2.2 Onshore Flare Stack Design
2.2.2 Model Setup
1. Start the Flaresim program through the Windows Start button in the usual way.
2. We will build our first model through the Case Navigator.
Close the Setup Wizard that opens automatically when Flaresim starts up.
3. Use the File/Preferences option on the main Menu. In the Units tab, select the European units set and close the view.
4. Create a new Fluid by selecting the Fluids branch in the Case Navigator view and then clicking the Add button or by selecting the Add - Fluid drop down menu option.
5. On the Properties tab of the Fluid view that opens enter the following data:
Name = Flare Gas GPF
Calculation Method = REFPROP Temperature = 160 C
Pressure = 1.5 bar a
6. Move to Options tab and enter the information below:
°
Correct Temperatures = Yes Isentropic Efficiency = 0%
Flash Method = PR (default)
When the Isentropic Efficiency is set to 0%, Flaresim will follow an isenthalpic thermodynamic path to bring the fluid from the reference T&P down to the pressure at the tip exit.
7. In the Composition tab add the following components and the fraction in Mole basis:
Methane 0.20
Ethane 0.20
Propane 0.30
i-Butane 0.10
n-Butane 0.15
i-Pentane 0.02
n-Pentane 0.02
n-Hexane 0.01
Flaresim calculates the fluid properties as shown below. Cp/
Cv and the critical properties will be displayed after running the model if REFPROP thermo package is selected.
Figure 2-13, Fluid View
8. Create a new Environment by selecting the Environments branch in the Case Navigator view and then clicking the Add button or by selecting the Add - Environment drop down menu option.
9. On the Overall tab of the Environment view that opens enter the following data:
Name = 9D - No Solar - No Aten.
Wind Speed = 9 m/s
Wind Direction = 0 (wind blowing from the North) Include Solar Radiation = No (box unchecked)
API 521 states that solar radiation should be considered on a case by case basis. Consideration should be given to: the frequency of the flaring event, the probability of personnel being present in the exposed location, the ease or difficulty of escape from the exposed location, etc.
Accounting for these criteria and the fact that the scenario represents an emergency scenario, the solar radiation will be excluded in our case.
Transmissivity Method = User Specified Transmissivity Value = 1
A value of 1 is the most conservative option as it does not take credit for atmospheric attenuation.
Other Parameters = leave as default
10. Move to Dispersion Data tab and enter the following data:
Correct W. Speed For Height = Yes
This option will use a wind speed vs height curve to correct the speed defined in the Overall tab and will have an effect on both radiation and temperature calculations.
Other Parameters = leave as default
11. Create a new Stack by selecting the Stacks branch in the Case Navigator view and clicking the Add button or by selecting the Add - Stack drop down menu option.
On the Details tab of the Stack view that opens enter the fol-lowing data:
Name = LP Flare
Stack located at the origin:
Northing = 0m Easting = 0m Elevation = 0m Length = 85m
Angle to Horizontal = 90 deg Angle From North = 0 deg
Size This Stack = No (box unchecked)
12. Move to Sterile Area tab and enter the following data:
Sterile Area Elevation = 2m (head height) Calculate Sterile Area = Yes
Update the radiation table with the following limit values:
1.6 kW/m2 (For continuous exposure from API 521) 3.2 kW/m2 (Allowed during emergency escape)
4.7 kW/m2 (For 2min emergency actions from API 521) 13. Create a new Tip by selecting the Tips branch in the Case
Navigator view and then clicking the Add button or by selecting the Add - Tip drop down menu option.
On the Details tab of the Tip view that opens enter the fol-lowing data:
Name = Pipe Flare - GPF 300t-h Tip Type = Pipe
Number of Burners = 1 Seal Type = None
Fraction Heat Radiated Method = Generic Pipe
Generic pipe F factor is a proprietary correlation based on refitting other methods across a range of exit velocities and molecular weights and represents a good approach when modelling gas pipe tips.
14. On the Noise Input tab of the Tip view, enter the following data:
Combustion Noise Method = Standard Reference.
15. Move to the Location & Dimensions tab and enter the following data:
On Stack = LP Flare Length = 0m
Angle to Horizontal = 90 deg Angle to North = 0 deg Exit Diameter = 36 in
Since the value we have been given is 36in, we first click the entry displaying "mm" and select "in" in the drop down menu before entering the value. If we wish to see the value in "mm" then click again in the units entry and select "mm"
to display the converted value of 914.4 mm Burner Opening = 100%
Riser Diameter = 36 in
Roughness = 0.025 mm (default)
Calc Burner Opening = No (box unchecked) 16. Click on the Fluids tab and enter the following:
Fluid Name = "Flare Gas GPF"
Mass Flow = 300,000 kg/h
At this point the Status Text at the bottom of the Tip view should indicate that the tip data is complete. Close the view.
17. Since we are interested in studying the radiation at head height, we will create a receptor grid to plot the radiation contours at this height. In the Case Navigator view, select
the Receptor Grids branch and click the Add button (alternatively select the Add - Receptor Grid drop down menu option) to create and open the view for a new Receptor Grid object.
On the Extent tab enter the following data:
Name = Grid @ Head Height Grid Plane = Northing-Easting Elevation Offset = 2m (head height) Northing Min = -250m
Northing Max = 50m Northing Points = 41 Easting Min = -150m Easting Max = 150m Easting Points = 41
18. We can customise the isopleth lines displayed on the plot.
On the Radiation tab change the display to Plot and click on the Customise button to open the plot properties view. Go to Contour Details tab and select the check boxes to show only the isopleth values for 1.6, 3.2 and 4.7 kW/m2 as shown below. Note the colours of each isopleth can be customised by clicking on the line colour panel and selecting the colour from the pop-up colour picker dialog. Assign a navy blue colour to the 1.6 kW/m2 isopleth.
19. Finally we need to select a radiation method to perform the calculations. Open the Calculation Options view in the Case Navigator, select "Mixed" radiation method and set the No.
Flame Elements to 25.
As discussed in the Methods chapter of the documentation, the Mixed method is a compromise designed to give the best accuracy for calculating radiation both close to and fur-ther away from the flame. As such it is a good default method. 25 flame elements are usually sufficient to calcu-late the flame shape with a reasonable degree of accuracy.
2.2.3 Initial Calculations
20. We are now ready to run the calculations. Click the large Calculate button at the top of the Case Navigator. Once Flaresim has finished calculating, check the Errors/
Warnings/Info log panel to confirm that the expected calculations have been completed.
Note that this window is colour coded:
Figure 2-14, Customise Isopleths
Green when calculations are completed without warnings Yellow when calculations are completed with warnings Red when errors are detected and results not generated 21. We are now ready to review the results. Open the "LP Flare"
view and go to the Sterile Area tab. The distances to meet each of the specified radiation limits are displayed on the table as shown below in Figure 2-15.
22. Open the Receptor Grid view to inspect the isopleths plot by clicking on the Radiation tab and then selecting Plot as the Display option, see Figure 2-16. It presents the contours for the radiation limits of interest at head height, the same as the sterile area calculation.
Figure 2-15, Sterile Area Results
23. This completes our initial evaluation. Save the case as “Ex2 - Onshore - Rating Results.fsw