At 7.33 am, the rupture of a 1″ branch pipe on the discharge tank of a propylene gas compressor in a plastics manufacturing plant caused the emission of a cloud of gas detected by the gas detector system. The reaction was stopped thus causing a pressure drop, and the amount of propylene released was 500 kg. The compressor, equipped with remote-controlled house load operation valves and that was operating at 70 °C and 32 bar, was immediately isolated. The technicians present wearing SCBA could then get closer to identify the device in question. The situation returned to normal 10 min later. There were no direct consequences, and only the damaged unit was shut down and restarted the next day after blowing down the installation, rinsing and regasification. Examination of the broken branch pipe confirmed that the rupture was in fact caused by mechanical fatigue, i.e. by progressive cracking. The examination of the fracture surface showed that an initial cracking phase had occurred several years earlier. Progressive cracking had then continued until total rupture of the branch pipe through ductile fracture, the remaining section of the tube could no longer stay in the elastic range. The blowdown pipe generated continuous mechanical vibration, with the embossing on the equipment acting as a recess creating a local stress zone which led to generating the fatigue fracture phenomenon. The design of the pipe was unsuitable for significant exposure to vibrations. The inspection organised on the pipe was also inappropriate: the inspection drawings of the “small branch pipes” that had been produced until then did not take into account fatigue cracking at the thread roots, which is specific to a screwed assembly. The thread roots had not been checked. Following this incident, the screwed branch pipe was replaced by a slip-on assembly. All of the branch pipes subject to vibrations were identified; the screwed assemblies were replaced where possible by a slip-on assembly or, failing this, the threaded tube was replaced by a new thicker tube; assemblies installed on equipment subject to surveillance by the inspection authorities for classified facilities were monitored in the inspection plans, and other equipment was monitored in the maintenance plans. Furthermore, the operator planned to modify this compressor and another neighbouring compressor, as quickly as possible, involving the reinforcement of this assembly using an additional weld bead on the tank, and connecting it to the torch system using hose connections isolated by valves on the compressor and torch system side, so as to eliminate a fixed point that was detrimental in environments subject to vibrations.