The massive April explosion at the Husky refinery in Superior could have been caused by a worn valve in a critical area where oil is transformed into gasoline, according to a federal analysis released Thursday.
The explosion caused a raging fire that burned for hours and prompted entire neighborhoods in Superior to evacuate the city for hours. Eleven workers were hurt, and 27 other people also sought medical attention.
The U.S. Chemical Safety Board and Hazardous Investigation Board on Thursday released a preliminary report on the incident, saying that air could have inadvertently flowed backward through the valve in question and mixed with flammable hydrocarbons, sparking an explosion.
The Chemical Safety Board, which investigates accidents and makes recommendations but issues no fines, termed Thursdays report a factual investigative update. The boards final report isnt likely to be completed until spring at the earliest.
At around 10 a.m. April 26, an explosion ripped through the refinery owned by Calgary-based Husky Energy. Debris from the blast flew about 200 feet and punctured a nearby steel tank filled with asphalt, spilling over 15,000 barrels, according to the report. Two hours later, the asphalt a product of the refining process caught fire.
Fortunately, neither the explosion or fire breached a tank filled with hydrogen fluoride, a highly toxic chemical used at some oil refineries that can vaporize into a deadly gas cloud.
The explosion occurred in the refinerys fluid catalytic cracking unit, which converts hydrocarbons in petroleum into various products, particularly gasoline. On the day of the explosion, the catalytic cracker was being shut down for routine maintenance.
Workers on the project were on break when the explosion occurred, and had moved from ground zero some to blast-resistant buildings, the report said.
At the heart of a fluid catalytic cracking unit are two components respectively called a regenerator and a reactor.
It is important to prevent air in the regenerator from mixing with hydrocarbons in the reactor and downstream equipment because of the potential for such mixing to create flammable (explosive) conditions, the report said.
Valves are used to maintain that separation. However, at times during the refinerys maintenance shutdown, conditions existed that could have allowed air from the regenerator to flow backward through a valve and into the reactor, the report said. Evaluation of the valve in question revealed internal wear.
Also, iron sulfide deposits in the catalytic cracker could have provided a source of ignition if exposed to air.
During the maintenance shutdown, Husky had been planning to treat equipment with chemicals to get rid of any iron sulfide deposits. However, at the time of the blast, the treatment hadnt yet been done, the report said.
The refinery isnt expected to reopen for at least 18 months, Husky said in a recent earnings report. The fire and explosion resulted in $27 million in damages and another $53 million in unspecified costs related to the incident.
The Superior refinery was built in the 1950s and was owned for years by Murphy Oil. In 2017, Husky bought the refinery from its then owner, Calumet Specialty Products.
Mike Hughlett 612-673-7003