Around 8:20 am, a sustained leak occurred on a pipeline located downstream of the reactors within a nitric acid production unit at a plant specialised in basic chemical products. The tripping of safety relays shut down the shop. An orange cloud of nitrous gas was released into the atmosphere for 30 seconds. Onsite fire-fighters installed water curtains at the level of the leak and conducted a number of toxicity measurements in the wind, all of which turned out to be negative. No one was injured or hurt. No production facilities were damaged. The operator issued a press release.

The investigation conducted by the site operator indicated that the leak occurred at the spot of a longitudinal weld to place a tap (DN 700, P=2.7 bar, T=85°C, type 304 L carbon steel), at the outlet of one of the unit’s nitrous gas / nitrous gas exchangers. The pipeline in question, built in 1991, was originally used by another Group plant that had since closed. After a 3-year idle period, this infrastructure was assembled on the given unit in 1994, with an increased diameter (from 500 to 700 mm) in order to limit head loss. By being waived of regulations relative to pressurised equipment (i.e. P<4 bar), the incident was not recorded (neither materials certificates nor specifications). Since this regulation changed in 2000, the equipment had to be recertified in 2004, although no construction or modification application was filed. A metallurgical appraisal of the tapping opening revealed that while the material was compliant, an acidic (intergranular) corrosion had developed at the site of the longitudinal welds performed using 2 rolled and welded metal sheets. The corrosion caused a significant loss of thickness in the thermally affected zones (TAZ) of these metal sheets: 0.4 mm residual vs. 2 mm originally. Excessive heat energy was used for the lateral welds during tap diameter modification. A precipitate of chromium carbides could also be observed outside these TAZ zones, which is characteristic of incomplete heat treatment of the sheet metal introduced. These two metallurgical processing errors caused a local decrease in the steel's chromium content, with the steel becoming more sensitive to intergranular corrosion by means of nitric acid condensation during transient phases. This corrosion wound up causing complete rupture of the pressurised tap adjacent to one of the welds. The loss of thickness at the point of the weld had already been detected in 2006 but only gave rise to a specific monitoring of the pipe; the next inspection was scheduled for 2008.

The operator also verified the thickness of other equipment along the unit’s pipe path, particularly longitudinal welds within the TAZ zones, yet no loss of thickness was recorded. The pipe inspection programme was enhanced and thereafter included a periodic thickness verification of TAZ zones containing longitudinal welds. Feedback on this accident was applied to the Group’s other plants by recognising this mode of equipment degradation.