Classification of Expansion Joints and Introduction to the Characteristics and Functions of Each Type

Release time:

2021-11-22

　An expansion joint consists of one or more bellows and auxiliary structural components, and is a device used to absorb thermal displacement in pipelines or equipment caused by thermal expansion and contraction. Expansion joints can be categorized into two main types: first, unconstrained expansion joints; and second, rod-type expansion joints equipped with limiting devices.

There are five commonly used types of expansion joints: universal type, lateral large-stroke type, straight-tube pressure-balanced type, curved-tube pressure-balanced type, and non-metallic expansion joint.

　　(1) A universal expansion joint consists of a bellows and two end fittings. It absorbs axial displacement and a small amount of lateral displacement through the flexible deformation of the bellows. The expansion joint itself cannot withstand the pressure thrust exerted by the medium; instead, this pressure thrust is borne by fixed supports located on either side. This type of expansion joint features a simple structure and low cost. Therefore, when the design pressure of the pipeline is relatively low, this type of expansion joint should be given priority.

　　(2) The lateral large-link expansion joint consists of a combination of two bellows, an intermediate connecting pipe, and a link mechanism, capable of absorbing lateral displacements in all planes. When displacement occurs, the spherical nut of the large lever rotates around the spherical washer, allowing the large lever to withstand a certain amount of internal pressure thrust. This type of expansion joint is particularly suitable when the installation space for the duct is limited, resulting in significant lateral displacements.

　　(3) The straight-tube pressure-balanced expansion joint features working bellows at both ends and a balancing bellows in the middle. It can only absorb axial displacement, while the lever mechanism can withstand a certain amount of internal pressure thrust. When dealing with large-diameter connections and high temperatures, and if greater axial compensation is required, this type of expansion joint can be selected.

　　(4) The bellows-type expansion joint with pressure balancing features two working bellows, one balancing bellows, and an elbow. It can absorb both axial and lateral displacements, and its lever mechanism is capable of withstanding internal pressure. These expansion joints are suitable for ductwork systems where the high-altitude elbow experiences both axial and lateral displacements simultaneously. However, since bellows-type expansion joints with pressure balancing are expensive, a combination of springs and lateral tie rods is typically used instead of these specialized expansion joints.

　　(5) Non-metallic expansion joints typically consist of a compensating element made from composite non-metallic materials and metal flanges. The composite non-metallic materials combine high-performance components such as fiber fabrics, silicone rubber, and plastics. These non-metallic expansion joints can absorb displacements in various directions, effectively reducing noise and vibration. They feature a simple structure that makes installation and maintenance convenient. To prevent excessive displacement of the expansion joint and reduce the reactive force caused by pressure, lever mechanisms are often added to non-metallic expansion joints, enabling them to withstand certain internal pressure thrusts. This type of expansion joint is commonly used at compressor and fan inlets, as well as in smoke ducts and natural gas pipelines, particularly when pipe diameters are large and operating temperatures and pressures are relatively low.

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