RICHMOND (CBS SF) — A “process upset” at Chevron’s Richmond refinery Monday morning triggered a flaring incident that forced the evacuation of less than 100 workers from the plant, according to Governor’s Office of Emergency Services.

The employees were evacuated as a precaution after the flaring began around 4:24 a.m. They were allowed to return later in the morning and the flaring was no longer visible. Residents living near the plant were not impacted.

A large black plume from refinery could be spotted for miles. Neighbors say it’s a sight that is becoming all too common.

Contra Costa County health officials said they had dispatched a hazmat team to the refinery to monitor the air quality and determine if more evacuations or a shelter-in-place is necessary.

Some Richmond residents received a level one warning, the lowest on the community warning alert scale .

The mayor of Richmond said he got the refinery warnings early Monday morning, but for others who live a few miles away from the refinery, the only warning they got was when they looked out their window.

Residents here say flare-ups are becoming all too routine .

“The refinery is right there …you can see how huge it is,” says Pennie Opal Plante, who lives 6 miles from the refinery. Around 9:45 a.m. Monday morning, she was on her backyard deck. She saw the flare-up and took video of it from her iPhone.

“There is black smoke coming out of the stack. There is also black smoke dissipating to the south. It always releases toxins, “ said Plante.

She says it’s what she smelled that worries her more.

“My nostrils started to and I could feel it in my throat . It was an acidic smell and taste. “

The Flaring was first reported internally at 4:26 a.m., forcing the evacuation of less than 100 workers who were inside the refinery at the time .

According to the Office of Emergency Services the plume of black smoke drifting through the air was identified as hydrogen sulfide .

Chevron issued a statment:

“Flaring is an important part of keeping the refinery running safely. Flares are highly regulated safety devices designed to relieve pressure during the refining processes and help keep our equipment and plants operating safely.”

Rob Dove works near the refinery and says every Wednesday the city does a practice drill.

“You can hear this alarm system – it’s huge. City alarm system. It’s a big pole outside so there is no escaping just how loud this thing is.”

He says even small incidents like the one that happened on Tuesday, are important to know about.

“We pay close attention to it because we also work at a chemical manufacturing facility that is downwind from that area and we have had it happen before when they have had major events where we have had to keep everybody inside “

Air District regulations require Chevron to submit a Flare Causal Report that details the root cause of each incident . Chevron has 60 days from the end of the incident month to provide reports to the Air District.