A fire in the hydrotreatment oven of a refinery

An explosion, followed by a fire, happened in an oven belonging to the hydro-treatment unit. Following a shutdown, the tube, containing only gaseous products and hydrogen, had been affected by instable flows. In presence of a flows lower than nominal, the tube temperature raised up to its mechanical resistance limit. Under internal pressure, it failed with a longitudinal crack of ca. 750 mm
The unit was shut down and the fire extinguished by itself.

The INITIATING CAUSE was the failure of a tube of the oven due to overheating.

The destabilisation of the operative conditions was due to shortcoming in the capacity to control the operative parameters, also due to lack of effective diagnostics by means of measuring gauges.

Hydrotreatment unit

The oven used for hydrotreatment of complex oils had been shut down 3 hours before.

It is probable that the oven affected by the explosion was a heat exchanger used to pre-heat hydrogen and other process gases before entering he hydro-treatment reactor.

As a maintenance was planned in January 2006, the unit will not restart before this date.

The unit could not restart before the conclusions of investigation and the subsequent updating of safety measures.

The Internal Operation Plan was launched. The unit was secured by using extinguishing means, isolatiing it, depressurising it and placing it under nitrogen.
The integrity of all equipment surrounding the oven was checked.

The plant operator:
1) verified the soundness of the range of allowed operative conditions,
2) ensured the correct functioning of the thermocouples measuring the temperature of the tubes,
3) improved the operative procedures, in particular those regulating the transitory phases.