A button used for testing an electrical switch was accidentally pressed, cutting off fuel to the boiler unit, according to the root-cause analysis Tesoro filed with the county's hazardous materials program earlier this month.
It's unclear who pressed the button.
"There were staging and electrical crews working in the area during the time of the incident," said James Jeter, an environmental health and safety manager at Tesoro, in the report. "In addition the area is congested and the button is at elbow height."
Tesoro may not have known the button could disrupt operations.
"I think they didn't realize if that button was pressed that it would shut down the whole system," said Randy Sawyer, Contra Costa County's chief environmental health and hazardous materials officer. "Someone could have been working around it, and they could have hit it with their elbow and not even realize they did it."
A boiler that produces steam is located inside the refinery's power plant. That plant connects to equipment throughout the facility.
When the button was pressed, power was shut off to a valve that controls how much gas is brought into the boiler unit. The boiler shut down, stopping steam from driving some of the refinery's compressors and pumps, causing them to turn off.
The high pressure gas in those units then went to the facility's vent system. Because the refinery wasn't able to produce steam, instead of just flaring, Tesoro sent a large plume of smoke into the air.
"It was a vulnerable point," said Eric Smith, associate director of the Tulane Energy Institute, who read the company's report and specializes in oil and gas production.
Initially, Tesoro put a physical covering around the test button to make it harder for it to be accidentally pressed. In January, it removed the button entirely, said company spokeswoman Patty Deutsche in an email.
"No more possibility of human factors for that issue," she said. "We took it one step further."
The Bay Area Air Quality Management District is investigating the incident.