HOUSTON (Reuters) - The U.S. Chemical Safety Board said on Wednesday that the events leading to a 2016 blast at ExxonMobil Corp’s Baton Rouge, Louisiana, refinery began when an operator disassembled a stuck valve on the alkylation unit, releasing a cloud of isobutane vapor.

Disassembling stuck valves was an “accepted practice” at the refinery, the board said in a video posted on its website on Wednesday. The vapor cloud exploded after reaching a welding unit 70 feet away that provided an ignition source.

Exxon spokesman Todd Spitler declined to comment on the video.

“We deeply regret the Nov. 22, 2016, incident at the Baton Rouge refinery and its impacts on the injured individuals and their families,” Spitler said. “We will continue to evaluate human factors associated with equipment design to mitigate identified hazards, and will evaluate and update procedures and training.

The U.S. Occupational Safety and Health Administration issued in May nearly $165,000 in fines to the company for safety lapses including inadequate training and equipment maintenance in the blast, which injured an Exxon employee and three contractors. Exxon was appealing the fines.

Safety board Chair Vanessa Allen Sutherland said the final report on the explosion would be released soon.

Sutherland said that among the recommendations the CSB was likely to make was consideration of human factors in equipment design and “making sure there are written procedures in place for operating equipment.”

The valve was supposed to be opened by gears in a gearbox operated by a handwheel, which failed to function on the day of the explosion.

Four bolts were removed to expose the valve stem.

“It was an accepted practice in the alkylation unit for operators to remove the gearbox so a pipe wrench could be used to turn the valve stem,” the board said in the animation.

The stuck valve was of a “30-plus-year-old design,” and only 3 percent of the valves on the alkylation unit were of that design.

Removing the bolts also removed a pressure-containing component of the valve, the board said.

When the valve stem was turned, the valve came apart, releasing 2,000 pounds (907 kilograms) of isobutane vapor into the atmosphere.

The Chemical Safety Board investigates chemical accidents and recommends improvements in operating practices to owners and operators. It has no regulatory or disciplinary authority.

The Baton Rouge alkylation unit uses sulfuric acid to convert isobutane to octane-boosting components of gasoline.