Rectangular cross-section flue is widely used in power plant flue duct, metallurgical dust removal duct and industrial ventilation system because of its convenient arrangement and manufacture. However, the problem of thermal expansion compensation of rectangular flue is more complicated than that of circular flue-its cross section is large, its stiffness is low, and its corner stress is concentrated. Flue rectangular metal expansion joint (also called rectangular metal bellows expansion joint) is a special flexible compensation device to solve this problem. This paper will systematically explain the selection and installation specification of rectangular metal expansion joint from four aspects: structural characteristics, waveform types, selection parameters and installation points.
1. Structural characteristics of rectangular metal expansion joint
The flue rectangular metal expansion joint adopts the structural design of rectangular cross section and rounded corner wave, and its core characteristics are as follows:
1.1 Fillet Transition Structure
Different from traditional right-angle bellows, the corners of rectangular metal expansion joints transition with large circular arcs to form a rectangular cross section. The advantages of this design are:
- Reduce corner stress levels: Rounded corner structure eliminates stress concentration at right angles and avoids regional high stress damage
- Uniform deformation ability: The product is uniformly deformed as a whole when running online, which improves the thermal compensation ability
- Improved corrosion resistance: In the flue gas environment containing SO₂, CO, H₂, the fillet structure can effectively improve the deterioration trend of stress corrosion cracking (SCC) and corrosion fatigue fracture (CFF)
1.2 Basic composition
The rectangular metal expansion joint consists of the following components:
- Corrugated pipe: Made of multi-layer stainless steel or Corten steel, it is the core elastic element that absorbs displacement
- End pipe/flange: Connection piece to flue, optional flange connection or direct welding
- Guide tube (optional): lined inside the bellows to protect the bellows from dusty fumes
- Tie rod/hinge: Used to restrain pressure thrust and protect the stability of bellows
2. Waveform type and compensation ability
The waveform design of rectangular metal expansion joint in flue directly determines its compensation performance and application range. Mainstream waveforms include:
| Waveform type | Wave height (mm) | Single wave compensation amount | Applicable scenarios |
|---|---|---|---|
| CE full height | 216 | ±24mm | Large displacement compensation requirement, large section flue |
| CE semi-high | 108 | ±12mm | Small and medium displacement, space constrained occasions |
| U shaped wave | Design on demand | larger | High pressure (≤0.25MPa) or requires long fatigue life |
| V-wave | Design on demand | smaller | Low pressure environment, economical scheme |
Selection principle: The single wave number of a single bellows should not exceed 6 waves. When it exceeds, it should be considered to increase the number of expansion joints or reduce the number of nodes by cold drawing 50%.
2.1 Corner structure form
The corner structures of rectangular expansion joints mainly include the following:
- Single bevel V-shaped: the most economical form for low-cycle, vibration-free applications, easy to bolt or weld
- Double-bevel V-shapes: cost slightly more than single-bevel, but have a longer cycle life
- Mirror-angled V-type: for low-pressure environments, cost in between, good cycle life
- Rounded U-shaped: Suitable for pressure up to 2 bar, high vibration and cycle life requirements, with the highest cost but optimal performance
3. Selection parameters and specifications
3.1 Basic Technical Parameters
According to GB/T12777-2019 General Technical Specifications for Metal Bellows Expansion Joints and related product standards, the typical technical parameters of flue rectangular metal expansion joints are as follows:
| parameter | scope |
|---|---|
| Nominal section | Any size (customized) |
| Design pressure | 0.01~0.25 MPa |
| Design temperature | -20℃～800℃ |
| Connection mode | Flange connection/welding |
| Material | Stainless steel (304/316L/321), Cotton steel, alloy steel |
3.2 List of necessary parameters for model selection
The user should provide the following information to the manufacturer when selecting the model:
- Flue section size: length × width (mm)
- Design Temperature and Pressure: Maximum Operating Temperature, Maximum Positive/Negative Pressure
- Media characteristics: flue gas composition, dust concentration, corrosiveness (SO₂, Cl⁻¹ content)
- Displacement requirements: axial, transverse and angular displacements
- Waveform selection: full height (216mm) or half height (108mm), desired wavenumber
3.3 Applicable Size Restrictions
Standard half-height rectangular corrugated expansion joints are suitable for:
- Sectional area less than 4.6 square meters
- Where one side of the outer dimension of the flue is less than 1.5m but larger than 0.6m
Beyond this range, a full-height or multi-node combination design is required.
Installation Specifications and Requirements
4.1 Preparation before installation
- Check that the model, specification and wave number of the expansion joint are consistent with the design drawings
- Check the bellows surface for no mechanical damage, indentation and scratches
- Verify that the parallelism and concentricity of flanges or pipes at both ends of the flue meet the requirements (deviation ≤3mm)
4.2 Cold Drawing
To reduce the number of bellows nodes, cold drawing should be considered by 50%, i.e. half of the design compensation amount of pre-compression or pre-tension during installation. Specific method:
- Flue with mainly hot elongation: 50% of the design compensation amount of cold pre-compression
- Flue with mainly thermal shrinkage: 50% of the design compensation amount of cold pre-stretching
- Fix to the installation length with temporary positioning pull rod, and then dismantle after all the flue system is installed
4.3 Guide Bracket Arrangement
Guide brackets are arranged at both ends of the expansion joint to ensure the expansion and contraction direction:
- The distance between the first guide bracket and the expansion joint is ≤4 times the flue equivalent diameter
- The distance between the second guide bracket and the first guide bracket is ≤14 times the flue equivalent diameter
4.4 Installation Protection Points
- Do not damage the bellows surface during installation
- It is strictly prohibited that welding current passes through the bellows (the grounding wire must be clamped in the same side flue)
- When welding the end pipe, cover the bellows with asbestos cloth to prevent the welding slag from splashing
- Installation direction of the guide tube: the fixed end is facing the incoming flue gas, and the free end is downstream
4.5 Post-Installation Inspection
- Remove the temporary positioning tie rod and confirm that the expansion joint can be freely expanded and contracted
- Check that the bolts are evenly tightened (diagonally, tighten in three times)
- Conduct airtightness test: No leakage under working pressure
5. Comparative selection of rectangular and non-metallic expansion joints
| Comparative Item | Rectangular metal expansion joint | Rectangular non-metallic expansion joint |
|---|---|---|
| Applicable temperature | -20~800℃ | -50~250℃ |
| Pressure resistance | ≤0.25MPa | ≤ ± 30kPa |
| Corrosion resistance | Select corrosion resistant material (316L/Cotton steel) | The inner layer of fluororubber is excellent in acid resistance |
| Compensation amount | Single wave ±12~24mm | Single piece 50~80mm |
| Applicable scenarios | High temperature, high pressure, dusty flue gas | Low temperature, wet flue gas, acid dew point working conditions |
Suggestions for selection:
- Rectangular flue with high temperature (> 250℃) or hard dust: Rectangular metal expansion joint for flue is preferred
- Wet flue gas with acid dew point corrosion: preferential use of non-metallic expansion joints
VI. Common faults and prevention
| Fault phenomenon | Probable cause | Precautionary measures |
|---|---|---|
| Trough corrosion perforation | Material is not resistant to corrosion under working conditions (chloride ion) | Upgrade to 316L or higher grade stainless steel |
| Bellows cracking | Over-limit displacement or fatigue | Check the actual displacement and increase the number of expansion joints |
| Guide tube falling off | Weak welding or erosion wear | Inspect regularly and reinforce with wear resistant welding electrodes |
| Corner leakage | Excessive corner stress | Use rounded U-shaped structure instead of V-shaped structure |
VII. SUMMARY
Flue rectangular metal expansion joint is the core component of rectangular section flue thermal compensation. The core points of correct selection and standard installation can be summarized as follows:
- Structural characteristics: It adopts rounded rectangular section and circular arc transition to effectively solve the problem of right-angle stress concentration and improve the thermal compensation ability and fatigue life
- Waveform selection: full-height type (wave height 216mm) single-wave compensation ±24mm, half-height type (108mm) single-wave compensation ±12mm, and the wavenumber of a single piece does not exceed 6 waves. When it exceeds, the number of nodes is reduced by cold drawing by 50%
- Corner structure: V-shaped corner is used in low-pressure economical applications, and U-shaped fillet is used in high-pressure or high cycle life requirements
- Configuration of guide tube: The dusty flue must be equipped with guide tube to protect the bellows from erosion and wear
- Installation specifications: Cold drawn 50% pre-compression/pre-tension, proper arrangement of guide brackets, protection of bellows during welding, removal of temporary tie rods after installation
Rectangular metal expansion joint plays an irreplaceable role in high temperature flue gas pipeline system in electric power, metallurgy, building materials and other industries because of its advantages such as high temperature resistance, reliable compensation and good structural rigidity. Following the above-mentioned selection and installation specifications, it can ensure its long-term stable operation and its service life can reach 6~10 years.