Two contractor employees were working with customer personnel to install 6" blind skillets between a natural gas valve and flange couplings. While attempting to spread flange couplings apart to install the blind skillet, gas began leaking from the flange separation. A contractor employee attempted to seal the leak by pumping grease into the 6" valve fitting using a battery-operated grease gun. While doing so, the escaping gas ignited and the fire engulfed the workers. One employee was hospitalized for burns to the arms, face, and the back of their neck.