A federal safety agency said safeguards at the Husky Energy refinery in Superior did not consider the possibility of a worn valve and were “ineffective” at preventing the April 26 refinery explosion that injured 36 people and resulted in the evacuation of much of Superior.

In a factual update released Wednesday, the U.S. Chemical Safety and Hazard Investigation Board said that while the refinery had “process hazard analyses” considering what would happen when an open valve would allow hydrocarbons and air to mix within the fluid catalytic cracking unit - where the explosion occurred - the safeguard did not consider what would happen when the valve was closed but unable to hold a seal. In an August update, the board said a worn - but closed - valve within the fluid catalytic cracking unit allowed air to mix with hydrocarbons, creating an explosive mixture that then ignited.

“The safeguards listed to protect against those scenarios were ineffective,” the Board wrote in Wednesday’s update.

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In August, the board said air likely entered through through the slide valve, mixed with hydrocarbons, then came into contact with iron sulfide deposits in the equipment, which can spontaneously ignite if in contact with air.

The April 26 explosion caused an initial fire that was quickly controlled, but then triggered an hours-long asphalt fire on the site.

In October, another federal agency, the Occupational Safety and Health Administration, also said the company’s Process Hazard Analysis - a document explaining risks involved in an industrial process, and operating procedures - failed to address what might happen if the valve seal failed.

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"The employer did not include the hazard of air introduction from the Fluid Catalytic Cracking Unit (FCCU) regenerator into the reactor due to loss of catalyst and or loss of seal on the spent catalyst slide valve," OSHA wrote in a citation.

The federal Chemical Safety and Hazard Investigation Board doesn't issue fines or citations, but does make recommendations and its investigation into the Husky refinery explosion is ongoing.

Lead investigator Mark Wingard said the investigation will likely be finished in late spring or early summer 2019.

The factual update was released as the board hosted a town hall meeting at the University of Wisconsin-Superior throughout the day Wednesday.