In a fertiliser plant, a leak of 62% nitric acid occurred at about 8:10 am on the cut-off valve at the saturator inlet of the ammonium nitrate workshop. Operating conditions were stable.

This leak, estimated at 1 t acid over 20 minutes, generated a considerable release of nitrous vapour in the workshop, especially when in contact with the heat insulator of a steam pipe running below.

The alert was signalled by an employee who, while moving around in the vicinity of the facilities, noticed dark smoke emanating from the workshop buildings. The IOP was activated at 8:12 am and then lifted at 9:47am.

The workshop was shut down. The acid discharge, and 2 m³ of water used to cool the installations and limit evaporation of the acid were recovered in a retention basin and channelled into a previously isolated drainage tank.

Only minor economic impacts resulted from the shutdown of this workshop.

Premature corrosion of the metallic joints on the cut-off valve caused the leak. The jointing material, implemented a few days prior, was not adapted and, according to the operator, did not correspond with the characteristics set forth in the merchandise checkout slip for this item.

Moreover, the Classified Facilities Inspectorate found that the nitric acid flow meter, located downstream of the shut-off valve at the saturator inlet, showed no change in flow entering the saturator despite the acid leak upstream. This flow meter, equipped with threshold alarms and acid supply cut-off valves, did not detect any leak nor did it alert plant technicians. No other safety device was available to warn the technicians of this leak. Lastly, the workshop was not equipped with sensors capable of detecting this type of discharge.

Following this accident and prior to restarting the workshop, the operator was required to carry out an exhaustive control of all joints similar to the faulty one (which has probably been replaced during recent maintenance interventions), perform a control and overhaul all installations that may have been adversely affected by the flow of acid.