An axial expansion joint takes movement along the pipe axis and nothing else. It is the simplest configuration, the most economical, and the one most often misapplied — because it passes the full pressure thrust straight into the anchors.
What it does
Compression and extension along the line. A straight run between two main anchors grows when it heats and shrinks when it cools; the axial joint absorbs that growth so the pipe does not have to find somewhere else to put it.
An axial joint will not take lateral offset or angular rotation. Asking it to is a design error, not a tolerance. Where the line also moves sideways or rotates, use a lateral or universal joint or an angular arrangement.
The anchor consequence
Because the bellows is free to extend, pressure acting on its effective area is reacted entirely by the anchors at each end of the run. On a DN300 line at 16 bar the thrust runs to several tonnes. Anchors have to be designed for it, and this is the point at which an axial joint stops being the cheap option — the civil work can cost more than the joint.
Where anchor capacity is limited, a pressure-balanced design removes the problem at the joint rather than at the foundation.
Guiding
An axial joint must be guided. The first guide goes within about four pipe diameters of the joint, the second within about fourteen, and further guides at the spacing the pipe stress calculation gives. Without them the pressurised bellows will buckle sideways rather than compress axially.
Envelope
| Property | Range |
|---|---|
| Nominal diameter | DN50 – DN3000 |
| Bellows material | SS 321, SS 316L standard; nickel alloys on request |
| Ends | Weld ends, or flanges to DIN PN6–PN40 or ANSI 150#–600# |
| Options | Internal flow liner, external cover, limit rods, pre-set (cold spring) |
| Design basis | EN 14917:2021, EJMA 11th Edition |
Cold spring
Where the movement is largely in one direction, the joint can be installed pre-set — extended or compressed at ambient so that it reaches its neutral position at operating temperature. This effectively halves the movement the bellows sees, and on the fatigue curve halving the movement per convolution is worth roughly six times the cycle life. Tell us the installation and operating temperatures at enquiry and we will state the pre-set on the drawing.