At 5:23 a.m., operators reported an irritating atmospheric release in a steel plant. The in-house emergency services located the leak on a manhole on one of the two 50 m³ 33% hydrochloric acid (HCl) storage tanks. The gaseous release was the result of the exothermal reaction of the acid leaking into the retaining basin containing rainwater.

Water curtains were installed by the in-house emergency services to contain the acid vapours. At 7:15 a.m., the valve connecting the two acid tanks was closed. The emergency services also installed clamps on the leaking manhole. At 10:15 a.m., a specialised company emptied the acid tanks’ containment area and then the contents were treated in the site’s treatment facility. The 2 HCl tanks were emptied.

Atmospheric measurements conducted to the south of the site indicated very low HCl values (0.5 ppm). The operator estimated the volume of HCl in the containment area at 8.1 m³ and the volume of evaporated water at 38 m³. The 400 m³ of water used to establish the water curtain was released into the natural environment after the pH was checked.

The annual maintenance on the tanks was carried out a few days prior to the accident. The manhole covers were checked, and the seals and bolts were replaced. The new tanks were filled in the days that followed. No leaks were detected, despite several inspections by the maintenance department.

The analysis of the accident highlighted other causes:

  • faulty tightening of 2 bolts on the 24 used for the manhole. The method for tightening these bolts is based on simple experience by the company that has performed this type of operation at the site for years;
  • flatness defect of the seal;
  • no watertightness test performed before the acid tanks were put back into service since filling was scheduled for the following day;
  • the presence of a level sensor in the containment area was used only to check the available volume remaining in the retention, especially in the case of rain. This so-called low-level sensor cannot be used to detect a leak;
  • instructions and maintenance procedures on faulty acid tanks;
  • maintenance on tanks not included in the site’s computerised maintenance management system (CMMS);
  • materials used for the manholes needing replacement.

Following this accident, the operator plans to undertake the following actions:

  • installation of fixed gas detectors and cameras around the storage facilities;
  • revision of operating procedures concerning manholes;
  • the installation of fixed water curtains and the possibility of installing translucent protection around the tanks;
  • the sharing of feedback internally and with the group’s other sites;
  • formalise the intervention of the in-house fire brigade and the monitoring of protective suits after being used to perform a procedure;
  • provision of gas measurement instruments for the inspectors.