Physikalisch-Technische Bundesanstalt

Tasks

Gas dispersion and explosion processes

The pressure wave developing during an explosion and, therefore, the effect of an explosion, strongly depend on the velocity at which the flame propagates. Compared to flame propagation in a mixture at rest or in a mixture at laminar flow, the flame in a turbulent mixture is accelerated due to the wrinkling of the flame front. As long as the propagation velocity remains below sound velocity, the combustion process is specified as a deflagration. Under certain conditions, for example in the case of flame propagation in a pipe of sufficient length, turbulence is generated in front of the flame by the blast effect of the combustion. Consequently - in a feed-back process - the flame can be accelerated to such a degree that transition to detonation takes place.The detonation is characterised by a combustion wave coupled to a shock wave propagating at supersonic speed (about 2000 m/s). Extremely high mechanical impulse loading then occurs in the direction in which the detonation propagates.

Deflagrations and detonations are classed under the general term explosions. The safe design of plants and the protection of plants by means of flame arresting devices require an intimate knowledge of the explosion phenomena. The PTB made important contributions in this field, in particular as regards turbulent flame propagation and deflagration to detonation transition.

Safety of plants

The experience gained in the above fields enables to assess the safety of plants. This is usually done at the request of authorities or supervisory boards and is normally restricted to plants whose safety is of particular importance, or to cases where novel technologies are applied. The plants concerned are intended above all for

• the storage and handling of flammable liquids (e.g. fuel depots, ports, filling stations)
• for the recovery of vapours of flammable liquids
• for technical after-burning.

In connection with the safety of plants, equipment and protective systems are type-tested. The conformity assessment procedures for equipment and protective systems intended for use in explosive atmospheres are regulated by the Directive 94/9/EG. Within this framework, the PTB is a notified body in compliance with EC Directive 94/9/EC. It carries out type examinations and inspects the manufacturers' quality assurance systems. Devices concerned are flame arresters (e.g. explosion, detonation and long-time burning arresters for the protection of tanks, Figs. 1 and 2) and some special non-electrical equipment of category 1 (for use in zone 0; e.g. ventilators). In addition, the PTB is concerned with safe tanks for the transport of dangerous goods.

Formation and dispersion of explosive mixtures

The problem of the formation and dispersion of an explosive mixture plays an important role in the assessment of explosion hazards (for example, evaporation of a pool of liquid and dispersion of the vapours). When large quantities of mixtures are set free as a result of an accident, the spreading of the mixtures under the prevailing atmospheric conditions can be determined only with the aid of complex computation models.

Depending on the likelihood of occurrence of hazardous explosive atmosphere the corresponding hazardous areas are divided into zones: zone 0 - continuously or for long periods; zone 1 - occasionally; zone 2 - only rarely and if so, for a short period only. In case that explosive mixtures are set free from apparatuses or plants, zones around these facilities have to be specified. With regard to enclosed spaces, the influence of the natural or artificial ventilation has to be taken into account in addition to the strength of possible sources of explosive mixtures. Only after the zones have been specified, a decision on suitable equipment can be taken. Concerning all these questions the PTB gives advice to authorities and industry.