The second release is from the same chlorine sphere, but the hazardous event is the rupture of a one-inch liquid line attached to the bottom of the sphere, where the initial liquid head will be 4.6 m. The line runs 4 m vertically downwards to 10 cm from the ground, then 5 m horizontally to an isolation valve; the rupture is assumed to occur just before the isolation valve.
Insert a Time varying short pipe Scenario
There are two types of Scenario available for modelling pipework rupture:
• The Short pipe Scenario, which models the release using the initial release rate for the start of the release, with a duration that is the time required to drain the inventory at this initial rate. This will normally give conservative results in the consequence calculations.
• The Time varying short pipe Scenario, which models the effect of the release on conditions in the vessel and the way that these conditions and the release rate change over time. These time-varying results can be represented either with a single rate (e.g. an average rate, or a rate at a particular time) or with a series of rates, depending on the options that are set for the Scenario.
For this tutorial, you will use the Time varying short pipe Scenario, perform an initial run of the discharge calculations, then examine the results and decide on the most appropriate way to represent the behaviour for the rest of the consequence analysis.
To add the Scenario, select the Pressure Vessel and select Insert > Time varying short pipe release from the right-click menu.
Name the Scenario Line rupture, liquid.
Setting the input data for the Scenario
The new Scenario will be shown as incomplete, as this type of Scenario does have mandatory data.
Open the input dialog and set the input data as follows:
Scenario tab section
Make sure the Scenario type is set to Line rupture (rather than Disc rupture or Relief valve).
Set the Pipe diameter to 25.4 mm, the Pipe length to 9 m, the Release height from vessel bottom to 0 m, and the Elevation to 0.1 m. With this value for Elevation, the liquid droplets will probably not evaporate inside the cloud, and will probably rain out and form a vaporising pool.
Note: the Pipe diameter is 1 inch, and the easiest way to set this is to type “1 in” in the input field and press [Tab]. The program recognises “in” as a defined unit for length, and will convert it to the default display units of mm when you press [Tab] or click in a different field.
The Scenario tab includes the Outdoor release direction field, which you should leave with the default value of Horizontal, which is the correct setting for this type of unobstructed rupture of horizontal pipework.
The list of directions includes a second horizontal option: Horizontal Impingement. You
Material tab section
When the vessel to which the Scenario belongs contains saturated liquid, the Phase to be released field in the Material tab section will offer a choice of release-phase for the line rupture: a vapour release from the top of the vessel, or a liquid release from the bottom of the vessel.
By default this will be set to Liquid, which is the value set for the Phase field in the Material tab section for the Pressure Vessel, and for this Scenario you should leave the field with the default value.
Short pipe tab section
The Short pipe tab section contains details for the modelling of frictional losses.
Leave the pipe roughness with the default value taken from the Parameters, and leave the numbers of valves as zero.
There is one bend in the 9 m of pipework, so you should set the Frequency of bends in pipe to 0.11 per m.
and decide on the most appropriate way to represent the behaviour for the rest of the consequence analysis, which may involve changing these settings.
This completes the input data for this stage, and you can click on OK to close the input dialog.
Supplying the tank shape data for the Pressure Vessel
The settings described above will complete the mandatory input data for the Scenario, but when you close the dialog, you will find that the Scenario is still shown as incomplete in the Study Tree. If you hover the mouse over the error icon for the Scenario node, a tooltip will appear saying that the “Tank shape” is missing. The Tank shape field is actually part of the input data for the Pressure Vessel, and it becomes mandatory if you have any Time varying Scenarios present under the vessel.
Open the input dialog for the Pressure Vessel again. You will find that two tab sections are now shown as incomplete:
Scenario tab section
The Release height from vessel bottom is shown as incomplete. You set this to 0 m for the Scenario, but the Pressure Vessel needs a value in order to perform checks on the data, and you should enter a value of 0 m here, as well.
Time varying releases tab section
Set the Tank type to Spherical and the Tank diameter to 6.74 m, as shown, and then press [Tab]. The program will use the process and inventory data from the Material tab to calculate the vapour and liquid contents of the vessel, and displays the results in the Inventory data section at the bottom of the tab section.
When you click on OK to close the input dialog, a message will appear warning you that the changes in the input data will make the results for the Catastrophic rupture Scenario out of date. After you have clicked on OK to proceed with the changes and return to the Study Tree, you will find that both the Pressure Vessel and the Time varying Scenario are now shown as complete.
Running the discharge calculations
Select the Time varying Scenario and then click on Run Discharge Only in the Home tab of the Ribbon Bar. This will run the discharge calculations alone, without performing the dispersion and effects calculations. The calculations may take several minutes, depending on the speed of your machine.
When the results are complete, view the reports and move to the TV Discharge Report.
The report shows that the rate drops by less than 3% in two hours of release, which means that the time-varying behaviour can be ignored for this release.
The time-varying discharge calculations are time-consuming, and the analysis will be easier to work with if you bypass the time-varying discharge modelling for this hazardous events. There are two possibilities in this situation:
1: Use the averaged discharge results to create a User-defined source Scenario
Most of the Scenarios for a Pressure Vessel perform in-built discharge calculations to determine the state of the material after expansion to atmospheric pressure, which is
When you performed the discharge calculations, the program calculated the average rate over the first 20 s, and this is the which you performed the discharge calculations. For this Scenario, the results will be the same for all Weathers, and when you select the Weather the program will create a User-defined source Scenario with the name User User-defined source for Category 1.5/F .
The dialog for the User-defined source Scenario does not
include the Short pipe or Time varying release tab sections, and instead of containing fields for the pipe diameter and length, the Scenario tab section contains a Release segments table in which you specify the discharge rate and conditions directly, since the User-defined source Scenario does not perform any discharge modelling itself. The Scenario will be created with discharge data taken from the averaged results from the
2: Insert a Short pipe Scenario and set up the equivalent input data
The Short pipe Scenario models the same type of hazardous event as the Time-varying short pipe release Scenario, but it calculates the initial discharge rate, without performing any time-varying discharge modelling. Inserting this Scenario involves repeating some of the data-input, but this is the approach taken in this tutorial as it will make the analysis clearer and easier to maintain: if you need to change some aspect of the input data you can edit the Scenario and rerun the calculations, whereas if you used a User-defined release Scenario you would have to edit and rerun the Time-varying Scenario first, then create a new User-defined release Scenario, and delete the previous User-defined release Scenario.
Before you insert the Short pipe Scenario, rename the Time varying Scenario to add “- time-varying not needed” at the end of the name, and then right-click on the Scenario and select Exclude from calculations from the menu. The Scenario will become greyed out in the tree and will not be included if you run the calculations for the Pressure Vessel or Study, which will make the calculations quicker.
Next, insert a Short pipe Scenario, name it Line rupture, liquid, and then edit it and set the values as follows:
Tab section Input field Value
Scenario Scenario type Line rupture
Pipe diameter 25.4 mm
Pipe length 9 m
Elevation 0.1 m
Material Phase to be released Liquid
Short pipe Frequency of bends in pipe 0.11 per m Run the consequence calculations and view the results
Select the Scenario and select Run from the Home tab of the Ribbon Bar or the right-click menu. When the calculations are complete, view the graphs for all of the Weathers.
You will see that there is a Pool Vaporisation tab in the Graphs View, which means that the liquid in the release did rain out. If you view the reports and look at the Commentary Report, you will see that rainout fraction is about 0.7 for all three Weathers, so the formation and behaviour of the pool will have an effect on the dispersion or toxic effects.
In the Toxic Lethality graph, the greatest effect distances are for the F 1.5 m/s weather outdoors, with a distance of 900 m to a lethality level of 10%, which is approximately a third of the distance reached by the catastrophic rupture. The least stable condition, D 5 m/s, reaches only 300 m for 10% lethality outdoors.
If you open a GIS Results View for all three Weathers and view the Toxic Outdoor Lethality Footprint for 10% lethality, you will see that the effects do not reach the village or the town.