The phosgene detection badges of nine external workers became coloured (concentration > 5 ppm) while performing maintenance work in the phosgene (COCl2) unit process enclosure of a chemical plant. The enclosure was physically open (watertight door unbolted), as the workshop was shut down for 10 days for maintenance. The gas alarm was activated. The workshop was evacuated and the staff confined to the other rooms and workshops onsite.
The nine plant technicians were placed on oxygen in the infirmary and then sent home. The measurements taken by fire-fighters and internal emergency services, one hour after activating the alarm, did not detect any more COCl2. The gas alert was lifted and the crisis centre disbanded. The operator assessed the amount of toxic gas emitted to be several tens of grams.
Prior to opening the process enclosure, the phosgene was removed from the installations, i.e. drained and washed with monochlorobenzene (concentration of COCl2 <50 ppm). The circuits were then washed with nitrogen (N2), treated with gas (washing soda) before being discharged into the chimney. Some devices, not inspected during this shutdown, remained under pressure, including a column (0.6 bar of nitrogen). A preheater had been removed from the process enclosure for replacement; three of the disconnected circuits were then sealed with plugs equipped with an ON/OFF valve and a hose connector for blowing out and purging.
The phosgene leak occurred on one of these valves, which was probably opened after a shock when handling the replacement heat exchanger during positioning. The upstream valve of the workshop gas treatment apparatus was closed, which probably favoured the leak.
To enable resuming work the next morning, the operator adopted several measures: a valve upstream of the chimney held open, verification of the proper working order of the phosgene detectors in the chimney, installation of ammonia “spraying”, a reduction of N2 pressure in the pipes, installation of steel cables and screwable plugs on the valves fitted to the buffers, and replacement of all removed devices.
