Tesoro Corp. blocked the U.S. Chemical Safety Board from investigating an incident at a Northern California refinery and downplayed the extent of workers’ injuries, the board said.

Tesoro, the U.S. West Coast’s largest refiner by capacity, refused to allow agency investigators to return to the site of a Feb. 12 chemical spill at the company’s Golden Eagle refinery in Martinez, the CSB said in a letter to Greg Goff, Tesoro’s chief executive officer. The company refused to preserve the accident site, prohibited certain interviews and indicated it wouldn’t comply with document requests.

Tesoro representatives didn’t immediately respond to requests for comment about the letter after normal business hours. The company said previously that it cooperated with CSB investigators.

“We have provided the agency documents, allowed the agency to inspect the incident location, and facilitated interviews with knowledgeable refinery personnel,” Megan Arredondo, a spokeswoman at company headquarters in San Antonio, said by e- mail Feb. 21. “We also asked the agency for a written basis for their authority to investigate and we have not received a response.”

The CSB is an independent agency that conducts root-cause investigations of chemical accidents at fixed industrial facilities. It doesn’t have authority to levy fines or penalties.

Two workers suffered first- and second-degree burns when their bodies were sprayed with sulfuric acid while putting a sampling station back in service at an alkylation unit at the 170,000-barrel-a-day refinery, CSB members said in the letter.

Unit shut

The refinery shut the unit, which makes high-octane blendstock for gasoline, after the incident. The unit remains shut until the California Division of Occupational Safety and Health, known as Cal/OSHA, approves a safety review and allows the restart, Erika Monterroza, an agency spokeswoman, said by e- mail Feb. 20.

Tesoro said the incident is clearly within Cal/OSHA’s jurisdiction. It questioned the CSB’s decision to investigate.

“We were surprised when the CSB notified the company that the agency intended to deploy a team to investigate, as the CSB is not charged with investigating a personal safety incident that did not result in serious injuries or substantial property damage,” Arredondo said Feb. 21.

Arredondo later said a personal safety incident involves “working surfaces, ladders, the use of personal protective equipment, and other things that may result in an individual injury.”

‘Inaccurate claim’

CSB members said Tesoro presented the “inaccurate claim” that the injuries were less than serious.

“Acid splashing on workers’ unprotected faces or other parts of the body, resulting in first- and second-degree burns requiring air evacuations to a hospital burn unit, treatment, and subsequent significant lost time at work, absolutely constitute serious injuries,” board members said.

The letter was signed by CSB Chairman Rafael Moure-Eraso and board members Mark Griffon and Beth Rosenberg.