Defining the second scenario: a liquid release from pipework

The second release is from the same chlorine sphere, but the hazardous event is the rupture of a oneinch 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 in Phast 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 timevarying 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.

Note: if you are using Phast Lite, the parts of this tutorial involving the Time varying short pipe Scenario will not be relevant to you, as this type of Scenario is only available in Phast. The tutorial includes a version of the liquid release that uses the Short pipe Scenario – which is available in Phast Lite – and you should move straight to page 33, which deals with inserting and defining this Scenario.

To add the Time-varying Scenario, select the Pressure Vessel and insert a Time varying short pipe release using either the right-click menu or the Insert gallery in the Home tab of the Ribbon Bar.

Name the Scenario Line rupture, liquid.

Supplying the tank shape data for the Pressure Vessel

When you insert a Time-varying Scenario under a Pressure Vessel, the tank shape input fields in the dialog for the Pressure Vessel become enabled and mandatory, when they are disabled when there is no Time-varying Scenario present. You should complete these fields before you work on the input data for the Scenario, as having the tank shape defined will make it easier to supply correct values for the Scenario.

Open the input dialog for the Pressure Vessel again and move to the Time varying releases tab.

Set the Tank shape to Spherical and the Tank diameter to 6.74 m, as shown in the illustration on the next page, 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 then display the results in the Inventory data section at the bottom of the tab section.