Expansion Joints

Externally Pressurized Expansion Joint: Large Axial Movement Without Squirm

Discover expansion joints that absorb large axial expansion in long pipelines without squirm, by applying pressure to the outside of the bellows.

ORFA Technical TeamUpdated: Oct 30, 20263 min read

In very long pipelines, axial expansion caused by temperature changes can easily exceed what a standard axial expansion joint can absorb. Adding more convolutions to the bellows increases its movement capacity, but long bellows under internal pressure become prone to squirm (buckling). An externally pressurized expansion joint applies the line pressure to the outside of the bellows, so large axial movements can be absorbed safely without squirm.

This article explains the operating principle, benefits, technical data and applications of externally pressurized expansion joints.

How Does an Externally Pressurized Expansion Joint Work?

In standard axial expansion joints, the fluid flows through the bellows and pressure acts from the inside. As the number of convolutions grows, the bellows behaves like a column and tends to buckle sideways. In an externally pressurized design, the bellows is enclosed in an outer shell and the flow path is arranged so that pressure acts on the outer surface of the bellows. A bellows under external pressure is far more stable, allowing many convolutions and therefore large axial movements.

The outer shell also protects the bellows from mechanical damage. This makes externally pressurized joints a preferred solution for systems carrying fluids that demand high safety, such as thermal oil.

Benefits

Large movement capacity: Absorbs high axial expansion in long pipelines.
Squirm resistance: Applying pressure externally eliminates the squirm risk of long bellows.
High safety: The outer shell protects the bellows from external impacts and suits critical fluids such as thermal oil.
Wide range of applications: Used especially in buried pipelines and geothermal lines.
Various movement values: Standard axial movement of 30, 60, 90 and 120 mm; project-specific values are possible.

Technical Data

Property	Value
Design and manufacture	To EJMA standards; pressure and temperature ratings to DIN 2401
Bellows material	Stainless steel to DIN 17440
End fittings	Stainless or carbon steel (other materials on request)
Connection types	Fixed flange, rotating flange, weld neck, threaded, grooved
Nominal size	DN25 (1″) – DN1500 (60″)
Pressure rating	PN16 standard; special designs for higher pressures
Temperature range	-196 °C to +600 °C depending on materials
Axial movement	Standard 30, 60, 90, 120 mm; special values possible

Operating pressure depends on nominal size and operating temperature; selection must consider all three together.

Applications

Absorbing thermal expansion and mechanical movement in long pipelines
Buried pipelines and district heating lines
Geothermal power plants and geothermal lines
Industrial plants operating at high temperature and pressure
Lines carrying safety-critical fluids such as thermal oil

Tip: Externally pressurized joints are still axial joints. Anchors are required at both ends of the section and guides near the joint; anchors must be designed to carry the pressure thrust.

Frequently Asked Questions

When should an externally pressurized expansion joint be used?

On long lines where a standard axial joint lacks movement capacity, where large axial expansion occurs, and for safety-critical fluids such as thermal oil.

Why does external pressure prevent bellows squirm?

Internal pressure pushes a long bellows to buckle sideways. When pressure acts from outside, the bellows is compressed and stays stable, so bellows with many convolutions can be used safely.

Can an externally pressurized expansion joint absorb lateral movement?

No. It is designed for axial movement and needs a proper arrangement of guides and anchors to work correctly.

Conclusion

Externally pressurized expansion joints reliably absorb large axial expansion in long pipelines without the risk of squirm. With a wide size range and various connection options, they can be adapted to many projects. For other models, see our metal bellows expansion joints page.

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