Dispersion

Accidental release of gaseous hydrogen or spillage of liquid hydrogen lead to evolution and dispersion of a gas could whose shape is influenced by the type and rate of release and by atmospheric conditions as well as by topography. There are several concentration graphs generated by PHAST, each of which shows a different aspect of the concentration profile for the cloud. These graphs are generated from the source models, and the dispersion report shows the details of the values plotted in the graphs. These include maximum concentration on footprint, side view, and centreline concentration.

Fig. 5.1 shows the movement of the cloud in the downwind direction both for continuous (steady state) and instantaneous dispersion. It presents different shapes of the contours inside the cloud, seen from the side through the centreline of the cloud for two example study objects (i.e. object 1 and object 5), and weather 1.5/F. Hydrogen is the lightest of all gases and tends to rise. For liquid hydrogen, however, the density of the saturated hydrogen vapour at the boiling point is higher than air density. It is therefore negatively buoyant when it first evaporates. The heat capacity of saturated hydrogen vapour is very small and the temperature is raised quickly and the buoyancy changes to neutral and positive.

Figure 5.1 Side view of the hydrogen release for the two study objects (1 & 5)

The concentration through the centre line of the cloud at a given time, as a function of distance downwind is shown in Fig. 5.2. All of the concentration graphs show the concentration at a given time, since the shape of the contours will change over the course of the dispersion, as the cloud moves with the wind and air is entrained.

Fig. 5.3 shows the maximum concentration footprint of the hydrogen release inside the cloud. It shows the calculated distributions of hydrogen concentration. The outer boundary indicates the LFL fraction of the lower flammability limit concentration (2%). Similar to the previous graph it is plotted for two study objects and weather 1.5/F only. Two different concentration profiles of the cloud from different release scenarios (i.e. continuous and instantaneous) can also be seen in this picture.

Figure 5.2 Centre line concentration for the two study objects (1 & 5)
Figure 5.3 Footprint of the hydrogen release for a concentration of 2% and different event

A minimum concentration of the dispersion results for a flammable material would normally specify some fraction of the lower flammability limit (LFL fraction). The LFL fraction to finish means the concentration (as a fraction of the LFL) that determines the maximum distance at which a flammable cloud can be ignited. Typical values are full (100%) and a half (50%) of the LFL (i.e. 0.04 fraction). The 50% will give more conservative results, and can be used to include the effects of imperfect mixing, which may give local concentration higher than those predicted by the dispersion modelling. Therefore, the hydrogen release with its LFL of 4% the concentration interest may be set to 0.02 fractions (2%). Alternatively, for flammable release, PHAST will automatically continue dispersion until it reaches the LFL fraction to finish set in the flammable parameter. Additionally, it can be used in setting the location for delayed explosions and the area covered by a flash fire [49].

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