An operator at ExxonMobil's Baton Rouge refinery whose disassembly of a pipe valve triggered a fireball that severely injured four workers was following "accepted practice" in stripping down the malfunctioning valve, the Chemical Safety Board said Wednesday.

But the particular malfunctioning valve the workers were trying to open — unlike roughly 97 percent of pipe valves in the sprawling plant — was a "30-plus-year-old design," which meant pulling it apart "can have catastrophic consequences," the board, a federal agency that investigates industrial chemical accidents, says in a three-minute video detailing the fire.

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When the operator tried to open the disassembled valve, it "immediately failed and came apart," releasing a cloud of pressurized isobutane, a highly flammable liquefied petroleum gas created during oil refining, according to the video. The cloud ignited when it reached welding equipment about 70 feet away and erupted into a large fireball that severely burned four people — one ExxonMobil employee and three contractors — working in the area.

The final report on the Nov. 22 fire is being reviewed and will be released "in the near future," Vanessa Allen Sutherland, chairwoman of the Chemical Safety Board, said during the board's meeting Wednesday.

Stephanie Cargile, a spokeswoman for ExxonMobil's Baton Rouge operations, declined to comment directly on the Chemical Safety Board's video but said an internal investigation by ExxonMobil identified the cause of the incident and the company has "taken appropriate corrective action."

"Refinery management will continue to evaluate human factors associated with equipment design to mitigate identified hazards," Cargile said in an email. "We will evaluate and update procedures and training as appropriate."

The Chemical Safety Board's video provides a detailed look at the events leading up to the blast but doesn't disclose the conclusions drawn during the agency's investigation. Sutherland, the chairwoman, said the board's recommendations will likely address equipment design and ensuring adequate operating procedures are in place.

A Chemical Safety Board official made clear during Wednesday's meeting that a worker's decision to manually open the valve wasn't the root cause of the fire, and the agency will identify a number of underlying factors in its final report.

In May, the federal Occupational Safety and Health Administration issued ExxonMobil preliminary fines of nearly $165,000 in the explosion. Eight of the nine safety violations noted by OSHA investigators were deemed "serious" while the ninth, for failing to inspect piping in the unit, was termed a "repeat" violation because of similar lapses at an ExxonMobil facility in Texas.

ExxonMobil is contesting the proposed OSHA fines, which were first reported publicly earlier this month.

Copies of the citations, which were obtained by The Advocate after reporting the fine, suggest federal regulators were alarmed by the lack of worker training or instructions on how to deal with a malfunctioning valve. For each of the eight "serious" violations, OSHA levied the maximum fine of $12,675. The more severe "repeat" violation, meanwhile, resulted in a larger $63,375 proposed fine, half of the maximum.

Cargile declined to specify which aspect of each citation the company is contesting.

In the moments before the explosion, workers were trying to open a plug valve in order to put a spare isobutane pump into service, but found the hand wheel and gear box used to open it weren't working, according to the Chemical Safety Board.

Taking off the gear box to open the valve with a pipe wrench was "an accepted practice in the akylation unit," the Chemical Safety Board said, and one of the workers unscrewed four bolts that connected the gearbox and its support bracket to the valve body.

"Unknown to the operators, these bolts also secured a critical pressure-containing component of the valve," the board said, something true of only some 3 percent of valves at the refinery. The other, newer valves are designed in ways that make it "less likely to remove the incorrect bolts."

Once the valve failed and isobutane began rushing out, the two workers warned other employees in the area to flee.

A February report by the Louisiana Department of Environmental Quality identified the disassembly of the malfunctioning valve as the trigger of the fire. But unlike the Chemical Safety Board video on Wednesday, the earlier report does not discuss whether taking apart troublesome valves is common practice.

"Hand wheels may not respond when turned, and there are several alternative ways of opening and closing a plug valve," Cargile, the ExxonMobil spokeswoman, said in an email at the time. "In this case, however, the assistant operator removed the support bracket with hand wheel and gear box still attached rather than just removing the hand wheel and gear box."

The Chemical Safety Board video and DEQ's February report include conflicting figures on just how much isobutane was released during the incident. In the hours after the fire, ExxonMobil officials told State Police as much as 2,500 pounds of butane and isobutane had been released.

But DEQ later concluded that far less — about 600 pounds — was released. The Chemical Safety Board, in the video released Wednesday, says approximately 2,000 pounds of isobutane escaped the pipe.

The Chemical Safety Board is also working on a "full safety video" addressing the Baton Rouge refinery fire that will be released shortly after the report, Sutherland said.

The board investigates accidents, and its reports highlight recommendations for safety improvements, but unlike OSHA and DEQ, it does not issue fines or craft industry regulations.

Follow Bryn Stole on Twitter, @BrynStole.