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Symmetry Flare Adds Flare Radiation Analysis

Symmetry Flare Workspace now enables users to evaluate and verify the safety of the entire relief system, from relief devices through the header network and to the flare tip.

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Figure 1: Flare tip unit operation in Symmetry is linked to Flaresim software for radiation calculations 

Symmetry Flare has now expanded its integrated scope to include a live link to Flaresim, the industry standard flare radiation analysis software. This link makes it highly efficient to consider stack and tip limitations early in the design process, and to verify the system adequacy to meet radiation and other flare safety limits through the project lifecycle.

PSVs, the header system and flare stack are all tightly interdependent, so any change in configuration or operation can impact the entire system. Flare radiation analysis has traditionally been calculated using either in-house spreadsheets or specialized software like Flaresim. This requires manual transfer of information between tools every time there is a change in the system. This is cumbersome, inefficient and obviously prone to errors.

The application example in this article shows how a localized change in the relief system has the potential to propagate through its different components, and how this impact is studied in the integrated workflow of Symmetry Flare.

Flaresim™

For over 30 years this highly specialized application from Flaresim Ltd has been used by the industry to estimate thermal radiation from a flare stack and has become the industry standard for many. Flaresim employs an intuitive set of workflows and extensive visualization tools to evaluate different scenarios and stack configurations.

Advanced capabilities include:

  • Noise calculations
  • Surface temperature of exposed objects
  • Flare gas dispersion
  • Isopleth plots, including the option to overlay on a plot plan
  • Flexible configuration for wind speed and direction, including wind rose analysis
  • Multiple receptor points
  • Multiple stacks and/or multiple tip support
Symmetry and Flaresim

Symmetry now includes a link to Flaresim through its Flare Tip unit operation. The user can enable the link and then simultaneously solve the entire flare system including relief valves, header network, and flare radiation across all scenarios and with consistent user experience and fluid property estimation. This flexible design allows for studies across the entire system or any one component at a time. The link itself requires minimal configuration and can be driven directly from the Symmetry simulation. The user may choose to load a Flaresim case to specify values in the link, and/or export results to perform further advanced analysis within Flaresim itself.

Some of the main advantages and features from the link are:

  • Consistent user environment and property estimation
  • Simulation model is kept in one place and the information is always up to date
  • Radiation constraints can be inspected immediately after running scenarios
  • Flaresim simulations can be combined with all the productivity tools from Symmetry including case study, OPC connectivity or optimizer
Application Example

This example showcases the benefits of having an integrated simulation tool for Flare analysis where all components of a relieving system are evaluated simultaneously. The following case study verifies the PSVs, header network and flare stack of a system under five emergency scenarios including a fire, cooling failure, and blocked outlet. 

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Figure 2: PFD of flare & relief system evaluated in this example 

Initially, the example indicates safety constraints being met for all scenarios in respect of backpressures, flows and flare radiation being safely below the specified maximum limits.
Relief loads for the fire scenario are then updated to match changes in the process design and the scenarios rerun resulting in several relief valves undersized for their new relief loads. The PSV are resized and the automatic scenario recalculation shows that all network and relief valve constraints are now met.

However, radiation exceeds the specified limit with the updated flows. The Flaresim link is used to calculate the minimum stack size required to meet the radiation limit at the receptor point. The fire scenario is confirmed to pass all constraints at this point. All other scenarios are then run, and one fails to meet the Mach number constraint due to the higher rated flow from the relief valve. The tailpipe is resized, and all scenarios meet the design constraints. A detailed description of each step follows.

The sample network in this example has been designed to operate safely for five defined scenarios.  Header pipe velocity calculations fall comfortably below the specified 0.7 Mach limit and tailpipes below 0.5 Mach. Tailpipes are evaluated at the rated flow to ensure they can safely relieve the peak flow from the relief valve.  RhoV2 limits are met for all pipes, and the calculated backpressure on all relief valves is below their MABP. 

Flare_3-0001.png Figure 3: Scenarios passing constraints on pipes and relief valves

Relief valves are sized and rated at the network-calculated backpressure. Each valve is sized for all scenarios and then the flow capacity evaluated for the selected orifice size. 

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Figure 4: Relief valve sizing and rating across scenarios

The stack is sized to meet the radiation limit at the receptor point for all the scenarios. This was done by setting the active scenario to the one with the highest total flow and activating the Flaresim Link in the flare tip. Operating conditions in this scenario were pulled into Flaresim radiation calculations. The calculated size was applied to the stack, and running all scenarios confirmed that the radiation limit was met in all scenarios. 

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Figure 5: Radiation analysis for the active scenario through the Flaresim link

In this case study, we will review the flare system performance after a design change is made in the process. The process design change is assumed to cause a 15-20% increase in the relief load of five relief valves in the fire scenario (PSV-01, PSV-05, PSV-10, PSV-11) and a change of composition in PSV-05.

The study begins by updating these relief loads and compositions in the fire scenario through the flare scenario manager and running it. Quickly we find that two of the relief valves are undersized for the newly required flows. These valves were close to the maximum capacity with the previous flows and are now undersized. The Model Audit reports that either a larger size should be selected for these valves or the setting should be changed to solve tailpipes at the required flow rather than the rated flow, in which case a valid size selection is not required. We select a bigger orifice size for these valves.

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 Figure 6: The Model Audit warns that relief valves are undersized for the new relief loads

The relief valve form shows sizing results for the active scenario, including the calculated minimum API orifice designation. Both PSVs now require a bigger orifice size.  

Once the newly selected sizes are updated, the active scenario runs automatically. Results in the scenario manager indicate that all pipe and relief valve constraints are met. The higher flows create increased velocities through the pipes, as well as higher backpressures on relief valves, but all values are still below the maximum limits.  

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 Figure 7: Scenario manager after increasing valve sizes

Next, we verify that the radiation limit is met under the new scenario conditions. In this instance, the higher flows have resulted in the radiation level exceeding the limit. The stack has to be re-sized and the new calculated length is 245 ft. This is a 7ft increase above the original size. 

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Figure 8: Radiation limit is exceeded with higher relief flows 

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Figure 9: Stack sizing results

The stack length is increased 7ft and the radiation limit is met.  

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Figure 10: Longer stack meets radiation limit

The modified fire scenario has now been fully verified. However, changes to valve and stack sizes require verification under all other scenarios. Larger valves increase rated flow through tailpipes, and a longer stack will change backpressure through the network, so the impact on constraints must be evaluated for each set of conditions. We run all remaining scenarios from the scenario manager.

The Governing Scenario of each relief valve is once again automatically calculated. 

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Figure 11: Relief valve sizing and rating across scenarios after system changes

A summary of sizing and rating results across scenarios provides an idea of valves that may be far too oversized and cause stability issues. A valve that appears to be in this region could be modeled in dynamics to gain a deeper understanding of the possibility of instability. 

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Figure 12: Larger relief valve may be unstable when relieving scenarios with smaller flow

The scenario manager displays a summary of results across scenarios. One scenario fails to meet constraints because of a tailpipe that is violating the maximum 0.5 Mach limit (calculated as 0.56). The bigger valve orifice creates a higher peak flow, and this particular tailpipe is slightly too small to handle the larger flow. All other scenarios pass all constraints. 

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Figure 13: Scenario results after system changes - one scenario fails

 The tailpipe size can be changed directly from the scenario manager, and the increase from a NPS of 4 into 6 in is more than enough to resolve this constraint violation and handle the bigger rated flow.  

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Figure 14: Constraints pass in all scenarios after design updates

Next, radiation is verified across scenarios. The maximum radiation was in the fire scenario, and the radiation calculated for all other scenarios is below the limit.

After a brief analysis and minimal changes, we have verified all components of the system for all scenarios and updated its configuration to meet all constraints. The integrated workflow in Symmetry Flare enables efficient updates so any investment in safety modeling can retain its value through the evolution of the project.

Please contact your local VMG office for more information.

Angela Solano, P.Eng., VMG Calgary

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