In a December factual investigative update, the U.S. Chemical Safety and Hazard Investigation Board drew parallels between an April 26 explosion and fire at Husky Energy Inc.'s Superior, Wis., refinery and a separate disaster that occurred at Exxon Mobil Corp.'s Torrance, Calif., refinery — now owned by PBF Energy Inc. — on Feb. 18, 2015, citing ineffective safeguards to prevent the the disasters.

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

"Both the Superior and Torrance explosions resulted from the inadvertent mixing of hydrocarbons with air inside the [fluid catalytic cracking unit, or FCCU] that found an ignition source," the report said. "In the Torrance explosion, hydrocarbons flowed backward into the air inside of the FCCU. In the Superior explosion, air flowed forward into the hydrocarbon side of the FCCU. In both cases, explosion debris impacted equipment in surrounding units and caused subsequent fires and releases to the atmosphere."

The FCCU uses a catalyst to convert high-boiling point, high-molecular weight hydrocarbon fractions of crude oil in to higher-value products such as gasoline.

The report said the ignition source in the Torrance incident was an electrostatic precipitator, which sits within the FCCU and removes catalyst particles using charged plates that produce sparks during normal operation.

In both incidents, the report said a valve designed to maintain a catalyst barrier to prevent the mixing of air and hydrocarbons within the FCCU eroded over time from exposure to catalyst and failed while the unit was not in normal operation, with the Superior FCCU being shut down for scheduled maintenance, and the Torrance FCCU operating in standby mode.

"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 report said.

The American Fuel and Petrochemical Manufacturers, an industry group, notified the CSB in August 2017 that it had held forums for its members to discuss the Torrance incident with the aim of preventing similar disasters.

Following the Torrance disaster, the report said the refinery operator updated operating procedures to address the potential failure of the slide valve but did not note any action taken to prevent the valve's failure.