On the first polyethylene production unit in a polymer production plant, fire broke out subsequent to a break in the connecting rod assembly of the hyper compressor. Gas (ethylene + air) content in the cylinders was released into the atmosphere and exploded, with ignition most likely being sparked due to friction of the various metal parts. The 500 litres of oil contained in the hyper compressor then ignited as well. A technician was slightly injured while exiting the stricken zone. The alarm was sounded by control room technicians 2 or 3 min after the explosion. On-site safety personnel set up a fire hose battery and relied on a powder extinguisher to contain the blaze, which was achieved 25 min after the explosion; external emergency teams thus did not need to intervene. The installation’s safety features responded appropriately: detection of vibrations exceeding the threshold at the hyper compressor, hyper compressor shutdown, cut-off of the ethylene feed line, isolation of the pertinent reactor, reactor decompression until reaching a 250-bar safety pressure. Unit 1 was idle, as was Unit 2 located nearby (the site contained 4 units in all). Water curtains served to separate Units 1 and 2. Damage was prevalent on: the Unit 1 hyper compressor and an adjacent motor, the roofing structure, and a portion of the roof itself. Extinction water was channelled to the confinement basin to prevent any pollution risk, then analysed and routed to the facility’s treatment plant. During the most recent 10-year inspection of the hyper compressor, a crack had been detected on the compromised part. Its repair had been performed by a specialist firm using a proven method. An expert appraisal was commissioned at the operator’s request. Given the specificity of this installation, the manufacturer had no spare parts in storage. Unit 1, which accounted for 20% of the site’s total HDPE output, could not be reactivated before several weeks or even several months.