In industrial flue gas pipe system, metal flue expansion joint is one of the most widely used flexible compensation devices. It makes use of the elastic deformation of stainless steel bellows to absorb the axial, transverse and angular displacement of the pipeline due to thermal expansion and contraction, and at the same time isolates the vibration of the equipment to protect the safe operation of the pipeline system. Compared with sleeve type and spherical expansion joint, metal expansion joint has the advantages of small footprint, simple installation, low engineering cost, no structural leakage link, no maintenance and so on. This article will systematically explain the technical knowledge of metal flue expansion joint from structural characteristics to installation and maintenance.

1. Structure and working principle of metal flue expansion joint

The core component of the metal flue expansion joint is a metal bellows, made of elastic stainless steel (SUS304, SUS316L, SUS321, etc.) with an annular or spiral corrugated structure. It consists of attachments such as bellows, end tubes, deflectors and flanges. When the pipe is elongated by heat, the bellows is compressed; When the pipe cools and shrinks, the bellows is stretched – this elastic deformation ability allows it to effectively absorb thermal displacement and reduce pipe stress.

The working process of metal flue expansion joint can be summarized as follows: the displacement generated by the thermal expansion of the pipeline acts on the expansion joint, and the bellows absorbs the displacement through its own elastic deformation, and converts the thermal stress into the elastic potential energy of the bellows, thus protecting the pipeline and equipment from damage. This design principle makes it widely used in power plants, petrochemicals, metallurgy and other industries.

Second, the core elements of the selection of metal flue expansion joint

The correct choice of metal flue expansion joint requires comprehensive consideration of the following five elements:

1. Analysis of pipeline working environment

• Temperature range: The corrugated pipe material should adapt to the working temperature range of the pipeline, and high temperature resistant alloy should be selected for the high temperature section
• Pressure grade: The design pressure of the expansion joint should meet the working pressure requirements of the pipeline system to avoid overpressure damage
• Media characteristics: The material must have good corrosion resistance and adapt to the chemical properties of the conveying medium

The criteria for selecting the material of the bellows according to the media are as follows:

2. Pipeline displacement requirements

The metal flue expansion joint needs to select the appropriate structure according to the pipe displacement type:

• Mainly axial displacement: select axial expansion joint
• Lateral displacement: transverse or universal structures should be considered
• Angular displacement: select hinged expansion joint

3. Select the material according to the working temperature

Specifications for the selection of bellows, couplings and flange materials based on operating temperature are as follows:

Typical procurement example: The technical requirements of metal expansion joints purchased by a chemical enterprise are: material 321, thickness 3mm, wave height 157mm, connector material 304, connector size Φ 1820×10, and the standard GB/T 12777-2019 is implemented.

4. Installation space and conditions

• The size of the expansion joint should be adapted to the limitation of installation space
• The installation method (welding or flange connection) shall conform to the actual situation on site

5. Special selection of rectangular metal expansion joint

For rectangular flue, attention should be paid to the selection of metal flue expansion joint:

• The waveforms are divided into full-height (wave height 216mm) and half-height (wave height 108mm)
• Half-height type is suitable for applications with cross-sectional area
• Full-height type suitable for flue with arbitrary cross section

III. Installation specification of metal flue expansion joint

The installation quality of metal flue expansion joints directly affects their service life, and the following core specifications should be followed:

1. Preparation and inspection before installation

• Check whether the model, specification and pipe support configuration of the expansion joint meet the design requirements
• Check the bellows surface for mechanical damage and corrosion
• Verify that the direction of the liner cylinder (guide cylinder) is consistent with the direction of media flow
• The end of the bellows expansion joint covered with a weld shall be located at the medium inflow end on the horizontal pipe

2. Critical controls during installation

According to industry specifications, installation of metal flue expansion joints shall comply with:

• It is strictly prohibited to use expansion joint deformation to adjust pipeline deviation: Except for pre-tension and compression as required by design, it is strictly prohibited to use the method of deforming bellows to adjust pipeline installation deviation
• Concentricity requirement: The expansion joint shall be concentric with the pipe and shall not be skewed
• Welding protection: Do not allow welding slag to splash onto the bellows surface
• Temporary restraint device: Temporary restraint device shall be installed during installation and removed after the pipeline is installed and fixed

3. Transportation Tie Rod Handling

After installation, the metal flue expansion joint must remove the transport protection tie rod (the positioning limit tie rod shall not be removed), so that the expansion joint can expand and contract freely. This is the most overlooked part of installation-if the transport tie rod is not removed, the expansion joint will lose its ability to compensate.

4. Cold drawing design of rectangular expansion joint

In order to reduce the number of bellows nodes, the rectangular metal flue expansion joint should be considered to be cold drawn 50%, that is, the expansion joint should be pre-compressed or pre-stretched during installation, so that it is in the best stress state during hot work.

4. Economy and maintenance considerations

On the premise of meeting the technical requirements of the metal flue expansion joint, the following economic factors should be comprehensively considered:

Qualification requirements: When purchasing metal flue expansion joint, the supplier must hold the Special Equipment Manufacturing License of the People's Republic of China and provide a valid type test report.

sum up

Metal flue expansion joints are indispensable flexible connecting elements in modern industrial piping systems. The correct type selection should start from the working condition parameters and be comprehensively determined according to the principle of "selecting material for temperature, selecting structure for displacement and selecting size for space". The installation process must strictly implement the specifications-the flow direction marks are consistent, the expansion joint is strictly prohibited to adjust the pipeline deviation, and the transportation tie rod is removed in time. Cold-drawn design should also be considered for rectangular expansion joints to optimize performance.

The metal flue expansion joint with reasonable design and standard installation can run stably for a long time under harsh working conditions such as high temperature and corrosion, and provide reliable flexible connection guarantee for flue gas system.