A crew of three employees had just repaired a heater/treater water leg 3" vitriolic coupling and had pressured it up with a pump. A leak appeared at a bolt on the unit and the two injured employees proceeded to tighten it. While the two were tightening the bolt, they heard a pop and something gave away. They were drenched with liquid and gas. The third employee saw this and shut the pump down and was going to isolate the unit when it caught fire near the coupling. The fire spread to the two drenched employees. One employee suffered third degree burns while the second employee suffered second degree burns.