In a chemical plant producing vitamins A and E, a 13-m³ tank, used for collecting methanol-laden water from the process, exploded while being cleaned. A technician was thrown against the protection barricades and sustained slight injuries: burns to the face and pain in his thorax. The vitamin A production unit was shut down and the operational control system activated, but the facility’s internal emergency plan was not triggered. No physical impacts were recorded onsite; moreover, the hazard potential present at the time of the accident was not of a type to damage the environment.

After the accident, the tank was filled with foam, which would complicate the resumption of cleaning due to clogging the suction pump. A follow-up control conducted on the lorry’s pump vents would indicate a concentration at 47% of the lower flammable limit (LFL).

The cleaning step, which consisted of suctioning the sludge followed by a washing-stripping operation using pressurised cold water, was conducted every 6 weeks; under normal operating conditions, the tank was being inerted with nitrogen, but nitrogen sweeping was halted to allow opening the manhole, controlling the explosimeter and disconnecting pipes prior to the actual cleaning. The explosion occurred 30 min into the cleaning step; an electric lamp, initially rated ATEX for its explosibility, might have caused the explosive cloud to ignite.

This accident demonstrates that sludge present at the bottom of a storage device, which had previously contained easily-inflammable liquids of the solvent-based effluent type, may salt-out an explosible gaseous phase. For any intervention taking place within an ATEX-rated zone, the effective management of tools and equipment used must be very rigorous with permanent atmospheric monitoring. Furthermore, after an explosion within a storage tank, the ATEX risk is still present and must therefore be managed on a continuous basis.