruptly burst. The internal contingency plan was activated at 12:15 am and significant human and material resources were deployed: a hundred firefighters, around forty vehicles and 2 helicopters. A toxic cloud was responsible for causing physical irritations to about 100 individuals at the industrial site (including 21 warehouse employees) and, 200 m from the drums, at a motorway rest stop where several cars were parked. 52 injuries were reported, of which 28 required hospitalisation for analyses into the evening, 5 of these individuals suffered from more serious cases: 2 gendarmes, 1 warehouse forklift driver, and 2 asthmatics.A 150-m safety zone was cordoned off and a street closed to traffic; highway message signs informed motorists that the rest stop was not open and that vehicle windows should be closed and ventilation systems turned off. Firefighters wearing masks dispersed the NH3 fumes using peacock tail spray nozzles. In order to maintain sufficient retention capacity, the dilution water collected in a 300-m³ catchpit was discharged into the network following a pH reading (8 to 9), 550 m³ were thus used. The intact NH3 barrels were then transferred into the refrigeration unit. The motorway rest area was reopened at 9:26 pm, and the incident was completely over around 10:00 pm.
An expert evaluation noted that the pressure equipment ruptured as a result of overfilling attributable “inappropriate” operating procedures and recommended that drums be systematically weighed. According to the CII (Classified Installations Inspectorate), the procedure is particularly difficult to perform: careful positioning of the “multi-purpose” drum on its side at an angle of 40° so that the 85 or 100% top-up limiting tube can serve its purpose. In addition, these drums are liable to contain liquefied gases or liquids according to 2 marks painted on its side… This setting is nevertheless imprecise as the end of the tube, based on its design, can vary in distance from the wall of the cylinder. The mixing of products (water / NH3…) can also not be disregarded due to the multipurpose nature of the arrangement.
Finally, systematic weighing may also be difficult: cramped machine rooms… A room adjoining the machine room and connected to the latter’s stack, capable of resisting a heat flux and equipped with an appropriate detection system, shall be built to store the containers. The internal contingency plan has been reinforced. A more formal approach has been taken in developing the mobile tank filling/draining procedures.
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