* Worker undergoes surgery for injuries
* Chemical Safety Board launches probe
* Other units operating at refinery
(Updates worker condition, investigations)
HOUSTON, July 20 (Reuters) - A fire continued to burn in an alkylation unit at Citgo Petroleum Corp's 163,000 barrel per day (bpd) refinery in Corpus Christi, Texas, on Monday, while other refinery units were still operating, a company spokesman said.
A worker, Gabriel Alvarado, 34, severely burned when the blaze broke out on Sunday morning underwent surgery at a San Antonio hospital on Monday, said sources familiar with his treatment.
The U.S. Chemical Safety Board said on Monday it was sending investigators to the refinery. Investigators from the U.S. Occupational Safety and Health Administration, U.S. Environmental Protection Agency and Texas Commission on Environmental Quality were already in Corpus Christi.
The fire in the hydrogen fluoride alkylation unit, which makes high-octane gasoline blending components, was contained by water cannons that surround the unit. Residual feedstock left in the unit was the source of the continuing fire.
A Chemical Safety Board spokesman said the board was most concerned about the release of hydrofluoric acid, which can cause burns, heart, lung and bone damage in humans.
"We take any incident in an HF alkylation unit very seriously," Horowitz said. This incident seriously injured one person. We don't have any evidence of injuries due to hydrofluoric acid, but we do plan to determine how much HF was released."
Citgo said its focus was assisting Alvarado and his family.
"Our primary concern right now is for the recovery of Gabriel and to support his family in their time of need," said Kevin Ferrell, vice president and general manager of the Corpus Christi refinery.
The president of the local United Steelworkers union, which represents workers at the refinery, said on Sunday employees had repeatedly pointed out problems to Citgo.
"The workers at this refinery have identified hazards the company has failed to fix," said John Warner, president of USW local 13-1647. "We have met with Citgo and told them there were problems and if they weren't fixed it was going to get bad. And it got bad."
A Citgo spokesman declined to comment about Warner's statement.
The U.S. Occupational Safety and Health Administration has issued 40 citations for violations of work safety regulations at the Corpus Christi refinery since 1999.
Citgo is the U.S. refining and marketing subsidiary of Venezuela's state-owned oil company Petroleos de Venezuela SA.