Instantaneous Release Instantaneous Release

Stage 1: Instantaneous Release

The previous chapter described a simple worst case analysis for the Anysite chemical installation. This analysis considered releases of four materials held on the site in significant quantities, and determined that the hydrogen cyanide release had the greatest potential to reach beyond the site boundary.

The scenario modeled in the worst case analysis for hydrogen cyanide was a release of the entire inventory as vapor over a period of ten minutes. These investigations will determine whether this is a realistic scenario, and examine the effects of several variables in the modeling.

In thefirst stage of the investigations, you will model the release of the entire inventory as an instantaneous release. The worst-case scenario was devised assuming that the ten-minute continuous release would give the greater dispersion distances and the greatest toxic effect, and this stage is intended to confirm that assumption, so that the other stages can concentrate on the factors affecting continuous releases.

Create a Separate Study for the

Create a Separate Study for the InvestigationsInvestigations

Before you can model the instantaneous release, you must make some preparations in the Anysite Study Folder.

Open the Study Folder, select the Study Folder in the Study Tree, and insert a new Study using the icon in the Toolbar or the option in the Insert menu.

Give the new Study the name HCN Investigation. You will be placing all of the Models for this tutorial in this Study.

Create a Special “Global” Study for Shared Weathers and Maps Create a Special “Global” Study for Shared Weathers and Maps

When you start to divide a Study Folder into multiple Studies, you have to make some adjustments in the design philosophy for the Study Folder. Some of these adjustments are essential, and you must make them to ensure that the Models in the various Studies are processed as you intended. Other adjustments are optional, but will make the purpose and design of the analysis clearer to another user.

This section describes how to create and use a “global” Study as a location for Weathers and Maps that are shared between the Worst Case Study and the HCN Investigation Study. This is an optional adjustment, but strongly recommended.

Chapter 4: Tutorial 2

Insert another Study, and give it the name Global Weathers and Maps. The program inserts each new Study at the bottom of the list of Studies. However, it will make the design of the analysis clearer if theGlobal Study is at the top of the list of Studies, especially if you are going to add more Studies to the list. PHAST does not allow you to simply drag the Global Study to the top of the list, and you must take the following steps in order to obtain the order that you require:

• Drag the icon for the Worst Case Study up to the icon for the Anysite Study Folder The program acts as if you have just “inserted” the Study into the Study Folder, and places it at the bottom of the list.

• Drag the icon for the HCN Investigation Study up to the icon for the Anysite Study Folder

Again, the program places the Study at the bottom of the list.

The Studies will now be in the more-natural order shown in the illustration.

Next, move to the Weather tab section of the Study Tree, select the 1.5/F Weather, and drag it to the Global Study. Then move to the Map tab section of the Study Tree and do the same with the Anysite surroundings Map. Finally, select Batch/

Weather Setup... from the Options menu, and se-lect the 1.5/F Weather under theGlobal Study.

The Global Study makes use of the fact that the program can run a Model in a given Study with a Weather in any other Study, and can display the results on a Map from any other Study. This means that you could have left the Weather and Map in the Worst Case Study, and the Models in theHCN Investigation Study would have been able to use them; however, it would not be obvious to another user that the Weather and Map were supposed to be shared, and this user might find it difficult to duplicate your results. When you use aGlobal Study, it is clearer which Weathers and Maps are intended to be shared, and which are specific to particular Studies.

The Rearranged Studies

Getting Started with PHAST

Move the Local Version of Hydrogen Cyanide to t

Move the Local Version of Hydrogen Cyanide to the Global Materialshe Global Materials

In this second tutorial, you are still only interested in the toxic effects of hydrogen cyanide, and not in the flammable effects. If you ran a Model in the new Study now, the program would not find a version ofHYDROGEN CYANIDE in theLocal Materials folder for the new Study, and would create a version in the Global Materials folder as a copy of the System Materials version, which is both toxic and flammable.

To ensure that the HCN Investigations Study uses the toxic-only version ofHYDROGEN CYANIDE, you can either copy the local version from the Worst Case Study to the HCN Investigations Study, or you can move the local version from the Worst Case Study to the Global Materials folder. The second approach is simplest, and is recommended here. To move the material, you can drag it from one folder to the other.

Making the toxic-only version into the Global version involves a slight shift in the design philosophy for the analysis. As a Local material, the toxic-only version was the exception, but as a Global material it is now the rule. If you want to investigate the flammable effects of hydrogen cyanide, you will have to create a further Study, insert a Local version of hydrogen cyanide into that Study, and then edit the input data to make the Local version both flammable and toxic. This is another example of the shifts in design philosophy that occur when you start to divide a Study Folder into multiple Studies.

Rerun the Worst Case Study Rerun the Worst Case Study

As you saw in the previous chapter, you can only compare the results of Models if they have been processed with the same Weather. In this tutorial, you will be comparing the worst case results for hydrogen cyanide with the results for the various investigations. Since you will be running the investigations with the Weather in the Global Study, this means that you must also run the worst case releases with this Weather. Select the Study (or just the Hydrogen cyanide Model), and press Ctrl+M to rerun. You do not need to view the results at this stage.

This may seem like a duplication of effort, since you previously ran the worst case releases with exactly this Weather when it was in theWorst Case Study. However, the program identifies Weathers by their location only, and does not know that the two Weathers are identical and that the results from theWorst Case Weather would be entirely valid for the Global Weather.

Chapter 4: Tutorial 2

Create and Run the Catastrophic Rupture Model Create and Run the Catastrophic Rupture Model

Select the Hydrogen cyanide Model from theWorst Case Study, copy it, and paste it into the HCN Investigation Study. Rename the copy Instantaneous.

Double-click on theInstantaneous Model to open the dialog, move to the Scenario tab section, and change the Scenario from 10 Minute Release to Catastrophic Rupture. This is the only change you need to make, and you can click on OK to close the dialog and save the change.

Select theInstantaneous Model and pressCtrl+M to run the calculations for the Weather in the Global Study.

View the Results View the Results

To compare the results for the two Models, move to the Weather tab section of the Study Tree and select the 1.5/F Weather from the Global Study. In the Plot Setup dialog, select the Worst Case/Hydrogen cyanide Model and theHCN Investigation/Instantaneous Model; don t forget to select the Map as well.

The Time and the Lethality Graphs give the best comparison. The instantaneous cloud does reach the town, and its peak concentration at the town is similar to that for the worst case release. However, the duration of exposure is much shorter, and the lethality is therefore much lower, as shown in the graph on the next page.

Getting Started with PHAST