B 1/2002Y

At Kaipiainen built-up area of Anjalankoski town, an accident involving hazardous substances occured on 13 September 2002 when an acrylonitrile storage tank exploded and caught fire. The incident caused no personal injury.

On a plant site, acrylonitrile was being conducted from a tank wagon through a fixed piping to a storage tank featuring a volume of 100 m³. The work had been going on for almost two hours (24 tonnes transferred) when suddenly a very loud bang was heard and the upper end of the storage tank flew about 70 meters, beyond the plant site fence. At the same time flames and smoke ex-truded from the upper part of the tank. In the explosion, the 11 meters high tank with a diameter of 3.34 meters remained in its upright position.

The Emergency Centre was advised of the incident in one minute from the explosion. Within 16 minutes altogether 21 rescue units were called out by the Centre. The fire was extinguished by foaming, in about one hour from the explosion.

Acrylonitrile is used in the manufacture of latex for the paper industry; it is an achromatic, easily evaporable and active liquid chemical. It is inflammable, toxic and may cause cancer. Acrylonitrile combustion generates hydrogen cyanide which is an extremely dangerous poison affecting the central nervous system. In terms of the propagation of the flue gases, the warning of the inhabi-tants in the area and the very rapid fire-fighting work were of crucial importance. There were seven police patrols and Anjalankoski Deputy Chief Fire Officer driving around Kaipiainen and advising the inhabitants to stay indoors.

The tank contained about 41 m³ acrylonitrile, with its gas space volume where the explosion took place, measuring about 59 m³. A specific quantity of air in the inflammable mixture was required for the explosive ignition to take place. Acrylonitrile and air mixtures in widely varying concentra-tions may ignite and explode, as the ignition range of acrylonitrile is remarkably broad (from 2.8% to 28 %). The structure of the tank that was charged with acrylonitrile allowed air to flow the space of the discharged liquid, via the valve on the top or the gas discharge pipe.

The investigation focused on where in the tank the inflammable gas encountered a spark or where the thermal energy source requisite for the ignition could be localized. Different possibilities were studied and analysed in the investigation, with its analysis chapter containing the relevant discussions conducted. However no certainty could be established as for the definitive factor hav-ing ignited the gas compound.

In order to prevent similar accidents, the Accident Investigation Board of Finland recommends that the presence of inflammable gas mixtures be prohibited in the tanks referred to. In practice, flow of air in the tanks may be prevented, e.g. by charging the gas space with nitrogen instead of air. Moreover the Accident Investigation Board recommends that the rules and regulations and instructions applicable to the planning and design, building and use of tanks for hazardous sub-stances be compiled to one user-friendly overall manual.

Download here the whole investigation report pdf-file (1,42 Mb), partly in English