UPDATE 1-Fire at Exxon's Beaumont, Texas, refinery injures 12 workers
* Exxon says fire extinguished on shut unit
* Six contract workers taken to area hospitals (Combines urgents, adds background)
HOUSTON, April 17 (Reuters) - Twelve contract workers were hurt on Wednesday morning when a fire broke out at a unit undergoing repairs at Exxon Mobil Corp's 344,500 barrel per day (bpd) Beaumont, Texas, refinery, a company spokeswoman said.
The fire was at a shut hydrotreater heat exchanger, according to sources familiar with operations at the refinery.
Exxon did not identify the unit but confirmed the fire broke out at 10:30 a.m. CDT (1530 GMT) on a shut unit undergoing planned maintenance work at the refinery. The blaze was quickly brought under control and extinguished, said company spokeswoman Rachael Moore.
Six of the 12 workers were taken to regional hospitals "for further medical evaluation and treatment," Moore said.
The sources said the workers were thought to have been welding on a section of the heat exchanger when the fire broke out.
All other employees and contractors working at the Beaumont refinery have been accounted for, Moore said.
The hydrotreater is part of the refinery's system for removing harmful substances from feedstock so the motor fuels produced will comply with federal environmental regulations. It uses hydrogen under high pressure in the process.
A heat exchanger heats feedstock going into a unit and cools product coming from a unit by having them pass close to each in dense system of piping.
In 2010, a hydrotreater exploded at Tesoro Corp's Anacortes, Washington, refinery, claiming the lives of seven workers. (Reporting by Erwin Seba in Houston and Naveen Arul in Bangalore; Editing by Gerald E. McCormick and Marguerita Choy)