Fire department officials say a carbon steel vent plug failed because of a chemical reaction.
This led to extreme pressure and high temperatures which caused the fire in a coker unit pump house.
No one was injured in the blaze, which was the third significant fire at the facility in 18 months.
In October 2011, corrosion in a pipe triggered a fireball that sent seven contract workers to hospital and forced 1,400 workers from the facility.
Last May, a much smaller fire occurred when an overheated crude oil pump ignited. (CJME)