The accident occurred in a chemical plant specialising in the manufacture of petroleum-based additives; an explosion occurred at 11:34 a.m. in a 28 m³ stainless steel dilution tank containing 600 kg of lubricating oil. The oil was from the rinsing phase of the synthesis reactor located upstream. The explosion occurred when the operator wanted to use compressed air to blow the oil contained in the 2nd filter module of the recirculation/filtration system into the tank. The discharge vent absorbed the wave of overpressure. A fire broke out in the tank, but the sprinklers were triggered and extinguished the flames. The internal emergency plan was activated, and the employees evacuated the unit. The internal response teams were able to secure the area, cool the tank and ventilate the building. At 5 p.m., the tank’s temperature had dropped to 40 °C. The sprinkler heads that had been triggered were replaced.

The fibrocement roof and the piping located above the tank and the insulated pipes above the tank were damaged by the pressure wave. Following the explosion, the workshop operator suffered from occasional buzzing in one ear. The damages were evaluated at €10,000.

The Inspection Authorities for classified facilities visited the site. The operator identified several causes of the accident:

• tracing on the tank and the recirculation/filtration lines, causing the oil temperature to rise from 50 °C (temperature at the reactor outlet) to more than 60 °C once the mixture had been re-circulated for several hours for a particular production sequence on the day of the accident;
• contamination of the rinsing oil with toluene (4%, i.e. 24 kg), lowering its flash point from 220 °C to 55 °C (the reactor is rinsed with toluene beforehand). This possibility of contamination was not considered in the process’ risk analysis;
• the presence of oxygen in the tank’s vapour space, aggravated by the compressed air;
• accumulation of electrostatic charges induced by the flow of compressed air used to push the rinsing oil in the recirculation lines to the tank (by blowing) and loading of the tank in “rain” mode (the tank was not supposed to receive highly flammable products).

The operator has planned to undertake the following actions:

• clearly identify the reactors 2 drain pipes: one for toluene, and the other for oil to eliminate the dead volume and to prevent toluene contamination;
• use nitrogen for cleaning this piping instead of compressed air;
• install an immersion tube in the tank instead of the “rain” loading;
• modify the reactor flushing procedure so that the potentially contaminated flushing oil is not drained into the dilution tank, or vice versa, no longer perform a washing operation with toluene before an oil filtration operation through the reactor;
• study the possible contamination of the manufacturing tanks by the flammable products used, particularly toluene.