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 boards 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 boards 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 Wednesdays meeting, Superior residents said they, too, want to see hydrogen fluoride removed from their citys refinery and replaced with a less toxic chemical process. Many people here didnt 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 Por...