An explosion in the gasoline-producing fluidic catalytic cracking unit (FCCU) set off a massive fire last Thursday at Husky Energy’s Superior, Wisconsin, refinery, an official of U.S. Chemical Safety Board (CSB) said on Tuesday.
The blast in the 13,000 barrel per day (bpd) FCCU at the 38,000 bpd Husky refinery damaged a nearby tank containing asphalt, said CSB Senior Advisor Tom Zoeller in a telephone interview with Reuters.
“The shrapnel from that explosion pierced the asphalt tank and the asphalt spilled out and found an ignition source,” Zoeller said.
The massive fire, which sent a giant plume of heavy, black smoke over Superior, forced the evacuation of the city of 27,000 residents on Thursday. They were allowed to return on Friday.
One person was hospitalized due to the explosion.
Zoeller said the board’s understanding of the accident came from descriptions by employees and contractors at the refinery preparing to shut the entire plant for an overhaul.
The cause of the FCCU explosion was unknown, he said.
Husky spokeswoman Kim Guttormson said the refinery was not operating as the company begins investigating the cause of the explosion in coordination with government agencies.
The CSB’s four investigators at the Superior refinery have not been able to inspect the site of the blast because refining units are considered too unstable to walk around.
“Husky said the cranes have started arriving and we expect to get in by the end of next week,” Zoeller said.
Husky shut down production at the refinery after the explosion.
The CSB, which has no regulatory or enforcement authority, may send parts from the FCCU and other refinery units for metallurgical testing, Zoeller said. Most board investigations take about a year to complete.