SUPERIOR, WIS. – A hole in a valve at the Husky Energy refinery in Superior, Wis., caused it to fail last spring, leading to an explosion that injured 36 people and required the evacuation of large sections of the city.

The erosion that created the hole in a slide valve allowed air to mix with hydrocarbons, setting off the blast that sprayed debris across the refinery and punctured an asphalt tank. Some 15,000 gallons of asphalt spilled, ignited and burned for several hours, according to the latest findings of the U.S. Chemical Safety and Hazard Investigation Board.

The update on the board's ongoing investigation was shared Wednesday at the outset of a town hall meeting in Superior, where residents have expressed concerns about safety at the oil refinery. The board's update linked the Superior explosion to one that occurred nearly four years ago in Torrance, Calif. In both cases, the board said, ineffective safeguards allowed an explosive mixture of air and hydrocarbons to form inside a fluid catalytic cracking unit. The unit is a common piece of equipment at oil refineries used to refine crude oil into higher octane fuels.

In both cases, the FCC unit was not in normal operating mode when the explosions occurred. And in both cases, the FCC units were due for routine upgrades and replacement of older equipment.

In the Torrance explosion, debris nearly hit a tank of hydrogen fluoride, a highly toxic chemical that causes severe burns and can kill on contact. The resulting public relations fallout has led to public calls for an end of the use of hydrogen fluoride at the refinery.

During a public comment period at Wednesday's meeting, Superior residents said they, too, want to see hydrogen fluoride removed from their city's refinery and replaced with a less toxic chemical process. Many people here didn't know that the refinery operating at the edge of town had drafted a worst-case scenario as required by the U.S. Environmental Protection Agency that said the release of hydrogen fluoride stored at the facility could endanger up to 180,000 people, essentially the entire Twin Ports population. The scenario is considered highly unlikely, but after the April 26 explosion and fire, people were rattled.

Kathryn McKenzie, who spoke at the meeting, said she lives about a mile from the refinery and understands that it provides jobs and a boost to the local economy. It still poses a risk to her and her family, McKenzie said, and its use of hydrogen fluoride makes her feel like she's "expendable."

Some people who lived closest to the refinery were told to evacuate due to the presence of hydrogen fluoride, but others should have been told as well, said Pastor Michelle R. Rowell of Concordia Lutheran Church in Superior.

"After that nearest evacuation was done, I think it was irresponsible that the rest of the community was not informed about the presence of that very dangerous chemical," she said.

Ginger Juel, co-founder of a local group opposed to the refinery's use of hydrogen fluoride, accused Superior Mayor Jim Paine of minimizing the risks posed by the chemical. The mayor spoke briefly at the town hall's start, saying he wants to see the Husky refinery stay in Superior but that citizens must be protected.

Susan Hedman, the former Great Lakes region director of the U.S. Environmental Protection Agency who is now the staff attorney for the Madison-based nonprofit Clean Wisconsin, said Superior got lucky last April and that things could have been far worse. Her organization called for an end to the use of hydrogen fluoride.

A petroleum industry lobbyist and a refinery worker said they support Husky and the industry. Atticus Larson, an electrician at Husky and a member of the International Union of Operating Engineers, Local 420, said the union doesn't have an official position on hydrogen fluoride. "I believe Husky will do it right and do it safely," he said.

Jared Hawes of the American Fuel and Petrochemical Manufacturers Association said about 50 refineries across the country use hydrogen fluoride, and that there's no one-size-fits-all solution for the industry.

It's not yet clear what the three-member CSB board will decide, but if its recommendations say anything about hydrogen fluoride, it could have implications for other refineries that use the chemical, including the Andeavor refinery in St. Paul Park in the Twin Cities metro area. The board's report isn't expected until next year.

U.S. Rep. Betty McCollum, one of several Congress members who called for the CSB town hall meeting, thanked the chemical safety board in a statement Wednesday for its work examining the risks posed by hydrogen fluoride.

CSB board member Rick Engler said in his closing remarks at Wednesday's meeting that hydrogen fluoride deserves a closer look. It was first introduced in refineries because it was cheaper than alternatives, he said, and it's proven to have a problematic track record. He pointed to a 2013 survey by the United Steelworkers union that found 131 hydrogen fluoride releases of near misses over a five-year period at refineries in the U.S.

Matt McKinney • 612-673-7329