2.3 Using Shields
2.3.1 Offshore Case - Add Welltest Burner
A welltest burner capable of burning 30,000 lb/hr of liquid is to be added to our design.
1. Use the File - Open menu option or the icon. In the File Open dialog that appears, browse to the Samples sub-folder in the Flaresim installation folder (usually [Public Docu-ments]\Softbits\Flaresim 4.0) select the file “Ex1 - Offshore Final Results.fsw” and click the Open button.
2. Change the units preferences to “Field” in the Preferences view if required.
3. In the Case Navigator view, select the Fluids branch and click the Add button to create a new Fluid and open its view.
Complete the view with the following entries;
Name = Welltest Liquid, Calculation Method = Flaresim Temperature = 100 F,
Ref Pressure = 14.7psi
The Tc and Pc fields can be left blank.
4. In the Case Navigator view select the Stacks branch and then click the Add button to create a new Stack and open its view. Enter data for the new stack as follows, leaving other entries at their default values;
Name - Welltest Boom, Location
Angle to Horizontal = 0 deg, Angle to North = 180 deg.
These entries define the new stack as a horizontal boom on the opposite side of the platform to the main flare stack.
5. In the Case Navigator, select the Tips branch and click the Add button to create and view a new Tip object. Name it
“Welltest Tip” and enter the following data;
Details tab
Tip Type = Welltest, Number of Burners = 3,
Fraction Heat Radiated Method = User Specified Specified Fraction Heat Radiated = 0.3
All other values should be left at their defaults.
Location & Dimensions tab On Stack = Welltest Boom, Length = 0ft,
Angle to Horizontal = 0 deg, Angle from North = 180 deg.
Exit Diameter = 4 in
Note the burner length and orientation fields serve to locate the precise location of the flame and the initial flame
direction. Even when the burner length is 0ft as here, the orientation fields must still be entered.
Fluids tab
Fluid = Welltest Liquid Mass flow = 30,000 lb/hr.
6. Add a new Receptor Point in the usual way. Define the following data to locate the receptor point at the base of the welltest burner boom;
Name - Base Welltest Boom, Northing = -200ft,
Easting = 0ft, Elevation = 0ft.
All other fields may be left at their default values. Close the view.
2.3.2 Offshore Case - Run Welltest Calculations
7. In the Case Navigator view, select the Stack 1 object. Clearthe Size This Stack check box. Now click the Ignore button.
This will exclude the two tips on the main flare stack from the calculations.
8. Run the calculations by clicking the large button labelled
“Click to Calculate”. Check in the Errors/Warnings/Info log panel that the case has run and calculated correctly.
9. Open the Receptor Summary view. The results show that the radiation limits for our original two critical locations that we have defined are met. The radiation at the base of the well test burner stack is 1405 btu/hr/ft2.
2.3.3 Offshore Case - Add Water Screen
The radiation calculated at the base of the welltest burner stack is acceptable for brief exposure only. Since more extended exposure might be required, it is necessary to reduce the radiation. While this
could be achieved by extending the length of the stack this would be an expensive option due to the added weight. It is normal to reduce radiation from welltest burners using water screens.
10. Add a Shield object, either by clicking the Shield branch in the Case Navigator view and then the Add button, or by using the Add - Shield menu option according to your preference.
11. Enter data in the Details tab of the new Shield view as follows;
Name = Water Curtain,
Radiation - Type = Water Screen
Radiation - Layer Thickness Calculation = User Radiation - Layer Thickness = 0.5 in
Noise - Transmissivity = 1.0 [default]
12. Select the Sections tab. The first section is already created for you. In the lower half of this view, click the Add Vertex button 4 times to create a rectangular shield section with 4 corners or vertices.
13. Enter the following data;
Name - Water Curtain
Vertex 1 = Northing -205 ft, Easting, 50 ft, Elevation 40 ft Vertex 2 = Northing -205 ft, Easting, 50 ft, Elevation -10 ft Vertex 3 = Northing -205 ft, Easting, -50 ft, Elevation -10 ft Vertex 4 = Northing -205 ft, Easting, -50 ft, Elevation 40 ft Note it is a requirement when entering the locations of the vertices that each point is directly connected to the next point in the list as shown below. Flaresim will attempt to sort the points to meet this criteria if necessary.
14. The Shield view should now show that the shield data setup is complete. Run the updated case and inspect the results.
The radiation value at the base of the welltest burner stack has been reduced to an acceptable value of 264 btu/hr/ft2. The radiation isopleth for the Receptor Grid, Grid 1 clearly shows the effect of the shield, see Figure 2-19..
15. Save the case as “Ex3 - Shields - Water Curtain.fsw
2.3.4 Onshore Case - Workshop Surroundings
After sizing the onshore flare to meet a radiation constraint at head height, we are now concerned about the surroundings of a workshop located in the vicinity of the stack. We will calculate the radiationFigure 2-18, Shield Section Input
Figure 2-19, Isopleth plot for Helideck Plan View
and temperature at a receptor located at the entrance of the workshop on the downwind side of the structure and study the shielding effects.
1. Use the File - Open menu option or the icon. to open the file "Ex2 - Onshore Sizing Results.fsw" and click the Open button.
2. If required, use the Preferences view to set the units to
“European”.
3. Add a new Receptor Point in the usual way. Define the following data to locate the receptor in the South-west direction from the flare base:
Name = Workshop Entrance Northing = -111m
Easting = -30m Elevation = 2m
All other fields may be left at their defaults. Close the view.
4. The resized flare with a new height of 106m will be used from this point onwards. Swap to rating mode as follows.
Open the "LP Flare" view and disable the Size This Stack check box under the Details tab. Set the stack length to 106m. Click the Calculate button to run the model.
5. Open the Workshop Entrance point to inspect the results.
The radiation at the workshop entrance is 3.8 kW/m2. Note the surface temperature calculated which is 46 C.
This equilibrium temperature value is based on the default material properties of the receptor which are appropriate for a steel plate 3mm thick exposed to radiation on one face.
We will use these properties as representative of exposed equipment at the workshop entrance.
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2.3.5 Onshore Case - Add Workshop
While the radiation received at the workshop remains below our sizing constraint of 4.7 kW/m2 it still exceeds the allowed limit for continuous exposure of 1.6 kW/m2 and the 3.2 kW/m2 allowed during emergency escapes.
In order to predict the radiation at the point of interest with more accuracy we should account for the fact that the workshop will act as a shield protecting the receptor from radiation.
6. Add a Shield object, either by clicking the Shields branch in the Case Navigator view and then the Add button, or by using the Add - Shield menu option according to your preference.
7. Enter data in the Details tab of the new Shield view as follows:
Name = Workshop Screen Type = Solid
Note the Radiation Specified Transmissivity is automati-cally set to 0. This is the value expected for opaque materi-als such as concrete or metal.
Noise Transmissivity = 1.0 [default]
8. Move to the Sections tab. Click on the Make Pit/Hut button and enter the following data in the popup window:
Select Hut radio button
This option automatically adds five sections to the shield:
four walls and the roof.
The Shield View status should now indicate that the shield is ready to calculate.
9. Click the Calculate button and review the results. The radiation at the Workshop Entrance is now 2.0 kW/m2 which allows safe escape during an emergency.
10. Open the “Grid @ Head Height” receptor grid and view the radiation isopleth plot. The shield sections representing the workshop will be shown on the plot but are rather small.
Click the Zoom button and when the zoom cursor icon appears, click and drag around the workshop region. The expanded plot is shown below.
This shows a symmetrical isopleth around the workshop which is unexpected given that the flare is to the North and East of the workshop.
This result is due to the fact that the isopleth curves are cal-culated by interpolation from the points in the grid. These points are too far apart to allow an accurate calculation of the isopleths around the small workshop.
Figure 2-20, Grid Radiation Isopleth
11. To plot the isopleths around the workshop in more detail an additional receptor grid is needed. Copy this receptor grid and specify the following data in the new grid:
Name = Workshop Surroundings Orientation = Northing - Easting Offset = 2m
Northing Min = -115m Northing Max = -95m
Northing Number of points = 41 Easting Min = -40m
Easting Max = -20m
Easting Number of points = 41
Click Calculate and inspect the radiation isopleths for the Workshop Surroundings grid. As shown below, this now reveals the expected lower radiation region to the South and West of the workshop.
2.3.6 Onshore Case - Add Local Environment
The workshop does not only protect the entrance area from radiation, it also protects it from the Northerly wind. This will reduce the convective cooling of exposed equipment and will result in higher equilibrium temperatures. We will extend our model toFigure 2-21, Workshop Surroundings Isopleth
include this effect by adding a local environment with 0 m/s wind speed for the workshop entrance receptor.
12. Select the existing Environment in the Case Navigator and click the Copy button. Rename the new Environment "No Wind-No Solar - No Aten." Change the wind speed to 0m/s.
13. The "9D" Environment was automatically ignored since only one can be active in the model. However we still want to use a 9m/s wind for the radiation calculations. Click on the Environment "9D - No Solar - No Aten." item in the Case Navigator and then click the Activate button.
14. Copy the Receptor Point "Workshop Entrance" and rename the new one "Workshop Entrance - No Wind". Move to the Properties tab and change the Local Environment to "No Wind - No Solar - No Aten." Creating a copy of this point will allow us to compare the temperatures with and without the cooling effect of the wind.
15. Run the case. Open the Receptor Summary view by double clicking on the Receptor Points branch in the Case
Navigator and compare the temperatures of the two points.
With the “No Wind” local environment the equilibrium tem-perature is 86 C as against 31 C with the base case wind speed of 9 m/s.
While higher windspeeds often lead to higher radiation val-ues due to greater flame deflection, this shows that studies of temperature should consider lower wind speeds if a receptor point is shielded from the wind. This result, at 0m/s wind speed, considers the worst possible case - it is likely that some wind will eddy around the workshop.
16. Save the case as “Ex3 - Shields - Structure”.
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2.4 Using Overlays
Flaresim provides Receptor Grid objects to visualise the thermal radiation around the flare. These calculate the radiation for a grid of points which are then used to generate isopleth charts showing lines of constant thermal radiation. Similar isopleth charts can be
displayed for noise and surface temperature results. The usage of these has already been explained in earlier examples.
The utility of these isopleth plots is greatly enhanced by plotting them on a plant drawing so that the radiation levels can clearly be identified at different locations.These examples show how two types of plant drawings, known in Flaresim as Overlays, can be integrated with isopleth plots.
2.4.1 Offshore Case - Flaresim Overlay
The first type of overlay available for adding to isopleth plots are known as Flaresim overlays. These are generated using the internal overlay editor. In this example, we will create a simple plan view within Flaresim for integration with the receptor grid isopleth plot in our offshore example.
1. Select the File - Open menu item or click the icon.
Open the case “Ex3 - Shield - Water Curtain.fsw” which you should find in the folder [Public Documents]/Softbits/
Flaresim 4.0.
2. In the Case Navigator, select the Overlay branch and click the Add button. A new overlay object called Overlay 1 will be created and displayed. Change the name to “Helideck Plan”.
3. In the “Update Details From Grid” section of the Details tab, select the “Grid 1” grid and click Update. The Overlay dimensions are updated with those from the chosen grid.
4. Select the Editor tab and click the zoom in and zoom out buttons to resize the view until you can see the full drawing. Check the Show Stacks check box to display the location of the stack in the drawing to act as a guideline.
Note this will not form part of the drawing.
5. Now click the Add Rectangle button and draw a rectangle to represent the platform outline from the top left corner [-200,0] to the bottom right corner [50,-200]. This is done by moving to the first point using the displayed X,Y coordinates at the left of the view as a guide, clicking and holding the left mouse button then dragging to the second point.
6. Add a second rectangle to represent the helideck from the points [-50,-100] to [30, -180].
7. Click the ellipse button and draw a circle within the helideck rectangle by moving to the point [-50, -100], clicking and holding the left mouse button and dragging to the point [30, -180].
8. Click the text button and then click the drawing in the middle of the helideck circle. A vertical flashing bar will appear to indicate the text insertion point. Type the letter H and then hit the enter key to complete the text entry.
If the text is too small, click the select button and then select the text you have just entered. A set of selection points will appear around it to indicate that it has been selected. Now click the properties drop down menu and select the Text Font option to open a standard font dia-log to allow the text size and style to be defined. A size of around 24 pt is probably suitable.
If required the selected text can also be moved by clicking the yellow dot and dragging with it the left mouse button - .
The overlay picture is now complete and should look some-thing like the view below.
9. Next open the “Grid 1” Receptor Grid and go to the Plot Overlay tab. Select the Use Flaresim Overlay radio button and then, in the drop down menu that appears, select the overlay we have just created, “Helideck Plan”. Finally tick the Show Overlay check box.
Now go to the radiation tab. The overlay is now displayed as the background picture to the isopleth as shown below.
Figure 2-22, Completed Overlay
10. Save the case as “Ex4 - Offshore - Flaresim Overlay”. The overlay file we have created will be automatically saved in the Flaresim case folder (i.e. Ex4 - Offshore - Flaresim Overlay) with the file extension “.fso”.
2.4.2 Onshore Case - External Overlay File
The other method of displaying an overlay with your isopleth plots is to link to an external graphics file. The best type of background drawing to import is a scaled vector drawing i.e. a Windows metafile (.wmf) or enhanced metafile (.emf). Bitmap files (.bmp, .png and .jpg files) can also be used. Given that the locations of the stacks etc.
in your Flaresim model are matched to the drawing on import, the isopleths will be correctly positioned in relation to the drawing.
The following example is based on the onshore example and shows how to import the plot plan of our facility as a .bmp file and integrate it with the radiation isopleths.
1. Use the File - Open menu option or the icon. In the File Open dialog that appears, browse to the [Public Docu-ments]/Softbits\Flaresim 4.0 folder. Select the file "Ex3 - Shields - Structure.fsw" and click the Open button.
Figure 2-23, Isopleth with Overlay
2. We know that the drawing represents an area 400m long and 300m wide of our facility. The base of the "LP Flare" is located at the origin in the Flaresim model (0m, 0m). In the drawing this is 247.8m North and 179.2m East from the left bottom corner that we will assume is at 0m North and 0m East.
3. Open the Plot Overlay tab of "Grid @ Head Height", ensure the Details radio button is selected in the External File Details section and enter the following values:
File Dimensions
Northing Minimum = 0m Northing Maximum = 300m Easting Minimum = 0m Easting Maximum = 400m
Location of Flaresim Origin in File Northing = 247.8m
Easting = 179.2m
4. Click the Browse button to import the background graphics file. The file to import is called Plot Plan.bmp and is located in the Samples\Ex4 - Onshore - External Overlay folder.
You will need to select "Windows Bitmap (*.bmp)" in the
"Files of Type" drop down in the File Open view to select this. Click Ok.
You can now click the Preview File radio button to see the imported graphic file together, with a blue outline rectangle which shows the extents of the current grid on the drawing.
5. Reselect the Details radio button and make sure the Show Overlay check box is enabled. Hit the Calculate button and move to the Radiation tab. You should see your overlay displayed together with the isopleths as shown below.
6. Save the case as “Ex4 - Onshore - External Overlay”.
The key aspects to the success of this process is the accurate understanding of the dimensions of the overlay file and the location of the flare stack within it. So for example if it was known that the Plot Plan bitmap covers the dimensions 150m to 450m N and 100m to 500m E, then the flare stack would be at 397.8m N and 279.2m E and this is the information that must be entered.
Be aware that if there is white space surrounding the drawing, this forms part of the drawing and its extent must be included in the drawing dimensions and the determination of the flare stack location.
Figure 2-24, Onshore Case - External Overlay File
2.5 Case Study
A new feature of Flaresim 4.0 is the ability to define one or more case study objects. These allow the selection of input variables and definition of alternate input values. A list of key result variables is also selected. When the case is calculated, each case study will be run, automatically updating the model with the different input data values and recording the key result variables selected. The results showing their variation with changes in input values are then available as a table or as a plot.
Two types of Case study can be defined: discrete variable studies based on single input values and incremental variables studies based on the variation in input values over a range. Examples of both types
Two types of Case study can be defined: discrete variable studies based on single input values and incremental variables studies based on the variation in input values over a range. Examples of both types