Bellows Type Deflections --- Axial Deflection

Bellows Type Deflections --- Axial Deflection

   In order to properly apply expansion joints to piping systems, it is necessary that both the piping specialist and the expansion joint designer each understand not only how the piping flexes, but how the various expansion joint types function and what they are capable of doing. It may be relatively easy to visualize that deflections may result from thermal expansion or the movements and vibrations of equipment and structures; however, all expansion joints do not accept the same types of deflection. Many can accept certain loads and movements, while others are incapable of resisting externally applied forces. Understanding the type, magnitude, and direction of these forces and deflections is critical, not only to the safety of the system, but to its cost.

        With today’s piping flexibility computer programs, determining where stresses are excessive, and therefore that expansion joints may be nessary, is a simple task. The placement of expansion joints , and their proper selection, still depends upon the designer’s experience and understanding of how expansion joints work, and how the piping must be anchored and supported when expansion joints are used. This section describes the different types of expansion joints available, how they function, what types of deflections they can accept, and what types of forces and movements they can and cannot resist.

        To aid in the explanations the following description of the types of movements are given.

Axial Deflection

        Axial refers to being parallel to the centerline of the expansion joint. COMPRESSION is the AXIAL deflection which will shorten the bellows length. Often confusion occurs because thermal EXPANSION in the piping will cause the expansion joint to be compressed. The specification for an expansion joint should always state the movements as they affect the expansion joint, and not as they are produced by the system. EXTENSION is the AXIAL deflection which stretches the expansion joint. Piping which is operating at temperatures lower than ambient, such as in cryogenic systems, will contract, causing the expansion joint to stretch or experience EXTENSION.