CSB issues investigation update, animated video on Wisconsin refinery explosion, fire

Washington — The Chemical Safety Board has released an update of its investigation into an April 26 explosion and fire at the Husky Energy refinery in Superior, WI, as well as an animated video that explores the cause of the incident.

The explosion occurred in the refinery’s fluid catalytic cracking unit while workers were on a break during a scheduled maintenance shutdown, CSB states in its report, released Aug. 2. Slide valves within the unit control material flow between a reactor containing flammable hydrocarbons and a regenerator containing air. Mixing hydrocarbons and air, however, may create flammable hazard conditions within fluid catalytic cracking units.

“During the shutdown, conditions existed that could have allowed air from the regenerator to flow backward through one of the slide valves into the reactor, and then into equipment down the stream of the reactor, which contained flammable hydrocarbons,” the video states.

Debris from the ensuing blast flew about 200 feet and punctured an aboveground storage tank containing asphalt, causing more than 15,000 barrels of hot asphalt to spill into the refinery. The asphalt ignited about two hours later, triggering a large fire.

Overall, 36 people sought medical attention, according to CSB, and officials evacuated a large portion of the northwestern Wisconsin city’s residents.

CSB’s investigation of the incident is listed as current.