Federal report: Worn valve in Superior, Wisconsin refinery explosion

SUPERIOR, Wis, (KMSP) - The April 26 explosion and fires at the Husky Refinery in Superior, Wisconsin may have been caused by a worn valve in process that converts oil to gasoline. The possible cause was part of a U.S. Chemical Safety Board factual update released Thursday as part of the board’s ongoing investigation.

“Disassembly and evaluation of the spent catalyst slide valve revealed internal wear that could have allowed catalyst flow through the valve even when the valve was in the closed position,” the CSB update stated. 

The incident happened during a planned maintenance shutdown and during a scheduled break time. According to CSB, “many workers previously in the unit before the explosion had moved either into blast resistant buildings or away from the process unit.”

The explosion injured 11 workers at the Husky refinery and 36 people total sought medical attention.

Investigators discovered one piece of debris from the explosion flew about 200 feet and struck a large, aboveground storage tank containing about 50,000 barrels of asphalt. The side of the tank was punctured, resulting in the release of more than 15,000 barrels of hot asphalt. Approximately two hours after the release, the asphalt ignited, resulting in a large fire.

The explosion and fires forced an evacuation of a large portion of the city of Superior. 

The CSB investigation found the explosion happened in refinery’s fluid catalytic cracking unit, which converts "high molecular weight hydrocarbons into more valuable, lower molecular weight hydrocarbons" like gasoline. The cracking unit has a regenerator and a reactor. According to the CSB report, “It is important to prevent air in the regenerator from mixing with hydrocarbons in the reactor and downstream equipment because of the potential for such mixing to create flammable (explosive) conditions within portions of the FCCU.”

Valves are supposed to keep the two air supplies separate, but during the scheduled shutdown, “conditions existed that could have allowed air from the regenerator to flow backward.”

Husky Energy reported in its second quarter earnings statement that the explosion caused $27 million in damages, which will be covered by insurance. The company has spent $53 million in relation to the incident.