The blast happened in a processing unit at the refinery in the 3700 block of 190th Street at 8:50 a.m. The explosion was followed by a ground fire, which was quickly knocked down.
The explosion was so strong that it registered as a 1.7-magnitude earthquake on a nearby seismometer operated by Caltech and USGS.
Employees say they first noticed a smell and then heard and felt the explosion.