The leak was reported on the 1660 block of West Anaheim Street around 10:15 a.m.
According to Los Angeles City firefighters, at least 11 employees were transported in fair condition to local area hospitals for minor respiratory issues.
Other workers were told to shelter-in-place inside buildings.
The problem was localized within the grounds of the refinery around 11 a.m. LAFD officials said there was no active leak and that the incident did not pose immediate danger to the surrounding community.
Air monitoring showed no health threat shortly after the accident, and none of the workers were admitted into a hospital.
"All other refinery employees and contract workers have been accounted for and are safe. The area has been secured and the refinery is running under normal operations," said Phillips 66 spokesman Rich Johnson.
CORRECTION: The location was initially reported as ConocoPhillips refinery. Eyewitness News has since learned the refinery is now owned by a new company called Phillips 66.