Initially, that caused a big plume of smoke to shoot into the sky, and also resulted in nearby evacuations and a shelter-in-place order on Friday morning.
Since Friday, on-and-off flaring has continued at the refinery.
Valero has blamed the power outage on PG&E, which said it was caused while crews were completing upgrades.
Original post from May 5:
Benicia authorities have lifted a shelter-in-place order at two elementary schools after a power outage at the city's Valero refinery early Friday morning, which led to flaring that sent black plumes of smoke into the air.
The order was lifted at approximately 1:20 p.m. at the Matthew Turner Elementary and Robert Semple Elementary schools, based on the results of air monitoring by the U.S. Environmental Protection Agency. Both schools are upwind from the refinery.
No students showed any signs or symptoms of health problems, said Charles Young, Benicia Unified School District superintendent. He said parents opted to keep many of their children at home.
Officials also ordered the evacuation of businesses near the plant, and the mandatory evacuation order for the industrial park remains in place, said Benicia Fire Department Chief Jim Lydon during a press conference.
He said two people had reported respiratory effects, but neither were transported to the hospital.
The outage, which began about 6:40 a.m., lasted for 18 minutes and took place as PG&E crews were conducting a "routine switching of an electrical transmission" device, said a spokesman for the utility.
The outage led to flaring at the refinery, officials said. Flaring is a process that allows the refinery to relieve pressure, but it can send out smoke and toxic gas.
At about 7:50 a.m., warning systems were activated in the city.
But some residents, like Marilyn Bardet, are concerned that warning systems were not triggered quickly enough -- and some did not hear sirens that went off around the city.
"The precautionary principle would’ve said, turn those sirens on as soon as you know that something dreadful is happening with that amount of flaring," Bardet said.
Benicia police advised all other areas of town to shelter in place, keep doors and windows closed, and bring pets inside.
Police also advised any business or structure downwind from the refinery to evacuate and avoid the area.
Local air regulators were on scene and investigating, including the Bay Area Air Quality Management District.
During the incident, toxic gases from the flares, such as hydrogen sulfide, peaked at more than 10 times normal background levels for the area, said Steve Calanog, a spokesman for the EPA.
"The flaring operation is designed to keep our employees and our neighbors and the community safe," said Don Cuffel, director of health, safety, environment and government affairs for Valero. "And they’re doing exactly what they need to do by burning the hydrocarbon that cannot be safely stored."
Valero said it's been 30 years since the last time the refinery lost total power.