Pollution
Humain
Environnement
Economique

Within an ammonia (NH3) production unit, a liquefied ammonia leak occurred on a scrubber used to eliminate residual non-condensable gases dissolved within the ammonia. Bothered by the smell, the control room operator sounded the alarm. A manual valve was closed and a water curtain deployed around the ammonia puddle in order to limit spreading of the aerosol cloud. An electrical malfunction had caused the accident: the erroneous relay of an ammonia compressor shutoff signal triggered closure of the liquid ammonia extraction valve on the scrubber, resulting in the scrubber being filled. Then, the local system regulating the release of non-condensable gases allowed liquid ammonia to escape. The installation was modified to improve safety by means of: replacing the local column level regulation system by a centralised device with an alarm for a more responsive diagnostic; replacing the compressor status relay system by an automated safety mechanism independent of the process automation controller; and improving control room confinement…