Pressure Thrust

Pressure thrust is the force created by pressure acting on bellows, it is a force equal to the media pressure multiplied by the effective cross-sectional area of bellow. All pipes with or without expansion joints have pressure thrust, which is reacted at each end by anchors.

When a pipe without an expansion joint is pressurized the whole system will not move as the pipe is countering the force in tension, whereas,  if an unrestrained expansion joint is installed in the system the force tends to pull the ends away from the expansion joint causing damage to the whole system as well as the bellows.  Therefore, the pressure thrust must be contained with either strong anchors or restrained expansion joints.

The main anchors must resist the sum of following forces:

  • Force due to the pressure (pressure thrust)
  • Force due to bellows deflection

Calculation of forces

Pressure thrust = P * Aeff

Effective Area = (Aeff) =

Dm2 = (Bellows ID + Bellows OD)/2


Pipe Size = 10” nominal

System Pressure (P) = 200 psig

Effective Area = (Aeff) = 126 in2

Pipe growth between anchors = (G) = 0.5 in (compression)

Axial Spring Rate of 10” Bellow = (S) = 5255 Lbs/in

Force due to the pressure = Pressure thrust = (P) x (Aeff) = 200 psig x 126 in2 = 25,200 Lbs

Bellows Spring Force = (G) x (S) = 0.5 in x 5255 Lbs/in = 2,628 Lbs


Hence the main anchors must resist the sum of above two forces:

Pressure Thrust + Bellows Spring Force = 25,200 Lbs + 2,628 Lbs = 27,828 Lbs