* Seven contractors hospitalized; Four being treated for burns
* Wednesday flash fire hit unit being overhauled
HOUSTON, April 18 (Reuters) - The overhaul of a hydrotreater hit by a flash fire on Wednesday at Exxon Mobil Corp's 344,500 barrel per day (bpd) Beaumont, Texas, refinery has halted while an investigation into the blaze is underway, said sources familiar with refinery operations on Thursday.
Seven contract workers remain in a hospital in Galveston, Texas, for treatment of their injuries, said a spokeswoman for the University of Texas Medical Branch. Four of the men are being treated for burns suffered in the fire.
Five other contractors injured in the blaze were treated at area hospitals and released, Exxon said on Thursday.
The U.S. Occupational Safety and Health Administration has taken control of the site of the blaze and won't release the unit to Exxon's control until it has completed its probe, the sources said.
An OSHA representative was not immediately available to discuss the investigation.
The hydrotreater, which uses hydrogen to remove pollutants from the feedstocks used to make motor fuels, had been shut for an overhaul.
The workers were cutting into a heat exchanger on the hydrotreater when the flash fire broke out. (Reporting by Erwin Seba; Editing by Gerald E. McCormick)