At 10:15 am during scheduled intervention on the main board at a refinery’s vapour production plant, a several-second outage on the 220-V network occurred, in turn upsetting the centralised control station. The safety features on flame detection systems, supplied from this station, were activated, which wound up triggering boiler shutdown. The lack of vapour altered the diesel hydrodesulfurization (HDS) unit before affecting the platform’s other units, in placing the installation in safe operating mode and routing the gases present to the flare. Spectacular plumes of black smoke were released at the flare for several hours.

The operator notified the Classified Facilities Inspectorate at 11:15 am, then activated the internal emergency plan at 11:30 and constituted a crisis unit. Sirens sounded to request personnel evacuation upon H2S detection.

The H2S content measured downwind of the HDS unit amounted to between 1 and 3 ppm inside the refinery, even though air quality control network sensors did not detect anything out of the ordinary. Around 1:30 pm, the vapour production station restarted and the alert was lifted at approx. 2:30 pm. The situation returned to normal at the end of the afternoon for vapours and the next day for all installations.

The quantity of H2S discharged was evaluated at 100 litres.

The operator published 2 press releases at 1:30 and 4 pm, respectively.

A circuit-breaker designed to ensure continuity between the battery/inverter assemblies and electrical boards, whenever switching produced an open position, caused this incident. The programmed safety controllers, equipped with an autonomous power supply, were not affected, which made it possible to place installations in safe mode in accordance with the standard sequence. As for the HDS part, given the absence of pressurised water vapour at the level of both stripper and ejector, the assembly entered into a pressure surge and hydraulic barriers were removed, thus enabling the H2S contained in the gaseous phase to escape to the outside.

Based on conclusions drawn from incident analyses, the operator adopted several remedial measures:

  • modification of the 220-V supply design according to a supply switching procedure that allowed verifying the circuit-breaker position;
  • hydraulic barriers placed on the HDS unit’s vacuum system to be redesigned and calculated, with vent diameter being raised from 2″ to 4″.

This event attests to the need to pre-emptively identify the consequences of electrical supply malfunctions on the various safety features, in addition to planning the supply of safety-related priority functions and implementing appropriate preventive measures. Moreover, let’s recall the importance of regularly testing and maintaining safety backup systems and specifying procedures and training modules for technicians potentially called during degraded operating conditions.

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