Pipe Expansion Compensators or Joints

When stainless steel bellows are provided with such restraints and/or with end connections they are called an expansion joint or compensator. A single axial expansion joint, which is the simplest expansion joint, does not carry any inbuilt restraints and the pressure thrust at the ends has to be absorbed purely by designing anchors of sufficient strength. This pressure thrust in expansion compensators is calculated as below:

Thrust Force Fp = Mean area of the Bellows A X internal pressure P


Effect of Thrust


Even stainless steel bellows can restrain the longitudinal pressure force only up to a certain limit, beyond which it can elongate under internal pressure as shown. Hence it becomes essential to provide expansion compensators with accessories such as Tie Rods, Hinges, Gimbal Rings etc. If the section in which expansion compensators are installed is not anchored properly at the ends.

Instability /Squirm Under Pressure

Over-pressurization or improper guiding of stainless steel bellows in the expansion joint may cause the bellows to become unstable and squirm. This is detrimental to bellow performance as it can considerably reduce the fatigue life and pressure capacity. The most common forms of Squirm in compensator are Column squirm and in-plane squirm as shown.


Inplane Instability / Squirm of Bellows Expansion Joints


Instability occurs when the bellows reach their critical pressure point and it’s of two types: Column Instability and in-plane instability which is when the convolution side wall gets deformed.

This critical pressure is a direct function of the bellow diameter and spring rate and an inverse function of length. An angulated or bent bellow’s centerline can begin to move away from the centre of curvature. As the effective length of the bellow increases, the material available to withstand pressure decreases increasing hoop stress and possibility of squirm.

Column Instability / Squirm of Single and Double Bellows Expansion Joints

Column squirm is defined as a gross lateral shift of the centre section of the bellows. This condition is mostly associated with bellows which have a relatively large length-to-diameter ratio and are analogous to the buckling of a column under compressive load. In-plane squirm in compensator is defined as a shift or rotation of the plane of one or more convolutions such that the plane of these convolutions is no longer perpendicular to the axis of the bellows.