COMUNICACIÓN DE LA AGRUPACIÓN

Viewing the Results

The Graphs give the most direct way of viewing the results. To view the Graphs for a Model, select that Model, then clickCtrl+G, or choose the Graph option from the View menu or the right-click menu. The Plot Setup dialog will appear, prompting you for the Weather and Map to use, and when you click onOK, the program will generate the Graphs, and display them in the Graph Window.

Getting Started with PHAST

The Results for Ammonia The Results for Ammonia

There are seven Graphs which show concentration results. For this analysis, the most important Graph is the Map. When you first move to the Map tab section, there will be two concentration contours shown on the Map, for around 200 ppm (the concentration of interest) and around 400 ppm, as shown in the illustration. These contours are some distance from the site, and show that the cloud has become detached from the release point because the time taken for the cloud to disperse to 200 ppm was much longer than the ten minute duration of the release.

This aspect of the release makes the results quite complex, and you may find them difficult to interpret at first, especially as the program gives much more detail in the results than in previous versions and provides many more options.

The first thing to notice in Graphs of this type is theTimeTime displayed in the legend.

In the Map in the illustration, the time is given as 2297.76 s, and this is the time after the start of the release at which the area covered by the 200 ppm contour (the contour for the concentration of interest) was largest.

The Concentration Contours on the Map

Chapter 3: Tutorial 1

To see the contours for other times, select Dynamic from the Graph menu or the right-click menu, and the Cloud Dynamics Control will appear. You use this Control to view an animation of the cloud dispersion.

Click on the rewind button at the left of the control to set the animation time to the beginning of the release, and then click on the play button at the right of the control to start the animation. You will see the development of the cloud displayed on the Map, and when the time reaches about 600 seconds (as shown in the legend), you will see the cloud become detached from the release point.

If you run the animation to the end of the release, you will see the 200 ppm contour reach the town, and disappear off the map. This shows that the worst case ammonia release does have the potential to reach populated areas offsite.

The effect of the cloud will depend on the time that it takes to pass over the town, and you can see this in the Time tab section. The Time Graph shows the time-dependent concentration profile at a particular distance from the release source.

When you first move to the tab section, the distance will be set as the mid-point of the cloud at the time that the contour covers the largest area (i.e. as in the first view of the Map Graph), but you can change this distance.

The distance from the release to the middle of the town is approximately 14,000 ft (4.3 km). To set this as the distance for the Time Graph, select Properties from the Graph menu or the right-click menu, and set the value as shown:

The Cloud Dynamics Control

Getting Started with PHAST

When you click onOK and return to the Time Graph, the Graph will change, and you can see the concentration profile at the town. The Graph shows that a person at that point would only be exposed to the cloud for about ten minutes, but the concentra-tion during this time would be over 400 ppm. The 200 ppm concentraconcentra-tion of interest is based on an exposure of an hour, so the effects from this cloud should be small, but could still be unpleasant.

The Results for Hydrogen Cyanide The Results for Hydrogen Cyanide

As with the ammonia release, the hydrogen cyanide release becomes detached from the source.

The 10 ppm contour does not reach its largest area until it is has passed over the town, so the cloud will not be visible in the Map Graph when you first move to the Graph.

If you use the Properties... option to set the distance of interest to 14,000 ft and move to the Time Graph, you will see that the duration of exposure at the town is ten minutes, as for the ammonia release. However, for most of this time, the concen-tration is over five times the concenconcen-tration of interest (i.e. over 50 ppm), whereas for the ammonia release, the concentration is twice the concentration of interest.

The Concentration Profile at the Outskirts of the Town

Chapter 3: Tutorial 1

The difference in the values for concentration of interest makes it difficult to compare the concentration results for ammonia and hydrogen cyanide using the Map and Time Graphs. However, the Lethality Graph allows you to compare the toxic effects directly, and you can also plot the results for the two Models on the same Graph.

In order to plot the combined results, you must open a third Graph window.

Move to the Weather tab section of the Study Tree, select the 1.5/F Weather, and then press Ctrl+G to generate the Graph. The Plot Setup dialog will open, with a Model tab section instead of a Weather tab section, and you can se-lect the two toxic Models, as shown:

The Concentration Profile for Hydrogen Cyanide

Plotting a Graph for more than one Model

Getting Started with PHAST

The Lethality Graph shows the results for both Models, and it shows that the toxic effects of the Hydrogen cyanide Model are worse than those for the Ammonia Model. At the town, the ammonia release gives zero probability of death, whereas the hydrogen cyanide release gives a significant probability, of about 10%.

If you look at the Graphs for concentration, you will find that they are plotting the results for 200 ppm, i.e. the concentration of interest for ammonia. The program cannot plot a comparison of the results for 10 ppm, because the calculations for ammonia stopped at 200 ppm, so it can only compare the results for 200 ppm.

This comparison at 200 ppm may be misleading, because the inventory for the Hydrogen cyanide Model is much smaller than for the Ammonia Model, and the cloud is diluted to 200 ppm much more quickly. This emphasizes that some Graphs are useful for some purposes (e.g. getting the details of the results for a single Model, or for comparing Models that involve the same material) whereas other Graphs are useful for other purposes (e.g. comparing Models that involve different materials).

The Results for Ethylene and Propylene The Results for Ethylene and Propylene

The two flammable Models have the same critical effect-level, i.e. 1 psig, and their results can be compared directly. Move to the Weather tab section, select the 1.5/F Weather, press Ctrl+G, and then select the two flammable Models in the Models tab section of the Plot Setup dialog, and make sure that the Anysite surroundings Map is selected in the Map tab section.

Comparing the Toxic Models in the Lethality Graph

Chapter 3: Tutorial 1

When you first move to the Map Graph, it will be displaying the concentration contours for the two Models. This is the default option for the results displayed on the Map, but you can use the Properties... option to change this. In the Display tab section,change the Display Option for MapDisplay Option for Map from Cloud Footprint to Early Explosion Overpressure, as shown. You should also move to the Overpressures tab section, and set the Primary OverpressurePrimary Overpressure to 1 psig, as shown.

When you have clicked on OK and the program has redrawn the Map Graph, you will see that the overpressure radii to 1 psig do not extend outside the boundary of the site, and pose no threat to the town, as shown in the illustra-tion.

Setting the Graph Properties for Viewing the Overpressures on the Map

The Explosion Results for

Getting Started with PHAST