A hotwork fire at a natural gas processing plant occurred while installing the 4 inch weld-a-let. Sweet dehydrated natural gas was released and ignited, causing a flash fire that lasted for approximately one second. As a result of the flash fire, hot gases and radiant heat were released through the 4 inch hole, burning the welder’s assistant on the neck and face. The welder’s assistant was transported by helicopter to a hospital for treatment. The hazards associated with welding in and around piping that contains flammable or explosive substances are well established as are the standards for preventing or minimizing fires while welding in such areas.