Fire in a refinery

A 1-metre high flame developed on a pipe at the inlet of the hydrodesulphurisation unit of a refinery. The fire was caused by the leak of the pipe content, consisting of hydrogen and diesel.

The fire was spotted visually and was located 25 m above the ground. The personnel was evacuated. The operator did not extinguish the flame to avoid the risk to generate a hydrogen cloud.
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.

The INITIATING CAUSE was a leak on a bolted flange, and specifically the sealing gasket.
During the work of replacing the flanges, the operator observed the good flatness of the flanges but a deformation of the gasket. This deformation could have occurred due to the over-tightening of the flange during the last maintenance shutdown.
Moreover, at the time of the event, the desulphurisation unit was arriving at the end of the production cycle. A replacement of the catalyst had been planned at 1 month after the event. According to the operator, the loss of catalytic activity at the end of the cycle was 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.
Therefore, the ROOT CAUSE could be attributed to installation error, and/or to a human error. A possible shortcoming of the procedural instructions cannot be excluded.

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.

The personnel was evacuated and a emergency operation room was installed.
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

After the fire, new bolts were used on the flange that leaked, and a controlled and incremental tightening of the assembly was executed throughout the restart of the unit.