In a chemical plant producing sweeteners, hot work on a pipe connecting two tanks (V1210/V1190) caused an explosion in a 15 m³ tank of toluene (V1190), which was empty at the time of the events, and caused its pressure relief valve to open. The explosion had no environmental impact. Both tanks had been connected until then using a hose, as the installation assembled for the tests was temporary. During the annual shutdown for preventive maintenance, the hose was supposed to be replaced with the installation of a rigid line. The 2 portions of line had been connected to the 2 storage capacities using flanges. The welding of the final connection, to be carried out 5 m from the empty tank and in an area defined by the operator as ATEX 2, was scheduled for 25/10. When the subcontractor pointed the torch at the line to create the weld, hissing and flames escaped from the pressure relief valve calibrated at 0.08 b. Toluene vapours still present in the tank ignited on contact with the hot spot. The presence of residual vapours in V1190 can be explained by the fact that the tank, only washed with water, had not been vapour stripped, or nitrogen-inerted even though it was equipped for this operation. For staff safety reasons, this system had been shut down on the equipment in the sector due to the performance of multiple maintenance operations. The valve located on the line, between the junction point of the operation in progress and tank V1190, had remained open during the welding. Over the next few days, the operator checked the tightness of the installation by filling the equipment that might have been affected by the overpressure caused by the internal explosion with water and replacing the pressure relief valve on tank V1190. This incident had no impact on the environment or staff and caused no damage to equipment. Several corrective measures are being implemented: a thorough risk analysis for all hot work on lines connected to other potentially hazardous equipment, the equipment line shall be isolated if potentially hazardous, and discussions are being held concerning the limitation of hot work in solvent ATEX areas with a preference for the installation of flanges.