In a petrochemical plant, visual inspection detects fumes under the heat insulation of a heat exchanger operating under hydrogen pressure. The first intervention team was mobilized and the unit stopped for depressurization and cooling. A risk analysis was conducted before erecting a scaffold, decalcifying the exchanger and inspecting it under safe conditions. After having defined the fire protection and fire prevention strategy, the operator decided to put the unit back under pressure. During this rise in pressure, a resumption of fire on the flange justified the triggering of the emergency pln. The intervention team managed the fire. No injuries or impacts outside the site are to be deplored. At the time of the accident, the plant was shut down due to a staff strike. The unit, usually operated under a mixture of butane and hydrogen, according to the supplier's recommendations, was filled with hydrogen only.