The reliability of expansion joints consists of design, manufacturing, installation, and operation management, and reliability should also be considered from these aspects. For the selection of corrugated pipes for heating networks, in addition to working temperature, working medium, and external environment, the possibility of water treatment agents, stress corrosion, and the impact of pipeline cleaning agents on the materials should also be considered. Based on this, the cost-effectiveness of the materials and the welding and forming of corrugated pipe materials should be combined to optimize the economical and practical corrugated pipe materials.
In general, the materials used for corrugated pipes should meet the following conditions: 1. tensile strength, high elasticity, and fatigue strength to ensure the normal operation of the corrugated pipe. 2. Good plasticity, easy to process and shape corrugated pipes, and sufficient hardness and strength can be obtained through subsequent processing techniques. 3. Good corrosion resistance, meeting the working requirements of corrugated pipes in different environments. 4. Good welding performance, meeting the welding process requirements of corrugated pipes in the manufacturing process. For the heat pipe network laid in the trench, when the expansion joint is located in a lower pipeline, the corrugated pipe will be soaked in rainwater or accidental sewage. Therefore, materials with strong corrosion resistance such as iron nickel alloy and high nickel alloy should be onsidered. When manufacturing corrugated pipes, it is possible to consider adding only a layer of corrosion-resistant alloy on the surface in contact with corrosive media, as this material is more expensive.
From the analysis of the failure types and causes of bellows expansion joints, it can be seen that the circumferential stability/planar stability and corrosion resistance of bellows are related to their displacement, that is, fatigue life. A low fatigue life can lead to a decrease in the corrosion resistance and stability of corrugated pipes. Based on experimental and application experience, the fatigue life of corrugated pipes used in heating engineering should not be less than 1000 times. Bellows cannot withstand loads and should be lifted separately. Except for the design requirements of pre stretching or cold tightening pre deformation, it is strictly prohibited to use corrugated pipe deformation methods to adjust the installation deviation of pipelines; During the installation process, welding slag must not splash onto the surface of the corrugated pipe or suffer other mechanical damage; All moving components of corrugated pipes shall not be jammed or restricted from normal operation by external components.
Most bellows failures are caused by external environmental corrosion, and the contact between external corrosive media and bellows can be considered in the design of expansion joint structures. A packing sealing device can be added between the outlet end ring and the outlet pipe. For the expansion joint, as a sleeve expansion joint, it can not only add a barrier to the corrugated pipe expansion joint, but also resist the invasion of external corrosive media. Even if the corrugated pipe is damaged, the expansion joint can still play a compensating role to avoid the failure of the corrugated pipe.
Expansion joints include non-metallic compensators and metal compensators. Depending on the different uses of the medium, they can also be divided into high-temperature compensators and professional anti-corrosion compensators.
