Abstract: Employee #3, #4, #5, and #6 were preparing to remove an aftercooler unit from a solvent recovery system, manufactured by Dedert Corp., for maintenance. Employees #3, #4, and #6 were approximately 17 ft 7 in. above the adjacent ground level, removing the bolts from the aftercooler unit. Employee #4 was using an oxygen and acetylene torch to cut off bolts that were difficult to remove. Employee #3 was on firewatch on the ground level. Employees #1 and #2 were watching the work from ground level, directly in front of a blower unit that was part of the recovery system. When the crane operator put tension on the valve for support to remove the unit, a seal was broken on the butterfly valve that separated the aftercooler from the main duct to adsorber beds. Employee #4 lit the torch and a fireball explosion occurred, injuring the six employees. Employee #1 and #3 sustained 2nd- and 3rd-degree burns and were in critical condition. Employees #2 and #4 sustained 2nd-degree burns and were in serious condition. Employees #5 and #6 sustained minor injuries. Improper purging of the system allowed toluene and lactol spirits vapors to remain and accumulate. Hot work permit procedures were not followed. The system was not effectively isolated from the hot work. Five of the six valves between the aftercoolers and adsorbers were open.