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.
Exxon Mobil said in a statement that four contractors were transported to Long Beach Medical Center for minor injuries, and everyone was accounted for.
PHOTOS: Explosion at Exxon Mobil refinery in Torrance
Some area residents reported feeling an earthquake-like jolt.
"We just heard this loud bang. We saw the walls cave in," said witness Daniel Wolfe. "It was kind of scary. We thought it was an earthquake, but it was too quick."
When the explosion happened, the system went into an immediate burn off, shooting flames 200 to 300 feet into the air and putting out extremely black smoke.
Residents saw ash falling in the neighborhood following the blast.
Torrance police said air quality readings are normal, but area residents were advised to shelter in place temporarily. The shelter-in-place order has since been lifted.
Weird ash like substance falling in Torrance! What's up?? pic.twitter.com/UjVcvDnhEA— PVReiki (@PVReiki) February 18, 2015
The Torrance Unified School District advised schools to shelter in their students. A total of 13 schools were affected, and parents have been notified.
Del Amo Boulevard was shut down between Maple Avenue and Crenshaw Boulevard.
A fire official said the product involved in the explosion was gasoline. The cause of the blast was not known.