FLEXIBLE BELLOWS AND
HOW THEY WORK
Flexible bellows, also known as expansion joints, are flexible elements that absorb movements in the pipe system. These movements they absorb are defined by axial, lateral, angular and universal movements. The flexible bellows can be designed to absorb one of these movements or to absorb more of these movements in combination.
Flexible bellows are designed and manufactured by Belman in sizes from DN 25 to DN 12000 and in all materials and types. Our product range can be seen here: Flexible bellows range
Axial movement is movement of the flexible bellows in the direction of the longitudinal axis. This movement can be compressive, where the bellows shortens in length, or extensive where the bellows extends in length. In the majority of applications, the flexible bellows is deemed necessary because of the increasing temperature of the pipe system. The expansion joint is fitted in pipe systems and installed between two fix points (anchors). The extension of the pipe is compensated by the compression of the bellows. In some cases, typically cryogenic and chilled water services, the pipe system contracts in service causing the expansion joint to extend in length. Thermal expansion of the pipe system results in an axial compression of the installed expansion joints. The specifications for flexible bellows should always state the movements as they affect the flexible bellows, and not those generated by the pipe system.
Lateral movement is movement perpendicular to the bellow’s longitudinal axis; it is a shearing movement of the bellows with one end offset from the other, usually with the ends of the bellows remaining parallel to each other. A single bellow expansion joint, working with a shearing action, can accept a relatively limited amount of lateral movement, especially when the flow characteristics of the system demand that an inner sleeve is necessary. For larger lateral movement capability, it is usual to utilise a twin bellows arrangement with an intermediate pipe between the bellows. The flexible bellows lateral movement is taken up by an angular rotation of the bellows in opposite directions. The amount of lateral movement available depends on the rotational movement capacity of each bellows and the distance between them. Increasing the distance between the bellows increases the lateral movement capability of the flexible bellows proportionally. Lateral movement can be applied in more than one plane; in such cases it is important that the expansion joint designer is made aware of the total lateral movement to be applied.
Angular movement is the rotation of the bellow’s longitudinal axis at one end relative to the other, the axis of rotation is taken at exactly the midpoint of the bellow and perpendicular to the longitudinal axis. Flexible bellows using angular movement to control pipe system expansion are almost always used in pairs, sometimes combined as part of a twin bellows unit and sometimes in sets of 2 or 3 in pinned restrained flexible bellows. The intelligent use of the angular capability of the bellows can enable a large amount of movement to be absorbed. In particular, pinned units used in 2-pin or 3-pin arrangements can convert pipe growth into angular rotation and control the expansion from 2 directions and in 2 planes.
It is important not to confuse angular rotation with torsion. Torsion is a twisting rotational movement around the longitudinal axis; it generates undesirable shear forces within the bellows and its influence on the bellows should always be avoided. Please refer to the section about torsion.
Universal flexible bellows, also known as universal expansion joints, can be designed and built to absorb applied axial, lateral and angular movements simultaneously. Such units usually require a lot of flexibility to absorb significant amounts of movements in combination. However, this often leads to a limited pressure containing capacity due to considerations towards the bellows’ stability.
Important to know about flexible bellows and movements
It is important that the designer of flexible bellows is fully informed of all the movements to that the expansion joint will encounter. Knowledge of the amount of movement, its direction and any combination of axial, lateral and angular movements occurring together is essential for the correct design of the expansion joints. Read more about Belman’s engineering of expansion joints here: Engineering & QHSE