Two mechanic workers had picked up a work permit for the removal of filters from a hydrogen line. An operator also visited the scene and answered the mechanics question as to whether the line should be de-pressurised and that it indeed should be so. The mechanics replaced the low-pressure side (4 bar) suction strainer at which point they noticed that there was pressure inside the pipe, which they released into the piping leading to the flare. In the afternoon, they carried out a similar filter replacement on the high-pressure side (34 bar). The standard procedure is to remove a flanged pipe section of 50 cm from the piping, after which the suction strainer is pulled out and cleaned. The flange is opened by first removing every second bolt. After that, the remaining bolts are undone. When starting to loosen the remaining bolts, the mechanics noticed that there was pressure inside the pipe. A flow of gas from the opened flange started immediately. When one of the mechanics went to check the pipe pressure from the gauge, the hydrogen was ignited, apparently due to an electrostatic charge. The other mechanic was caught in the flames, and the lower part of his overalls caught fire. He was taken to an occupational health clinic to receive first aid, after which he was taken to a hospital by an ambulance. The burns sustained by the mechanic were of classes I and II, covering about 3035% of his skin.