HOUSTON (Reuters) - An “ineffective” safeguard failed to prevent an explosive mixing of air and fuel at a Husky Energy refinery in Superior, Wisconsin, leading to a blast and fire in the plant’s gasoline-producing unit in April, a U.S. industrial safety group said on Wednesday.

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Air seeped through a hole in a valve within a fluidic catalytic cracking unit (FCCU), the U.S. Chemical Safety Board (CSB) said, causing an April 26 explosion that led to a massive fire and a 24-hour-long evacuation of residents living within miles of the plant.

The board said 36 people sought medical treatment after the blast, including 11 working at the refinery. The plant was undergoing maintenance at the time.

Husky, according to the CSB, had only considered a failure of the valve when locked open, not a failure when it was closed, according to an updated report the board presented of its months-long investigation at a meeting in Superior.

The failures leading to the Superior refinery explosion were similar to those that caused a 2015 explosion in an FCCU at a Torrance, California, refinery then owned by Exxon Mobil Corp. Exxon sold the refinery to PBF Energy in 2016.

“Prior to both incidents, the process hazard analyses identified scenarios in which hydrocarbons flowed into the air side of the FCCU and vice versa due to a failure of the spent catalyst slide valve (SCSV), but the safeguards listed to protect against those scenarios were ineffective,” the board said.

Husky did not dispute the CSB’s finding, and will continue to work with the board while it conducts its own investigation into the explosion, said spokesman Mel Duvall.

The CSB’s “investigation is important to understanding what happened,” Duvall said. “We are also continuing with our own investigation.”

The CSB, created by the U.S. Clean Air Act, has no regulatory or enforcement authority but is charged with determining the causes of chemical plant explosions and fires and making recommendations to government and industry.

“Given the similarities between these two incidents, the CSB will be examining areas of further improvement that need to be taken by industry,” the board said.

FCCUs use a fine, silica catalyst in high heat to make gasoline from gas oil and the passage of the sand-like catalyst over the slide valve at the Superior refinery wore a hole in it.

A mixture of air and hydrocarbon within the unit can easily find an ignition source in the 1,300 degree Fahrenheit (715 Celsius) operating temperature of the FCCU.

“While we don’t expect to resume normal operations at the refinery until 2020, we remain committed to our employees and the community,” Duvall said.

Duvall said the company is reviewing alternatives to hydrogen flouride as it prepares to rebuild the refinery for restart in 2020.