The gas leak containing hydrogen (H2) and hydrogen sulfide (H2S) occurred at the upper isolation valve located at a high-pressure separator (300 bar) of an oil production unit in a refinery. DESCRIPTION of the PROCESSAfter hydrocracking under high pressure and temperature, the oil viscosity is adjusted by means of reaction with a catalyst, hydrogen and a load of distillates. At the outlet, the high-pressure separator enables gases to be separated from liquids, with the latter channelled towards the low-pressure section by opening a valve. The uncondensed gases in the high-pressure separator are mainly composed of H2 and H2S.The SEQUENCETo control this valve, two measurements of the level of the mixture are necessary to prevent the entry of gas into the low-pressure section. A maintenance intervention was scheduled to replace one of the faulty level sensor. During the operation, the piece used to operate the valve broke without causing a leak. Thirty minutes later, the upper part of the valve was ejected under the effect of pressure, causing the release of about 25 litres of hydrogen.The CONSEQUENCEThe leak occurred 4 m aboveground and had no direct impact on staff. After depressurisation by gas flare, the equipment was purged by inert gas (nitrogen), which caused several successive sudden rises in pressure within the gas flare network along with the valve release. The amount of gas released by the gas flare during the day was estimated at 13 tonnes.