An ammonia leak occurred on a 50 t wagon during its unloading in a confined building of a chemical plant. The automatic detection of an ammonia content of around 100 ppm triggered the alarm and switch over of the unit to safety mode: the transfer was automatically stopped and the high-flow ventilation system of the building was switched on. This system diluted the ammonia discharged and thereby limited the risk of a concentration higher than the irreversible effects threshold at ground level. The drip-by-drip leak was located on a joint between the tank and the bottom valve. Closure of this valve could therefore not stop the leak. Under these conditions, the operator decided to terminate unloading of the remaining 20 t after disengaging the control linked to the ammonia detection system. The amount of ammonia released was estimated at 11 kg. No human or environmental consequences were reported.
A sealing fault on the joint of the wagon’s bottom valve caused the accident. The operator asked its ammonia supplier to reinforce the inspection and reliability of the packaging supplied.
Similar incidents that had occurred in the past had led the operator to carry out dynamic containment of the ammonia unloading station at the end of 2005, that proved effective during this incident.