Expansion Joint Selection: First Read the Movement Correctly

An expansion joint is a component that takes the unavoidable movement of a pipeline onto itself. When the temperature changes the pipe grows; when a pump runs it vibrates; when the ground settles the line shifts. That movement has to go somewhere. Without an expansion joint it goes into flanges, welds and equipment nozzles.

Three types of movement

Axial movement is the pipe growing and shrinking along its own axis; this is the most common case and comes from temperature difference. Lateral movement is displacement perpendicular to the axis of the line; it appears at expansion joint crossings, in ground settlement and in building movement. Angular movement is the line changing angle at a point.

The critical point is this: not every expansion joint accommodates every movement. When a bellows designed for axial movement is forced to take lateral displacement, the convolutions are loaded unevenly and the product fatigues far earlier than expected.

Axial thrust: the load that gets overlooked

In a pressurised line the bellows produces a force equal to the fluid pressure times the effective bellows area. This force pushes the line in both directions and, if it has nowhere to go, lands on the nearest fixed point. That is why, on lines with axial expansion joints, the position and capacity of the fixed points are calculated together with the joint. Selecting the right joint but leaving the anchor weak produces a result similar to fitting no joint at all.

Limited-rotation (tied) types carry this force within the unit. The rods both limit over-extension of the bellows and take the axial thrust, so the load on the fixed supports is significantly reduced.

Choosing the connection type

• Fixed flange: Seats directly on the bolt holes of the mating flange. Requires alignment during installation.
• Rotating flange: Because the flange turns on its own axis, aligning bolt holes is easy; it shortens installation time in tight, hard-to-reach locations.
• Weld end: Flangeless, welded directly into the line. Reduces the number of sealing points, but is not removable.

What to watch during installation

1. Do not use the expansion joint to correct an installation error. Pulling two misaligned flanges together with the joint consumes its movement capacity on day one.
2. Remove the shipping brackets before commissioning the line — but not before installation is complete. If the line heats up with the brackets still fitted, the movement is not accommodated.
3. Fit the inner liner in the flow direction. The liner prevents flow from entering the convolutions and causing turbulence and erosion; fitted backwards it does not do that job.
4. Protect the bellows during welding. Weld spatter starts pitting corrosion on a stainless bellows surface.

Summary

Selecting an expansion joint is not a catalogue comparison but an exercise in reading the behaviour of the line: which movement, how much, in which direction. A selection made before those three are clear gives the wrong result even with the right product.