Abstract: A 24-inch diameter crude oil pipeline was going to be rerouted as alternative relief for a 124,000 gallon storage tank which was going to be inspected and tested in accordance with API 653 requirements. The employees had drained the crude oil from the line and locked it out. Then, they cold cut the pipe to prepare to connect a "T" in the line to divert it. After making the cut, the original line was filled with bentonite clay to build a barrier between the opening and the remaining length of pipe. Continuous monitoring of the atmosphere was performed to ensure that hot work could begin. The two employees who entered the excavation to perform the welding were both wearing Nomex clothing as well as a welding hood. Employee #1, the welder, and Employee #2, a helper, entered the excavation to begin welding the "T" into position. As they began, after about two to three minutes, a fire ball erupted from behind the clay wall. The flash fire caused Employee #1 to get burned and he was hospitalized. Employee #2 was also burned but was released after staying in the hospital overnight. The other three employees on site also received minor burns and were sent to the hospital, treated, and released.