(Reuters) — An explosion set off a huge fire at Valero Energy Corp.'s 225,000 bbl/d Texas City, Texas, refinery on Thursday afternoon but the fire was quickly contained, according the City of Texas City Emergency Management office.

No injuries were reported and the blaze, which was burning light hydrocarbons, was contained about an hour and a half after it broke out, Texas City Emergency Management said.

Sources familiar with plant operations said the explosion at about 6 p.m. EDT was on a 12,000-bbl/d alkylation unit. It was unclear if other units had been affected.

Alkylation units convert refining by-products into high-octane components blended into gasoline.

Valero spokeswoman Lillian Riojas said the company was co-operating with local, state and federal agencies in responding to the fire, which was being fought by the refinery's firefighting team.

"At this time, Valero has very little information as to the cause of the fire," Riojas said. "Our primary concern is for the safety of our workers."

Firefighting units from Texas City and the neighbouring Marathon Petroleum Corporation refinery were called to the Valero plant, the sources said.

Workers at the Valero and Marathon refineries, located near each other on the south side of Texas City, were told to shelter-in-place, though no such order was issued for the entire city, 68 kilometres south of Houston.

The explosion was heard eight kilometres away from the Valero refinery and rocked buildings within 1.6 kilometres of the plant, according to local media reports.

Texas City was the site of the deadliest industrial explosion in U.S. history when on April 16, 1947, a ship carrying more than 2,000 tons of ammonium nitrate blew up, setting off blasts on other ships and nearby oil storage tanks and killing 581 people.

The deadliest U.S. refinery explosion in the 21st Century took place on March 23, 2005, at the BP plc Texas City refinery (now owned by Marathon), killing 15 workers and injuring 180 people.