Explosions and fire on the catalytic reforming unit of a refinery

A series of explosions occurred in thee desulphurisation and catalytic reforming unit of the refinery, followed by fires in the same unit. The establishment is classified as Seveso Upper Tier Class. The emergency, consisting in a series of domino effects, lasted almost one day.
DETAILS of the INCIDENTS
The fire, associated with muffled explosions, started at the desulphurisation unit section of the "cracker No. 7". It was caused by the rupture of a connection at the suction pipe of a pump, followed by ignition of the released cloud (about 200 kg of the substance) and the creation of a torch fire. The torch fire caused the rupture of an exposed 3-inch mineral oil pipe creating 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 part of a pump.

Following the ignition of the 200 kg of products, and the failure to isolate the leak by not activation of the shut-off valve. Therefore a series of CONTRIBUTION CAUSES triggered a domino effect consisting in the fire-damage of other components with consequent release of additional flammable substances (liquid and gases).

The ROOT CAUSE was related to a safety design which had not been able to take in account the possibility of domino effects.

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 the accident of 3 September 2001 was only of the several crackers.
It is a catalytic reforming unit, which is used to produce high-octane gasoline fractions. This unit can process 4200 t/day of product. The unit consists of 3 successive sections: a desulphurisation section, a catalytic reforming section, and a fractionation section. In the reforming section, the reaction occur exothermic at a temperature of approximately 500 C and at pressure range 15 - 20 bar

Two firefighters from the refinery were slightly injured, effected 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).

[reformulated based on 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, and can have extremely serious consequences. On of this aggravating factor, is the delay in closing the shut-off valve which could have avoided escalation. 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 treating the theme of the domino effects between units on petrol installation 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 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.