Three employees for J&R Transport were onsite when a flash fire occurred at an oil/gas wellsite. J&R Transport was hired to clean heater treater equipment at the wellsite. The heater treater had been removed from the oil/gas well system and was placed adjacent to the well head. A roustabout crew from another employer was onsite and removed the bolts and fire tube from the heater treater. When the tire tube was removed, a black liquid came out of the heater treater and spilled onto the ground and surrounding area. J&R Transport performed vacuum truck operations to remove the liquid from the ground. When that was complete, two J&R Transport employees began power washing the inside of the heater treater. One employee operated the power washer wand while the other employee operated the vacuum truck's suction hose. During this operation, a flash fire occurred inside the heater treater vessel resulting in fire shooting from the open end of the vessel where the two employees were working. One employee was thrown backward and suffered fractures to his collar bone, ribs, and shoulder and burns to face and arms. The other employee suffered second degree burns to his hands and first degree burns to his face. One employee was hospitalized.