Casqade Tutorial
The program has two modes of operation, the ‘automatic mode’ and ‘manual mode’. The automatic mode is the recommended method and is based on the concept of a ‘scenario’. The scenario is a release of material into the atmosphere resulting in a physical effect such as a vapour cloud, a fire or an explosion. The scenario may be one of three types:
- A transient release of material into the atmosphere from a breach in a container of material, where the container can be a tank, a pressure vessel or a pipeline. Such a breach may result in a vapour cloud or fire.
- A steady state release of material from, for example, pipework, a vent or a flare, where the release may result in a vapour cloud and/or jet fire.
- An explosion of flammable vapour in an obstructed region. Only explosions where the mechanism is flame acceleration due to turbulence is modelled by the code. Other mechanisms such as confinement in a physical boundary or detonation of a dense phase explosive are not modelled by the code. Our training course will provide further details of these differences.
A scenario is developed by selecting the material which is to be modelled, the containment type, and the temperature and pressure of the containment.
Different scenarios can be included together in a ‘project’ folder. Project folders and their associated scenarios can be created, accessed and deleted from the initial screen.
Creating Projects and Scenarios
Following installation, you will be presented with the ‘select project and scenario screen’. In this screen you can create new project folders and scenarios, create new scenarios in existing project folders or retrieve existing scenarios from existing project folders. Later in the tutorial we will explain how to delete projects and scenarios.
As an example scenario, we will show you how to model a release from a storage tank containing liquefied methane where the pressure in the tank is close to atmospheric pressure.
In the ‘Select/Create project’ area click on the ‘New’ button, then type in “Tutorial Examples” as the project. Move to the ‘Select/Create scenario’ area, click on the ‘New’ button and type “Release from a hole in a liquefied methane tank”. Move to the ‘Material’ selection box and scroll down to select “Methane” (the first material in the list). Leave the default ‘Automatic mode’ option. Your screen should look like this:
Now click ‘Continue’ and you will be presented with the ‘Scenario Definition Screen’.
The scenario definition screen is where you define the type of containment you are interested in; a tank, a pressure vessel, a pipeline, or a steady flow from a pipe, vent or flare. This is also where you define other parameters such as the size of the hole in the containment, whether the release is onto land or water, whether a pipeline is above ground or below ground and the ambient weather conditions prevailing at the time of the proposed scenario. As examples we will demonstrate modelling a hole in a tank containing liquefied methane and a hole in a vessel containing pressurised propane
An Example of a Release from a Tank at Near Atmospheric Pressure
The scenario definition screen is shown below.
The containment options in Casqade are:
- Atmospheric Tank
- Pressure Vessel
- Pipeline
- Steady Release or Vent/Flare
- Explosion of Vapour in Obstructed Volume
Firstly select ‘Atmospheric Tank’ as the containment type.
You will then be presented with the screen which is shown below which asks for details of the release type and details of the tank geometry. Select ‘A release from the tank wall’. Input the tank diameter as 30m, the tank height as 20m, and the level of liquid in the tank as 15m. Now define the type of release. Select ‘hole’, put the hole at a 1m height above the tank base and give the hole a diameter of 75mm.
Your input should look like this:
Now click ‘OK’ and continue with the scenario definition. Leave the default ‘Above Land’ as the release location, ‘Unconfined’ as the confinement of the released liquid and ‘Unconfined’ as the confinement of the vapour which evaporates from the liquid.
Enter the Material temperature as -162°C, i.e. below the boiling point of methane, as we wish to model a release of liquid methane. The pressure will be automatically set as ‘atmospheric pressure’ as the containment type was a tank at near atmospheric pressure. Leave the ambient conditions as the default ‘Day’ and ‘Rural’ values; i.e. we are modelling a release during daytime in a rural location. Your screen should look like this:
Now click ‘Dispersion’.
Your next screen should look like this:
On this screen you can select the concentration of interest for the contour of the vapour dispersion cloud. The default in this case is the Lower Flammable Limit of methane, 4.4%vol:vol. Leave this as the concentration of interest and click ‘Continue’.
The software will then show you the representation of the vapour cloud in the horizontal plane. As you can see the cloud is predicted to go 134m downwind of the release location as shown below. In this case the centre of the tank which is represented by the zero co-ordinate on the x axis.
Now click ‘back’ to determine what could happen if this cloud was ignited and there was a flash back to form a pool fire around the tank.
Click ‘back’ on the concentration of interest screen to return the main scenario screen as shown below.
Now click the ‘Fire’ button as shown below.
You will now see the screen to define the radiation contours of interest. For the present leave the defaults as shown below and click on ‘Continue’.
The calculated fire radiation plots for the plane height of interest (in this case 0m) will be plotted as shown below. In this case the downwind distance to a radiation distance of 6.3kW/m2 is 172m.
If you want to save a copy of this graph, you can use the save to clipboard (for example to paste into a word document or similar), or alternatively use the ‘print’ command on the graph menu to print the plot.
If you want a copy of the input and output data, click on the ‘text file’ button. You can either copy this file to the clipboard or print it directly if required.
Congratulations. You have now modelled a scenario of a release of liquefied methane from a 75mm hole in a tank to give the vapour dispersion distance to the lower flammable limit and the pool fire radiation distance to a flux level of 6.3kW/m2.
You can now go back and model various options, for example to determine the differences which would result if you were to put a berm/bund around the tank. To do this you need to go back (twice) to return to the original scenario screen.
In this scenario we modelled a small release from a 75mm hole. As most of this release will evaporate rapidly, the spreading of this release will be limited and a bund/berm would have to be very close to the tank to affect the results. So change the scenario to that of a release from a larger, 300mm hole.
To do this, click on the ‘Atmospheric Tank’ containment type, elect ‘Atmospheric Tank’ to bring up the tank parameters. Change the 75mm hole to a 300mm hole as shown below.
Click ‘OK’. Run the dispersion model as previously by clicking on the ‘dispersion’ button. Accept the default LFL value of 4.4%vol:vol as the concentration of interest. You will get a plot showing the downwind dispersion distance of 370m as shown below.
Now modify the input data to put a 50m diameter bund/berm around the tank. To do this you need to go back to the scenario input screen. On the way the program will ask you if you want to save your new run. Click ‘save as’ and save this run as “Release from a 300mm hole in a liq methane tank” (note you need to abbreviate ‘liquid’ due to the limit on the number of characters in file names). Now click on ‘OK and ‘Back’ (twice) to go back to the scenario screen.
At the scenario screen, identify the ‘Confinement of Released Liquid’ dropdown box. Click inside this box and select the ‘Confined’ option. A box will now pop up and ask for a ‘Bund/Berm Diameter’. Use a 50m Bund/Berm diameter as shown below and click ‘OK’.
The bund/berm of 50m diameter is located beyond the 30m diameter tank to prevent the full extent of the spreading of the pool and hence limiting evaporation.
Now Click ‘OK’. Run the dispersion model as previously by clicking on the ‘dispersion’ button. Accept the default LFL value of 4.4%vol:vol as the concentration of interest. You will get a plot showing the downwind dispersion distance of 302m as shown below.
This set of modelling indicates that, in this case, the bund/berm has restricted the predicted spreading and evaporation of the liquid methane and has resulted in a predicted reduced vapour dispersion distance.
An Example of a Release from a Pressure Vessel
For this example let us imagine that we have a small sphere of 4m diameter containing propane. The propane is stored at an ambient temperature of 20°C. If sufficient propane is stored in the vessel, then part of the propane gas will liquefy, and the pressure in the vessel will rise to the saturated vapour pressure (SVP) of propane at 20°C. This is the pressure where the vapour and liquid are in equilibrium. (For further information on phase equilibrium and vapour pressure, please attend one of our training courses).
For the purposes of the scenario let us assume that the vessel is half full of liquid, and we wish to model the physical effects which could occur if a 75mm hole were to be created in vessel wall:
- Above the liquid level to release vapour to the atmosphere.
- Below the liquid level to release liquid to the atmosphere.
From the project selection screen click on your existing project and then click on ‘New’ in the Select/Create title box. Type in a new scenario name, for example, ‘Release from a hole above the liq level in a propane vessel’. (Note that you have to use ‘liq’ instead of ‘liquid’ to prevent the title exceeding the allowed 60 character limit).
Select ‘Propane’ from the ‘Select Material’ dropdown box. Your Select project screen should look something like this:
Now click ‘Continue’.
From the scenario screen select ‘Pressure Vessel’. In the pop-up box put in the details of the pressure vessel, the hole location and the hole size.
Note that Casqade has two options; release from the vessel wall and release from pipework attached to the vessel. If the former is chosen the initial angle of the released material will be that normal to the vessel wall at the height of the hole. This angle will depend on the shape of the vessel. For example, a hole one third up a spherical vessel will give a release pointing downwards, whereas a hole one third up a vertical cylindrical vessel will give a horizontal release as shown below. Note that if a ‘release from pipework attached to the vessel’ is selected, then Casqade will model the release as horizontal and at a height from grade which is at the same height as the hole in the vessel from grade.
Choose to model a release directly from the vessel wall, choose the shape to be a sphere, the diameter to be 5m, the height of the sphere above the ground to be 1m, the height of liquid in the sphere to be 2.5m above the base of the vessel (i.e. half full), the release type to be a hole located 5m above the base of the vessel (i.e. at the top of the sphere) and the release orifice to be 50mm in diameter. Your input screen should look like the one shown below.
Click ‘OK’ to return back to the scenario definition screen. As we wish to model a release of material from a vessel at 20°C, put ‘20’ as the material temperature at the release point. We wish to model a release from the vessel when the vapour and liquid are in equilibrium at this temperature, so click the ‘select pressure’ button and then click the ‘saturated vapour pressure’ button. Casqade will calculate a vessel pressure for this case and write it to the screen. You scenario data should look like this:
If you now click the ‘Dispersion’ button you will see the dialog asking you to input the concentration of interest. In this case leave the default value of 2%vol:vol which is used to represent the lower flammable limit of propane in air.
You will also notice a button to allow you to change between an ‘Unobstructed Release’ and an ‘Obstructed Release’. In the ‘Obstructed Release’ option, Casqade will model the vapour dispersion as if it were a ground level release with low initial momentum. For our case leave the default ‘Unobstructed Release’ option. This option will model the dispersion of a free jet in air.
Click ‘Continue’ to run the dispersion model.
Your output should look like this:
This is a representation of a flammable gas plume resulting from the release.
To assess the predicted thermal radiation flux if this plume was to be ignited you will need to go back and re-run the scenario as a fire.
Therefore click on ‘Back’(twice) to return to the scenario screen and click on ‘fire’ from the scenario screen. Leave the output flux levels of interest at their default values; 6.3kW/m2, 12.5kW/m2 and 35kW/m2. Click ‘Continue’ and your output should look like the following:
This is a representation of the thermal radiation contours to the flux levels of interest. You will notice that in this representation, the shadow cast by the physical obstruction of the sphere itself has not been included. The radiation contours have been calculated as if the sphere itself does not exist.
Supposing you wanted to know what release rate was used in the calculation of these effects. You would go back to the scenario screen and graph the outflow calculations. So if you click the ‘Back’ button, you will be asked to save your new jet fire scenario. Save this as ‘Jet fire from a hole above the liq level in a propane vessel’. Go back to the scenario screen and click on ‘Graph Release Rate’. You will see an output which looks like this:
As you can see, Casqade predicts the release rate for this propane vapour release from this scenario to be a steady release of 4.7kg/s for a long period of time. This is because the pressure in the vessel is assumed to be maintained by evaporation of the liquid in the vessel.
If you now run case b), a release from a hole below the liquid level in the sphere, you will see a different form of release, as the represented outflow will be initially liquid, followed by a frothing liquid and finally a gas.
To run case b), you will need to go back to the scenario screen, click on the ‘Pressure Vessel’ containment type and modify the geometry definition screen. Modify the location of the hole to 0m from the base (i.e. a hole in the bottom of the sphere). Your screen should look like this:
This scenario is representing a hole in the bottom of a propane sphere resulting in a jet of liquid directed vertically downwards into the ground. If you continue and model the vapour dispersion of this scenario, (click ‘OK’ and then ‘Dispersion’) and accept the default concentration of interest, you will receive the following message:
Casqade will not allow you to run this scenario as a free jet in air. It is actually represented as a liquid release which evaporates rapidly from the ground. The vapour released spreads as a heavy gas cloud. Click ‘OK’ and you should get the following representation of the vapour dispersion cloud.
As you can see, the representation of a release from a hole in the bottom of the sphere indicates that cloud will disperse to a distance to the lower flammable limit in the region of 230m downwind. If you click on the vertical projection you will see that the prediction is that a low flat vapour cloud. This is a significantly different result to that of dispersion of vapour from the hole in the top of the sphere.
If you want to take a look at the prediction of the fire formed around the sphere if the release is ignited, go back to the scenario screen, on the way save the case you have just run as ‘Release from a hole below the liquid level in a propane sphere’. At the scenario screen click on ‘Fire’ as described in the previous examples.
Summary of Automatic Mode
This is the end of the tutorial on ‘automatic mode’. Congratulations, you have modelled a release of liquefied methane from a tank and a release of propane liquid and vapour from a pressurised sphere. You have looked at both the unignited vapour dispersion distance to the lower flammable limit, which upon delayed ignition could result in a flash fire, and the fire case if the release were to be ignited immediately.
As you have seen, Casqade will also model releases from other containment types; for example above ground pipelines, buried pipelines, pipework etc. To receive instruction on using these other containment types, and an introduction to the chemistry and physics which you should be aware of when representing releases of materials using physical effect models, we would encourage you to attend one of our training courses.