(12b) Dispersion Of Heavy Gases - Experimental Results and Numerical Simulations

The hazardous potential of accidental heavy gas releases, especially those involving flammable and toxic gases, is widely known. In order to predict the area in which these gases are in hazardous concentrations, an estimation of the dispersion of these gases must be carried out. While the hazardous area for flammable heavy gases is determined by the lower explosion limit (ca. > 1 vol.%), the release of toxic heavy gases can result in a much larger hazardous area. Toxic gases, even in very low concentrations (ca. < 3000 ppm), have the potential to be highly damaging.

State-of-the-art dispersion models, such as the VDI guideline 3783, can be used to estimate the dispersion of heavy gases. However, VDI 3783 gives no method for the prediction of the height and width of a heavy gas cloud, which are both required for quantitative risk analysis as well as for a possible coupling of a Lagrangian particle model with the VDI 3783 heavy gas dispersion model. Therefore, further calculation methods were used to describe these dimensions and were evaluated against, experimental studies of the length, width and height of the heavy and neutral gas field.

Most of the heavy gas dispersion models assume the point of release at ground level (x = 0 m). However, in reality many of the known heavy gas releases took place from elevated positions. As part of the experimental investigations, the influence of the source height on the heavy gas dispersion was also examined. It was found that elevating the source leads to a reduction of the length of the heavy gas area. Once the source reaches a critical height, a heavy gas area at ground level no longer exists. Therefore, for release heights above the critical height, heavy gas dispersion effects can be neglected and the calculation of the heavy gas area according to VDI 3783 part 2 is therefore no longer necessary. A calculation formula for determining the critical source height has been developed and will be introduced.

All of the experimental investigations were carried out at the Test Site Technical Safety of the BAM Federal Institute for Materials Research and Testing. The numerical simulations of the heavy gas dispersion were carried out using the models VDI 3783, AUSTAL 2000 and the Computational Fluid Dynamics program ANSYS CFX. The experimental results and numerical simulations will be presented.