California's Division of Occupational Safety and Health (Cal/OSHA) inspected the refinery shortly after the incident, closed its investigation the same month and decided not to issue any violations.
The lack of punitive action outraged Benicia's mayor and environmentalists.
"No violations of existing rules does not mean we are safe," Mayor Elizabeth Patterson said in an email. "Sleeping on inadequate protection does not make us safer -- doing nothing to correct these deficiencies does not extinguish the risk."
Patterson has been calling for the City Council to develop regulations that would give Benicia more oversight of the refinery, a proposal Valero opposes.
"This report raises disturbing questions about how unprepared Bay Area refineries and PG&E are for electrical outages that can lead to dangerous air pollution," said Maya Golden-Krasner, an attorney with the Center for Biological Diversity.
"Regulators seem reluctant to hold anyone truly accountable for this massive release of pollutants, but what else will prevent something like this from happening again?" Golden-Krasner said. "It's a systemic failure to protect the air we breathe, and it shows why we need to move away from dirty fossil fuels."
The energy giant expressed optimism about the state of the investigations into the outage and reiterated its blame of the entire episode on PG&E, which it has sued, seeking at least $75 million in damages and lost revenue.
"We are pleased that Cal/OSHA concluded there were no violations by Valero arising from the May 5 PG&E power outage nor has Solano County issued any violations to date," said Lillian Riojas, a company spokeswoman, in a statement.
"PG&E caused this outage and significant damages. Valero, like others, is waiting on answers from PG&E, which are still not forthcoming," Riojas said.
PG&E hired Exponent, a third party engineering firm, to conduct a view of the outage. A utility spokeswoman said Tuesday that Exponent's report on the incident has been completed and sent to the California Public Utilities Commission.
"The safety of our customers, employees and the general public is always our top priority," said PG&E's Deanna Contreras in an email. "We continue to partner with Valero and the City of Benicia to prevent similar power disruptions," Contreras said.
Another agency, the Bay Area Air Quality Management District, issued several notices of violation due to the flaring in the days after the outage. Its investigation into the incident is ongoing.
The refinery has two power sources, both operated by PG&E. When the utility put both of those sources offline on May 5, it caused an "immediate and full shutdown of the facility," the Solano County report states.
Valero also has a cogeneration plant, but it does not provide enough power to fully supply the facility. County investigators point out that the plant must maintain a line to PG&E's power circuit to remain online.
That's a problem, according to Eric Smith, associate director of the Tulane Energy Institute, who read the county's report and specializes in oil and gas production.
"The on-site emergency power supply could have been robust enough to allow for an orderly shutdown," Smith said.
The outage led to pressure inside the refinery that had to be relieved by the use of its flaring system. But, the loss of power shut the facility's steam boilers and cooling tower down. That meant the flaring did not operate normally, which led to flames and black smoke shooting out of the refinery, according to the report.
"Their system got overwhelmed," Schmidtbauer said.
Firefighters were brought in. "The dump stack ignited and was extinguished during the first hour of the incident," the report said.
The city's fire department imposed shelter-in-place and evacuation orders for parts of the city. At least a dozen people sought medical treatment for breathing difficulties.
Three days after the initial outage, the refinery underwent another malfunction as it slowly restarted the facility, causing more flaring, this one lasting more than five hours.
Valero initially thought the May 8 problem was tied to the wrong refinery unit, according to county investigators. It turned out the malfunction was connected to its Coker unit, which makes gasoline through the use of high temperatures.
A week later the same unit malfunctioned, leading to yet another round of flaring. This time, it covered cars near MRC Global, a company on Bayshore Road close to the refinery, with an "oil-based" substance.
That second problem was caused by trapped moisture in the piping system as a result of the unit being shut down because of the initial outage.
"The refinery could have done a better job of minimizing subsequent releases that occurred during the restart," Smith said.
The outage led to an increase in the state's gasoline prices, hurt the company's bottom line and damaged one of the refinery's flares.
Recently it has prompted extra scrutiny from the U.S. Environmental Protection Agency into the power issues at the Benicia facility.
The refinery released more than 80,000 pounds of sulfur dioxide on the day of the outage and in the weeks afterward.