An explosion at 12:28 p.m., followed by a fireball, occurred in an upper-tier Seveso industrial gas plant. The incident happened on the oxygen conditioning pump located at the base of a full liquid oxygen tank. The fireball spread to two nearby electric cabinets. The resulting fire threatened two other nearby argon and nitrogen tanks as it spread through the technical ducts. The employee closest to the incident raised the alarm. Two first responders arrived on the scene with three fire extinguishers and two fire hoses. Three others, equipped with self-contained breathing apparatus, took over with fire hoses connected to the site’s two fire hydrants. The source of the fire, fuelled by an oxygen leak, proved difficult to control.

Equipment involved: 25 m³ cryogenic tank storing up to 23 m³ of liquid oxygen (8,500 l) manufactured by the site, at low temperature (-180 °C) and under pressure. Thermally insulated, the tank delivers oxygen in either liquid (liquid side) or gaseous form via an evaporator or thermosiphon (gas side). The latter allows excessively hot product to be recycled. A high-pressure piston vacuum pump is used to draw in the O2, which has become gaseous as a result of an increase in the tank’s temperature, toward the cylinder conditioning racks. Its cold head prevents cavitation phenomena which could damage the pump.

The internal emergency plan was triggered at 12:35 p.m., 10 min. before the emergency services arrived. The neighbours were informed by remote

alarm and by an employee who went door-to-door, although no confinement order was issued. The gas cylinder filling operations were stopped, and the power supply to the filling unit was switched off. The 72 employees were confined for two hours. The fire was finally brought under control at 1:45 p.m. The oxygen tank continued to drain until 2:30 p.m. Neither the firemen nor the operator were able to close the liquid outlet valves as they had been damaged by the heat flows. On the gas outlet side, the emergency services closed the valves, but during the closing operation, the leaking oxygen formed thick white smoke upon contact with the air. This smoke was visible from outside the site. At ground level, the liquid phase leak resulted in the formation of an ice layer.

The pipes (DN 5-8) of the transfer network from the three tanks to the cylinder conditioning unit were damaged by the heat flows. These pipes, still under load, released nitrogen and argon. Fearing the spread of fire to the buried portion of the system, firefighters sprayed it down with water and conducted a visual inspection. They took two employees, suffering from smoke inhalation, to the hospital, then plugged the leaks and cooled down the storage tanks. The intervention came to an end at 3:30 p.m. The extinguishing waters were directed to the SAUDRUNE. The site was monitored over the weekend. Eighty percent of the site’s special and industrial gas conditioning activities were shut down for two months. Deliveries to customers were ensured from other sites throughout France. The non-temporary employees of the site were not placed on technical unemployment. The plant operator issued a press release.

The expert assessment showed that a heat surge had occurred on the cold head of the rack filling pump, located at the base of the tank, resulting in a fireball which ignited the fire. The tank’s thermosiphon had little effect due to a control deviation on the temperature probe. This lack of efficiency led to a cavitation phenomenon in the pump and the creation of particles. These internal particles were drawn between the piston and the pump’s low-pressure chamber, resulting in a friction ignition mechanism. This “heat surge” scenario had been taken into account in the hazard study, and the accident does not call into question the probability of an associated occurrence (10-2).

These failures could have been avoided through preventive maintenance of the cold head. This maintenance was not carried out owing to a lack of a maintenance plan and the lack of spare cold heads enabling the dismantling operation to be carried out. Also, the pump (cryogenic high-pressure piston pump + supercharger) was an unusual model designed for higher pressures (300 bar), which degraded its tightness.

The operator reconstructed the damaged slab by moving the electrical cabinets away from the two oxygen conditioning pumps. A system for isolating the oxygen tank in the event of a nearby fire was installed by adding remote shut-off valves. The temperature sensor was adjusted according to the manufacturer’s recommendations. A preventive maintenance plan for pumps of the same type was implemented at all the group’s sites.