Fire in a refinery

The accident originated in the isomerisation unit and propagated to the benzene saturation unit. The accident source was a mechanical rupture of one of the two compressors used to transfer hydrogen mixtures to the isomerisation unit.
The rupture caused the release of a gas mixture containing hydrogen as main component (70%), which ignited by mixing with air, forming a jet fire. The jet fire was directed towards the adjacent benzene saturation unit, hitting a pressure vessel located 14 m from the release point (the jet flame could have been 30 m long in absence of obstacle). The weakening of this vessel caused the release benzene, which contributed to extend the fire.

Due to the fact that the process control records did not register significant changes in the process parameters it has to be assumed that the failure of the crank mechanism was caused by aspects linked to maintenance or by a material defect.

Off-site fire brigade teams intervened extinguishing the fire within approximately 1 hour and a half of work. No damage was reported to persons or to the environment.

The INITIATING CAUSE was the mechanical failure of a piston at the base of one of the cylinders of one of the compressors.
However, the cause of the accident was not identified wit certainty. One hypothesis was the breakage of a crankshaft component inside the compressor, causing the piston to strike the lower part of the cylinder. The combined effects of the impact and the internal pressure (20 bar) led to the failure of the nuts securing the lower part.
Maintenance was performed regularly.

The root CAUSE was the failure in considering the possibility of domino effects from the fire in the isomerisation unit (whose risk had been considered in the risk assessment).

piston, compressor, supply line

DESCRIPTION OF THE PROCESS
The aim of a isomerisation unit is to converts linear molecules (e.g. pentane) into higher-octane branched molecules for blending into the end-product gasoline. It is also used to convert linear normal butane into isobutane for use in the alkylation unit. it requires the feed of hydrogen.
The release occurred on one of the two compressors working in parallel at a pressure of 20 bars, conveying the gas mixture needed for the isomerisation process.

PLANT QUANTITIES
Maximum amounts of substances registered in the plant inventory were:
- approximately 25,000 m3 of hydrogen rich gas mixture, therefore 1,500 m3 of pure hydrogen (approximately 1,5 tons).
- 147000 t of liquid automotive petrol.
The establishment extends over a surface of 100 ha.

No damage was reported to persons or to the environment.
The damage to the equipment has been evaluated around 2 to 3 MEuro, depending on the source.
The benzene saturation unit was destructed; only a small part of the equipment could be recovered. In the isomerisation unit the loss of the compressor was registered, the compressor had practically to be completely substituted and was taken back into service in six months later.

Total estimated released quantities :
3 t gas mixture from the isomerisation unit + 0.3 t gas mixture of the benzene saturation unit, for a total of approximately 3.3 tons of gas mixture with 70% volume of H2, corresponding to approximately 10% in weight.
6 t gasoline from the isomerisation unit + 6 t gasoline present in the benzene saturation unit for a total of 12 tons of gasoline.

An official systematic investigation into the failure cause was expected to be started, subject to appointment of a consultant by the civil court . The company had already requested a technical expert opinion.

15:00 - A loud abnormal noise coming from the isomerisation unit was heard by the personnel working near that unit. Having identified the in the compressor the cause of the noise, they tried to stop it via the emergency shut-off switch.
In the control room the emergency shut-down procedure was activated, triggering the a rapid depressurisation by blowing down the system.
Immediately afterwards a violent release of a hydrogen rich gas mixture occurred from the compressor bottom, the jet ignited almost instantly and hit the adjoining part of the benzene saturation unit.
The refinery personnel closed the manual valves installed on the pipe mains and started to extinguish the fire, activating the fixed cooling systems and the foaming system of the units involved and nearby units.
15:03 - the onsite Emergency Control Centre team gathered and established communication with the Advanced Control Centre.
15:12 - Off-site fire brigade alerted. Meanwhile the on-site emergency response team fought the fire with a fixed and mobile extinguishing units from all sides.
15:16 - Emergency shut-down procedure started for the entire establishment.
15:30 - First off-site fire brigade team on-site, supporting response operations.
15:50 The size of the fire greatly reduced by because the compression section of the plant no longer fed the fire; fire continued in the benzene unit engulfing it with flames.
16:25 the fire almost extinguished; cooling operations continued.
16:50 the emergency called off.

The accident could have caused more severe damage without the adoption of effective response measures and the joint effort of the on-site personnel and the off-site fire brigades; even if the on-site emergency response organisation could be improved.

The post-incident analysis could not establish an evident, direct link between the actual causes of the accident and failures or inadequacies in the safety management system of the plant, particularly regarding inspections, controls, or maintenance of facilities.

Nevertheless, the accident highlighted generic topics which played a significant role, even if they had already been identified on several other occasions:
(1) To improve the plant design to reduce domino effect (escalation).
(2) To identify control parameters which may predict anomalies or failures in the facilities, and to use them for early detection of malfunctions.
(3) To improve the performance of the emergency systems (flow rate and autonomy of water reserves, number and location of water/foam tanks and nozzles, etc.)
(4) To facilitate accessibility of critical areas by rescue teams/external firefighters (e.g. suitability of spaces for appropriate equipment, number of intervention points).
(5) To strengthen communication means during rescue operations.

According to the eMARS report (see references), the company adopted the following measures based on the experience gained and the studies carried out company:
A) Measures related to structures and equipment:
a1) replacement of the reciprocating compressors with a centrifugal compressor.
2) Replacement of the old, manually driven shut-off valves with electrically driven valves at the intake near the compressor, to minimize gas leakage.
a3) More strategic placing of the hydrants and fixed detectors.

B) Measure concerning management and documentation aspects:
b1) Better location of personal protective equipment.
b2) Better training (including frequent drills) of personnel in management of emergencies.
b3) Clear sequence of operations to secure the equipment.
b4) Updated risk analysis for the safety report.