Knowledge about pipe expansion joints

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An expansion joint can also be referred to as a pipe expansion joint, expansion fitting, compensator, or flexible joint. The expansion joint is a new product used to connect equipment such as pumps, valves, and pipes to pipelines; it connects them together using bolts, forming a single unit that allows for a certain amount of displacement, thereby facilitating installation. It can withstand the axial pressure of the pipeline. This allows for adjustments based on the actual installation dimensions during installation and maintenance; it not only improves work efficiency but also provides some protection for pipeline equipment such as pumps and valves. The connector of the expansion joint is of flange type, with one side being a flange and the other side being welded. ▲A brief analysis of common types and applications of pipe expansion joints (compensators). Functions of expansion joints: 1. To compensate for and absorb the thermal-induced expansion and contraction of pipes in the axial, lateral, and angular directions. 2. Absorb equipment vibrations and reduce the impact of such vibrations on the pipes. 3. Absorb the deformation of pipelines caused by earthquakes and land subsidence. Due to the thermal expansion and contraction of pipes, stress as well as pushing and pulling forces are generated in the pipe walls ； The magnitude of the wall stress affects the strength of the pipeline; as the forces of thrust and pull increase, the fixing supports required for the pipeline must be larger in order to withstand these forces resulting from the expansion and contraction of the pipeline ； Therefore, the method of varying the opening degree using expansion joints is employed to reduce wall stress and thrust. Expansion joints are classified according to their structural design. They are primarily used to compensate for the thermal expansion and contraction of pipes due to temperature changes, as well as to provide the necessary length adjustment for pipes during installation or other adjustments. Based on their structural form, they are mainly divided into three types: elbow-type expansion joints, bellows-type expansion joints, and sleeve-type expansion joints. A bent-tube expansion joint is a type of expansion joint in which the pipe is bent into a U-shape or another shape (as shown in the figure below), and the elastic deformation capacity of this shape is utilized for compensation. Its advantages are good strength, long service life, and the ability to be manufactured on-site; its disadvantages are large space consumption, high steel usage, and high frictional resistance. Such expansion joints are widely used in various steam pipes and long pipelines. Bellows expansion joint: An expansion joint made of a metal bellows. It can extend and contract along its axis, and it also allows for a slight amount of bending. The figure below shows a common axial bellows expansion joint, which is used to provide axial length compensation in pipelines. To prevent the compensation amount from exceeding the allowable limit, protective tie rods or rings are installed at both ends of the bellows, and guide supports are provided on the pipes connected to them. There are also elbow-type and transverse expansion joints, which can be used to compensate for the angular and lateral deformation of pipes. The advantages of this type of expansion joint are space savings, material conservation, and ease of standardization and mass production; the disadvantage is its shorter lifespan. Bellows expansion joints are generally used in pipes with moderate temperatures and pressures and relatively short lengths. With the improvement of bellows manufacturing technology, the application scope of this type of expansion joint is expanding. It consists of a bellows (an elastic element) that forms the main part of its structure, along with accessories such as end tubes, brackets, flanges, and conduits. It is mainly used in various pipelines, where it can compensate for thermal displacement and mechanical deformation of the pipelines, as well as absorb various mechanical vibrations. This helps to reduce the stress caused by pipeline deformation and extend the service life of the pipelines. The connection methods for bellows compensators are flange connection and welding. Buried pipeline compensators are generally welded. A sleeve expansion joint consists of an inner sleeve and an outer sleeve that can move relative to each other axially (see figure below). The space between the inner and outer casings is sealed using a stuffing box. Keep the tubes at both ends moving on the same axis while in use. Guiding brackets are installed at both ends of the expansion joint. Its advantage is that it has low flow frictional resistance and a compact structure ； The disadvantage is poor sealing performance and high thrust on the fixing bracket. Sleeve expansion joints are mainly used in water pipelines and low-pressure steam pipelines. Expansion joints are classified by material; mainly, they are divided into rubber pipe expansion joints and metal pipe expansion joints. Features of rubber pipe expansion joints: 1. Small size, light weight, good elasticity, and easy installation and maintenance. II. During installation, axial, lateral, longitudinal, and angular displacements can occur, without being restricted by misalignment of the user’s pipes or non-parallel flanges. III. It can reduce vibration and noise during operation. IV. Using special synthetic rubber, it can withstand high temperatures, acids, alkalis, and oils, making it an ideal material for corrosion-resistant pipes in the chemical industry. The features of metal pipe expansion joints include a large amount of expansion compensation, the ability to withstand high temperatures, and the capacity to handle high pressures. Non-metallic expansion joints and rubber compensators for air ducts are composed of rubber, rubber-fiber fabric composites, steel flanges, sleeves, and thermal insulation materials. They are primarily used for providing flexible connections between various fans and air ducts. Their functions include shock absorption, noise reduction, sealing, resistance to chemical agents, and ease of displacement and installation. They represent an excellent solution for shock absorption, noise reduction, and smoke/dust removal in the field of environmental protection. Fabric compensators are mainly made of high-temperature resistant materials such as fiber fabrics and rubber. It can compensate for the vibrations generated by the operation of fans and air ducts, as well as pipe deformation. Fiber fabric expansion joints can compensate for axial, lateral, angular, and other types of movements. They feature no thrust generation, simplified support design, corrosion resistance, high-temperature tolerance, as well as sound and vibration reduction properties, making them particularly suitable for hot air ducts and dust exhaust ducts in power plants. The fiber fabric and insulation material used in non-metallic compensators possess sound-absorbing and vibration-isolating properties, enabling them to effectively reduce the noise and vibrations generated by systems such as boilers and fans. It has a simple structure, is lightweight, and is easy to maintain. Features and application ranges of various pipe expansion joints. Non-metallic expansion joints and non-metallic flexible compensators: also known as non-metallic expansion joints or non-metallic fabric compensators, they can compensate for axial, lateral, and angular movements. They have advantages such as no thrust generation, simplified support design, corrosion resistance, high-temperature tolerance, as well as sound and vibration reduction; they are particularly suitable for hot air ducts and dust exhaust ducts. Features: 1. Compensates for thermal expansion: It can provide compensation in multiple directions, **outperforming metal compensators that can only offer single-direction compensation. 2. Compensating for installation errors: Systematic errors are inevitable during pipe connection, and fiber compensators effectively compensate for such installation errors. 3. Noise and vibration reduction: Fiber fabrics and insulation materials possess the ability to absorb sound and prevent the transmission of vibrations, thereby effectively reducing the noise and vibrations generated by systems such as boilers and fans. 4. No reverse thrust: Since the main material is a fiber fabric, there is no force transmission. Using fiber compensators can simplify the design, eliminate the need for large supports, and save a significant amount of material and labor. 5. Good high-temperature and corrosion resistance: The fluoroplastic and silicone materials used possess excellent high-temperature and corrosion resistance. 6. Good sealing performance: It features a well-developed production and assembly system, and the fiber compensator ensures no leaks. 7. Light in weight, simple in structure, and easy to install and maintain. 8. The price is lower than that of metal compensators, and the quality is superior to imported products. Stainless steel: There are four types, namely straight tube type, double type, angular type, and square type. Stainless steel compensators can compensate for axial, lateral, and angular movements; they feature no thrust generation, simplified support design, corrosion resistance, high-temperature tolerance, as well as sound and vibration reduction properties. They are particularly suitable for hot air ducts and dust exhaust ducts. Metals: The reliability of metal bellows compensators depends on various factors such as design, manufacturing, installation, and operational management. Reliability should also be considered from these aspects. When selecting materials for bellows used in heating pipelines, in addition to considering the working medium, operating temperature, and external environment, factors such as the possibility of stress corrosion, as well as the impact of water treatment agents and pipeline cleaning agents on the material, must also be taken into account. On this basis, and by considering the weldability and formability of the bellow material along with its cost-performance ratio, an economical and practical material for manufacturing bellows can be selected. Under normal circumstances, the material chosen for bellows should meet the following requirements: (1) a high elastic limit, tensile strength, and fatigue strength to ensure the proper functioning of the bellows. (2) Good plasticity, which facilitates the processing and shaping of the bellows, and it is possible to achieve sufficient hardness and strength through subsequent treatment processes (cold working hardening, heat treatment, etc.). (3) Good corrosion resistance, meeting the operational requirements of bellows in various environments. (4) Good weldability, meeting the welding requirements during the manufacturing process of bellows. For thermal piping systems laid in trenches, when the pipeline where the compensator is located is at a lower elevation, rainwater or accidental sewage can soak the bellows; in such cases, materials with greater corrosion resistance, such as iron-nickel alloys or high-nickel alloys, should be considered. Due to the high cost of such materials, when manufacturing bellows, it is possible to consider adding a layer of corrosion-resistant alloy only on the surfaces that come into contact with corrosive media. As can be seen from the analysis of the failure types and causes of bellows compensators in fatigue life design, the planar stability, circumferential stability, and corrosion resistance of the bellows are all related to its displacement amount, that is, its fatigue life. An excessively low fatigue life will lead to a decrease in the stability and corrosion resistance of the metal bellows.