 November 22, 2016, the Exxon mobile refinery in Baton Rouge, Louisiana, a fire erupted during maintenance activities, severely burning four workers. The fire occurred when operators inadvertently removed bolts that secured a piece of pressure-containing equipment to a type of valve known as a plug valve. When the operators then attempted to open the valve, it came apart and released flammable isobutane, which formed a vapor cloud that quickly ignited. The fire at Exxon mobile's Baton Rouge refinery occurred in the sulfuric acid alkylation unit. There, isobutane and olefins are converted into a major component of gasoline through a reaction involving sulfuric acid. On the day of the incident, two operators were preparing to put a spare isobutane pump into service. To do so, the operators needed to open the pump's inlet plug valve. That plug valve was operated manually. A hand wheel and a piece of equipment called a gearbox were connected to the valve by a support bracket. The gearbox uses gears to make it easier to turn the valve stem into the open or closed position. The operators turned the hand wheel on the spare pump's inlet plug valve, but the gearbox was not working properly and the valve would not open. When the gearbox was malfunctioning, it was an accepted practice in the alkylation unit for operators to remove the gearbox so that a pipe wrench could be used to turn the valve stem and open the valve. The gearbox was connected to the valve body by the support bracket using bolts. The operators removed four vertical bolts from the valve. But unknown to the operators, these bolts also secured a critical pressure-containing component of the valve known as the top cap. Approximately 3% of the numerous plug valves in the refinery's alkylation unit were designed so that the gearbox support bracket attached to the top cap. With this 30-plus year-old design, removal of the gearbox support bracket can have catastrophic consequences. Approximately 97% of the valves were newer models with updated design that did not connect the gearbox support bracket to pressure-containing equipment, making it less likely to remove the incorrect bolts. With bolts removed from the top cap, one of the operators attempted to open the valve by rotating the valve stem with a pipe wrench while the second operator stood by and observed. The valve immediately failed and came apart. Pressurized isobutane escaped from the valve, forming a flammable white vapor cloud. The two operators warned others in the area and directed them to evacuate. Approximately 2,000 pounds of isobutane released to the atmosphere. The vapor cloud likely found an energized welding machine, roughly 70 feet away from the initial release and ignited. The resulting fire seriously injured one Exxon mobile employee and three contractors working in the area. The CSB also found that although refinery operators did receive training on hard to operate valves, the training did not specifically address how to remove an inoperable gearbox from the older plug valve model. The first key lesson is to establish detailed and accurate written procedures for workers performing potentially hazardous work. This is especially important when different types of equipment or configurations exist that could cause confusion. A second key lesson is to provide training to ensure that workers can perform all anticipated job tasks. Training should include a focus on processes and equipment to improve hazard awareness and help prevent chemical incidents. The CSB found that the 30 plus year old valves left in the refinery were not explicitly required to be updated to the safer design. That is because the older design did technically adhere to current industry standards, which call for items like gearboxes to be capable of removal without affecting pressure-containing parts. That could be accomplished by removing two horizontal bolts that connect the support bracket directly to the gearbox rather than the four vertical bolts that secure the support bracket to the top cap. But on the day of the incident, the operator did not remove the two horizontal bolts. A third key lesson from the CSB's safety bulletin is to apply the hierarchy of controls to mitigate identified hazards. The CSB also found that in its most recent alkylation unit process hazard analysis, the refinery did not evaluate the human factors associated with valve operation and maintenance.