On November 9, 2010, an explosion killed one contract worker and seriously injured another at the DuPont facility in Tonawanda, New York. Contract workers were performing welding on top of a 10,000-gallon slurry tank that contained vinyl fluoride — a highly flammable and toxic gas used in the production of polyvinylidene fluoride (PVDF). Sparks from the welding operation fell through an opening in the tank roof, igniting vinyl fluoride vapors inside the tank.

DuPont engineers had reviewed and approved the hot work permit for the welding operation. However, neither the engineers who approved the permit nor the contractors who performed the work recognized that the slurry tank contained vinyl fluoride vapors at concentrations within the flammable range. The tank was believed to be purged and safe for hot work based on a surface reading that did not detect the vapors accumulating at depth inside the tank.

The CSB found that DuPont hot work permit system failed in multiple ways: the hazard of vinyl fluoride in the slurry tank was not adequately characterized in the permit, the atmospheric testing was inadequate, and DuPont engineering staff who reviewed the permit lacked sufficient knowledge of the flammable properties of vinyl fluoride in a slurry tank environment. The case demonstrates that a hot work permit signature is only as reliable as the hazard knowledge behind it.