Before 8:00 a.m. one morning, a leak occurred on a non-return valve at a refinery’s flue-gas desulphurisation unit. At 8:13 a.m. a hydrogen sulphide (H2S – a flammable, toxic and odorous gas in very low concentrations) detection alarm with a threshold set at 10 ppm was activated at the unit. Technicians unsuccessfully searched for the leak. At 9:30 a.m. a contractor noticed a pungent smell and alerted the refinery’s employees. Other H2S alarms were activated. A survey was conducted, but nothing unusual was found. At the same time, firefighters received hundreds of calls from people in the French departments of Essonne, Yvelines and Seine-et-Marne reporting a gaseous odour. At 11:02 a.m., a second-level H2S alarm with a threshold of 40 ppm was triggered. The unit’s scrubbing tower was shut down 30 minutes later. At 12:30 p.m., the operator reported a sulphurous water vapour leakage to the authorities. The surrounding municipalities were alerted. The internal emergency plan was implemented at 1:52 p.m. Air-monitoring tests conducted near the site did not reveal any health risks. The internal emergency plan was lifted at 6:25 p.m. An estimated 187 kg of gas, 90% of which consisted of H2S, had been released.

Frost-cracking of the valve

The leak occurred on a valve on the gas line between the scrubbing tower and the gas-flaring system. Frozen condensate (outdoor temperatures below –5° C) had caused the valve cover to buckle. During normal operation, the gas inside this line does not circulate. The pipe had been protected with heat tracing to prevent condensation, but the operator observed that the system was not functioning properly. The expansion of the frozen water caused the valve to crack. Gases then leaked out of this crack when the ice thawed. The absence of lagging and the fact that the valve was positioned at the bottom allowed water to build up inside the valve.

Management of the event

The unit’s operating staff searched for the leak without informing any other teams at the refinery. Unaware of the situation, the safety department and shift supervisor were unable to provide satisfactory information when contacted by emergency services that morning. However, 50 fixed H2S alarms, including two second-level alarms, at the refinery went off at between 8:13 a.m. and 11:53 a.m.

Corrective actions

Since the event, the operator has:

  • repaired the lagging and the heat tracing system on the flanges and small appliances;
  • lowered the H2S alarm thresholds to 5 ppm and 10 ppm;
  • revised the procedures for responding to H2S leaks (alarm management and safety shutdown of facilities) by setting new thresholds and clarifying the expected responses and transmission of information about detected leaks;
  • revised the procedures for implementing the internal emergency plan by adding an emergency-response matrix according to the type of event (including odours) and adding a scenario involving off-site detection of odours.

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