This incident occurred at a linear alkylbenzene manufacturing plant. It affected dehydrogenation reactor of normal paraffin, during the operation of catalyst exchange in the reactor. This is usually happening by isolating the reactor and its peripheral part from the slightly pressurised section by closing a 20-inch remotely controlled valve. The hydrogen pressure of the peripheral part was set at 20 KPa, and the reactor was opened to the atmosphere. Assuming the possibility of small hydrogen leakage, a vacuum device was used to evacuate the piping and a nitrogen purging was performed. After the exchange of the catalyst, the piping section had to be reconnected to the reactor. To this purpose, a top flange with an elbow was mounted, and a nitrogen purging of the reactor and piping started. At this moment hydrogen leaked into the piping and flames spouted from the clearance of the flange.The detailed incident development: 1st day, 05:30 on the LAB plant was shut down for a catalyst exchange.13:10, the remotely controlled air-driven valves at the inlet and the outlet of the dehydrogenation reactor were closed. The top and bottom piping of the reactor was kept at 20 KPaG with hydrogen gas.2nd day, the reactor was left in a sealed condition at 40 KPaG with nitrogen gas.09:00 on April 14th, the reactor depressurized, the top cover (the inlet side) was removed, and the blind flange (a partition board installed at the end of the piping) was set at the end of the inlet piping, and evacuation started with an ejector.11:00, catalyst extraction started.14:00, catalyst extraction finished.16:00, new catalyst filling finished.16:10, the blind flange was detached.16:30, the top cover of the reactor was mounted, and substitution of air in the reactor with nitrogen started.16:40, Flames spouted out through the flange, and two plumbers got burnt during tightening flange bolts.