All metal bellows expansion joints exert force, or pressure thrust, that pushes the pipe ends apart
Bellows Pressure Thrust
Bellows pressure thrust force is the result of the system operating or test pressure being applied to the effective area (based on the mean diameter) of the bellows and is larger than the pressure thrust force of the connected pipe.
Pressure thrust forces are typically higher than all other system forces and are usually restrained by system anchors. If anchoring is insufficient or absent, bellows pressure thrust forces can also be restrained by externally-attached hardware such as Tie Rods, Hinges, or Gimbals.
Pressure Thrust Force
F = P·a
Fs is Force (lbf)
P is Pressure (psig)
a is the Effective Area of the Bellows (in.2)
Shown here, pressure thrust is restrained by the strength of the pipe between the blind flange attachments
When blind flanges are attached to the ends of unrestrained metal bellows, the bellows response to pressure thrust results in the bellows convolutions stretching out until it returns to its original tube form.