A worn-out valve may have allowed air to contact various chemicals within equipment at the Husky Energy refinery in Superior, potentially triggering the April 26 explosion, the U.S. Chemical Safety and Hazard Investigation Board reported Thursday.

The explosion occurred within the fluid catalytic cracking unit, which was previously reported. The "factual investigative update" released by the board offers further information on what may have caused the explosion, which injured 36 people and led to the evacuation of most of Superior. The investigation is ongoing.

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Typically, a series of three valves controlling the flow of catalyst would prevent air from the FCC's regenerator from reaching the hydrocarbons in the reactor, but one of the valves - the spent catalyst slide valve - showed damage, the board said.

"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 report said.

At a press conference held by the board in Superior on Thursday, lead investigator Mark Wingard said air likely entered through through the slide valve, mixed with hydrocarbons, then came in contact with iron sulfide deposits in the equipment, which can spontaneously ignite if in contact with air.

"At this point, it seems the most probable source of your ignition is this iron sulfide deposit," Wingard said.

While Husky had planned on treating the iron sulfide to "mitigate" it, that had not yet happened when the explosion occurred, the report said.

Testing of the valve by the board showed it "wasn't sufficient to maintain that barrier for that time period," Wingard said, but details on the valve's inspection routine and maintenance record were not offered Thursday.

"At this point, the factual update, we can't speak to the inspection and maintenance that's been done, but that's something we're certainly looking into as we move forward," Wingard said.

The board also did not say how old the valve was.

Debris traveled 200 feet

The board reiterated Thursday that the hydrogen fluoride tank was not damaged in the incident. However, cracking unit debris from the explosion flew 200 feet while the tank containing hydrogen fluoride sat 150 feet from the FCC.

"Going forward, we may be looking at the HF unit, but it is too premature to say whether that will be a focus of the investigation ... We will be looking at the debris that was strewn across the site and determining whether there were additional potential hazards that resulted from that," interim executive Dr. Kristen Kulinowski said.

Breathing in hydrogen fluoride at high levels, or in combination with skin contact, can cause death from an irregular heartbeat or from fluid buildup in the lungs.

The gas can also cause blindness by rapid destruction of the corneas.

When asked if the release of hydrogen fluoride or damage to its tank was a "near miss," Kulinowski stressed the board's recommendations and analysis will come later.

"We can't say that yet. This is not the analytical portion of our investigation update," Kulinowski said. "This is really just the facts as we know them today."

The board has identified which pieces struck the asphalt tank 200 feet from the cracking unit, causing the tank to spill 15,000 barrels of hot asphalt throughout the refinery.

That asphalt ignited at 12:15 p.m., over two hours after the initial explosion, and burned into the evening.

It is not known what ignited the asphalt, a board spokesperson said in an email to the News Tribune on Thursday afternoon.

"At this time we do not know the exact ignition source," the spokesperson said. "There were several possible sources at the refinery."

The evacuation of Superior was based on concern that the hydrogen fluoride tank could be damaged.

Investigation ongoing

While the board is finishing their on-site field investigations, it could take 18 months or more until a final report outlining root causes and recommendations is available.

"That's really just the start of our investigation process," Kulinowski said.

Last month, Husky said that once the investigation is complete, it could take another 18 to 24 months to "resume normal operations."

The explosion and fire at Husky Energy's Superior refinery in April resulted in $27 million in damage and $53 million in expenses, which is expected to be covered by insurance.