The company has been served with 19 citations, 18 of which were for serious offenses that could have resulted in of worker death or serious injury.
Six of these serious violations were classified as wilful because California's regulator, the Division of Occupational Safety and Health (Cal/OSHA), found ExxonMobil intentionally ignored known hazardous conditions and failed to comply with state safety standards
ExxonMobil has 15 working days to appeal the citations to the Occupational Safety and Health Appeals Board.
"Petroleum refineries have the responsibility to keep workers safe, and to also protect nearby communities and the environment," Department of Industrial Relations director Christine Baker said.
"This investigation revealed severe lapses in Exxon's safety protocols."
February's blast was the result of a hydrocarbon release from the refinery's fluid catalytic cracker unit into its electrostatic.
The hydrocarbons ignited inside the precipitator, causing the entire unit to explode.
Eight workers were decontaminated after the incident, and four were sent to hospitals for treatment of minor injuries.
Cal/OSHA's investigation concluded that ExxonMobil's management new about the risks of flammable vapour leakage after an internal audit in 2007 but chose to ignore them.
The company also knew that the spent slide valve on the cracker unit was leaking before the accident occurred and did nothing.
Further, the unit had not been working properly for up to nine years, with no functional pressure transmitter, so the company could not monitor hydrocarbon pressure build-up.
Cal/OSHA issued an order prohibiting use of the FCC unit on February 18, and that order remains in effect.
The agency has investigated ExxonMobil's Torrance facility twice in the last five years for accident-related incidents that resulted in serious workplace injuries in 2011.
In March 2011, a refinery operator was working on the FCC unit, attempting to shut down a failed pump when a motor in the pump suffered mechanical failure and exploded.
The worker suffered a fractured jaw and lost six teeth in the accident.
Another incident in September did not occur in the same cracker unit.
Cal/OSHA issued three serious and five general citations in those cases.
Both the February 18 explosion and the August 2012 Chevron Corporation refinery fire in Richmond have led to proposed improvements in petroleum refinery regulation.
An Interagency Refinery Taskforce, formed in the aftermath of the 2012 Chevron refinery fire, is working to improve regulations and coordinated interagency inspections will reduce risks that can lead to serious accidents.
