Bellows expansion joint

A bellows expansion joint consists of a thin-walled, corrugated metal bellows. The convolutions deform elastically and absorb movements that would otherwise need an expansion loop. The bellows has to be thin to stay flexible – and that is exactly what makes it the most delicate component in the line.

Every movement bends the convolutions. The life of a bellows is therefore stated in cycles and depends on the size of the movement, the pressure and the temperature. Bellows are designed by the manufacturer on the basis of EN 14917 or the EJMA standards.

A bellows is soft in the axial direction, so internal pressure tries to push it apart – with a force equal to pressure times the effective bellows area. With an unrestrained axial expansion joint this force, the pressure thrust, has to be carried entirely by the anchors of the line. On large diameters it is usually the largest force in the whole system.

Restrained types – lateral joints with tie rods, hinged and gimbal joints – contain the pressure thrust within themselves. The anchors then carry only spring and friction forces.

In the analysis model the expansion joint appears with its spring rates for axial, lateral and angular movement and – for unrestrained types – with the pressure thrust as an external force. The calculation yields the movements at the bellows; they are compared with the manufacturer’s allowable values in combination, not one by one.

An axial expansion joint needs guides on both sides so the line cannot buckle sideways. Whether and how far it is installed pre-set is decided in the design. A bellows tolerates virtually no torsion about the pipe axis.

Unlike a pipe bend, a bellows is designed for a certain number of cycles. It therefore belongs in the inspection plan of the plant: visual inspection for corrosion, deformation of the convolutions and the condition of the restraints. Where there is room for an expansion loop, the loop is often the more robust solution for this reason.