Defining the Ammonia Release
The first worst-case release is the 40,000 lb (18.1 tonne) ammonia release.
Inserting the Model Inserting the Model
Select the Study, and then insert a Vessel or Pipe Source Model, either by clicking on the icon in the toolbar, or by selectingVessel or Pipe Source from the Insert menu. The new Model will be given the nameNew Vessel/Pipe Source, and you should rename it immedi-ately to Ammonia, as shown.
When you insert the new Model, you will find that red boxes appear around all of the icons in the Models tab section of the Study Tree. The box appears around the new Model to show that it does not have a
complete set of input data, and you will therefore not be able to process it through the calculations; when you have completed the data input for the Model, the box will disappear. The box appears around the Worst Case Study to show that a Model inside the Study has incomplete data, and similarly for the Study Folder; this effect on the higher levels of the tree can be useful in a large analysis with many Studies.
Setting ppm as Unit for Concentration Setting ppm as Unit for Concentration
Before starting work on the input data, you should set ppm as the default unit for cloud concentration. The previous chapter described how to choose a system of units from the four that are supplied with the program, and this is the next level of refinement—changing the selection of units for a particular system.
To change the system of units, chooseEdit Current System... from the Units cascade in the Options menu, and the Edit dialog will appear. The concentrations use the
“SmallFraction” unit-type, so you must selectSmallFraction in the main list at the left of the dialog, then selectppm in the drop-down list as shown in the illustration on the next page. Next, click on the Replace Unit button, and you will see the unit for SmallFraction change in the main list, and you can then click onOK to implement the change.
Chapter 3: Tutorial 1
The Edit Dialog for Units
Selecting a Material
Setting the Material Data Setting the Material Data
Double-click on the Ammonia icon to open the dialog for the input data, and set values in the Material tab section as shown in the illustration on the next page.
To set the Discharge MaterialDischarge Material, click on the button with three dots at the far right of the dialog, and select AMMONIA from the list which appears, as shown. You will see that the list contains many materials, and not just the two materials that you inserted in the Local Materials folder. The Scope column shows these two materials as Local, whereas all of the other materials are shown asSystem. If the Scope is System, then there is currently no Global or Local version of the material. If you selected one of these System materials (e.g.
BENZENE ), the program would automatically create a version of
Getting Started with PHAST
The Input Values for Material Data
When you select the material, the program automatically sets the Material toMaterial to Track
Track to AMMONIA. You only have to choose a material to track if the DischargeDischarge Material
Material is a mixture.
Note that you can use scientific notation when entering values, so you can enter the inventory as “40e3”.
Chapter 3: Tutorial 1
Setting the Scenario Data Setting the Scenario Data
Move to the Scenario tab and set the following:
Scenario
Scenario 10 Minute Release Release Phase
Release Phase Vapor
For most other types of Scenario, you have to give additional data that will enable the discharge calculations to calculate the release rate. However, for the 10 Minute Release, the release rate is given by (inventory/600 seconds) and not by any discharge calculations, and the Scenario input data are very simple.
Setting the Location Data Setting the Location Data
Move to the Location tab, skipping the Vessel tab, and set the values shown in the illustration:
Getting Started with PHAST
The ElevationElevation has a default value which is greater than zero, and you should leave it with this default value. If a release is located at ground level (i.e. theElevationElevation is zero), the program omits the detailed modeling of liquid droplets and their evaporation and possible rainout, and simply assumes that all of the liquid in the release rains out immediately; this is a reasonable assumption, since liquid droplets will have no opportunity to evaporate during the fall to the ground if they are released directly onto the ground. However, most releases will be at some elevation above ground level, and the program is supplied with a defaultElevationElevation that will give a treatment of the liquid droplets that is more typical of a real release.
This worst case ammonia release is a vapor-only release, so the elevation is not as important as for a liquid or two-phase release, but it is still more realistic to place the release at some distance above the ground.
Leave the NorthNorth and EastEast coordinates with the default coordinates of zero, which will place the release at the srcin for the Map, which is in the middle of the Anysite facility.
You can leave the bund data unset, since they are not relevant to this vapor release.
For a liquid release, however, the presence and size of the bund can have a very large effect on the results: if there is no bund, then the pool from any liquid rainout can spread to cover a very wide area, giving a high evaporation rate from the surface of the pool; whereas if there is a bund, then it limits the area of the pool and the evaporation rate, as you will see in the next chapter.
Leave the three DistancesDistances blank. You can set a distance if you are interested in the effect levels at a particular location, but for this analysis you are interested in the maximum dispersion distance to a concentration of 200 ppm.
Check the box for Concentration of interestConcentration of interest, set a value of 200 ppm, and set UsesUses averaging time
averaging time to Toxic. The significance of the Averaging time is described in detail below.
Averaging Times in PHAST: an
Averaging Times in PHAST: an IntroductionIntroduction
The averaging time is important in PHAST , and is more prominent now than in previous versions. It is used to take into account the effects of changes in the wind direction over the course of the release, and the way that the changes cause the plume to meander from side to side. In order to interpret concentration results correctly, you must know the averaging time that was used in calculating the concentration, and the program allows you to specify different averaging times for different types of concentration results.
Chapter 3: Tutorial 1
The wind does not blow steadily in a straight line; its direction varies with time, which causes a cloud plume to meander from side to side. If you are standing downwind, at one moment you are in the centre of cloud, experiencing the peak concentration, and the next moment the peak has moved away to the side, and you are experiencing a much lower concentration—and in the moment after that, the peak comes back over you and off to the other side, and so on.
The average concentration you receive over, say, 5 minutes will be much less than the peak concentration; if you stood at the location for 30 minutes, the average would be lower still. This factoring down of the peak concentration is carried out by the Averaging Averaging Time Time AdjustmentAdjustment—the longer the time window, or Averaging Averaging Time
Time, the lower the calculated average concentration will be.
For the Concentration of InterestConcentration of Interest, you can choose between several averaging times, depending on the type of release. For a toxic-only material, there are five choices: aUser-Defined time that you set in theUser-definedUser-defined field at the bottom left of the dialog. group below; a Toxic time that is set in the Toxic Parameters; and the ERPG, IDLH and STEL times that are set as part of the definitions of these measures of toxicity, and cannot be changed. When you select a type of averaging time from the list, the value of the averaging time will be displayed in the field to the right of the list; the default toxic averaging time is 600 seconds, which is also the duration of this release.
Setting the Indoor/Outdoor Data Setting the Indoor/Outdoor Data
Next, move to the Indoor/Outdoor tab and set the Release DirectionRelease Direction to Horizontal.
You can model a release as out of doors, where the only obstruction is the ground, or as inside a building, where the size and ventilation of the building affects the initial stages of dispersion.
Ignoring the Other Tab Sections Ignoring the Other Tab Sections
You skipped the Vessel tab section, and you can ignore all of the remaining tab sections and clickOK to save the changes you have made.
For a vapor release, the Vessel tab section is only relevant if you want to perform time-dependent discharge modeling, in which case you must give information about the dimensions of the vessel and the liquid level. Such modeling is not applicable to the 10 Minute Release scenario, which requires only the simplest
Getting Started with PHAST