RICHMOND (CBS SF) — Black smoke hanging over the East Bay Wednesday evening was the result of flaring in a unit at the Chevron Refinery in Richmond, officials said.

Chevron said the flaring started around 7:30 p.m and was taking place at a process unit due to an upset. Refinery employees promptly handled the cause and the flaring stopped before 9 p.m.

Chevron notified the Contra Costa County Fire Department and issued a Level 1 Community Warning System to keep residents informed.

A Chevron official said flaring is an important part of keeping the refinery running safely.