The leak occurred at the hydro-isomerisation unit of a refinery. Following the observation of a pressure drop between two heat exchangers, controls were carried out on site. A noise was heard, but no gas detection was activated on the unit. A hydrogen leak was detected but the source was not identified. Following the detection of hydrogen, the access to the unit was forbidden and emergency services and the fire department alerted. The unit was decompressed using the safety flare. When the pressure was sufficiently reduced, two operators looked for the origin of the leak. They located it on a flange at the level of a heat exchanger/beam gasket. The line was depressurized under nitrogen.The pressure drop was accompanied by a rise in temperature between the 2 exchangers, causing fragility to the flange assemblies. The hydrogen leak is estimated at 100 kg.The bundles of the 2 exchangers were dismantled. It was then noticed that the baffles were mounted upside down on the 2 exchangers (baffles not mounted in the direction of fluid flow: baffles located on the outlet and not on the inlet of the load). Maintenance on these exchangers had been carried out during the previous major shutdown. The deflectors were deformed at the tube plate. In addition, an imprint of the outlet line of the heat exchanger on the deflector was noted. The deflectors are repositioned in the good direction and reintroduced in their respective heat exchanger.