 April 26, 2018, the Husky Superior Refinery in Superior, Wisconsin. At 5.40 a.m., operators took steps to shut down the refinery's fluid catalytic cracker, or FCC unit, for periodic maintenance and inspection. The FCC unit uses heat and a solid catalyst to break or crack heavy hydrocarbons from crude oil into smaller hydrocarbons, which can then be blended into gasoline and other products. Slide valves control the flow of catalyst between a reactor, which contains flammable hydrocarbons, and a regenerator, which contains air. It is important to prevent mixing the hydrocarbons in air, since if they mix, a dangerous explosion can occur. To avoid this hazard, the slide valves maintain a catalyst level between the reactor and regenerator, which acts as a barrier. To initiate the shutdown of the FCC unit, refinery workers stopped the flow of hydrocarbons to the unit and closed the two slide valves connecting the reactor to the regenerator. But unknown to the workers, one of those slide valves was eroded, and even though it closed, it could not maintain a catalyst barrier in the reactor. As a result, the catalyst from the reactor fell through the valve to the regenerator, leaving no protective barrier between the two pieces of equipment. This allowed air from the regenerator to flow backwards through the slide valve into the reactor, and then into equipment downstream of the reactor, which contained hydrocarbons, forming a flammable mixture. At approximately 10 a.m., the hazardous mixture of air and hydrocarbons found an ignition source, and there was a large explosion. Debris from the explosion flew about 200 feet and hit a large above-ground storage tank, puncturing the side of the tank and releasing hot asphalt. Asphalt poured out of the tank over the containment ground and into the refinery's operating units, where it spread along the ground. At around 12.15 p.m., the released asphalt ignited near the storage tanks, and a fire broke out. The fire traveled along a stream of asphalt to the refinery's crude and FCC units. The fire produced a huge plume of black smoke, leading to the evacuation of a portion of the town of Superior. Typically, this type of asphalt fire can burn for days. A plan to extinguish the fire was developed by the refinery's emergency response team. Around three o'clock, they began to address the asphalt fire together with the Superior Fire Department. A firetruck drove to the edge of the crude unit, and emergency responders sprayed dry chemical fire extinguisher inside to fight the fires that had broken out there. Responders also used two stationary hose loops called monitors to shoot water into the crude unit in an attempt to extinguish the flames. All these fires were being put out, over by the storage tanks, emergency responders began to approach the large area of burning asphalt. The emergency responders' plan was to push the asphalt fire into an empty area near the storage tanks and then aggressively attack the fire from multiple directions. First, they used two monitors, one to the north and another to the west, to sweep water to push the asphalt fire toward the designated containment area. Then, when the fires within the refinery's units had been extinguished, the firetruck relocated so that it could spray the burning asphalt in the containment area with foam from the east. Around 5.30, an additional team began to attack the fire from the south, spraying foam from a monitor. At this point, the asphalt fire was completely surrounded within the containment area. With the fire contained, emergency responders were able to put out the asphalt fire at around 7 p.m. The asphalt fire had been extinguished in a matter of hours, as opposed to days as had been initially feared. The evacuation order was lifted and superior residents were allowed to return home. Nevertheless, 36 Husky employees and contract workers sought medical attention for injuries.