Phast

DNV SOFTWARE PHAST Getting Started Manual Chapter 2: Performing a Worst-Case Analysis

PHAST Getting Started Manual

Chapter 2: Performing a Worst-Case Analysis

Table of Contents Introduction..................................................................................................................................... 1 Starting PHAST .............................................................................................................................. 2 Introduction to the Analysis............................................................................................................ 3 Creating the Anysite Study Folder.................................................................................................. 6 Setting the Materials Input Data ..................................................................................................... 7 Setting the Weather Input Data..................................................................................................... 10 Setting the Map Data .................................................................................................................... 12 Defining the Ammonia Release .................................................................................................... 16 Defining the Hydrogen Cyanide Release...................................................................................... 21 Defining the Ethylene Release...................................................................................................... 22 Defining the Propylene Release.................................................................................................... 25 Viewing the List of Global Materials ........................................................................................... 26 Running the Calculations.............................................................................................................. 27 Viewing the Results ...................................................................................................................... 28 Summary of Worst Case Analysis ................................................................................................ 34

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Introduction

This manual will lead you through the mail features of PHAST and PHAST Micro, by opening a pre-defined case so that you can view both input data and results.

All Examples are from PHAST Micro

There are two versions of PHAST, and this manual covers both of them. PHAST Micro is the simpler version, containing DNVs sophisticated dispersion modelling in full, but with some limitations to the options in other areas of the modelling. PHAST is the fully-featured version, offering control over most aspects of the modelling, and including stand-alone versions of the fire, explosion and pool vaporization models that are built into the integrated dispersion modelling. All of the examples in this chapter are based on PHAST Micro and are fully applicable to that version. If you are using PHAST, you will see some features in your program that do not appear in the illustrations and are not described in the text. Instruction on these features are given in the PHAST Introductory Training Course. For details on the PHAST training courses please contact your local Technical Support centre.

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Starting PHAST

When you install PHAST, the installation routine places a DNV folder under Programs in your Start menu, and you can start PHAST running by selecting the icon from the folder.

The DNV folder in the Start Menu The installation routine also places a PHAST icon on the desktop so you can also start PHAST running by clicking on the desktop icon.

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Introduction to the Analysis

In this chapter, you will perform a simple worst-case analysis for the Anysite chemical installation, to determine whether releases on the site have the potential to reach populated areas beyond the site boundary.

Hazardous Materials There are four hazardous materials present on the site in significant quantities:

Material Type of Hazard Mass Present lb tonne Anhydrous ammonia Toxic 40,000 18.1 Hydrogen cyanide Toxic 5,000 2.3 Ethylene Flammable 50,000 22.7 Propylene Flammable 75,000 34.0

Hazardous Inventory for Anysite Facility

Storage Conditions The ethylene is stored under supercritical conditions, and the three other materials are stored under saturation conditions. For the worst-case analysis, the materials will be modelled at the maximum temperature experienced at the facility over the last five years, which is 90F (32C). At this temperature, the storage pressures for the materials are as follows:

Material Conditions Storage Pressure Psig barg Anhydrous ammonia Saturation 180.1 12.4 Hydrogen cyanide Saturation 18.7 1.3 Ethylene Supercritical 700.0 48.3

Storage Conditions

Release Scenarios Different scenarios will be modelled for the toxic and the flammable materials, since different types of release cause the worst long-range effects. For the two toxic materials, the release scenario will be a release of the entire inventory over ten minutes, and for the two flammable materials, the scenario will be an instantaneous release of the entire inventory. For toxic releases, the duration and concentration profile at the populated areas are more important than the total mass in the cloud at any given time. A large continuous release will give

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a greater duration of exposure than the equivalent instantaneous release. It may also take longer to disperse to harmless concentrations, since air is mixed into the cloud from the sides only, whereas air is mixed into an instantaneous release across all exposed surfaces. For flammable releases, the greatest effect distances are usually produced by vapour cloud explosions, and the size of these explosions depends on the flammable mass in the cloud at the time of the explosionwhich will be greater for an instantaneous release than for a continuous release.

Critical Effect Zones For the toxic materials, the calculations will obtain the dispersion distances to the Emergency Response and Planning Guidelines (ERPG) Level 2 concentration, which is the concentration that nearly all individuals can be exposed to for up to an hour without experiencing any irreversible adverse health effects or symptoms which could impair the ability to take protective action. For ammonia, this concentration is set at 200 ppm, and for hydrogen cyanide, it is set at 10 ppm. For the flammable materials, the calculations will obtain the explosion distances to an overpressure of 1 psig, which is an overpressure that may cause injuries as a result of minor structural damage (e.g. broken windows), but is unlikely to cause fatalities.

Weather Conditions The calculations will use a windspeed of 5 ft/s (1.5 m/s) and an atmospheric stability of F, which are common night-time conditions for the location. These conditions give low levels of atmospheric turbulence, and the release may travel long distances before being diluted to a harmless concentration. The average humidity for the location is 70%, which is typical for a temperate, maritime location. The calculations require a value for surface roughness, which is a measure of the turbulence induced in the air as it moves over the ground, and will be set conservatively to 0.06, a value for sea or for flat, treeless land. This assumes that the wind is blowing towards the town, and that the surface conditions upwind of the release determine the surface roughness.

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Location of the Anysite Facility

As shown in the map, Anysite is a large, ocean-side facility, located in an industrial area, and nearly two miles from the nearest residential area.

The Anysite Chemical Facility

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Creating the Anysite Study Folder First, you must create a new Study Folder to store all of your work on the Anysite facility. Close any Study Folder that is currently open in PHAST, and then select New from the File menu.

The program will create a new Study Folder called Untitled with an empty Study called New Study.

Saving the Study Folder

You cannot save a Study Folder with the name Untitled. Use either the Save or the Save As... The New Study Folder options in the File menu to save the new Study Folder in the study pane to the DNVuser directory with the name Anysite.PSU.

Renaming the Study

Click on the Study to select it, and then choose Rename from either the Edit menu or the right-click menu. An insertion point will appear in the name of the Study, and you should edit this to change it to Worst Case.

Using Program Preferences to Open the Study Folder Automatically

All of the tutorials in this manual use the Anysite Study Folder. If you do not perform the tutorials in a single session, you will be returning to the Study Folder several times. The list of recently-used Study Folders at the bottom of the File menu makes it easy to re-open a Study Folder that you have been working on, but you can also use the Preferences for the program to make this even easier.

Select Preferences... Installation from the Options menu. The Preferences dialog will appear, and you should set the option in the Startup tab section to Try to open most recently used file, as shown in the illustration on the next page. If the file has been deleted or moved, the program will display a File Open dialog instead, so you can locate the file yourself.

Setting the Preferences for Opening a Study

Folder Automatically

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Setting the Materials Input Data

In the database of System Materials supplied with the program, ammonia and hydrogen cyanide are defined as being both flammable and toxic. However, for the worst case analysis, you are only interested in the toxic effects, and you can simplify the input data and the results if you define them as toxic only for this analysis. You do this by creating local copies of the materials, and editing the property data. If you wish, you can omit this stage, since it is not essential. However, you may find it useful as a quick and straightforward introduction to the properties system.

Creating Local Versions of the Toxic Materials Move to the Materials tab section of the Study Tree, select the Local Materials folder under the Worst Case Study, and select Material... from the Insert menu. The Insert Material dialog will appear, as shown in the illustration below.

Inserting a Local Version of Ammonia

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The dialog offers three ways of inserting a material. The New option allows you to create a completely new material, with no pre-defined properties data. The Existing and Copy options both allow you to create a copy of a material that is already in the materials database at a higher level (i.e. at the System or Global level): the Existing option keeps a link to the original material, and if the values for the original material are changed, the program will automatically update the values for any fields that are still using the original, default values; the Copy option does not keep a link, and the local version will not be affected by any changes to the original material. Select the Existing option, locate and select AMMONIA in the list of materials, and then click on OK to add the material to the Local Materials folder. Next, repeat the process, selecting HYDROGEN CYANIDE as the material.

Editing the Materials Data for the Local Materials When you expand the Study Tree below the Local Materials folder, you will see the icons for the two materials. Double-click on the icon for AMMONIA to open the input dialog, and set the Flammable/Toxic field in the General tab section from the default value of Both to Toxic only, as shown in the illustration on the next page. Click on OK to save the changed data, and then repeat the process with the local version of HYDROGEN CYANIDE.

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The Changed Data for Ammonia

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Setting the Weather Input Data

Before defining any of the worst-case releases, you will define the other aspects of the input data, which will be the same for all four releases: the Weather data, and the Map data. Each new Study Folder is created with a set of default Weather conditions defined for the default Study, as shown in the illustration. These default Weathers are representative of the range of common conditions, and they enable you to obtain results for a new Study Folder very quickly. For this Worst Case analysis, you are only going to model one condition1.5 m/s with F stability which is one of the default conditions.

The Default Weather Conditions for a New Study Folder

Delete the Unwanted Conditions First, delete the other default conditions. You delete an icon from the tree by clicking on it to select it, and then using the Delete (Del) key or the Delete option in the Edit menu or the right-click menu. You could leave the conditions in the tree, but it will make the design of the analysis clearer if you delete them.

Set the Detailed Weather Data Next, double-click on the Weather 1.5/F icon to open the dialog for input data, and set the following values in the Atmospheric Parameters tab section:

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The Atmospheric Parameters Weather options

All of the fields in the Atmospheric Parameters tab section take their initial values from the defaults system, which is shown by the green border around each field. When you change the values to those required for this analysis, you will see that the border disappearsthe colour-coded borders mean that you can see at a glance which fields in a dialog are using the default values directly, and which have been changed.

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Setting the Map Data

In the Map tab section of the Study Tree, select the Raster Image Set option under the Anysite Plant, and then select the option to insert a new Raster Image using the icon in the Toolbar, the Insert menu, or the right-click menu.

Selecting the Bitmap Image A bitmap image of the Anysite facility and its surroundings is supplied with PHAST. This can be selected by browsing to the C:\Program Files\DNV\PHAST_6_4_2\Examples\ folder and selecting the Anysite.bmp file.

The Insert Raster Image Screen

Once you have selected this raster image the Interactive Placement Mode option will become enabled. This means that you can place your raster image onto your Map Window interactively. The remaining Placement Mode options are not available because the Anysite.bmp file does not contain GIS data. If you use a GIS raster image, with Header or Georeference data, PHAST will place your map automatically for you. For further details on the GIS system in PHAST please refer to the Online Help system. When you press OK a Coordinate System Wizard will appear. Press Cancel to exit this wizard, as it is for use with true GIS raster images. Again, for further details on the Co-ordinate System in PHAST please refer to the Online Help system.

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The Coordinate System Wizard

A blank Map screen will now appear, for you to draw your raster image on to, using your mouse, which will appear as a cross. Draw your map onto the blank Map screen, ignoring the actual coordinates on the screen as you will set the origin and scale of the map in your next step. NOTE: After pressing cancel you may have to wait a while before the blank map appears. Do not

click elsewhere on the screen during this time or the program will switch to a new action and you will have to repeat the Insert Raster Image step.

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Setting the Scale

You set the scale by drawing a line between two points whose distance apart is known, then typing in the distance. In this case you will draw a line across the entire map, which is 5 km wide.

The Map Scale and Origin menu

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To set the scale, select the map by clicking on it, then choose Map Scale and Origin... Set Scale from the Map menu or the right-click menu. Then draw a line across the entire width of the map. A dialog box will appear asking for the length of your line, so you should type in 5 km. When you press OK to close the scale window you will see that the horizontal and vertical axes have been rescaled for the new value.

Setting the scale

Setting the Origin The location of the map is defined when you first draw the raster image onto the blank map screen. The co-ordinates at that time are unlikely to be correct for the map, whether you are setting them with global co-ordinates or reference co-ordinates (in relation to a specific place). Therefore, you must set the raster image origin manually after placing it on your screen and setting the scale.

In this example you will set a reference origin of (0, 0) in the middle of the Anysite Facility. You will later set the co-ordinates of the release to (0, 0), which will place them at this origin.

Setting the Origin To set the origin, select the map by clicking on it, then choose Map Scale and Origin... Set Origin from the Map menu or the right-click menu. The cursor will change to a cross-wire, and you simply click on a point on the map to set that point as the origin. A dialog box will appear for you. For this worst case analysis, you do not have to place the origin with great precision, and any location near the middle of the site will be suitable.

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Defining the Ammonia Release

The first worst-case release is the 40,000 lb (18.1 tonne) ammon

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 selecting Vessel or Pipe Source from the Insert menu. The new Model will be given the name Vessel/Pipe Source, and you should rename it immediately 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 The ntherefore not be able to process it through the calculations. When you have completed the data input for the MThe box appears around the Worst Case Study to show that a Mincomplete data, and similarly for the Study Folder; this effect obe useful in a large analysis with many Studies.

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 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 ia release.

ew model in the study tree

odel, the box will disappear. odel inside the Study has

n the higher levels of the tree can

Selecting a Material

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of the other materials are shown as System. 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 the material in the Global Materials folder for the Study Folder, and the next time you opened the Select Material dialog, you would see that the Scope for the material had changed from System to Global; you will see this later, when you are defining Models for the two flammable materials. Now enter the remaining data in the Material tab, using the data shown in the screen below.

The Input Values for Material Data

When you select the material, the program automatically sets the Material to Track to AMMONIA. You only have to choose a material to track if the Discharge Material is a mixture. Note that you can use scientific notation when entering values, so you can enter the inventory as 40e3.

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Setting the Scenario Data Move to the Scenario tab and set the following:

Scenario 10 Minute Release Release Phase Liquid

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 calculated as (inventory/600 seconds) and not with any discharge calculations, so the Scenario input data are very simple.

Setting the Location Data Move to the Location tab, skipping the Pipe and Vessel tab, and set the values shown in the illustration:

Location Input Data

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The Elevation 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. the Elevation is zero), the program omits the detailed modelling 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 default Elevation that will give a treatment of the liquid droplets that is more typical of a real release. This worst case ammonia release is a vapour-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 North and East coordinates with the default coordinates of zero, which will place the release at the origin for the Map, which is in the middle of the Anysite facility. Leave the three Distances 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 interest, set a value of 200 ppm, and set Uses averaging time to Toxic. The significance of the Averaging time is described in detail below.

Averaging Times in PHAST: an Introduction 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. 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 concentrationand 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 Time Adjustmentthe longer the time window, or Averaging Time, the lower the calculated average concentration will be. For the Concentration of Interest, you can choose between several averaging times, depending on the type of release. For a toxic-only material, there are five choices: a User-Defined time that you set in the User-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

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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 Bund Data You can leave the bund data unset, since they are not relevant to this vapour 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.

Setting the Indoor/Outdoor Data Next, move to the Indoor/Outdoor tab and set the Release 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. All continuous releases you must set the direction. Instantaneous releases do not require a direction as the inventory will be released in all directions.

Ignoring the Other Tab Sections You skipped the Vessel tab section, and you can ignore all of the remaining tab sections and click OK to save the changes you have made. For a vapour release, the Vessel tab section is only relevant if you want to perform time-dependent discharge modelling, in which case you must give information about the dimensions of the vessel and the liquid level. Such modelling is not applicable to the 10 Minute Release scenario, which requires only the simplest discharge modelling. The remaining tab sections allow you to change the default settings for explosion, fire and discharge modelling. For a 10 Minute Release of a toxic-only material, these tab sections are not relevant.

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Defining the Hydrogen Cyanide Release

The ammonia and the hydrogen cyanide releases have the same data for the Scenario and Indoor/Outdoor tab sections, and differ only in the Material and Location data. To take advantage of this, you will create the Hydrogen cyanide model as a copy of the Ammonia model, and then edit the Material and Location data.

Copying the Ammonia Model Select the Ammonia icon, and then select Copy from the Edit menu or from the right-click menu. Then select the Worst Case Study, and select Paste from either menu. The program will give the copy the name Ammonia(1), and you must rename it to Hydrogen cyanide.

Setting the Material Data Double-click on the Hydrogen Cyanide icon to open the dialog, and change the values in the Material tab section to the following values:

Discharge Material HYDROGEN CYANIDE Inventory 2300 kg (2.3 tonnes)

Setting the Location Data Move to the Location tab section and change the values to the following:

Concentration of interest 10 ppm These changes complete the data for the release, and you can click on OK to save the edited data.

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Defining the Ethylene Release

The input data for the flammable releases are significantly different from those for the toxic releases, and there is nothing to be gained from copying one of the existing releases. Create the ethylene release by inserting a new Vessel or Pipe Source Model, and give it the name Ethylene.

Setting the Material Data Double-click on the Ethylene icon to open the dialog, and set the values in the Material tab section as follows: Discharge Material ETHYLENE Inventory 22,700 kg (22.7 tonnes) Process Conditions Temperature Pressure Temperature 32 oC Pressure 43 barg The ethylene is stored under supercritical conditions, and you must specify both the temperature and the pressure. Before you complete the entry of the temperature and pressure, you will see that the program gives the Vessel Type and Phase as Unknown. At this point, the program has not checked the state of the material at these conditions, and does not know that it is supercritical. Once you enter the temperature and pressure then click or tab in another field the Phase will change to show the material is stored as a Vapour and the Vessel Type will change to show Pressurised Gas.

Setting the Scenario Data In this case you will accept all the Scenario tab default values, including the catastrophic release settings, for which there is a choice. You will see that in this tab the release phase has been set to Vapour. This happens because the program has checked the phase and determined that the material is supercritical (which the program models as vapour). Because you have set your release as a Catastrophic Vapour you can ignore both the Pipe and Vessel tabs.

Checking the Location Data Move to the Location tab section and view the data as shown in the illustration on the following page:

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The Location Input Data for the Ethylene Model

Unlike the toxic cases, you do not need to set a Concentration of interest or choose or set an associated Averaging Time. For flammable releases, the program automatically performs the dispersion to a fraction of the lower flammable limit (where the fraction is set in the Flammable Parameters), using the Flammable Averaging Time (also set in the Flammable Parameters). If you are interested in the details of the concentration results for a flammable material, you might set an additional concentration of interest and a user-defined averaging time, but for this analysis the effects from an immediate explosion are likely to be more significant than any later cloud dispersion.

Checking the Flammable Data Move to the Flammable tab section and check that the Explosion Method is set to TNT. PHAST now has three explosion models available.

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Leave the Early Explosion Mass Modification Factor with its default value of 3. This factor is used in calculating the mass involved in an early explosion. The program calculates the mass of vapour in the cloud after it has expanded to atmospheric pressure, and then multiplies this mass by the Modification Factor to obtain the explosion mass, with an upper limit set by the flammable mass released.

Setting the TNT Data Move to the TNT tab section. The tab section is to the right of the Flammable tab section and may not be immediately visible when the dialog first opens. If you cannot see the tab section, use the navigation button at the far right of the tabs to reveal the other tabs in the dialog. Leave the TNT Explosion Efficiency with its default value of 10%. This determines the fraction of the combustion energy in the explosion mass that is converted into explosion energy. Set Air / Ground Burst to Ground Burst, which means that the explosion occurs near the ground, i.e. at the same elevation as the release. For this type of explosion, the effects of reflection from the ground are assumed to double the amount of energy involved in an explosion, so this type will give the worst case results. These changes complete the data for the release, and you can click on OK to save the edited data.

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Defining the Propylene Release The propylene release differs from the ethylene release only in the Material data, so you can create it as a copy of the Ethylene model, using the method described for creating the hydrogen cyanide release. Give the copied model the name Propylene.

Setting the Material Data Double-click on the Propylene icon to open the dialog, and change the values in the Material tab section to the following values: Discharge Material PROPYLENE Inventory 34,000 kg (34.0 tonnes) Process Conditions Temperature Saturated Liquid Temperature 32 oC When you change the material, the program performs flash calculations to check the current process conditions and updates the reported Phase and Fluid Type if necessary. At 32 oC and 48 barg, ethylene is a supercritical vapour but propylene is a liquid. Therefore, you will see the Phase change from Liquid to Two-Phase and the Fluid Type change from Pressurised Liquid to Saturated Liquid.

Checking the Scenario Data Move to the Scenario tab section, and you will see that the only choice for Phase to be released is Liquid. The presence and design of a bund or dike can have a significant effect on liquid releases, but you should leave the bund data in the Bund Data tab section unset, as with the vapour releases, since this will allow the liquid pool to spread indefinitely, giving a larger evaporation rate than with a bund. This completes the release data, and you can click on OK to save the edited values.

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Viewing the List of Global Materials

Move to the Materials tab section, and expand the tree under the Global Materials folder. You will see that icons for ETHYLENE and PROPYLENE have been added to the folder; each was added as a copy of the System version when you selected the material in the input dialog. The Ethylene and Propylene Models obtain their materials data from these Global versions. If you add Models for further ethylene or propylene releases, these Models will also use these versions. The global & local materials lists

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Running the Calculations

To run the calculations for all of the Models, move back to the Models tab in tab in the Study Tree and select the icon for the Worst Case Study, then start the run. There are three ways of starting a run: you can select Model(s) from the Run menu or Run Model(s) from the right-click menu, or you can press Ctrl+M. You can follow the progress of the run in the Progress Meter, and also in the Message Log tab section of the Log Window.

Running the calculations

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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 click Ctrl+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 to use, and when you click on OK, the program will generate the Graphs, and display them in the Graph Window. In this example we will view the Category 1.5/F weather, though you can view more than one weather if you wish.

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 facility, 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.

The map graph view

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 the Time displayed in the legend. In the illustration above, the time is given as 1881 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.

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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. Note: The Dynamic option will not be available if you have selected more than one weather from the Plot Setup screen. 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.

The cloud dynamics control screen

If you run the animation to the end of the release, you will see the 200 ppm contour go into the ocean, and disappear off the map. In this direction the cloud does not have reach any populated areas. However, the wind could also be moving from the south, and we should examine the effects from this direction.

To model the release from the south select Wind Direction from the Graph menu or the right-click menu, and the Wind Direction control screen will appear. Move the bar to 180 degrees, as shown, to indicate the direction the wind is coming from.

The cloud dynamics control screen After setting the Wind Direction you should again run the animation to see if the cloud reaches the town. You will then see 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 Concentration vs Time graph. This 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 4.3 km. To set this as the distance for the Concentration vs Time Graph, right-click on the centre of the graph image and select Properties from the right-click menu, and set the value as shown:

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Setting the downwind distance for the concentration vs time graph When you click on OK 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 concentration during this time would be over 400 ppm. The 200 ppm concentration 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 concentration vs time graph

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This graph shows many vertical lines from 0 to 400 ppm. These represent the end points of different cloud segments which are produced because the Ammonia release forms a pool which vaporises at different rates. For further details on cloud segments please refer to the on-line Help system.

The Results for Hydrogen Cyanide Unlike the ammonia release, the hydrogen cyanide release does not become detached from the source. This is because the cloud rains out close to the source and the pool vaporises about an hour. The wider cloud positioned further from the source is produced by pool segments with a high vaporisation rate. The thinner cloud closer to the source is produced by pool segments with a lower vaporisation rate. If you use the Properties... option to set the distance of interest to 4.3 km and move to the Time Graph, you will see that the duration of exposure at the town is just under one hour. In addition, for some of this time, the concentration is almost four times the concentration of interest of 10 ppm. This indicates a much more significant hazard than the ammonia release, which reached twice the concentration of interest for only ten minutes. However, the difference in the values for concentration of interest makes it difficult to correctly compare the concentration results for ammonia and hydrogen cyanide using the Map and Time Graphs. However, the Lethality Graph (stored under the Toxic tab) 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 from the Weather tab section of the Study Tree. To do this move to the weather tab and 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 select the two toxic Models, as shown:

Plotting a graph for more than one model

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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. However, neither gives a significant probability of death at the town.

The Probability of Fatality Graph

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 two flammable Models have the same critical effect-level, i.e. 0.02068 bar, 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. 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.

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In the Display tab section, change the Map Event from Dispersion to Late Explosion, as shown.

Setting the graph properties for viewing the late explosion overpressures on a map

When you have clicked on OK and the program has redrawn the Map Graph, you will see that the overpressure radii to 0.02068 bar do not extend outside the boundary of the site, and pose no threat to the town, as shown in the illustration.

The explosion results for the flammable models

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Summary of Worst Case Analysis

The Worst Case analysis shows that the hydrogen cyanide inventory poses the greatest offsite risk, although no scenarios are capable of causing fatalities at the town. If you require any further information on any of these cases please contact your local Technical Support desk or sign up for one of our training courses.

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