June 15, 2023

“A Serious Near Miss”: CSB Video Explains Causes of 2018 Refinery Explosion

The April 2018 refinery explosion in Superior, Wisconsin, could have been much worse if debris had punctured a storage tank containing highly toxic hydrofluoric acid, according to an animated video of the incident released by the U.S. Chemical Safety and Hazard Investigation Board (CSB). The incident injured 36 workers, caused more than $500 million in damage to the refinery, released 39,000 pounds of flammable hydrocarbon vapor, and led to the evacuation of 2,500 Superior residents. Nearby Duluth, Minnesota, also issued a shelter-in-place order.

The events that led to the incident occurred during a shutdown of the refinery’s fluid catalytic cracking (FCC) unit for planned maintenance. A worn valve and the lack of appropriate safeguards allowed air to mix with hydrocarbons within the FCC unit, forming a flammable mixture that spread throughout the refinery until it found an ignition source. The resulting explosion destroyed two pressure vessels and launched steel fragments, one of which punctured an asphalt storage tank 200 feet away. Hot asphalt leaked from the tank and accumulated within the refinery for two hours before catching fire and producing a huge plume of black smoke.

The storage tank of hydrofluoric acid, located just 150 feet from the explosion, was undamaged, a stroke of luck that a CSB news release characterized as “a serious near miss.” HF can cause severe burns when it contacts skin, and inhalation exposure can be fatal. The NIOSH IDLH (immediately dangerous to life or health) value for HF is 30 ppm.

Cenovus Energy, which acquired the Superior refinery in 2021 when it merged with Husky Energy, states on its website that “our layers of safety and risk mitigation protocols worked as designed during the 2018 incident” and notes that “no HF was released.”

CSB’s investigation identified many deficiencies at the refinery that contributed to the incident. According to the agency, the refinery lacked procedures and safeguards for “transient” operations such as equipment shutdowns, which present different hazards than normal operations. In some cases, the refinery’s FCC procedures contradicted guidance from the entities that commissioned the equipment and licensed the technology. Operating procedures were unclear and operators were not properly trained, CSB said.

The video notes that the process vessels were made of a grade of steel that is no longer approved for such structures. A higher grade of steel likely would have torn open instead of shattering into projectiles, according to the agency. CSB also found that emergency responders waited too long to address the leaking asphalt and could have minimized damages if they had treated the asphalt as a flammable liquid.

The CSB video is available on YouTube. For more information, read the agency’s press release.

Related: The Synergist has published recent articles on modernizing process safety, the IH’s role in process safety management, and using OSHA’s process safety management standard to enhance EHS programs.