At a site producing industrial gases, in the unit storage of gas petroleum liquefied, methane is released in atmosphere. This accident follows a change in the source of supply cooling tank methane liquefaction. At 17h30, a technician on site observed that process relief valves had lifted on tank 2, filled a few months earlier. Pressure steadied after relief valves had opened and closed a few times to let off gas. All valves are shut at 19h30 after being scraped off. A small crack 0,5 m long in outer skin may have been caused by valve hammer. Due to the absence of ignition, the accident has only led to the shut-down of all the units of liquefaction. No people were injured. The cost of refilling (liquefaction) is estimated at 0.5 M£. Insufficient and inadequate procedures and loss of process control are the causes induced.