The U.S. Chemical Safety Board (CSB) has released an animation describing events leading to the November 22, 2016, fire at the ExxonMobil refinery in Baton Rouge, LA, which severely burned four workers. The fire erupted during maintenance activities in the refinery’s sulfuric acid alkylation unit.
Unknown to the operators, the valve was one of 3% of the total number of valves in the alkylation unit that were of an older design. This 30-year-old design used bolts to connect the gearbox and its support bracket to the valve and also secured a critical piece of pressure-containing equipment called the top-cap. When removed, the bolts leave the top-cap disconnected and the valve vulnerable to pressure.
With the bolts and gearbox removed, and the top-cap not secured, the operators attempted to open the valve using a pipe wrench, and the valve immediately failed and came apart.
Pressurized isobutane escaped from the valve, forming a flammable white vapor cloud, which likely found an energized welding machine roughly 70 feet away from the initial release, and ignited. The resulting fire seriously injured one ExxonMobil employee and three contractors working in the area.