Friday, July 12, 2013

Toxics Primer

This post tries to explain how FACET3D handles toxic dispersions and what the various combinations of settings achieve. Toxic results are dependent on a number of settings, primarily found in two places. First, there is the dispersion object itself (Figure 1), which holds the dispersion data imported from PHAST at the core averaging time*. Second, there is the toxic options under the options form analysis tab (Figure 2).


Figure 1


Figure 2
FACET3D calculates two toxic results. First is the toxic concentration variable, which is a direct mapping of the plume concentration at the effect height onto the ground plane contour grid. Second is toxic lethality, either due to cross wind evacuation, stationary evacuation for a specified duration, or shelter in place lethality.
Starting with #2, the lethality calculation, all different types of lethality are calculated using the averaging time specified in the dispersion object form (Figure 1). This averaging time defaults to the PHAST toxic averaging time and is used to transform the dispersion cross section data (in the table) before it goes to the lethality calculations. Therefore, about 5 minutes is usually good since this is on the order of the time needed to evacuate. A larger value may be used if you are doing shelter in place lethality over 1 hour or a stationary evacuation using a duration longer than 5 minutes.
Back to #1, the toxic concentration, which also can use the averaging time in the dispersion form but may instead use the averaging time from the toxic options form (Figure 2) if either the Toxic Conc. Results as Equiv. Conc. checkbox or Concentration Transformation combo box options are selected.
Equivalent concentrations are simply the concentrations at each downwind distance that give the same dose for the supplied reference time as the dose given by the original concentration over the duration of the toxic plume. This feature allows plotting equivalent ERPG values when the release duration is less than 1 hour. If the release duration is >= 1 hour, equivalent and original concentrations will be the same.
Concentration Transformations map the original or equivalent concentrations (ppm) to a common baseline so chemicals with different ERPG values can all be shown on the same contour plot. For example, ERPG[1-3]->1,2,3 option will map a curve of decreasing concentrations in ppm vs. downwind distance to a curve passing through the integer values 1, 2, and 3 vs. downwind distance where the ERPG-1 concentration (ppm) gets mapped to the integer 1, the ERPG-2 concentration (ppm) gets mapped to the integer 2, etc. Then, if we request a contour plot of the value 3, we are effectively contouring through the ERPG-3 concentration for all chemicals.
* Averaging time with respect to dispersions is a way to account for the fact that the wind direction varies and does not blow directly downwind toward a target 100% of the time. A short averaging time (18.75 sec in PHAST) represents a near instantaneous snapshot of the dispersion and the centerline concentrations will be the maximum possible as if the wind was blowing directly toward the target. A higher averaging time will result in lower average centerline concentrations (beyond the passive transition zone) to account for the centerline of the plume only being directly over the target some of the time while at other times the target is slightly off centerline resulting in lower concentrations. Many toxic endpoint criteria (ERPG, IDLH) have an implied reference time (duration of exposure), and the averaging time should be set to match.

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