A crew of three Bassler Energy Services employees were about to clean out a heater treater vessel bottom. The vessel had been depressurized and its contents had been evacuated below the manway. When the crew opened the vessel to begin pressure-washing the manway face, fumes ignited and the resulting flash fire burned two of them. One suffered burns to the face, cheeks, and neck; the other suffered burns to the hands. One employee was hospitalized.