Insulated pipe of DN200 of 6-7 m length composed of 3 sections, including a central bent.

Carbon steel hot finished seamless tube.

Pressure: 8 bar

Flow rate 285 m³ / h

Nominal thickness: 12 mm

Equipment not subject to the MA of 15/03/2000

Product transported: hot solution (150 ° C) water – soda (200 g / l) and bauxite (60 g / l dry matter).

During the night, in a metallurgical plant located in an urban area, a pipe connecting two regulators and transporting heated caustic soda and bauxite ruptured. The 15 m³ released was initially contained on site. But the Mistral wind dispersed the product beyond the site’s boundaries in the form of a spray. Whitish crystal deposits had an impact on 34 ha, including schoolyards. The town hall was contacted at noon because the operator was late in realizing the impact of the discharge outside the site’s boundaries. At 2:30 p.m., a crisis unit was set up to coordinate the clean-up operations. The operator informed the inspection authorities for Classified Facilities at only 7:20 p.m.

pH measurements, conducted at various points around the city, made it possible to map out 34 ha affected by the solid deposition. The operator had additional pH measurements performed in the streams and rivers.

The piping was opened up over a length of 60 cm, along its upper axis. The pipe has a flat spot in this location. The piping that had caused the release was voluntarily monitored by the site’s inspection department during short (every 10 weeks) and long shut down periods (every 18 months). It was subjected to strong erosion phenomena owing to the high rate of flow and the abrasive nature of the product conveyed. During the previous checks performed on this pipework, the inspection department measured a residual thickness of 3.6 mm without issuing an anomaly report to the maintenance department, as the internal procedure recommends. Similarly, the 2.5 mm loss in thickness between the last two checks did not alert the maintenance department. Furthermore, the production department did not react despite observing pressure drop in one of the regulators and a temperature drop shortly before the accident. The assessments and measurements carried out by the inspection department also revealed that the diaphragm head in the regulator downstream from the pipe rupture was no longer in serviceable condition. It may have accelerated the wear of the upstream piping.

Following the accident, the operator planned to do the following:

  • modify certain procedures;
  • undertake work and checks before restarting the unit;
  • recalculate the diameters of diaphragms;
  • modify the operating standards;
  • integrate the history of previous checks while associating the thickness/wear rate into the anomaly reports;
  • specify the level of uncertainty of the thickness measurements;
  • study the installation of piping protections.