Pollution
Humain
Environnement
Economique

A gas leak containing hydrogen (H2) and hydrogen sulphide (H2S) occurred at the upper isolation valve located at the high-pressure separator (300 bar) of an oil production unit in a refinery. After hydrocracking under high pressure and temperature, the oil viscosity was adjusted by means of reaction with a catalyst, hydrogen and a load of distillates. At the outlet, the high pressure separator enabled 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 were mainly composed of H2 and H2S.

To control this valve, two level measurements were necessary to prevent the entry of gas into the low-pressure section. A maintenance intervention was scheduled to replace one of the faulty level sensors located on a standpipe of the high-pressure separator. During the operation, the square of the upper quarter-turn 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 leak occurred 4 m aboveground and had no direct impact on staff. After depressurisation by gas flare, the equipment was inerted with nitrogen, which caused several successive and 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.

The Inspection for Classified Facilities noted these facts and requested from the operator an analysis of the causes of the event, as well as the corrective actions taken or planned.

The operator found a defect with the valve flaps and decided to replace all identical valves on the site with valves of a different design.