Gas release from the hydrocracking unit of a refinery

A gas leak containing hydrogen and hydrogen sulphide (H2S) occurred at the upper isolation valve located at a high-pressure separator (300 bar). This unit spearates the process gases from process liquids. The liquid is then sent to the low-pressure separator, while the gases leave via the upper isolation valve.

To 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 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.

INITIATING cause of he leak was the failure of a valve in the high-pressure part of a gas separator.
Root cause was a design defect of that type of valve.

high-pressure separator (300 bar) of an oil production unit

The incident occurred after repairs

DESCRIPTION of the HIGH-PRESSURE SEPARATOR PROCESS
After 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 gases leave the high-pressure separator through the upper isolation valve.

No inury, negligible equipment damage, 13 tons of gas released.

The Classified Facilities Inspectorate 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.

After depressurisation by gas flare, the equipment was purged with nitrogen, which caused several successive and sudden rises in pressure within the gas flare network along with the valve release.

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.