Explosions and fire on the catalytic reforming unit of a refinery

A series of explosions and fires occurred at a catalytic reforming unit of the refinery, followed by fires on the same unit. The establishment was classified as Seveso Upper Tier Class. The emergency, consisting in a series of domino effects, lasted almost one day.

It started with the rupture of a connection at the suction pipe of a pump at the desulphurisation section of the hydro-cracker. The released cloud (about 200 kg of the substance) ignited forming a torch fire, which caused the rupture of an exposed 3-inch mineral oil pipe. The ignited oil formed another fire jet of larger size. By further domino effects, collector, pipes and valves broke contributing to release of combustible substances.
The ignited releases kept on burning until the material being processed in the unit was exhausted and the flare system could be isolated without risk.

The INITIAL CAUSE was the rupture of a connection on the suction line of a pump, with a consequent release of the process gas contained in the pipe.
The pump was part of a pump / steam turbine unit, used as backup of another pump which was used during normal operation. This backup pump in function because of a periodic test of the equipment. Several shortcomings in the design of this pump led to strong vibrations, which caused:
- the rupture of a ¾ inch connection located just above the pump, through fatigue stress and
- a misalignment of the turbine/pump, which caused a rupture of the shaft and deterioration of the connection.

It followed the failure to isolate the leak because the shut-off valve was not activated. This triggered a domino effect consisting in the fire-damage of other components and consequent release of additional flammable substances (liquid and gases).
An aggravating event was the abnormal delay in closing shut-off valve of the supply line to the pump.
The operation of this valve was occurring by a programmable safety controller, which closed it after 10 minutes from the rupture. If the valve would have closed within 1 minute, as expected, the domino effects could have been prevented.

The accident occurred during a periodical test phase carried out with a turbine, which constituted an emergency relief unit.

DESCRIPTION OF THE FACILITY
The refinery was a large refinery with a capacity of crude oil distillation of 17 millions tons per year. The unit involved in this accident was only one of the several crackers.
It was a catalytic reforming unit, used to produce high-octane gasoline fractions. This unit could process 4200 t/day of product and consisted of 3 successive sections: a desulphurisation section, a catalytic reforming section, and a fractionation section. In the reforming section, the reaction occurred exothermic at a temperature of approximately 500 C and at pressure range 15 - 20 bar.

Two firefighters from the refinery were slightly injured, affected by the heat during the intervention.
No significant impact on the environment. The water used in fire-fighting was stored in a 60,000 m3 buffer storage tank designed for that purpose.
The unit was shutdown for around 7 months. The amount of the damage is in the order of 14 M€ for rehabilitation and 68 M€ in operating losses.

The accident has been classified as level 4 on the international accident gravity scale. Considering the material damage, which was high, the criterion in Annex VI is exceeded: the accident has to be notified to the European database (MARS).

The emergency was executed by means of the refinery's fire-fighting equipment and the local administration fire brigade.
The strategy for fighting the fire consisted in limiting the dangers of the fire spreading and cooling the superstructures that were exposed to heat fluxes, in order to prevent collapse. In addition, the fire was confined by using an extremely high flow rates of water (in the order of 2300 m3/h, at 7 to 8 bar, taken from the site's fire system).

[general lessons learnt, reformulated from the ARIA report]
This accident is a clear illustration of how the simultaneous occurrence of several causal events which, if taken separately, can appear of secondary importance, can have extremely serious consequences.
The sub-event carrying the major responsibility for the escalation was the delay in closing the shut-off valve which would have stopped the fuel supply to initial fire. It reminds of the importance of the timing of the activation of mitigating measures.
The timeline of the series of events demonstrates the great difficulty in handling the possibility of domino effects propagating between units on petrol-chemical plants of the type involved in this event.
Finally, this accident could have had serious consequences in terms of human life if one or more operators had been located at the site where the leak occurred. This element is important when performing a correct quantitative evaluation of the risks.

(proposed by the plant operator and approved by a team of experts):
(1) Modification of the type of pump and the vertical positioning of the tube, to prevent cavitation phenomena.
(2) Replacement of the motorised suction valves B, in order to obtain a quicker closing time, with the objective aiming at a closure ability in less than 30 seconds.
(3) Modification of the speed regulation device of the turbine, in order to obtain an "isochronous" regime,
(4) Replacement of the ¾ inch connections of the pumps concerned by 1 inch connections fitted with reinforcement gussets.
(5) Modification of the purge system of the turbines concerned.
(6) Implementation of an awareness campaign concerning the strict application of the operating procedures for turbines in the process of heating.