The copyright in this manual and its accompanying software are the property of Softbits Consultants Ltd with all rights reserved. Both this manual and the software have been provided pursuant to a License Agreement containing restrictions on use.
Softbits Consultants Ltd reserves the right to make changes to this manual or its accompanying software without obligation to notify any person or organisation.
No part of this manual may be reproduced, transmitted, transcribed, stored in a retrieval system or translated into any other language in any form or by any means, or disclosed to third parties without the prior written consent of Softbits Consultants Ltd.
WARRANTY
Softbits Consultants Ltd or its agents will replace any defective manual, program disks within 90 days of purchase of the product providing that proof of purchase is evident. Neither Softbits Consultants Ltd nor its agents or dealers make any warranty, implied or otherwise, with respect to the software or results generated by the software.
This program is intended for use by a qualified engineer to aid the design and analysis of flare systems. The results calculated by this program may not be reliable if the input data has not been appropriately specified or if the program is used without regard to its documented limitations.
It is the responsibility of the user to interpret the results generated by this program. Softbits Consultants Ltd shall bear no liability for special, indirect, incidental, consequential, exemplary or punitive damages arising from use of this software.
The governing law of this warranty shall be that of England.
ACKNOWLEDGEMENTS
Softbits Consultants Ltd would like to thank Mr. John F. Straitz III and the National Airoil Company and GBA Ltd of Slough for assistance with some algorithms within the software. Windows XP, Vista and Windows 7 are registered trademarks of Microsoft Corporation. Copyright Softbits Consultants Ltd, 1989, 1990, 2002, 2006, 2008, 2010, 2013
1 Introduction... 1-1
1.1 Features . . . 1-4 1.2 Program Overview. . . 1-8 1.3 Documentation Overview . . . 1-14
2 Getting Started... 2-1
2.1 Offshore Flare Stack Design . . . 2-4 2.2 Onshore Flare Stack Design . . . 2-24 2.3 Using Shields . . . 2-35 2.4 Using Overlays . . . 2-45 2.5 Case Study . . . 2-51 2.6 Dynamics . . . 2-59 2.7 Gas Dispersion . . . 2-64 2.8 KO Drum . . . 2-73
3 Interface... 3-1
3.1 Terminology . . . 3-3 3.2 Menu Bar. . . 3-8 3.3 Multiple Case Views . . . 3-11 3.4 Tool Bars . . . 3-12 3.5 Log Panels . . . 3-15 3.6 File Dialogs . . . 3-16 3.7 About View . . . 3-21 3.8 Radiation Limits View . . . 3-22 3.9 Flaresim Update View . . . 3-234 General Setup ... 4-1
4.1 Case Navigator View. . . 4-4 4.2 Case Summary View. . . 4-9
4.5 Component Management View . . . 4-46
5 Fluids ... 5-1
5.1 Fluid View . . . 5-4 5.2 Assist Fluid View . . . 5-15
6 Environment... 6-1
6.1 Environment View . . . 6-4 6.2 Environment Summary View . . . 6-15
7 Stacks ... 7-1
7.1 Stack View. . . 7-4 7.2 Stack Summary View . . . 7-10
8 Tips ... 8-1
8.1 Tip View. . . 8-4 8.2 Tip Dynamic View . . . 8-35 8.3 Size Tip View. . . 8-40 8.4 Tip Summary View . . . 8-42
9 Receptors ... 9-1
9.1 Receptor Point View . . . 9-5 9.2 Receptor Point Dynamics View . . . 9-20 9.3 Receptor Point Summary View . . . 9-22 9.4 Receptor Grid View . . . 9-25
10 Shields... 10-1
10.1 Shield View . . . 10-4 10.2 Rectangle Builder . . . 10-11 10.3 Polygon Builder . . . 10-13 10.4 Pit / Hut Builder . . . 10-15 10.5 Transform View . . . 10-17
11.2 Implementation Details . . . 11-12
12 Overlays And Isopleths... 12-1
12.1 Overlay View . . . 12-4 12.2 Zoom View . . . 12-15 12.3 Isopleth Customise View . . . 12-17
13 KO Drums... 13-1
13.1 KO Drum View. . . 13-4 13.2 KO Drum Summary View . . . 13-23
14 Case Studies ... 14-1
14.1 Case Study View . . . 14-4 14.2 Select Variable View . . . 14-18
15 Calculations ... 15-1
15.1 Calculation Sequence . . . 15-3 15.2 Calculation Options View . . . 15-4
16 Printing ... 16-1
16.1 Report View. . . 16-4 16.2 Output Graphic Report View . . . 16-9 16.3 Select Graphic Report Printer . . . 16-13 16.4 Graphic Report Page Settings. . . 16-14
17 Calculation Methods ... 17-1
17.1 Thermal Radiation . . . 17-4 17.2 Surface Temperature. . . 17-20 17.3 Noise . . . 17-21 17.4 Purge Gas . . . 17-28 17.5 Water Sprays. . . 17-31 17.6 Gas Dispersion . . . 17-33A Graphic Report Layout... A-1
A.1 Introduction to XML . . . A-4 A.2 Layout File Structure . . . A-6
Page
1 Introduction
1.1
Features . . . 4
1.2
Program Overview . . . 8
1.2.1 Flaresim Objects . . . 9 1.2.2 Object Definition . . . .11 1.2.3 Running a Model . . . 131.3
Documentation Overview . . . 14
Flaresim is a computer program designed to assist professional engineers in the design and evaluation of flare systems. The program calculates the thermal radiation and noise generated by flares and estimates the temperatures of exposed surfaces. It also performs dispersion analysis of the combustion gases or relieved fluid in flame out conditions.
Flaresim provides a user friendly interface with program actions accessed by menu and toolbar options. Data entry is through a series of data views controlled from an overall Case Navigator view. Context sensitive help is available at all points to assist the user in the use of the program and selection of appropriate design
parameters.
Output from the Flaresim is highly customisable with the user having the freedom to select summary or detailed output. The reports also include graphical output where appropriate.
Experienced flare system engineers should read the remainder of this chapter for an overview of the way that Flaresim performs calculations. They may then find that they will be able to use the program with assistance from the help system without further reference to the manual. However we would advise study of the manual to become familiar with the full range of options and recommendations for using the program.
Engineers new to flare system design should work through the examples in the Getting Started section of the manual after first reading this chapter. The examples provide a step by step guide to using Flaresim for flare system design and highlight some of the critical parameters that must be determined.
1.1 Features
The following features highlight the main capabilities of Flaresim. • Equally applicable to the design of flare systems for offshore
platforms, gas plants, refineries and chemical plants.
• Data may be entered and reported in the users choice of units and may be converted at any time.
• Correlations are available for modelling a range of flare tips including sonic tips, pipeflare tips and steam or air assisted tips. For assisted flares the quantity of steam or air required for smokeless operation can be calculated.
• A number of correlations are provided to predict the fraction of heat radiated from flames of a range of hydrocarbon fluids with different types of flare tip.
• Liquid flaring systems can be handled.
• A wide range of algorithms for calculation of thermal radiation. These include integrated multipoint methods and the Chamber-lain (Shell) method in addition to the Hajek/Ludwig and Brzustowski/Sommer methods which are described in the API guidelines for flare system design.
• Full three dimensional flame shape analysis with complete flexi-bility in specification of the location and orientation of multiple stacks.
• Thermodynamic flash routines from NIST to calculate change in fluid properties with pressure.
• Dynamic calculation option to evaluate results as flare flows vary with time.
• Case study manager to allow multiple comparitive results to be generated within a single Flaresim model.
• Calculation of combustion gas composition. • Calculation of purge gas flows required for tips.
• Jet dispersion model to analyse flammable gas concentrations close to flare in flame out conditions.
• Gaussian dispersion model to analyse longer distance dispersion of the relieving fluid or combustion gases.
• A range of options for defining and analysing the noise spectrum generated by flare systems including user defined spectra. • Ability to define multiple environmental scenarios to allow rapid
evaluation of flare system performance under different wind speeds and directions.
• Multiple stacks/booms each accomodating multiple flare tips. • Calculation of radiation, noise spectrum and surface
tempera-tures at multiple receptor points.
• Calculation of radiation variation with wind direction and speed at a point and display of results on a wind rose chart.
• Ability to define multiple receptor grids in multiple planes for calculation of radiation, noise or surface temperatures.
• Plotting of grid results as isopleth contours for sterile area definition.
• Receptor point characteristics for calculating surface tempera-tures include mass, absorbtivity, emissivity, area, specific heat, orientation and initial temperature.
• Option to define local environmental conditions at receptor points for calculating temperatures.
• Sizing and rating of knock out drums.
• Modelling of water curtains or solid shields to reduce radiation and noise transmission.
• Sizing of stack or boom length to meet radiation, noise or sur-face temperature limits at defined receptor points.
• Sterile area calculations to allow the safe distance from flare stack at different radiation limits.
• A setup wizard to allow new users to set up an initial model rapidly with appropriate defaults.
• Expert mode to control access to less commonly used options. • Import of files from Flaresim 2.0 and later.
• Multiple reports can be created and compared as updates are made to a model and the data corresponding to any report can be saved.
• Quality Assurance options are included in the reports. • Customisable HTML reports
• Customisable graphic reports
• Multiple Flaresim cases can be open at the same time.
The wide range of calculation options available within Flaresim may lead to the possibility of selecting inappropriate correlations for a particular combination of fluid type and flare system configuration. While we have tried to prevent the use of the more obvious problems we have also tried to allow flexibility for “one off” situations. As
with all engineering computer software, Flaresim is a tool which cannot replace sound engineering judgement.
Softbits Consultants Ltd are always interested in continuing product development to ensure that Flaresim meets the needs of our clients. Should you wish to see any feature incorporated in Flaresim, please feel free to contact us at [email protected]. If the request is reasonable we will endeavour to include it in future releases of the program.
1.2 Program Overview
The Flaresim program has been developed to provide great flexibility in modelling by breaking down the flare system into a number of objects such as fluids, stacks, tips etc. These individual objects are then linked together to define the complete system. Flaresim provides a Case Navigator view, see Figure 1-1, that shows a tree structure of all the objects that have been defined in a given model and provides a rapid overview of which ones are currently complete and in use.
Figure 1-1, Case Summary view
Case Navigator Icons
Required object present and ready
Required object missing or not ready
Optional object Permanent object Object ready Object not ready Object ignored
1.2.1 Flaresim Objects
The objects that can be defined
are:-Case Summary
Each model contains a single Case Summary object which defines descriptive information.
Fluids
A model can contain multiple fluid objects. Each object describes the physical properties of a fluid to be flared such as density, lower heating value, lower explosive limit etc. Fluids may be defined either by entering bulk properties or by defining the composition of the fluid to allow calculation of its properties from pure component data. A single fluid can be flared through multiple tips.
Environments
A model can contain multiple environment objects each of which describes a combination of wind speed, direction, humidity etc. The variation of wind speed with direction can also be defined to support wind rose calculations. Environment characteristics can also be defined for use in dispersion calculations. Only one environment object can be active for a set of calculations.
Stacks
Multiple stack objects can be defined which may be active or ignored in any set of calculations. Stack data includes length, location and orientation. Each stack may support multiple flare tips. The distance from each stack to defined radiation and noise limits can be calculated to evaluate the sterile area required around each stack.
Tips
Multiple tip objects can be defined and set active or ignored in a set of calculations. Tip data includes tip type and associated calculation methods, dimensions and stack location data and the flow and selection of the fluid being flared. Tip objects provide access to flame shape and other tip specific results such as combustion gas composition and purge gas requirements. Tip objects also have a
dynamic view that allows the variation in flare flow with time to be defined and modeled.
Receptor Points
Multiple receptor point objects can be defined and then set active or ignored in a set of calculations. Receptor point data includes location, characteristics for surface temperature calculation and constraints for sizing calculations. Receptor point objects provide access to results calculated for the point. The effect of wind speed and direction on the radiation can also be calculated and displayed as a wind rose plot. Receptor point objects also provide a dynamics view that displays the variation of results as the flare flow varies with time.
Receptor Grids
Multiple receptor grid objects can be defined and then activated or ignored in a set of calculations. Receptor grid data includes
orientation, location and coarseness data as well as characteristics for surface temperature calculations. Receptor grid objects provide access to their calculated results including contour plots of radiation, noise, surface temperature and gas dispersion.
Assist Fluids
Multiple assist fluid objects may be defined and selected for one or more flare tips. Data includes assist fluid type and calculation method to be used.
Shields
Multiple shield objects may be defined to model the reduction in radiation and noise through the installation of water sprays and solid shields. The transmissivity of water sprays can be specified by the user or calculated using an internal correlation. Shields can also be defined to model burn pits or protective locations.
Dispersions
Multiple dispersion objects may be defined to model the dispersion of combustion gases and flare fluids over long distances using a Gaussian dispersion model. Either concentration contour plots for a single pollutant or a downwind plot for multiple pollutants can be calculated.
Overlays
Overlay objects allow simple drawings to be created to act as background pictures for contour plots produced by the Receptor Grid and Dispersion objects.
KO Drums
KO Drum objects may be defined to perform Sizing and Rating calculations for knock out drums. Vapour and liquid properties can be entered directly or a composition specified to allow them to be calculated by the NIST flash package. Calculations may be run for either horizontal or vertical drums with a variety of end types. Either API or GPSA settling velocity correlations can be selected.
Case Studies
Case study objects can be created to run comparitive calculations to be run alongside the main calculation case. Two types of Case Study are available. A discrete input Case Study allows a set of input variables to be selected and case by case values defined.. An incremental input Case Study allows values for one or two input variables to be varied in steps over a range of values. Any result variable can be selected for output in either type of Case Study.
Calculation Options
A single calculation options object defines the correlations to be used in the calculations. It also provides for control of stack sizing options, heat transfer options to be used for temperature calculations and default emissions data. A data fitting option is also available.
Component Management
A component library manager object allows maintenance of the pure component database.
1.2.2 Object Definition
Flaresim objects are created by selecting the branch in the Case Navigator view and then clicking the Add button. Alternatively the Add dropdown menu in the Case Navigator can be used.
Creation of an object automatically opens its view to allow its data to be entered. When all the required data has been entered the status text at the bottom of the view will indicate Ready as shown in Figure 1-2.
Some objects have more data items than will fit on a single form so their views have been divided into multiple tabs.
For example the Stack view as shown in Figure 1-2 has tabs for Details and Sterile Area. Individual tabs are selected by clicking on their name.
Existing objects can be updated by double clicking them in the Case Navigator view or selecting them in the Case Navigator view and clicking the View button. When the Case Navigator is closed existing objects can be displayed by selecting them in the View dropdown menu.
1.2.3 Entering Values
When new values are entered in Flaresim they are checked to ensure that they lie between a minimum and maximum value designed to protect the Flaresim calculations from unreasonable values. The fact that a value falls within the range allowed by Flaresim does not mean that it is thereby valid - the validity of all values entered are the responsibility of the user.
1.2.4 Running a Model
In order to run calculations a Flaresim model must contain at least one of each of the following objects in an active and ready state. • Fluid object
• Environment object • Stack object
• Tip object
While this is sufficient to perform calculations this will not calculate any radiation, noise or surface temperature results without addition of at least one active Receptor Point or Receptor Grid.
Calculations are started by clicking the button at the top of the Case Navigator. This button is also used to display the progress of calculations and the status of the model. When the Case Navigator is closed the icon can be clicked to run the model. Progress of calculations and any problems encountered are reported in the right hand Message window at the bottom of the Flaresim screen.
Results from the calculations may be viewed through the
appropriate tabs in the Tip view, Receptor Point view or Receptor Grid view. Results may be viewed in tabular or graphical format where appropriate. Alternatively results can be viewed and printed through the Print or Print Graphic Report buttons in the Case Navigator tool bar.
Once complete a case can be saved using the Save and Save As buttons in the Case Navigator tool bar.
1.3 Documentation Overview
The printed Flaresim manual contains the following
chapters:-Chapter 2 - Tutorial with detailed worked examples.
The electronic documentation in the file Flaresim.pdf contains this material and the following additional chapters which provide a full detailed description of the program features.
Chapter 3 - Concepts, Flaresim Interface, Menu structure, Log
Panels and File Dialogs.
Chapter 4 - General Setup including Case Navigator, Case
Summary, Preferences and Component Management.
Chapter 5 - Fluid and Assist Fluid views. Chapter 6 - Environment view.
Chapter 7 - Stack view. Chapter 8 - Tip view.
Chapter 9 - Receptor Point and Receptor Grid views. Chapter 10 - Shield view.
Chapter 11 - Dispersion view. Chapter 12 - Overlay editor view. Chapter 13 - KO Drum view. Chapter 14 - Case Study view.
Chapter 15 - Calculation Options view.
Chapter 16 - Report options inc. Print Reports and Graphic Reports. Chapter 17 - Calculation methods.
Page
2 Getting Started
2.1
Offshore Flare Stack Design . . . 4
2.1.1 Objective and Data . . . 4
2.1.2 Initial Setup. . . 4
2.1.3 Initial Calculations . . . 13
2.1.4 Print Results . . . 15
2.1.5 Sonic Tip Design . . . 17
2.1.6 Run Sonic Tip & Review Calculations . . . 18
2.1.7 Compare Results . . . 19
2.1.8 Two Tip Design . . . 21
2.1.9 Update Pipe Tip . . . 23
2.2
Onshore Flare Stack Design . . . 24
2.2.1 Objective . . . 24
2.2.2 Model Setup . . . 24
2.2.3 Initial Calculations . . . 30
2.2.4 Sizing Setup . . . 32
2.2.5 Run Sizing Calculations . . . 33
2.3
Using Shields . . . 35
2.3.1 Offshore Case - Add Welltest Burner . . . . 35
2.3.2 Offshore Case - Run Welltest Calc’s . . . . 37
2.3.3 Offshore Case - Add Water Screen. . . 37
2.3.4 Onshore Case - Workshop Surroundings 39 2.3.5 Onshore Case - Add Workshop . . . 41
2.3.6 Onshore Case - Add Local Environment . 43
2.4
Using Overlays . . . 45
2.4.1 Offshore Case - Flaresim Overlay . . . .45 2.4.2 Onshore Case - External Overlay File . . . .48
2.5
Case Study . . . 51
2.5.1 Offshore Case Study - Discrete Variable . .51 2.5.2 Onshore Case Study - Increm. Variable. . .55
2.6
Dynamics . . . 59
2.6.1 Offshore Case . . . .59
2.7
Gas Dispersion. . . 64
2.7.1 Objective and Data. . . .64 2.7.2 Load or Create Base Case . . . .65 2.7.3 Jet Dispersion Calculation . . . .65 2.7.4 Gaussian Dispersion, Contour Plot . . . .67 2.7.5 Gaussian Dispersion, Line Plot . . . .69 2.7.6 Dispersion Analysis Comments . . . .71
2.8
KO Drum . . . 73
2.8.1 Initial Sizing . . . .73 2.8.2 KO Drum Rating . . . .75
The purpose of this chapter is to provide an introduction to the use of Flaresim. The examples show how Flaresim may be used to calculate thermal radiation, noise and exposed surface temperatures arising from flaring at one or more flare stacks. Examples of case studies, dynamic and dispersion calculations are also given. The examples begin with simple flare stack designs for offshore and onshore situations which are then refined and expanded. The examples attempt to highlight some of the critical parameters to be considered when designing a safe flare system.
The examples build up in stages. If you wish to skip a particular stage, the folder [Public Documents]\Softbits\Flaresim 4.0\Samples has model files saved at each stage.
2.1 Offshore Flare Stack Design
2.1.1 Objective and Data
The objective is to design a flare stack for an offshore platform. It is assumed that an inclined flare boom will be used, mounted on the side of the platform which faces the prevailing wind. The design is to be based on thermal radiation limits as
follows:-• 1,500 btu/hr/ft2 at the base of the flare stack.
• 600 btu/hr/ft2 at the helideck located 150 ft from the side of the platform and 30 ft above the base of the flare stack.
The following design data is available Fluid
Material Hydrocarbon Vapour
Flow 100,000 lb/hr
Mol Wt. 46.1
Vapour Temp. 300 F
Heat of combustion 21,500 btu/lb Heat Capacity ratio 1.1
Tip Diameter 18 in
Wind Velocity 20 mph
2.1.2 Initial Setup
1. Start the Flaresim program through the Windows Start button in the usual way.
2. We are going to build our first model through the Setup Wizard. For a new installation of Flaresim this will open automatically, ready to build a new model. If this does not appear then you should select the File - Preferences menu option and select the “Use Setup Wizard for New Cases” check box on the Files & Options tab. Then select File -
°
New or the New File icon on the tool bar to create a new case with the Setup Wizard.
3. In the opening view of the Setup Wizard, set the unit set to Default Field as shown. Then click the Next button to move to the Fluid definition tab.
4. In the Fluid tab of the Setup Wizard, enter the following data items, using the tab key or the mouse to move from field to field.
Temperature = 300 F Mole Weight = 46.1 LHV = 21500 btu/lb Cp/Cv = 1.1
Figure 2-1, Setup Wizard Opening View
Note that some of these values (e.g Temperature or Cp/Cv) are originally displayed in purple colour denoting a default value. When you enter a value the colour changes to blue denoting a user specified value.
The full list of colours used by Flaresim to display values
is:-Purple for a fixed default value Red for calculated default values Blue for a user specified value
Grey for a fixed, unchangeable input value Black for a calculated result
The remaining values for Ref Pressure, LEL and Saturation can be left at their default values. The finished view is shown below
Figure 2-2, Setup Wizard Fluid Tab
LEL is used only by the Brzustowski flare radiation method.
Note that Flaresim requires the lower heating value for a fluid within its calculations. We are assuming that the value we have been given is the lower, net heating value rather than the higher, gross heating value.
Advice on the usage of each input value and the allowable input range is displayed in the advice panel as you move through the input fields.
When the entries are complete click the Next button. 5. In the Tip tab, select the radio button to set the tip type to a
Pipe Tip. In the table including the selection of F Factor method, select the check box to select the Generic Pipe method.
The F Factor, i.e. the fraction of heat radiated by the flame, is a critical design parameter for flare system design. The Generic Pipe correlation has been developed to predict F Factors across a range of exit velocities and fluid molecular weights and is generally recommended for initial calcula-tions. For final designs, we would always recommend con-sulting a flare system vendor for advice on the appropriate F Factor for a specific fluid and specific flare tip.
6. Still in the Tip tab, enter the Fluid Mass Flow Rate as 100,000 lb/hr. After this entry has been completed, the Tip Diameter field is updated to show the tip diameter required for the default Mach number of 0.45. In our case we know the tip diameter is 18 in so we update the calculated value to 18 in. The Mach number will be updated to 0.199 to
indicate the velocity for the new diameter.
When complete the view should be as shown in Figure 2-3. Click the Next button to move to the next tab.
7. In the next tab, the Environment tab, enter the wind speed. Since the value we have been given is 20 mph, we first click the entry displaying ft/s and select mph in the drop down menu before entering the value. If we wish to see the value in ft/s, click again in the units entry and select ft/s to display the converted value of 29.33 ft/s.
The remaining items can be left at their default values namely Wind Direction as 0 (i.e. North), Temperature 59
F, Humidity 10% and the User Transmissivity 1.0, with the Transmissivity Method set to “User specified”. Note this default transmissivity method with a specified transmissiv-ity value of 1.0 is the most conservative option.
The final input is to remove the tick from the check box labelled “Include Solar Radiation” which means that the
Figure 2-3, Setup Wizard Tip Tab
° The humidity value is only
used when calculating the transmissivity.
specified solar radiation value will NOT be added to the cal-culated value of flare radiation.
Including solar radiation leads to a more conservative design. API 521 states that its inclusion should be consid-ered on a case by case basis. Solar radiation can have a sig-nificant impact on the flare design when low radiation values are considered. Since we are considering a low design radiation for the Helideck, in this case we will exclude solar radiation for this example.
The completed view is shown as Figure 2-4. Click the Next button to continue.
8. In the Stack tab, select the radio button to set the Vertical Orientation to 60 degrees from horizontal. Then set the Stack Horizontal Orientation angle to 0 (i.e. North). The Stack Length will be left unspecified to let Flaresim calculate it.
Click the Next button to continue
9. In the Receptors tab, click on the default receptor point “RP_1” and rename it to “Stack Base”. Set its Distance Downwind from Stack to 0 ft and confirm that the Allowable Radiation for the point is 1500 btu/hr/ft2.
Click the Add button to create an additional receptor point for the radiation at the Helideck. Change the default name “RP_2” to “Helideck” and enter the location as Northing -150ft, Easting 0ft, Elevation 30ft. and the radiation limit as
600 btu/hr/ft2. The completed form is shown as Figure 2-5 above. Click the Next button to continue.
10. In the Calculations tab, set Calculation Method check box to Mixed and the Flame Elements to 25.
As discussed in the Methods chapter, the Mixed method is a compromise designed to give the best accuracy for calculat-ing radiation both close to and further away from the flame. As such it is a good default method. 25 flame elements is usually sufficient to calculate the flame shape with a reason-able degree of accuracy.
The completed view is shown as Figure 2-6. We have com-pleted the Setup Wizard so click the Finish button.
11. When the Finish button is clicked, the Setup Wizard takes the data we have supplied and uses it to create the Flaresim
objects that we need for our initial model. The Case Navigator view will be displayed to list all of these objects as shown in Figure 2-7. Note that the icon is shown against each object indicating it is ready to calculate and that the icon is shown against the key object branches to indicate that the model has the minimum information needed to run calculations.
At this point you can open each object’s view by double clicking on them in the Case Navigator to see how the Setup Wizard has initialised the values.
12. This is a suitable point to save the data we have entered so far. Click the button in the tool bar at the top of the Case Navigator or main tool bar. Since we have not yet saved the file, a File Save Dialog window will appear to allow us to specify the location and name of the file. Use the name “Ex1 - Offshore - Ready To Run”.
2.1.3 Initial Calculations
13. We are now ready to run the calculations by clicking the large button labelled “Click to Calculate” at the top of the Case Navigator. The button will change to show a progress bar as the calculation runs. Messages will be output to the Error/Warnings/Info log as the calculations progress. When calculations are complete the colour of the log panel will change to summarise the status of the calculations. A green colour represents success, yellow represents some warnings were generated and red represents errors were encountered.
The scroll bars can be used to review earlier messages. The log window can be resized by dragging the separator bar above it.
We can now review the results. Click Stack 1 in the Case Navigator view and click the View button. The view will show that the stack length has been calculated as 247ft. Double click the Grid 1 item in the Case Summary view and then click the Radiation tab. Then select Plot in the Display drop down. The radiation isopleths are displayed as shown below.
Finally open the Receptor summary view by double click-ing the “Receptor Point” branch label in the Case Navigator. As shown below, the Radiation Results line shows that our design radiation limit of 600 btu/h/ft2 has been met for the Helideck receptor, while the radiation value at the Stack Base receptor is lower than its allowed value limit at 767 btu/hr/ft2.
14. This completes our initial design. Save the case as “Ex1 - Offshore - Initial Results”.
2.1.4 Print Results
15. Select the Print button in the Case Navigator tool bar. The Report Preview view shown below in Figure 2-11 opens. Note that this will open in a new window, independent of the main Flaresim view.
16. Select the report elements you wish to see printed. To see what the report will look like with the current set of elements, you will need to click the Refresh button to update it.
In order to allow us to compare these results with future results you will need to ensure that the Stack Configuration, Tip Results (General and Flame Shape elements) and the Receptor Point results are included. Once you have set your preferred report options you can click the Save Options button to save your report options to a configuration file. Your chosen options will also be saved with the case. 17. When you are happy with the options you have chosen click
the Print button to send the report to your default printer. The standard Printer Dialog view will appear to allow the printer and other options to be selected.
2.1.5 Sonic Tip Design
The design that we have produced meets our design radiation limits but requires a long 247ft stack. Since we are designing a flare stack for an offshore platform, we wish to minimise the length and hence the weight of the flare stack as much as possible. Therefore we will attempt to reduce the required flare stack length by redesigning the system with a sonic flare tip.
The fluid data, environmental data and radiation limits are the same as for Example 1.
1. If you are continuing from the previous section, you should save your case before continuing using the button from the tool bar at the top of the Case Navigator. Skip to step 3. 2. Otherwise use the File - Open menu option or the icon.
In the File Open dialog that appears, browse to the Samples folder created by your Flaresim installation. This will usually be in the Softbits\Flaresim 4.0 folder in your configured “Documents” folder. Select the file “Ex1 - Initial Result.fsw” and click the Open button.
3. Create a new tip by selecting the Tip branch in the Case Navigator view and then clicking the Add button or by selecting the Add - Tip drop down menu option.
4. After the Tip View opens, enter the following data on the Details tab:
Name = “Sonic Tip” Tip Type = Sonic Number of Burners = 1 Seal Type = None
Fraction Heat Radiated Method = High Efficiency
5. On the Noise Input tab of the Tip view enter the following data:
6. Move to the Location & Dimensions tab and enter the following data: On Stack = Stack_1 Length = 3.0ft Angle to Horizontal = 90 Angle to North = 0 Exit Diameter = 18in Riser Diameter = 18in
Contraction Coefficient = 1.0 (default) Exit Loss Coefficient = 1.0 (default) Roughness = 9.843e-4in (default) Calc Burner Opening = Selected
7. Click on the Fluids tab and enter the following: Fluid Name = Fluid 1
Fluid Mass Flow = 100,000lb/hr
8. At this point, the Status Text at the bottom of the Tip view should indicate that the tip data is complete. Close the view. 9. In the Case Navigator, select the branch labelled Tip 1 and
then click the Ignore button. The icon beside the label should turn to a icon to confirm that the tip will not be included in the calculations.
2.1.6 Run Sonic Tip & Review Calculations
10. We are now ready to run the calculations. Click the large 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 calcu-lations for the two Receptor Points have been completed. Note that if any earlier messages in the log panel are caus-ing confusion, you can click the right mouse button over the log panel to access a pop-up menu. This provides a Clear option to remove the current log messages.
11. We are now ready to review the results. Open the Stack view for the Main Stack. The new length calculated for the stack is 68ft.
12. Open the Receptor Summary view. This indicates that the Stack Base receptor point is now the controlling limit since the thermal radiation at this point is calculated as 1500 btu/ hr/ft2. The radiation at the Helideck receptor point is 543 btu/hr/ft2.
13. Save the new design to a new case name, “Ex1 - Offshore - Sonic Tip Results”.
14. Generate a report for this new case using the Print tool bar button.
2.1.7 Compare Results
Our new design with the sonic flare tip is clearly better since it leads to a much shorter stack. This will save a great amount of weight and hence cost over our initial design using the pipe flare tip. It is worth doing a detailed comparison to understand the difference between the designs:
15. Reopen the original case “Ex1 - Offshore Initial Results.fsw” and click the Print tool bar button. Since reports are generated in separate windows then you will now have two report windows that you can compare side by side. Note that both cases are open simultaneously in Flaresim and you can switch between them using the Windows menu option.
Alternatively you can use your Internet browser to view the saved report files “Ex1 - Offshore Initial Results.html” in the “Samples\Ex1 - Offshore Initial Results” sub-folder and “Ex1 - Sonic Tip Results.html” in the “Samples\Ex1 - Off-shore Sonic Tip Results” sub-folder (usually in [Docu-ments]\Softbits\Flaresim 4.0).
16. Find the Tip Data - Results section in the reports. The fraction of heat radiated value for the Pipe flare design is 0.35 while that for the Sonic design is 0.1.
The fraction of heat radiated by a flare is a critical parame-ter in the design. Pipe flares exhibit relatively poor mixing of air with the flared fluid and as a result the flame contains many partially combusted luminescent carbon particles that give it an orange colour and a relatively high fraction of heat radiated. Sonic flare tips are designed to maximise the mixing of air and the flared fluid and so burn with a clearer flame with lower heat radiation.
By selecting the appropriate F Factor method to calculate the fraction of heat radiated in both our designs, we have allowed the program to calculate an appropriate value for the different tips. However since this is such an important factor in the design, the heat radiation factor to be used should be confirmed with your flare system vendor prior to the final design. Should you wish to use a heat radiation factor supplied by a vendor you should set the method to User Specified and enter the value.
17. Still in the Tip Data - Results section of the reports find the flame length. For the Pipe flare design this is 173 ft, while for the Sonic flare design the flame length is 88ft. Note that the flame length calculated by the API method is the same in both cases.
Sonic flare tips by their design and by their greater gas exit velocities lead to a flame shape that is shorter and stiffer compared to that of a pipe flare. As a result the flame is less affected by wind and stays closer to the tip and thus further from the platform. This can be seen most clearly by compar-ing the 3D plot of the Flame Shape in the reports.
Finally in the Tip Results section of the reports, find the tip back pressure i.e. tip inlet pressure. For the Pipe flare this is 14.7 psi while for the Sonic flare it is 26.0 psi.
The fact that the sonic tip is operating at choked conditions means that the pressure drop over this type of tip is much higher than for the pipe tip. Thus a sonic tip can only be used if the resulting back pressure on the flare system is not so high as to prevent safe relief of the gas.
Comparison of our two designs using the pipe tip and the sonic tip shows that the sonic tip is much better since it produces a shorter, stiffer flame with a lower F Factor than the pipe flare. This means that the flare stack can be much shorter while still meeting radiation limits. Given the advantages of the sonic tip, it might appear that we should always specify this type of tip.
However we have also seen that the sonic flare tip results in higher back pressures on the flare system. In many cases, this additional back pressure will be too high to allow safe relief from all the possible relief sources in the process. Therefore it is common to see designs with both high and low pressure flare systems relieving through different tips.
2.1.8 Two Tip Design
The relieving sources in our process have been reviewed to check the new back pressure resulting from the sonic tip is acceptable. The review has shown that 10,000 lb/h of the material being flared cannot be relieved safely at the new higher back pressure. As a result we have decided to split our design so this 10,000 lb/h is relieved through a low pressure flare system, leading to a pipe tip with the remaining material flowing through a high pressure flare system to a sonic tip.
1. If you are continuing from the previous section you should save your case before continuing using the Save tool bar button in the Case Navigator. Skip to step 3.
2. Otherwise use the File - Open menu option or the icon. In the File Open dialog that appears, browse to the Samples sub-folder of your Flaresim installation (usually [Public
Documents]\Softbits\Flaresim 4.0), select the file “Ex1 - Offshore - Sonic Tip Results.fsw”and click the Open button. 3. In the Case Navigator view, double-click the Sonic Tip
branch to open the view for this Tip. On the Fluids tab, change the flow rate to 90,000 lb/h. Close the view. 4. Open the view for the Tip 1 by double-clicking this in the
Case Navigator view or by selecting it and then clicking the View button. Rename the tip to “Pipe Tip”. On the Fluids tab, change the flow rate to 10,000 lb/h. Then clear the tick from the Ignore check box to activate this tip again. Close the view.
5. We are now ready to run the calculations. Click the large button at the top of the Case Navigator.
Check the Errors/Warnings/Info log panel to confirm that the expected calculations for the two Receptor Points have been completed.
6. Open the Stack view for the Main Stack. The new length calculated for the stack is 96ft.
7. Open the Receptor Summary view. This indicates that the Main Stack receptor point is still the controlling limit since the thermal radiation at this point is still calculated as 1500 btu/hr/ft2.
2.1.9 Update Pipe Tip
In reducing the flow through the Pipe tip we have changed its performance:
8. Open the Tip view for the Pipe tip. You will see on the Details tab that the fraction of heat radiated from this tip has been calculated as 0.38 whereas before it was 0.35. The reason for this is the greatly reduced velocity, 0.02 mach, through the tip which reduces the tips efficiency. For efficient operation the velocity should be 0.2 mach or higher.
9. On the Location & Dimensions tab, click the Size Me button. Set the Mach number to 0.3 and set “Use Nominal Diam” to “No” and the tip size will be calculated as 4.6 in. Set “Use Nominal Diam” back to “Yes” and a nominal diameter of 5 inch will be selected. The calculated Mach Number which be automatically updated and shows 0.25 Mach. This is acceptable, so click the Ok button. The tip size and riser diameter will automatically be updated to the new selected diameter.
10. Now recalculate the case. The new exit velocity is 0.25 Mach and the fraction of heat radiated is now 0.34. The improvement in efficiency of this flare reduces the calculated size of the stack to 90ft.
11. Our two tip design is complete so save the case as “Ex1 - Offshore - Final Results”.
2.2 Onshore Flare Stack Design
2.2.1 Objective
The objective of this tutorial is to calculate the sterile area around an existing vertical flare located in an onshore facility and evaluate whether the current design is acceptable during a General Power Failure (GPF) scenario. The sterile area will be calculated at an elevation of 2m, which represents the typical head height for personnel.
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.
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.
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
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:
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.
23. This completes our initial evaluation. Save the case as “Ex2 - Onshore - Rating Results.fsw
2.2.4 Sizing Setup
The model that we produced for the existing flare calculated a distance of 120m from the flare base to the 4.7 kW/m2 radiation limit. Due to the proximity of process equipment and activities taking place in the vicinity of the flare, the extent of the calculated sterile area is not acceptable. The flare height needs increasing to meet a maximum permitted radiation level 4.7 kW/m2 on a horizontal plane elevated 2m from ground (head height).
1. If you are continuing from the previous example you should save your case before continuing using the Save button in the Case Navigator. Skip to step 3.
2. Otherwise use the File - Open menu option or the icon. In the File Open dialog that appears, browse to the Samples folder created by your Flaresim installation. This will usually be in the Softbits\Flaresim 4.0 folder in your configured "Documents" folder. Select the file "Ex2 - Onshore - Rating Results.fsw" and click the Open button. 3. Open the "LP Flare" view and enable the Size This Stack
check box under the Details tab.
4. Open the "Grid @ Head Height" view, select the Max Radiation tab and enter a Sizing Constraint of 4.7 kW/m2. Close the view.
5. We will also create a grid for the vertical cross-section through the axis of the flare to visualise radiation levels at different elevations.
In the Case Navigator view add a new Receptor Grid. On the Extent tab enter the following data:
Name = Elevation Grid
Grid Plane = Elevation-Northing
Easting Offset = 0m (section through the axis of the flare) Elevation Min = 0m Elevation Max = 300m Elevation Points = 41 Northing Min = -250m Northing Max = 50m Northing Points = 41
Customise the isopleth lines to show only the isopleth val-ues for 1.6, 3.2 and 4.7 kW/m2 as before.
2.2.5 Run Sizing Calculations
6. Hit the Calculate button. The log panel is red indicating that there is an error in the calculations. The flare height needs to be higher than the maximum height set by default.
7. Open the Calculation Options view from the Case Navigator, go to the Sizing & Pressure Profile tab and change the Stack Maximum Length to 150m.
Rerun the case. The sizing calculations are now successful. 8. Check the results. Open the "LP Flare" view, the stack
height has been increased to 106m to meet the 4.7 kW/m2 at head height. The location of the maximum radiation point (at 2m of elevation) is displayed in the Max Radiation tab of "Grid @ Head Height". The location of this point is at 51m downwind as shown below.
9. Open the Radiation tab of "Elevation Grid" and select the plot option. The 4.7 kW/m2 isopleth is above head height (2m from ground).
10. Finally open the Sterile Area tab under the "LP Flare" view. The 4.7 kW/m2limit shows a distance indicating that this value is reached. This is due to the sterile area calculation using a different solver routine to the sizing calculation. Increase the radiation limit to 4.701 kW/m2 and recalculate to remove this discrepancy.
11. This completes the stack sizing tutorial. Save the case as “Ex2 - Onshore - Sizing Results.fsw.
2.3 Using Shields
Flaresim includes the ability to model shield sections that will protect specific locations from the flare radiation. The shield sections may model solid obstructions blocking all of the radiation or water curtains that provide a partial block.
Two examples are presented here extending the base examples. For the offshore example, a welltest burner is added which requires the use of a water curtain shield to protect the platform. For the onshore example, a structure and its surroundings are modelled.
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 Mole Weight = 52.9 , LHV = 19,550 btu/lb, Cp/Cv = 1.2, LEL = 1.7%, Saturation = 100%. °
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 Northing = -200ft, Easting = 0ft, Elevation = 0ft, Dimensions section Length = 55ft,
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. Clear the 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 radiation
Figure 2-18, Shield Section Input
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.
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 Length = 10m Width = 4m Height = 4m Northing = -105m Easting = -30m Elevation = 0m Click OK.
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.
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 to
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”.
