In the glyoxylic acid manufacturing unit of a chemical plant composed of 5 x 15.5 m³ oxidisers, the rupture disc (DN 500) of one of the oxidisers ruptured at 3:10 p.m., causing 45 kg of nitrous vapours to be released through an outlet located at a height of 45 m. The unit was confined until 3:25 p.m. The sound of the disc’s rupture was heard from outside the site. Two residents, fearing that they had been exposed to fumes, went for a medical consultation that concluded that they had not been poisoned. The facility operator replaced the damaged disc the next day, and the 4 others in the unit within a few days. Production was not affected because 4 oxidisers are sufficient to meet demand. The Classified Installations Inspection authorities visited the site the following day.

The production unit was operating in batch mode and was in the habitual process phase at the time of the incident. The nitric oxidation reaction of glyoxal to produce glyoxylic acid takes 8 hours at 40 °C with a pressure of about 4 bar. The rupture disc failed at a pressure of 3.9 bar, although it is designed to hold up to 5 bar. The disc had been installed in 2011 (2.5 years); the normal replacement cycle was usually 2 years. This was a fatigue failure.

The facility operator plans to implement a maintenance frequency of no more than 2 years and to review the inspection schedule for the other equipment on site. The Inspection authorities for classified facilities requested that the operator include this incident in the site’s internal feedback during its five-year review of its hazard study.