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RODEO — Residents near the Phillips Petroleum Co. refinery were ordered to stay in their homes for about an hour and a half Wednesday afternoon after black smoke began billowing from the facility. 

A loss of power caused part of the plant to shut itself down. As part of that process, it’s programmed to flare and burn off excess material, said Michael Marchiano, of the Contra Costa County Office of Emergency Services. It was unclear what caused the plant to lose power. 

The black smoke was the result of an overflare involving unburned hydrocarbon that spewed from the refinery about 4 p.m., he said. Phillips was working to put the plant back online Wednesday evening. 

Residents of nearby Crockett were asked not to leave their homes unless absolutely necessary, and were advised to stay inside with the windows and doors shut. They also were told to turn off air conditioners and fans, and to close fireplace dampers and vents and cover cracks around doors and windows with tape or damp towels. 

The shelter-in-place order was in effect from about 4:24 p.m. to 5:40 p.m. Officials then advised residents to open their windows and doors and let fresh air inside. 

The release comes during a record-breaking heat wave with a high pressure system that has reduced winds and sent temperatures soaring into the triple digits. Normally, the wind would blow the smoke out over the bay, said Paul Andrews, a county hazardous materials specialist.