Case Study: Flange Tightening Sequence Optimization

Modified on Wed, 14 May at 12:17 PM

1. Introduction

The AKSELOS Structural Performance Management (SPM) for Flanges tool is a specialized simulation tool that supports engineers in evaluating the structural behavior and sealing performance of bolted flange joints during hot bolting procedures, refers to the sequential loosening and retightening of bolts while the flange remains pressurized, which introduces complex interactions between gasket compression, bolt preload, and flange deformation.

In this article, is to guide users through the process of creating a complete flange joint model using the tool and performing a full integrity assessment based on a specific real-world use case.

This article is part of a larger tutorial series designed to help users build confidence and proficiency in using the tool through practical, step-by-step examples:

To help you better understand how the SPM for Flanges tool is used in practice, the following workflow outlines the full modeling and assessment process for this case study — from geometry input and load definition to results interpretation and performance evaluation of the gasket, bolts, and flange.

Figure 1.1. SPM for Flanges user journey

Note: This article is based on the SPM for Flanges tool – Version 2025.04. If you need assistance with installation or setup, please don’t hesitate to contact our support team.

2. Problem Statement and Objectives

Flange Tightening Sequence Optimization for Uniform Stress Distribution

This case study addresses the challenge of ensuring proper bolt tension and stress distribution in flanged joints by applying an optimized tightening sequence. Incorrect or inconsistent tightening can lead to gasket damage, flange misalignment, leaks, or even joint failure. By simulating a structured tightening pattern, such as a cross-pattern, the study aims to reduce these risks and improve the reliability of flange assembly under operational conditions.

Figure 2.1. FLANGE tightening sequence

Summary of Model Configuration

Geometry: Flange assembly with 8 bolts, spiral wound gasket, and use bolt root diameter
Materials: SA-105 for flange, SA-193 B7 for bolts, graphite-based spiral wound gasket.
Boundary Conditions & Loads: Cross pattern with three rounds (30% - 60% - 100%). No external loads are included.

Focus of This Case Study

This case study is designed to help users understand the importance of bolt tightening order and how it affects the structural performance of a flange joint. Using the SPM for Flanges tool, users will apply a multi-stage tightening sequence and evaluate its effect on bolt stress uniformity and sealing performance. The focus is on verifying whether a well-planned sequence can prevent over- or under-tightening, reduce stress concentration, and support longer equipment life through more reliable pressure containment.

Figure 2.1. THE STRESS RESULT OF THE FLANGE

3. Before we start

Before diving into model creation, there are a few essential steps to ensure you have the proper setup and access required to work with the SPM for Flanges tool. Follow these steps carefully to avoid issues during the modeling and assessment process.

Step 1: Set Up Your Account and Access. You’ll need an Akselos account with appropriate access permissions to the correct location. This is necessary to download the tool, create new model collections, and manage your simulations. If you haven’t already, review the following articles:

[Create Akselos Portal Account] – Required to access the platform and use the tool.
[Akselos Portal for New Users] – Provides an overview of the platform’s features and interface.

Step 2: Install the Latest Akselos Modeler. Ensure you have the most up-to-date version of the Akselos Modeler installed. If you’re unsure or need help obtaining it, please contact Akselos support for assistance.

Step 3: Organize Your Working Folder. Efficient data management is key to a smooth workflow. Follow the best practices outlined in the article below to set up your simulation folders properly:

[Working Folder and Data Management] – Recommendations for organizing files and managing simulation data on your local drive.

Step 4: Start with a New Model Collection. Once your environment is ready, you can begin building your model by creating a new collection and importing it for workspace preparation. Refer to:

[Start Building Your Model with a New Collection] – Step-by-step instructions on how to set up and manage a new model collection in the portal.

Completing these steps and familiarizing yourself with the articles above will help ensure a stable starting point for building and analyzing your flange joint models using the SPM for Flanges tool.

Bonus Track
As an additional resource, we’ve prepared a sample collection that demonstrates how the setup and simulation are structured within the Akselos environment. This example can serve as a helpful reference as you follow the tutorial or build your own model. You can access the sample collection here:

Case Study 3: Flange Tightening Sequence Optimization

4. Model Creation

Create a New Collection on Akselos Portal

On the Akselos Portal, a blank collection is the starting point for building any simulation model. A collection acts as a container for your project data and exists in two locations:

Locally on your drive (where you build the model)
On the cloud (where the solver engine runs)

These two are kept synchronized through a sync process (see Step 6).

To get started:

Step 1: Log in to the Akselos Portal (https://portal.akselos.com) using your account.

Step 2: Go to your workspace (e.g., your organization or team space like: https://portal.akselos.com/SPM_for_Flanges_Trial/<Your _Folder>).

Step 3: Click New Collection, enter a name (e.g., Case_study_1).

Note: Avoid using spaces in the name. Use underscores (_) instead to prevent errors.

FIGURE 4.1. CREATING NEW COLLECTION

Step 4: Wait for the collection to be successfully created.

Connect SPM for Flanges to the Cloud

To import collections, sync data, and submit simulation jobs, you need to connect your software to the cloud.

Step 1: In the SPM for Flanges software, go to the top left corner: Cloud ➔ Server Authentication.

FIGURE 4.2. OPENING THE SERVER AUTHENTICATION WINDOW

Step 2: In the authentication window, enter your Akselos username and password/token.

Step 3: Click Check and wait for the indicator light to turn green

FIGURE 4.3. CHECKING THE STATUS OF SERVER CONNECTION

Common Connection Errors:

Yellow light : Internet connection issue
Red light : Incorrect username or password/token

Import the Collection into SPM for Flanges

Once connected to the cloud:

Step 1: Navigate to Collections ➔ Import Collection.

FIGURE 4.4. OPENING THE IMPORTING COLLECTION WINDOW

Step 2: In the pop-up window, locate your newly created collection (e.g., Case_study_1) and click Import Collection.

FIGURE 4.5. IMPORTING the collection to the local machine

Step 3: Wait for the import to finish. You’ll know it’s complete when the collection name appears in the title bar of the software interface.

FIGURE 4.6. The importing collection process notification

Download the Material Library

The material library is stored on the cloud and must be downloaded each time you launch the software (more details in Step 3).

Note: The material library requires special access permissions. If you’re unable to download it at this step, please contact our support team at [email protected] to request access. We’ll assist you in the shortest time possible.

Step 1: Geometry setup

This example demonstrates how to create a flange joint model based on ASME B16.5 (2020) with a Class 150 rating and a Nominal Pipe Size (NPS) of 8 inches. The resulting flange has an outer diameter of 698 mm and includes 8 bolts. This model does not include the pipe and weld line.

FIGURE 4.7. SPM for flanges - define geometry

How to Input:

In the Akselos Modeler, select Flange Hot Bolting Tool from the tool drop-down list.
Navigate to the Geometry Tab.
Input the following parameters:
- Standard: ASME B16.5
- Class: 150
- NPS: 8
Select use Root Diameter options

Step 2: Define Materials

assigned specific material properties to accurately represent typical industrial conditions. The flange, pipe, and welds are modeled using carbon steel forgings, following the ASTM SA 105 specification with UNS designation K03504. The bolts are defined using ASTM SA 193 Grade B7 alloy steel, which is suitable for high-temperature and high-pressure environments. The gasket is modeled as a spiral wound type from the graphite group, chosen for its ability to maintain sealing performance under elevated temperatures and in contact with process fluids.

FIGURE 4.8. SPM for flanges - download material database

To input material data in the Akselos Modeler:

Step 1: Navigate to the Material tab

Step 2: Download Material Library

Go to the Material tab.
Download the full materials library.
Once the download is complete, the Define button will become available.

Note: Access to the material library requires permission. If you are unable to download it at this step, please contact our support team at [email protected] to request access. We’ll assist you in the shortest time possible so you can continue your setup without interruption.

Step 3: Assign Material to Pipe, Weld, and Flange.

FIGURE 4.9. SPM for flanges - define and assign material to pipe, weld, and flange

Click Define and enter the following:
- Behaviour: Elasticity Materials
- Product Form: Forgings
- Spec. No: SA-105
- Type/Grade: undefined
- Alloy desig./UNS No: K03504
- Class/Cond./Temper: undefined
Click Apply to save.

Step 4: Assign Material to Bolt.

FIGURE 4.10. SPM for flanges - define and assign material to bolt

Behaviour: Elasticity Materials
Product Form: Bolting
Spec. No: SA-193
Type/Grade: B7
Alloy desig./UNS No: G41400
Class/Cond./Temper: undefined

Step 5: Assign Material to Gasket.

FIGURE 4.11. SPM for flanges - define and assign material to gasket

Behaviour: Gasket Material
- Material Group: Graphite
- Name: Spiral Wound Gasket

FIGURE 4.12. SPM for flanges - materials assigned to parts

You can view detailed material properties such as thermal expansion or Young’s modulus by clicking the information button in the material library window. The corresponding values will be automatically shown in the table and chart panel.

Step 3: Defining Loads and Boundary Conditions

In this simulation, the focus is on replicating the bolt tightening process, so no external loads such as pipe loads or clamp loads are applied. The only load representing operational conditions is the internal pressure, which will be used to simulate the normal working state of the flange joint. The primary emphasis is on the bolt pretension load, applied through a controlled tightening sequence to observe stress distribution across the joint. Contact settings remain at their default configuration, frictional between the flange and gasket, and bonded between the bolt and flange.

FIGURE 4.13. SPM for flanges - set up boundary conditions

To input the simulation conditions:

Set Internal Pressure
- Apply 0.45 MPa on the inner surfaces of the flanges and pipe only.
Set Temperatures
- Environment temperature: 22°C
- Assembly temperature: 22°C
- Operating temperature: 100°C
Set Bolt Pretension: 80 kN
Set Contact Conditions: Leave general contact settings as default.
- Flange–Gasket contact: Frictional
- Bolt–Flange contact: Bonded

Note: In this case, pipe loads and clamp loads are excluded to isolate the effect of pressure and bolt pretension.

Step 4: Load Sequence Setup

In this simulation, the tightening sequence is configured using the Cross Pattern (Legacy) method to replicate a balanced and controlled bolt tightening process. The sequence begins with one Snug Tight (Install) step, where all bolts are engaged at 15% of the full pretension load. This is followed by three main tightening rounds, each divided into two sub-steps: one for odd-numbered bolts, and one for even-numbered bolts. This split approach allows bolts to settle gradually and reduces uneven gasket compression. Each round increases the pretension load incrementally, 30% in Round 1, 60% in Round 2, and 100% in Round 3, ensuring uniform load distribution across the flange and minimizing the risk of flange rotation or misalignment.

FIGURE 4.14. SPM for flanges - set up load sequence

To set this up:

Navigate to the Load Sequence tab
Set the Tightening Option

Choose Cross Pattern from the tightening options.
Define the stages as follows:
- Snug Tight: 0.15
- Tightening Rounds: 0.3, 0.6, and 1.0

FIGURE 4.15. tightening sequence visualization in spm for flanges software

Operation Step

Turn off both clamping load and piping load by entering 0 in their respective load factor fields.

Hot Bolting Table

Remove all entries from the Hot Bolting Table, as hot bolting is not part of this case study.

FIGURE 4.16. SPM for flanges - remove the hot bolting step

This case does not include hot bolting, so you can remove Step 10 (if auto-generated).
To do this, select Step 10 and click Remove Row.

Step 5: Final Check

Review the setup using the Load Sequence Summary Table to ensure all steps and values are correctly defined.

FIGURE 4.17. SPM for flanges - check the load sequence summary table

Step 5: Automatic model creation

Once all input parameters are completed, users should review all settings before clicking Create Model. The software will then automatically generate a fully configured model, eliminating the need to manually go through the CAD and meshing stages.

After the model is created, users can review all predefined settings directly in the Model Structure Tree under the Model Ribbon. This is a good opportunity to confirm that all setup parameters have been correctly applied before proceeding.

FIGURE 4.18. Model created from the Spm for flanges tool

Final Checks After Model Creation:

Check Created Components

There will be three main components in the model: Flange, Bolt, and Gasket.

Check Materials

Go to Components, then select a component (e.g., Bolts).
Under Subdomain, choose the specific part you want to inspect.
Material details will be shown in the property panel below.

Check Boundary Conditions

Constraints: Navigate to Boundary Conditions > Geometry Constraints. Selecting a constraint will highlight its location in the model.
Contact Settings: Go to Contact Interactions, then click a contact surface to view its type and assigned settings.

Check Loads

In the Load Cases section, use the drop-down menu to browse through all defined loads.
Although the tool may automatically create multiple load cases (typically five), in this tutorial, you only need to focus on the internal pressure case.

5. Analysis Results & Assessment Metrics

Step 6: Data synchronization

Once the model is fully created and all settings have been verified, the next step is to save the model and sync it to the cloud, where the Akselos solver engine operates. This step is essential for ensuring that the solver has access to the complete model data. After the solve, results will be automatically sent back to the SPM for Flanges software for visualization and further examination.

Saving the Model

The simulation model will be saved within the aks_files folder, with the format of *.aks: Partial_bolting.aks for example.

To save the file: Go to File > Save, or press Ctrl + S

Important: Do not use spaces in the file name, as this may cause errors. Instead, use underscores (_). Example: model_name.aks

Syncing the Model to the Cloud

Step 1: Go to Collections ➔ Sync with Portal.

Step 2: Click Commit and Close to complete the sync process.

Step 3: Log in to the Akselos Portal and open your collection to verify that the model has been uploaded successfully and is ready for solving.

Submit solve

Step 1: Navigate to Solution ribbon

Step 2: Click Solve to send the job to the cloud solver.

FIGURE 4.19. Solving model

This model is relatively detailed, with over 800.000 degrees of freedom (DOFs), so the simulation may take approximately 15 minutes to complete. Once finished, the solution will automatically download and display in the graphics window for visualization.

The simulation generates a total of nine child solutions, each named according to the steps defined in the load sequence, corresponds to a specific tightening action or operational state, allowing users to analyze the progressive effects of the tightening sequence on bolt stress and gasket pressure:

Tightening: Cross Pattern - Install
Tightening: Legacy Pattern - Round 1 - Applied Bolts Indices: (1, 5, 3, 7)
Tightening: Legacy Pattern - Round 1 - Applied Bolts Indices: (2, 6, 4, 8)
Tightening: Legacy Pattern - Round 2 - Applied Bolts Indices: (1, 5, 3, 7)
Tightening: Legacy Pattern - Round 2 - Applied Bolts Indices: (2, 6, 4, 8)
Tightening: Legacy Pattern - Round 3 - Applied Bolts Indices: (1, 5, 3, 7)
Tightening: Legacy Pattern - Round 3 - Applied Bolts Indices: (2, 6, 4, 8)
Tightening: Cross Pattern - Lock
Operation Load

Step 7: Gasket Pressure and Risk assessment

In this assessment, the focus is on evaluating the gasket normal pressure throughout the tightening process. The goal is to ensure that the gasket is uniformly compressed and remains within the optimal pressure range to maintain sealing performance. Using the Gasket Risk Matrix, users can identify whether any areas of the gasket are under- or over-compressed, which could lead to leakage or material damage. In addition, the gasket compressive pressure distribution chart helps visualize how the pressure is spread across the gasket surface. From this, users can assess whether the applied tightening sequence has achieved an even load transfer and make informed decisions to improve flange assembly procedures.

FIGURE 4.20. Checking the gasket normal pressure result of the model

Checking Normal Pressure on Gasket

To examine the pressure distribution:

Select a Solution Step
Go to the Solution Fields, select Gasket Normal Pressure (or any field) as the result type.

Using the SPM for Flanges Tool - Results tab

The Gasket Risk Matrix in the SMP for Flanges tool is a visual assessment framework used to evaluate sealing reliability based on gasket compressive stress. It helps engineers identify the likelihood and consequence of gasket failure during different stages such as assembly, operation, and hot bolting. By presenting this information in a structured format, the matrix supports safer decision-making and highlights areas that require further attention.

FIGURE 4.21. SPM for flanges - the risk matrix

Steps to check the Gasket Risk Matrix:

Navigate to the Result tab.
In the Component dropdown, select Gasket, in the Results dropdown, select Risk Matrix.
On the left panel under the Solution section, choose a child solution (e.g.,Round 3 - Applied Bolts Indices: (1, 5, 3, 7)).
Click Compute Gasket Risk Matrix to generate the chart.

The x-axis in each graph represents the gasket radius from inner to outer diameter, while the y-axis shows the compressive pressure in MPa. Each colored line reflects pressure measured at different radial positions around the gasket. The shaded background areas provide important reference thresholds:

Red Zone (Not Operate): Below the minimum pressure needed for sealing; operation in this zone risks leakage.
White Zone (Industry Acceptable Range): Represents the optimal range of compressive stress for reliable gasket performance.
Green Outline (Assemble Stress): Indicates the target range for gasket compression during bolt tightening.

FIGURE 4.22. SPM for flanges - the risk matrix

What the Charts Show

From Round 1 to the Lock step, we observe a progressive and uniform increase in compressive pressure across all positions:

In Round 1 (30%), the compressive pressure is still within the Not Operate zone, indicating insufficient stress to seat the gasket properly.

FIGURE 4.23. SPM for flanges - the risk matrix of round 1, step 3

In Round 2 (60%), stress levels rise noticeably but remain below the industry-acceptable range, still signaling a risk of leakage if operation begins at this stage.

FIGURE 4.24. SPM for flanges - the risk matrix of round 2, step 5

In the Lock step (100%), all positions reach a consistent pressure level well above the required sealing threshold, entering or nearing the Industry Acceptable Range.

FIGURE 4.25. SPM for flanges - the risk matrix step 8 - lock

These results demonstrate that the applied tightening sequence is effective. The pressure is distributed consistently across all gasket positions and increases steadily with each tightening round. By the final Lock step, the flange joint achieves proper gasket compression, which helps ensure long-term sealing integrity and reduces the risk of under-compression or uneven stress.

This confirms that the tightening procedure used, cross pattern with staged loading, is appropriate and performs as intended, delivering a controlled, uniform build-up of gasket pressure while preventing overloading or damage during assembly.

6. Conclusion

This tutorial guided users through the process of simulating a flange joint tightening sequence using the SPM for Flanges tool in Akselos. By configuring multiple tightening rounds and comparing stress distribution outcomes, users can evaluate how bolt tightening order influences gasket pressure uniformity and sealing performance. The gasket pressure results showed that certain tightening patterns failed to achieve acceptable compressive stress during early rounds, while the final locking step produced a more balanced and sufficient stress profile across the flange face. This case study highlights the importance of defining an optimized tightening sequence to ensure reliable flange sealing and demonstrates how the tool supports users in identifying and validating effective assembly procedures.

Disclaimer:
These tutorials are intended for instructional purposes only, to help users understand how the FHB tool operates within the SPM for Flanges software. The models and results presented here are not based on any real-world flange assemblies, and should not be used as the basis for evaluating or validating any actual engineering designs.
All input parameters used to create and simulate the model are conceptual examples. Users are responsible for independently verifying and validating all input data and ensuring its suitability for their specific use cases or engineering assessments.