A hydrochloric acid (HCl) leak was detected at 5:30 a.m. on a scrubber on the 4th floor of a production building in a chemical plant. As the floor’s recovery trap was blocked, the product spilt onto the concrete slab and then made its way down to the lower level. The operator triggered the internal emergency plan. A safety perimeter was established, and the firefighters were assisted by the site’s first response crews. The leak was stopped by closing the installation’s supply valves. The product was recovered with absorbent materials by three-man crews wearing protective suits. The personnel unclogged the drainage trap and flushed it with water. The absorbent materials were packed into drums, then discharged to an approved processing centre. An estimated 100 litres of acid had leaked. The intervention ended at approximately 5:00 p.m. after the toxicity measurements in the building’s air were determined to be negative.

The day before, at 10 a.m., the production equipment was shut down following a power outage test to check the operation of the site’s emergency generator system. Most of the equipment was restarted automatically by the unit’s PLC, but certain elements had to be tested and manually restarted. Following a successful manual test, the water recirculation pump of the building’s first “acid” scrubber remained disconnected from the PLC controlling the scrubbers without anyone restarting it. At around 3 p.m., and in compliance with the instructions, a production operator conducted his rounds to check the 4th floor to monitor the condition of the gas scrubbers. He noted that the pump of the 1st scrubber was stopped, but neglected to restart it or to relay the information to his co-workers in the control room. At around 5:30 p.m., the production operators resumed production without knowing that this pump was stopped. The HCl vapours released by the production process were not absorbed.

Earlier the same day, at around 8:30 a.m., work had been carried out on the production building’s technical alarm transmission network. This work led to the deactivation of the transmission of the operating and safety alarms on specific equipment, including those on the 2nd so-called “basic” gas scrubber. The last scrubber recovers the acid effluents from the 1st scrubber to neutralise them with caustic soda. This situation was not identified by the maintenance department, which thus did not notify the production operators. The effluents from the 1st scrubber were too highly concentrated for the 2nd scrubber which eventually became saturated. Its solution became acidic as the injection of caustic soda began to run out and rapidly corroded the stainless steel flange on the recirculation valve. It finished by rupturing due to the lack of alarms. The leak was detected only when the corroded valve was found on the floor during rounds at around 3:30 a.m.

The operator replaced the valve and its steel fitting on the 2nd scrubber with a blind polyethylene flange (acid corrosion-resistant). The building’s alarm transmission network was put back into service. The operator reinforced the scrubber monitoring instructions by setting up rounds every 2 hours, with mandatory reading of the critical operating parameters of this equipment (pH, pressure of the recirculation pumps, water flow rates, tank levels, temperature, and HCl and NaOH solution parameters). An “upstream” system for the purification of acidic vapours from the production process was set up to avoid excessive overloading of the scrubbers. Finally, a safety device designed to close the valve used to inject the main reagent into the lubricant production process was installed. This device operates in the event of a significant deviation from the critical reaction parameters or in the event of a malfunction on the new purification system.