A significant portion of Shell's Martinez refinery lost power on Monday afternoon, leading to a loud flaring operation that sent flames and black smoke into the sky, prompting an hours-long health advisory.
To relieve pressure the facility pushed close to 39,000 pounds of gas, which included hydrogen sulfide, to its flares, the company said in a preliminary report sent to Contra Costa County officials on Thursday.
As gas was pushed through the refinery's flaring system, nearby residents and first responders could hear it, according to Randy Sawyer, the county's chief environmental health and hazardous materials officer
"It was a high pressure system with a lot of gas going through at one time that the whole thing rumbled," Sawyer said in an interview. "People could hear it off-site."
The outage began at 1:15pm on Monday, according to Shell. One of the three main electrical substations that feeds power to the refinery "tripped", the company's 72-hour report said.
The brief loss of power led to the shut down of several sections of the refinery including a unit that processes light oil and in turn, prompted flaring.
"The multiple units shutdown caused flaring at the LOP and FXG flares, which resulted in multiple odor complaints from the community," wrote Ha Nguyen, the refinery's process safety manager, in the report.
Contra Costa Cost Health Services issued a health alert an hour after the outage began. The alert was aimed at keeping people with preexisting respiratory conditions away from the refinery.
Shell did not disclose a chemical breakdown of its gas release - that information is expected to come out in a more in-depth report to be released next month.
"It would be good to know how much sulfur dioxide was created," Sawyer said, noting that when hydrogen sulfide, which smells like rotten eggs, goes through a refinery's flares it burns, producing sulfur dioxide which has a "burnt matches" smell.
People near the refinery were smelling a strong odor of sulfur dioxide that afternoon, Sawyer said.
Short-term exposure to sulfur dioxide can harm the human respiratory system and make breathing difficult, according to the U.S. Environmental Protection Agency. People who suffer from asthma are particularly sensitive to the effects of the gas.
In its report, though, Shell said it couldn't detect high levels of the chemicals on and near refinery property. Shell's ground level monitors located on the facility's fence-line showed no detection of sulfur dioxide and hydrogen sulfide "above background levels", the company's report said.
On Monday the Bay Area Air Quality Management District launched an investigation into the outage and its impacts.