Practical knowledge of expansion joints
Why use expansion joints?
According to the physical characteristics of metals, most metals expand when the temperature increases and contract when the temperature decreases. In the pipeline system, this kind of physical phenomenon will lead to the change of the length of each part of the pipeline.
However, the pipe is usually fixed on at least two joints. Any length change will cause thermal stress on the material and increase the force and moment on these joints.
Therefore, it is necessary to make full use of the inherent flexibility of the pipeline to compensate for the expansion of the pipeline (natural compensation). If the expansion exceeds the inherent flexibility of the pipe, expansion joints are needed to compensate for these movements.
In addition, the mechanical vibration of pumps, compressors, turbines and motors will damage adjacent pipes, bearings, or any installed equipment, and the vibration shall be attenuated. However, the force and moment that the nozzles, vessel nozzles and other industrial components of compressors and turbines can bear are limited and cannot exceed their critical values.
In all these cases, expansion joints should be used.
The core component of each expansion joint is the metal bellows (*), which can act as a spring according to its shape and wall thickness. However, as an elastic element, bellows must meet the following basic requirements:
- 1. Withstand the operation and test conditions (pressure, temperature) of the pipeline system.
- 2. Ensure corrosion resistance to internal (fluid) and external (atmospheric) environment.
- 3. Provide sufficient flexibility to compensate for thermal expansion and mechanical vibration and have required durability (life).
- 4. Sufficient creep stability.
(*) Except for rubber expansion joints.
Why is the material of expansion joint often made of stainless steel?
- (1) Due to the small corrosion desire of stainless steel, the thickness of expansion joint can be designed thinner and the single wave compensation is larger;
- (2) The plasticity of stainless steel is about twice that of carbon steel, which is good for cold forming;
- (3) Under the same service life, the allowable stress amplitude of stainless steel is 1.5 times higher than that of carbon steel.
What is the selection principle of expansion joint?
- 1. Corrugated expansion joints made of carbon steel and low alloy are only suitable for t ≤ 375 ℃; corrugated expansion joints made of austenitic stainless steel are suitable for t ≤ 500 ℃.
- 2. The corrugated expansion joint made of carbon steel or low alloy steel shall have a corrosion allowance of not more than 1mm, otherwise austenitic stainless steel shall be used.
- 3. For medium prone to corrosion such as chloride, sulfide, acid and alkali or with high working temperature (over 550 ℃), corrosion-resistant alloy or superalloy shall be selected to make expansion joint, such as domestic materials fn-2, ns111 and b-315 or Incoloy800, 825, etc.
What are the precautions for installation and pressure test of expansion joint?
Before installation, the model and specification of the bellows expansion joint and the support configuration of the pipeline shall be checked to ensure that they meet the design requirements.
For the expansion joint with inner liner, attention shall be paid to make the direction of inner liner consistent with the flow direction of medium (installed according to the flow direction sign of expansion joint). The hinge rotation plane of the plane angular expansion joint shall be consistent with the displacement plane.
For expansion joints that need to be “cold tight”, the auxiliary components used for pre deformation shall be removed after the installation of piping system.
The auxiliary positioning mechanism and fasteners used for installation and transportation protection on the expansion joint shall be removed immediately after the installation of the pipe system, and the limit device shall be adjusted to the specified position according to the design requirements, so that the pipe system has sufficient compensation capacity under the environmental conditions.
In addition to the design requirements of pre tension and compression or “cold tight” pre deformation, it is strictly prohibited to adjust the installation deviation of the pipe by the way of deformation of the bellows, so as to avoid affecting the normal function of the expansion joint, reducing the service life and increasing the load of the pipe system, equipment connecting pipe and supporting components.
All moving elements of the expansion joint shall not be blocked by external components or limit the normal operation of its moving parts.
During the installation process, it is not allowed to splash the welding slag on the surface of the group corrugated pipe and cause other mechanical damage to the corrugated pipe.
For the expansion joint and its connecting pipes used for gas medium, during the hydrostatic test, it is necessary to consider whether it is necessary to add temporary support to the connecting pipe of expansion joint to bear the load during water filling.
The water for hydrostatic test must be clean and non corrosive, and the content of chloride ion in the water shall not exceed 25ppm.
After the hydrostatic test, the accumulated water in the corrugation shall be drained as soon as possible, and the inner surface of the wave shell shall be dried quickly.
Source: China Expansion Joints Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(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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