A pressure-balanced expansion joint cancels its own pressure thrust internally. It is the configuration you reach for when the equipment at the end of the line cannot accept the load — a turbine, a compressor, a pump nozzle, a column.
How the balancing works
A second, balancing bellows is built into the joint and connected so that the pressure force acting on it opposes the force on the working bellows. The two are tied together, so the thrust is reacted inside the assembly and very little of it reaches the anchors or the equipment nozzle. The line still gets the movement it needs; the nozzle does not get the force.
In-line and elbow
| Type | Where it goes | Movement absorbed |
|---|---|---|
| In-line pressure-balanced | In a straight run | Axial, with thrust neutralised |
| Elbow pressure-balanced | At a change of direction, built around the elbow | Axial and lateral at the turn |
The elbow version is the more common on plant, because a change of direction is exactly where a sensitive nozzle usually sits and where the thrust would otherwise be most awkward to anchor.
What it costs you
Honesty is more useful than a sales claim here. A pressure-balanced joint is longer, heavier and more expensive than an axial one, and the force it applies at the nozzle is higher because there is more than one bellows in the load path. You specify it when the alternative — designing an anchor or a nozzle for tens of tonnes of thrust — is worse. On a turbine exhaust or a compressor suction it usually is.
Envelope
| Property | Range |
|---|---|
| Nominal diameter | DN50 – DN3000 |
| Bellows material | SS 321, SS 316L; Alloy 625 (UNS N06625) where temperature or strength requires it |
| Ends | Weld ends or flanges to DIN or ANSI |
| Options | Flow liner, external cover, limit rods, instrumentation bosses |
| Design basis | EN 14917:2021, EJMA 11th Edition |
Tell us the nozzle allowable
If the equipment vendor has given you a maximum permissible force and moment at the nozzle, send it with the enquiry. It is the governing input for a pressure-balanced design, and it changes the bellows selection more than the pressure does.