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What is an expansion joint?

What is an expansion joint?

Expansion joint can also be called pipe expansion joint, compensator. The expansion joint is a new product connecting the pump, valve, pipeline and other equipment with the pipeline. It is connected by all bolts to make it a whole with a certain displacement, which is convenient for installation. It can bear the axial pressure of pipeline. In this way, the installation and maintenance can be adjusted according to the installation size on site. When working, it not only improves the working efficiency, but also plays a certain role in protecting the pipeline equipment such as pumps and valves.

The connection form of the expansion joint is flange connection, with one side flange and one side welding.
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Function of the expansion joint

  1. Compensate the expansion deformation caused by the axial, transverse and angular heating of the absorption pipeline.
  2. Absorb equipment vibration and reduce the impact of equipment vibration on pipeline.
  3. Absorb the deformation of pipeline caused by earthquake and subsidence.

Because of the thermal expansion and cold contraction of the pipeline, the stress and pushing force of the pipe wall will be generated for the pipeline; the stress of the pipe wall will affect the strength of the pipeline, and the pushing force will increase, so the fixed support of the pipeline will do a lot to bear the pushing force generated by the expansion of the pipeline; therefore, the method of variable opening compensated by the expansion joint will be used to reduce the stress and pushing force of the pipe wall.

Classification of expansion joints by structure

The expansion joint is mainly used to compensate the expansion deformation of the pipeline due to the temperature change, as well as the length compensation of the pipeline due to the installation and adjustment. According to the structure, it is mainly divided into three types: elbow expansion joint, bellows expansion joint and sleeve expansion joint.

Elbow type expansion joint

An expansion joint that bends a pipe into a U-shaped or other shape (as shown in the figure below [elbow expansion joint]) and compensates for it by using the elastic deformation capacity of the shape. It has the advantages of good strength, long service life and can be made on site, but it has the disadvantages of occupying large space, consuming more steel and large friction resistance. This expansion joint is widely used in various steam pipes and long pipes.
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Bellows expansion joint

An expansion joint made of metal bellows. It can stretch along the axis direction and also allow a small amount of bending. The following figure [bellows expansion joint] is a common axial bellows expansion joint, which is used for axial length compensation on the pipeline. In order to prevent exceeding the allowable compensation amount, a protection rod or a protection ring is arranged at both ends of the bellows, and a guide bracket is arranged on the pipes at both ends connected with it. In addition, there are corner type and transverse expansion joints, which can be used to compensate the corner deformation and transverse deformation of the pipeline. The advantages of such expansion joints are saving space, saving materials (official account: pipe circle), which is convenient for standardization and batch production, and the shortcoming is short life. Bellows expansion joints are generally used for pipes with low temperature and pressure and short length. With the improvement of bellows production technology, the application scope of this kind of expansion joint is expanding. The utility model is composed of a bellows (an elastic element) which forms the working main body, an end pipe, a bracket, a flange, a conduit and other accessories. Mainly used in various pipelines, it can compensate the thermal displacement of pipelines, mechanical deformation and absorb various mechanical vibration, play a role in reducing the deformation stress of pipelines and improving the service life of pipelines. The connection mode of corrugated compensator is divided into flange connection and welding. Welding method is generally adopted for compensator of directly buried pipeline.
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Sleeve expansion joint

It is composed of inner and outer casings that can make axial relative motion (see figure [casing expansion joint]). Stuffing box is used to seal the inner and outer casings. When in use, keep the pipes at both ends moving on an axis. A guide bracket is arranged at both ends of the expansion joint. The advantages of the utility model are small flow friction resistance to the fluid, compact structure, poor sealing performance and large thrust to the fixed bracket. Casing expansion joints are mainly used for water pipes and low-pressure steam pipes.
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Classification of expansion joints by material

According to the material, it is mainly divided into rubber pipe expansion device and metal pipe expansion device.

Characteristics of rubber pipe expansion joint

  1. Small size, light weight, good elasticity, convenient installation and maintenance.
  2. During installation, it can produce axial, transverse, longitudinal and angular displacement, which is not limited by the user’s pipe is not concentric and the flange is not parallel.
  3. It can reduce vibration and noise during operation.
  4. It is an ideal product for chemical corrosion-resistant pipeline, which can resist high temperature, acid and alkali, oil and so on.

Characteristics of metal pipe expansion joint

Large expansion compensation, high temperature and pressure.

Nonmetal expansion joint

The air duct rubber compensator is composed of rubber and rubber fiber fabric composite material, steel flange, sleeve, heat preservation and insulation material. It is mainly used for flexible connection between various fans and air ducts. Its function is shock absorption, noise reduction, sealing, medium resistance, convenient for displacement and installation. It is an ideal matching kit for shock absorption, noise reduction, smoke and dust removal in the field of environmental protection.
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Fiber fabric expansion joint

Fabric compensators are mainly fiber fabrics, rubber, and other high temperature resistant materials. It can compensate the vibration of fan and air duct and the deformation of pipeline. The fiber fabric expansion joint can compensate axial, transverse, angular and other products. It has the characteristics of no thrust, simplified support design, corrosion resistance, high temperature resistance, noise reduction and vibration reduction, and is especially suitable for hot air pipes and smoke pipes in power plants. The fiber fabric and thermal insulation cotton in the non-metallic Compensator have the functions of sound absorption and vibration isolation, which can effectively reduce the noise and vibration of the boiler, fan and other systems. The utility model has the advantages of simple structure, light weight and convenient maintenance.
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Characteristics and application scope of various expansion joints

Nonmetal expansion joint

Nonmetal flexible compensator: also known as nonmetal expansion joint and nonmetal fabric compensator, it can compensate axial, transverse and angular direction. It has the characteristics of no thrust, simplified support design, corrosion resistance, high temperature resistance, noise reduction and vibration reduction, especially suitable for hot air pipeline and smoke and dust pipeline.

  • 1. Compensation for thermal expansion: it can compensate for multiple directions, which is much better than the metal compensator that can only be compensated in a single way.
  • 2. Compensation for installation error: due to the inevitable system error in the pipeline connection process, the fiber compensator can better compensate the installation error.
  • 3. Noise reduction and vibration reduction: the fiber fabric and insulation cotton have the functions of sound absorption and vibration isolation, which can effectively reduce the noise and vibration of the boiler, fan and other systems.
  • 4. No thrust: because the main material is fiber fabric, it is unable to transmit. The use of fiber compensator can simplify the design, avoid the use of large bearings, and save a lot of materials and labor.
  • 5. Good high temperature and corrosion resistance: the selected fluoroplastics and organosilicon materials have good high temperature and corrosion resistance.
  • 6. Good sealing performance: a relatively complete production and assembly system, fiber compensator can ensure no leakage.
  • 7. Light weight, simple structure, convenient installation and maintenance.
  • 8. The price is lower than the metal compensator and the quality is better than the imported products.

Stainless steel

There are four types: straight cylinder type, duplex type, angular type and square type.
Stainless steel compensator can compensate axial, transverse, angular, thrust free, simplified support design, corrosion resistance, high temperature resistance, noise reduction and other characteristics, especially suitable for hot air pipe and smoke pipe.
Metal: the reliability of metal corrugated compensator is composed of design, manufacture, installation and operation management. Reliability should also be considered from these aspects. Material selection of bellows used in heating pipe network, in addition to the working medium, working temperature and external environment, the possibility of stress corrosion, the influence of water treatment agent and pipeline cleaning agent on the material should also be considered, and on this basis, combined with the welding, forming and material performance price ratio of bellows material, the economic and practical materials for bellows production should be optimized.
Generally, the materials of bellows shall meet the following conditions:

  • (1) High elastic limit, tensile strength and fatigue strength to ensure the normal operation of bellows.
  • (2) Good plasticity, easy to process bellows, and can obtain enough hardness and strength through subsequent processing (cold work hardening, heat treatment, etc.).
  • (3) Good corrosion resistance, to meet the requirements of bellows in different environments.
  • (4) Good welding performance, to meet the requirements of bellows welding process in the production process.

For the heat pipe network laid in the trench, when the pipeline where the compensator is located is low, the corrugated pipe will be soaked by rainwater or accidental sewage, so the materials with stronger corrosion resistance, such as ferronickel alloy and high nickel alloy, should be considered. Due to the high price of such materials, when manufacturing bellows, only one layer of corrosion-resistant alloy can be added on the surface contacting with corrosive medium. The fatigue life design can be seen from the failure type and cause analysis of the bellows compensator that the plane stability, circumferential stability and corrosion resistance of the bellows are all related to its displacement, that is, the fatigue life. Too low fatigue life will lead to the decline of metal bellows stability and corrosion resistance.

Source: China Expansion Joints Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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