Fire in a refinery

The fire, 1 m high fire occurred in a pipe at the inlet of the hydrodesulphurisation unit of a refinery. The fire was caused by the leak of the pipe content, hydrogen and diesel fuel. According to the first elements of observation, a leak on a bolted assembly (flange) at the level of an insulation gasket of the reactor would be at the origin of the incident.
The fire was detected visually and was located 25 m above the ground. The personnel was evacuated. The operator did not extinguish the flame in order not to generate a hydrogen cloud, which is highly explosive.
The unit was gradually shut down with decompression to a site flare. The gas contained in the unit, feeding the leak, was progressively replaced by nitrogen. A preventive foam mat was put in place under the reactor to protect against a possible ground pool fire.

During the work of replacing the flanges, the operator observed the good flatness of the flanges but a deformation of the gasket. The operator considers that this deformation would have occurred due to the over-tightening of the flange during the last maintenance shutdown. Moreover, at the time of the event, the desulfurization unit was at the end of its cycle with a replacement of the catalyst initially planned 1 month after the event.
According to the operator, the loss of catalytic activity at the end of the cycle is compensated by an increase in the reaction temperature. This temperature increase, combined with the over-tightening of the flange, would be the cause of the leak.

hydrodesulphurisation unit, pipe

At the time of the event, the desulfurisation unit was at the end of its cycle with a replacement of the catalyst initially planned 1 month after the event.
Due to the normal losses in catalytic activity at the end of the cycle, the reaction temperature was usually increased i nespect to the prescribed value att the star tof the cycle.