Project Variables | Flue Duct Expansion Joint

It is critical to understand that a flue duct expansion joint is a highly specialized product that needs special attention to the design, installation and operation phases for a correct and safe performance.
In most flue duct expansion joints applications, a careful analysis of the system is required prior, in order to determine the type, materials of construction needed and other required components.
The flue duct expansion joints are designed for gas applications with pressures between -0.4 and +0.4bar.

Following are the main variables that should be considered regarding flue duct expansion joint design and manufacturing.


Duct dimensions, the geometry of the unit, the flow direction and expansion joint overall length (OAL) influence its design and ability to absorb movement.
There are specific metallic frame designs for each application and will have a dramatic effect on the ease of accessibility for installation and future maintenance.

Types of Media

The products in contact with the expansion joints should be carefully analyzed. The correct specification of the media (gas or liquid) is very important for a successful project.
The fluid pH, especially with extremely acid or caustic fluids is an important factor to know. The presence of sulfur (S), should be informed, as well.
In flows with saturated fluids, the condensation probability should be considered, comparing the fluid dew point with normal operation temperatures. Even with systems that operate with high temperatures, starting and stopping can generate condensation.
Very long pipeline systems, with deficient external thermal insulation, can generate a condensation by fluid temperature loss from duct walls.
Abrasive media, such as dust, particulate materials, catalysts, etc., need to be clearly identified. Cleaning procedures used for pipeline systems, such as internal washing or “pigging”, must be known up front to allow for accommodation during design of the expansion joint.

Internal Pressure

The operational pressure should be stated and within the allowable limits for flue duct expansion joints, i.e., between -0.4bar and +0.4bar. However, sudden pressure changes in operation (flutter) are also quite harmful and should be considered in any design parameters. This flutter, particularly in expansion joints installed close to large draft fans or gas turbines. It is important to stress that flue duct expansion joints are not leak-proof or leak-free. They are designed to have a high, but not absolute tightness, which is appropriate and expected for industrial applications.


The accurate operation temperature specification of the system is needed, as well as its maximum and minimum peaks. These will directly influence the bellows selection.
Special care to determine the temperature constants is needed so as not to design in excessive safety coefficients.


The determination of real movements’ to be absorbed is critical to specify the breach opening correctly. This, ultimately, is directly responsible for the flue duct expansion joint’s capability to absorb movement.
Besides thermal movement, it should be verified other types of movement, as vibration and misalignment.
Movements in different directional planes should be analyzed to determine whether they are concurrent or not.

The movements absorbed by a flue duct expansion joint could be:

freeflex freeflex freeflex freeflex
Axial Compression and
Axial Extension
Lateral Angular Torsion

Stresses generated by the expansion joint

The stresses generated by flue duct expansion joints are insignificant to the design. The necessary force to flex the bellows is so insignificant that it is not considered. The pressure force (figure 6) is a condition created by the installation of a flexible element (expansion joint) in rigid and pressurized tubing. The pressure force is a function of the system’s internal pressure and the bellows dimensions. Its amplitude is calculated by multiplying the bellows effective area (transversal) by the system’s internal pressure.


Pressure Force


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