CSB Safety Video: Transient Hazards - Explosion at the Husky Superior Refinery

U.S. Chemical Safety and Hazard Investigation Board (CSB), June 9, 2023

The U.S. Chemical Safety and Hazard Investigation Board (CSB) has released a new safety video on its investigation into the April 2018 explosion and fire at the Husky Superior Refinery in Superior, Wisconsin. The incident injured 36 workers, caused roughly $550 million in damage to the facility, and released 39,000 pounds of flammable hydrocarbon vapor into the air. Over 2,500 residents of the City of Superior were evacuated from their homes, and the City of Duluth, Minnesota, issued a shelter in place order.

At the time of the incident, the Superior Refinery was owned by Husky Energy, which had purchased the refinery less than six months earlier in November 2017. In 2021, Husky Energy merged with Cenovus Energy.

The safety video includes an animation of the series of events leading to the incident, and interviews with both the CSB’s Chairperson Steve Owens and Lead Investigator, Melike Yersiz.

The incident at the Superior Refinery occurred while the refinery was shutting down its fluid catalytic cracking (FCC) unit to perform planned maintenance. Two vessels within the FCC unit exploded sending debris flying into the air. One piece traveled about 200 feet and hit a large aboveground storage tank, puncturing the side of the tank, and releasing hot asphalt. The asphalt later ignited causing a huge fire. 

In the safety video Chairperson Steve Owens states, “Refinery shutdowns, as well as startups, can be particularly dangerous because processes are not in normal operation mode. Our investigation found that critical safeguards were not in place during this shutdown, and the procedures followed at the refinery were not correct. The result was a massive explosion and fire that injured dozens of workers, caused hundreds of millions of dollars in damage, and threatened surrounding communities.

The CSB’s final report notes that in addition to smoke from the fires at the refinery, the City of Superior’s evacuation was based on the potential risk of a release of highly toxic hydrofluoric acid (HF), which was stored and used at the refinery. Although no HF release occurred, the HF storage tank was about 150 feet away from the two vessels that exploded, about 50 feet closer than the asphalt storage tank that was damaged. Therefore, the CSB considers this incident to be a serious near miss that, had an HF release occurred, could have been much worse.

Chairperson Owens closes the video by saying, “The incident at the Superior Refinery should serve as a wakeup call to other refineries, especially those that have hydrofluoric acid alkylation units. During transient operations, refineries must have necessary safeguards in place -- and follow correct procedures -- to prevent disasters from occurring.


About the CSB

The CSB is an independent, non-regulatory federal agency whose mission is to drive chemical safety change through independent investigations to protect people and the environment. The agency’s board members are appointed by the President and confirmed by the Senate. CSB investigations look into all aspects of chemical incidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems. The CSB does not issue citations or fines but makes safety recommendations to companies, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA. For more information, please visit

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