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Stage 3: Modeling the Release with the User-Defined Model

You now have the information that you need in order to model the release in PHAST Professional

PHAST Professional.

Creating the User-Defined Source Creating the User-Defined Source

You can avoid some of the effort of data input if you create the User-Defined Source Model from the existingPipeline Rupture Model.

Select the Pipeline Rupture Model, then pressCtrl+D to run the discharge calculations, and then selectCreate Source from the Edit menu. The program is only able to create a User-Defined Source from a Vessel or Pipe Source immediately after the discharge calculations have been run; at any other time, the option is disabled in the Edit menu.

The program will add a User Defined Source Model with the name Calculated Discharge. Change the name to 3 Regime Release, as shown.

Setting the Input Data for the Source Setting the Input Data for the Source

Double-click on the new Model to work on the input data.

Most of the tab sections in the input dialog for the User-Defined Source are the same as for the Vessel or Pipe Source, and for these tab sections the program simply copies the values when it creates the source.

However, there are some tab sections that are not the same, and these are the tab sections that deal with discharge calculations or results. In the Material tab section, the fields for process conditions are all disabled, and the Scenario and Vessel tab sections are replaced by the Discharge tab section—which you will use

to define the three regimes.

First, set the inventory in the Material tab section to 41,000 lb, to add the pipeline inventory to the inventory in the vessel. For this release, the pipeline inventory is small compared with the vessel inventory, but this may not be the case for other releases that you model.

Next, move to the Discharge tab section. The program has defined the User Defined Source as a leak, with the discharge results for the Base Case for the Pipeline Rupture Model (which had a pipe length of 0.1 ft), as shown in the illustration on the next page.

Chapter 6: Tutorial 3

The Initial Discharge Input Values

This Base Case is not relevant to any of the regimes, and you must edit the values to set them to the values for Regime 1, and then add columns to the table for Regime 2 and the Steady State.

First, set the values in the first column to those shown below:

Getting Started with PHAST

You can leave the droplet diameter with the value from the Base Case.

Next, click on Add Segment to add another column to the table after the column for Regime 1. The program always adds columns to the end of the table.

This new column will have theRelease PhaseRelease Phase set as Liquid, with all of the other fields blank. Set the phase toVapor , and then set the values for Regime 2 as follows:

To change the phase, click in the cell, then use the scroll bars that appear to select Vapor from the list, and then click on the cell again before clicking on any other cell in the table. If you do not click on the cell for a second time (i.e. after selecting the phase), then the program will not process your selection, and will return the phase to its initial value the first time you click on another cell.

Next, click on Add Segment again to add a column for the Steady State regime, and set the values as follows:

You do not need to change any values in any of the other tab sections, and can click on OK to close the input dialog.

Discharge Data for Regime 2

The Completed Discharge Data

Chapter 6: Tutorial 3

Run the Calculations and View the Results Run the Calculations and View the Results

Select the 3 Regime Release Model and press Ctrl+M to run the dispersion and effects calculations, and then view the results.

The program models the sequence of segments in the same way as for a liquid release that includes rainout and evaporation, as described in detail in Chapter 4.

It calculates the dispersion distances for each segment assuming that the segment has sufficient duration to become fully developed, and that is a very conservative assumption for this release, since the segment with the largest discharge rate has a very short duration.

The illustration below shows the state of the cloud at 30 seconds, when the cloud for Segment 1 has reached about 1000 ft downwind, and the discharge has not yet reached steady state conditions.

The segment for Regime 1 has a long dispersion distance and reaches the town, but with a duration of 1 second, this part of the cloud appears on the Map simply as a

The State of the Cloud at 30 Seconds

Getting Started with PHAST

This effect becomes more pronounced with time, since the cloud for Segment 1 takes over an hour and 30,000 ft to reach its full dispersion distance, while Regime 2 takes 200 seconds and 2,500 ft, and the steady state regime takes 180 seconds and 1,700ft.

The illustration below shows the state of the cloud after 300 seconds, when steady state conditions have been reached in the pipeline, and the cloud from Regime 2 has finished dispersing. The gap between Segment 1 and the later segment has widened.

In reality, the change in the discharge rate, although rapid, is smooth, and the cloud would be continuous. If you introduced additional intermediate regimes between Regime 1 and Regime 2 in order to reduce the discharge rate more gradually, then the gaps between the segments would not be as large, and might disappear entirely;

however, the modeling of a fully-developed cloud for the short-duration regimes is still extremely conservative.

The State of the Cloud at 300 Seconds

Chapter 7: Tutorial 4

Chapter 7

Chapter 7