CUSTOM
BELLOWS

TECHNOLOGY

Single Miter Corner of Rectangular Expansion Joints

This configuration of the corner is relatively simple to produce when it comes to convolutions manufacturing but the biggest disadvantage is the stress concentration at the corners near to pipe welds and HAZ ( Heat Affected Zone ) present at the stress concentration area. Below picture represents the stress result of the convolution subjected to axial movement of 10 mm. It can be easily observed that the highest stress occurs at the transition part between the duct and bellows convolutions. The area of the highest stress concentration will lead to fatigue damage.

Why Multilayer Rectangular Expansion Joint solution is better than common Welded Corners Configuration?

Current state of technology for producing rectangular expansion joints for heavy industry is still based on single miter 90 degrees welded corners. This old fashioned technology is enforcing specific convolution geometry and making the use of advantages of multilayer configuration impossible. Nowadays it is becoming even more important to reduce the energy consumption and Co2 emissions prolonging the life time of the components used in heavy industry and so we do. In this document we will try to explain basic principles behind the Custom Bellows improvements.

Corner configuration vs. Equivalent Stress Concentration

It is well known that the geometrical characteristics plays the biggest roles when it comes to fatigue strength of the bellows. As long as the round bellows manufacturing processes are well developed we have seen the lack of performance in rectangular expansion joints commonly used for thermal stresses and movements compensation in rectangular ducts of all types. Due to its geometry rectangular duct is subjected to a different stress distribution that is highly concentrated at the corners, therefore it is not a surprise that rectangular expansion joints that are produced as the welded at the corners single layer structure will behave quite similar. In general we could divide the most common corner configuration as follows :

• single miter corner (90 degrees welded)

• double miter corner (45 degrees welded)

• single miter corner with rounded top convolution (90 degrees welded with rounded cap)

• rounded corners (rounded welded corners sections but rarely used)

For better understanding, you will find below more information together with graphical representation of the stresses made with use of Finite Element Method in Nastran Software.

Rounded corners of Rectangular Expansion Joints

Above examples of Nastran FEA of the rectangular bellows convolutions are indicating what is the most critical area for stress concentration. Due to fact that geometry pays the crucial role in fatigue, the best choice for the corner configuration should be the rounded transition between the duct and the bellows. However, due to current manufacturing capabilities it is not always possible considering the convolution geometry, and for most of the companies will be hard for manufacturing. Stress distribution at the rounded corners could be observed at the picture below.

Double Miter Corner of Rectangular Expansion Joint

Another very common type is the double miter corner configuration. Use of this specific type of corner configuration will help to improve the life time of the expansion joint however its main disadvantage will still be the sharp edge geometrical transition between the duct and the bellows. Below is the plot stress indicating the same convolution as above that was divided into two sections and subjected to the same axial movement.

Single Miter Corner of Rectangular Expansion Joints with Rounded Top

When it comes to bellows stress evaluation, we have seen that engineers are focusing on the top of the convolutions and are trying to make the performance improvement with adding the rounded top to the convolutions, however in the rectangular expansion joints designs same approach as to round bellows is not applicable. Below screen from Nastran FEA is indicating that even though the top convolution was redesigned, the stress remains the same as with single miter corner configuration. Moreover the presence of the crossed weld seams will make the fatigue performance even worst.

Multi Layer Rectangular Expansion joints vs. Single Layer

Above described distribution of the stresses at the bellows convolutions are strictly realated to the duct bellows transition. As could be seen on above FEA results, it is obvious that rounded corner configuration is of the best choice. However, it is not all…

We have developed the manufacturing process allowing us to use the advantage of multi layer forming for rectangular expansion joints making their fatigue performance similar to round expansion joints.

But what is the difference between the single layer design vs. multi layer design ?

The best explanation would be comparison of the FEA results of single layer vs. multilayer.

Single layer convolution stress results in tension (red) and compression (blue) of the inner and outer material fibers subjected to 5 mm movement is presented on below picture.

For comparison the same convolution geometry and movements were used and the results can be seen on below stress plot.

The main conclusion from the Finite Element Analysis is that the tension and compression stress was decreased for more than half related to single layer design. This situation will lead to improved fatigue performance to lower stress concentration.

The last but not least is the spring rate parameter, which is one of the most important right after fatigue performance. Choosing multilayer rounded corners configuration you may expect huge decrease in spring rates values compared to single layer and therefore lower reaction forces and moments that will be transferred to your duct.

Why to choose Multilayer Rectangular Bellows Technology ?

Based on the above technical analysis of the rectangular expansion joints deigns, it is obvious that multilayer, rounded corners rectangular bellows configuration will be the best when it comes to fatigue performance. In case you would have more questions regarding advantages and disadvantages of different expansion joint design do not hesitate to contact us and send the design request or commercial inquiry. Preventive maintenance is a key for many companies from energy sector and other heavy industry segments to increase the OEE (Overall Equipment Effectivness) together with extending lifetime of their assets. With multilayer rectangular expansion joints companies are able to take their maintenance strategies to the next level. The first major improvement with our product in the service and operability of the plant processes will be possibility to avoid unscheduled shut downs of the production lines. Supported with the extensive analysis and calculations multilayer rectangular expansion joints will make possible to have a timely and punctual understanding of the operational capacity of the duct, and change whole concept of execution of maintenance from firefighting to failures prevention.

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