In the containment of a chemical unit, a phosgene (COCl2) sensor was triggered at 6.40 am after maintenance operations resumed as part of a technical shutdown.

This detection is further to a COCl2 leak that occurred (ARIA 32965) the previous day in a valve upstream to an exchanger being replaced. Only one out of the thirty sensors fitted in the containment reacted. The staff was evacuated.

None of the detection badges worn by the technicians reacted. Operations were suspended. The working group set up the previous day was reconstituted and the facilities were inspected again.

The sensor that sounded the alert was close to the outtake of an air duct whose air intake was near the leak site. The accident occurred due to the release of COCl2 at the joint upstream to the valve that leaked.

Several corrective actions were implemented:

• strengthening bolting of stoppers (during stop phases, bolting is reduced to save time)
• coating joints with adhesive to increase air-tightness
• stopping supply of nitrogen to pipelines to reduce pressure (weather conditions reduce the chances of humidity setting in facilities).