scope:

Introduction

Many gases, due to their large molar mass or due to the low temperature which they have in the case of accidental release, are heavier than air. They are often Uquefied by pressure or stored and transported at a very low temperature.

The dispersion of heavy gas emissions cannot be calculated by the procedures which have been developed for density-neutral or light gases. This is due to the fact that a heavy gas cloud has a strong inner dynamic, and that the turbulent mixing with the ambient air is impeded by the density step at the upper edge of the cloud. Due to negative buoyant forces, the heavy gas cloud disperses in a flat layer near the ground.

In general, complex, structured obstacles (such as buildings, plants etc.) are located in the flat layer near the ground, which, at least in the near field of the source, may not be parametrized as roughness, but must be regarded as individual obstacles; therefore, simple analytical or numerical models cannot be used. For this reason, the present Guideline is based on results which have been gained by systematic wind tunnel experiments [5]. By the application of dimensional analysis relations, the results which have been gained in the small-scale model can be transferred directly to full-scale released masses and to the environmental and ambient conditions, which are of practical interest. This procedure has been verified by comparing the results with data of the large number of field experiments, and, in principle, they correspond satisfactorily [6].

The wind tunnel experiments, however, cover only the area near the source, in which the gas concentration is reduced to about 1% of the concentration at the source by mixing with the ambient air. Typically, infiammable gases have lower fiammability concentrations of 2 to 10%. Thus, the area which is potentially endangered by a faire hazard can be determined even in the case of non-diluted released gases.

The area which is potentially exposed to accidents with toxical gases of the same quantity is considerably larger. In this case, a procedure in two steps has been elaborated. For the first step, as in the case of combustible gases, one must refer to the results of the wind tunnel experiments. These results constitute the input data for a numerical far-field model. This model, however, can be rather simple because the initial density of the gas must not be taken into consideration, and because obstacles may already be parametrized as roughness if the vertical dimension of the gas cloud is considerably higher than the height of the respective obstacle. The model which is described in Guidehne VDI 3783 Part 1 has been used; thus it has been made sure that the investigations of the dispersion which are to be carried out in the safety study lead to consistent results for buoyant, density-neutral, and heavy gases.

Up to now, effects of orography and obstacles are determined by a rather large grid. At present, one takes care of the peculiarities of Single source configurations and dispersion investigations to be better accounted for by including further obstacle configurations. The results of these detailed investigations are updated in the latest Computer program ²⁾.

²⁾ The application of this Guideline is facilitated by a menu-controlled program which is written in FORTRAN 77, and which runs on personal Computers with the operating system MS-DOS. The program is stored on a disk which may be obtained at the VDI-Kommission Reinhaltung der Luft, Postfach 1 1 39, 4000 Düs¬ seldorf 1, West Germany. When using this program, all the criteria mentioned above are automatically asked, and it is decided whether Part 1 or Part 2 of this Guideline should be applied.