The reliability of expansion joints is comprised of design, manufacturing, installation, and operational management.

Release time:

2021-05-17

The reliability of expansion joints is comprised of design, manufacturing, installation, and operational management; therefore, reliability must be considered from these aspects as well. When selecting bellows for heating pipeline networks, in addition to factors such as operating temperature, working medium, and external environment, material selection should also take into account the effects of water treatment agents, the potential for stress corrosion, and the impact of pipeline cleaning agents on the material. On this basis, the material’s cost-effectiveness and the weldability and formability of the bellows material should be considered to optimize the selection of an economically viable and practical bellows material.

In general, the materials used for bellows should meet the following requirements: 1. High tensile strength, high elasticity, and excellent fatigue resistance to ensure the normal operation of the bellows. 2. Good ductility to facilitate the fabrication and forming of the bellows, while subsequent processing steps can impart sufficient hardness and strength. 3. Excellent corrosion resistance to meet the operational demands of bellows in various environments. 4. Good weldability to satisfy the welding requirements during the manufacturing process of the bellows. For thermal pipeline networks laid in trenches, when the pipeline section containing the expansion joint is located at a lower elevation, the bellows may be submerged by rainwater or accidental sewage. Therefore, it is advisable to consider using highly corrosion-resistant materials such as iron-nickel alloys or high-nickel alloys. During the manufacture of bellows, it may be feasible to apply a layer of corrosion-resistant alloy only to the surfaces that come into contact with corrosive media, given that such materials are relatively expensive.

From the analysis of failure modes and causes of bellows expansion joints, it can be seen that the circumferential stability, planar stability, and corrosion resistance of bellows are all closely related to their displacement—thus determining their fatigue life. A fatigue life that is too low will compromise both the corrosion resistance and stability of the bellows. Based on experimental and practical experience, the fatigue life of bellows used in heating systems should not be less than 1,000 cycles. Bellows must not be subjected to loads during installation and should be lifted and installed separately. Except for design requirements involving pre-stretching or cold-pre-deformation, it is strictly prohibited to use deformation of the bellows as a means of adjusting installation deviations in pipelines. During installation, welding spatter must not be allowed to splash onto the surface of the bellows, nor should the bellows suffer any other mechanical damage. All movable components of the bellows must not be jammed by external parts, nor should their normal operation be restricted in any way.

Most bellows failures are caused by external environmental corrosion. Therefore, when designing expansion joints, it is advisable to consider the contact between the external corrosive media and the bellows. A packing seal device can be added between the outlet flange and the outlet pipe. For sleeve-type expansion joints, this not only provides an additional barrier for bellows expansion joints but also effectively resists the intrusion of external corrosive media. Even if the bellows itself is damaged, the expansion joint can still serve as a compensator, thereby preventing complete failure of the bellows. Expansion joints include both non-metallic and metallic types; depending on the specific application and medium, they can further be categorized into high-temperature-resistant expansion joints and specialized anti-corrosion expansion joints.

Jiangsu Yaxing

Address: Qingdun Industrial Park, Hai'an City, Jiangsu Province

Phone: 86-513-88483622 ，86-513-88480666,

86-513-88480999

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