A welding contractor was killed while repairing a 3 cm by 1,3 cm crack on the bottom of a water clarifier tank at a food processing plant. The tank was used to separate dirt and debris from wastewater in a potato-washing process area. The tank was open at the top and had a metal skirt around its cone-shaped base. While the welder was working inside the tank, an explosion occurred; the internal tank structures collapsed, resulting in his death.

The investigation determined that approximately 40 cm of debris-laden water had leaked through the crack in the tank and accumulated in the hidden space under the tank skirting. Bacterial decomposition of the organic matter presents in the waste water likely produced flammable gas, which was then ignited by the welding activity. The employee had tested for combustible gas inside the tank prior to the hot work, but only from the entrance of the tank and no flammable gas was detected. Monitoring for combustible gases was not conducted in the immediate area of the crack just prior to the initiation of the welding or in the adjacent space where flammable gas was present. Personnel were inadequately trained on the use of the specific combustible gas detector that was used and no hot work permit had been issued prior to commencing the welding (Source: Chemical Safety Board – USA)