Abstract: At approximately 9:30 a.m. on July 14, 2010, Employees #1 and #2 were installing a big hole and little hole blind flange between a gate valve and a 30-in. pipe containing coke oven gas. The work was part of an attempt to isolate the Preheater from the underfire system for cleaning. When the contractors removed the spacer ring connecting the two flanges, coke oven gas was released. Shortly thereafter, portable and fixed alarms began to sound, warning of potential hazards. Subsequently, an ignition source was found and an explosion occurred. Following the explosion, US Steel shutoff the supply of gas by closing and placing a water seal on top of the Pad valve located outside the battery. A nitrogen truck was brought in and used to purge the under fire system. As a result of the explosion, Employees #1 and #2 sustained first, second, and third degree burns as well as other unspecified serious injuries. The employees were hospitalized and treated for burns. Coworkers were then able to enter the battery wearing supplied air respirators and tighten the bolts securing the blind flange. This was a feasible option prior to the explosion instead of the procedure attempted that morning. The failure to follow recognized energy control requirements contributed to the incident.