Around 11:30 am, when restarting a unit inside a Seveso-rated atmospheric gas plant (nitrogen, argon and oxygen), a pressure surge occurred in the cold box case (exchanger cases and distillation columns). The 3 ballasted safety valves protecting the unit opened at a height of 25 m, producing a discharge of white dust containing perlite (a crushed volcanic rock used to insulate a cryogenic liquid pipeline, small doses of which irritate the eyes and respiratory tract). The start-up procedure was halted, the site placed in safe operating mode and the unit depressurised. The internal emergency plan was activated so that staff could assemble and a safety perimeter be installed around the affected unit. At noon, fire-fighters inspected the unit and found no abnormally high oxygen (O2) concentrations. The dust formed a cloud that moved with the wind towards the adjacent motorway. Traffic was not suspended, but messages were displayed asking drivers to slow down as the pavement was made slippery by perlite deposits. Once the unit had been restored to atmospheric pressure, some employees drained the cryogenic liquid while others wearing goggles and masks sprayed water on perlite deposits on the ground in order to prevent any more clouds from forming. The incident was closed at 4 pm, with the discharge evaluated at 63 tonnes of perlite released within a few minutes out of the 360 tonnes present in the unit (700 m³ out of 3,000 m³).

The inspection authorities for classified facilities visited the site. This installation would only restart upon completing all necessary investigations and tests, while the 2nd production unit was placed into service during the corresponding time interval. The plant operator wrote up a press release, revised the unit start-up procedure and updated the site’s internal emergency plan.

Steps to remove the Perlite from the cold box led to the observation that the exhaust pipe of a turbine and the nearby coupling bellows had been left open, with signs of hot spots visible on the turbine discharge pipe. The investigations pointed to an excessive gaseous O2 content in the turbine when the unit was turned on. The start-up process for the damaged unit was modified, and a study was initiated to instrument the ballasted safety valves on the reversible cases with a unit safety mode feature.