Abstract: Employee #1 and coworkers on site were replacing piping between the pressurized separator and the earthen overflow pit at an oil/gas well. The workers had already loosened a hammer union near ground level and removed that riser on the east side of the separator. Employee #1 went around to the west side of the tank and hit the hammer union at the top of the next piping to be removed. This union was directly above the tank and as he tapped it, the tank began to leak gas. Employee #1 immediately tapped it closed again. The coworkers did not know that there was a ruptured disk within this union and that it had been disturbed. Moments later the disk blew, the pipe threads sheared off, and a section of piping over 50 ft long struck Employee #1 in the head and killed him. The section of piping then flew 50 to 60 ft out into the field.