Abstract: The employees of two subcontractors were 19 ft above floor level, soldering a condensate line to the vent of two 36,000 lb combustible liquid storage tanks that were in use. One employee was using an acetylene turbo torch. Ink varnish that had just been transferred into the tank was approximately 45 degrees Fahrenheit above its 215 Fahrenheit flash point. Twenty minutes later, during the soldering process, flammable vapors produced by the elevated temperature of the stored combustible liquid (which was mixed elsewhere in the plant) were ignited, causing a flashback in the tank and an explosion. Employee #1, who had not been provided with fall protection, was thrown from the top of the tank to floor. Employee #2 was on a mechanical lift that was next to the tank. Several employees of the link company were affected by the blast and were hospitalized. No precautions had been taken or attempts made to eliminate ignition sources. The tanks were not emptied or inerted. Employees of both subcontractors were unaware of the hazards of combustible liquids and vapors at elevated temperatures.