In the flue gas conveying systems of electric power, metallurgy, chemical industry and environmental protection industries, circular flue is widely used because of its strong pressure bearing capacity and low flow resistance. However, circular flues also face the challenges of thermal expansion, vibration, and corrosive media. Circular flue non-metallic fabric expansion joints have become the preferred solution for flexible connection of circular cross-section flues due to their excellent compensation, sealing and corrosion resistance. This paper will systematically analyze the structural characteristics, selection parameters, installation points and maintenance strategies of non-metallic fabric expansion joints of circular flue, so as to help engineers and technicians to give full play to their technical advantages.
1. Why do circular flues need non-metallic fabric expansion joints
The circular flue is usually connected to the boiler outlet, denitrification reactor, dust collector, induced draft fan and absorption tower. During operation, the smoke temperature can rise from normal temperature to above 400℃, resulting in significant thermal expansion displacement. Taking a section of circular carbon steel flue with a length of 20 meters and a diameter of 2 meters as an example, when the smoke temperature rises from 20℃ to 350℃, the axial expansion can reach about 80mm and the radial expansion can reach about 1.5mm.
Compared with the metal bellows expansion joint, the circular flue non-metallic fabric expansion joint has three outstanding advantages in the application of circular cross section:
- Stronger compensation ability: a single piece can absorb axial compression 50~80mm, transverse displacement ±30mm, angular displacement ±5°, while metal bellows can usually only absorb axial displacement.
- The effect of vibration isolation and noise reduction is remarkable: the multi-layer fabric structure can absorb 60% ~80% of mechanical vibration and protect precision equipment such as induced draft fans.
- Excellent corrosion resistance: The inner layer of fluororubber or PTFE can resist the long-term attack of SO₂ and SO₃ condensate acid, and the stress corrosion cracking common to metal expansion joints will not occur.
Therefore, non-metallic fabric expansion joints have become the mainstream choice in circular flues with a diameter of 300mm to 3000mm.
2. Structural composition of non-metallic fabric expansion joint of circular flue
A standard circular flue non-metallic fabric expansion joint comprises the following functional layers sequentially from the inside to the outside:
| Structural layer | Material selection | Functional role |
|---|---|---|
| Inner isolation layer | Fluorine rubber (FKM), silicone rubber (VMQ) or PTFE coated fiberglass cloth | Direct contact with smoke, sealing and anti-corrosion. The fluororubber is acid-resistant and temperature-resistant at 200℃; PTFE is resistant to 260℃ and resistant to adhesion |
| Intermediate reinforcement layer | Alkali-free glass fiber cloth (2~4 layers), which can be clamped into stainless steel wire mesh | Provide tensile strength and compression resistance to prevent the inner layer from being sucked through by negative pressure |
| Thermal insulation (optional) | Ceramic fiber felt (thickness 10~30mm) | Reduce external surface temperature below 60°C to reduce heat loss and prevent scalding |
| Outer protective layer | Silicone rubber coated glass fiber cloth | Weather-resistant, water-resistant, UV-resistant, protecting internal structure |
| Metal connectors | Circular flange ring (carbon steel or 316L stainless steel) and plates, bolts | The skin is fastened to the flue flange to transmit the load |
The key difference between the circular structure and the rectangular structure is that the flange ring of the circular expansion joint is usually rolled integrally to ensure that the roundness deviation does not exceed ±3mm; The rectangular expansion joint is welded in segments. In addition, the guide tube of the circular expansion joint should be designed into a conical or cylindrical shape to ensure the smooth transition of air flow.
3. Key selection parameters and calculation
Proper selection requires the following core parameters to be provided to the manufacturer:
3.1 Flue diameter and wall thickness
DN (nominal diameter) can be customized from 300mm to 6000mm. It is necessary to specify whether it is the inner diameter or the outer diameter, and the wall thickness of the flue (used as a flange).
3.2 Design temperature and instantaneous overtemperature
- Continuous working temperature: determine the material of the inner layer (choose fluorine rubber at 180 ℃; choose high temperature resistant silicone rubber or PTFE at 180~250 ℃).
- Instantaneous overtemperature (e.g. boiler fuel to support combustion): The peak temperature and duration should be indicated in order to add ceramic fiber insulation layer.
3.3 Corrosivity of media
- Wet flue gas (containing acid dew point): fluororubber or PTFE inner layer must be selected, and ordinary silicone rubber can not be used.
- Dust concentration> 20g/m³: Stainless steel wear-resistant guide tube with thickness ≥4mm is required.
3.4 Displacement
- Axial displacement: Δ L = α × L × Δ T (α is the linear expansion coefficient of steel 12×10⁻⁶/℃, L is the spacing between two expansion joints, and Δ T is the difference between smoke temperature and installation temperature).
- Lateral displacement: usually caused by thermal expansion of the vertical section of the flue, which needs to be determined by structural stress analysis.
- Angular displacement: Near the elbow needs to be calculated separately.
3.5 Pressure and Negative Pressure
The pressure resistance of non-metallic fabric expansion joints is usually ±5kPa ~ ±30kPa. If the flue is located behind the induced draft fan and is running under negative pressure, the maximum negative pressure value should be indicated to prevent the skin from being sucked in.
IV. Key points of installation and construction
The installation quality of circular flue non-metallic fabric expansion joint directly affects its life. The following are the key control points:
4.1 Inspection before installation
- Check whether the expansion joint specifications: diameter, length, center distance of flange bolt hole and quantity are consistent with the design drawings.
- Check skin for shipping damage (scratches, creases, delamination).
- Measure the flange parallelism at both ends of the flue: the radial deviation should not exceed ±5mm, and the angle between the flange surfaces should not be greater than 3°.
4.2 Cold Pre-compression
When the non-metallic expansion joint is installed in the cold state, it should be compressed by 5% ~8% of its free length. For example: free length 300mm, compressed to 276~285mm during installation. Use temporary positioning rod to fix, and after all the flue system is welded and insulated, remove the positioning rod to make the expansion joint enter the working state.
4.3 Guide tube alignment
The deflector is a critical component for protecting the skin. Installation must ensure that:
- The fixed end of the guide tube is located upstream of the flue gas, and the free end extends downstream.
- An annular gap of 10~15mm is kept between the guide cylinder and the inner wall of the expansion joint, which is used to absorb thermal expansion and avoid dust blockage.
- The guide tube must not be in contact with the inner layer of the skin, otherwise the high temperature will directly ablate the skin.
4.4 Bolt tightening
- Adopt high strength bolts above grade 8.8, hot-dip galvanized treatment.
- Fasten in three times in diagonal order: 50% → 75% → 100% design torque.
- Typical torque value: 120~150N·m for M16 bolts; M20 bolts are 200~240N·m.
- Immediately after tightening, sample 20% of the bolts with a torque wrench.
4.5 Concentricity Correction
For long-distance circular flue, the concentricity of flue between two adjacent expansion joints should be controlled within ±3mm. Concentricity excess can cause the expansion joint to bear additional bending stress and accelerate fatigue failure.
V. Common faults and treatment
| Fault phenomenon | Probable cause | Treatment method |
|---|---|---|
| Acid leakage from bottom of skin | Long-term immersion of condensate acid; The inner layer is made of acid-resistant silicone rubber | Replace the inner layer with fluororubber and add the bottom drainage port |
| Bulging or tearing of skin | Over-limit displacement; Direct impact of airflow caused by the detachment of the guide tube | Correcting flue brackets, repairing or replacing guide tubes |
| Flange bolt loose | Failure to tighten diagonally or attenuation of preload force after thermal cycling | Re-tighten to design torque after hot shutdown |
| Local high temperature on external surface (> 80℃) | The ceramic fiber insulation layer collapses due to moisture or the inner layer burns through | Incision inspection after shutdown, replenishment of insulation or replacement of skin |
| Circular flange deformation | Improper transportation or hoisting resulting in ellipse | Field jack correction to roundness deviation ≤3mm |
Maintenance and Life Management
- Monthly inspection: Visually inspect the skin surface for bulging, cracking and delamination; Check the bolts for looseness or rust.
- Quarterly infrared temperature measurement: scan the surface temperature distribution of the expansion joint. If the local temperature difference exceeds 30℃ and the temperature is higher than 80℃, it indicates that the internal heat insulation layer is damaged.
- Annual shutdown inspection: Enter the inside of the flue to check the wear of the guide tube and whether the inner isolation film is carbonized and hardened. The inner wall of the skin was observed with an endoscope from the outer flange gap.
- Replacement cycle: The design life of non-metallic expansion joints is usually 5~8 years. After 5 years of operation, samples shall be taken annually for tensile strength and elongation at break testing. If the value drops below 60% of the initial value, it must be replaced in the next overhaul.
VII. Comparison of circular and rectangular type selection
In practical engineering, some designers will struggle with whether rectangular expansion joints can be used in circular flues. The brief conclusions are as follows:
- Circular expansion joint is preferentially selected for circular flue: the circular structure is uniformly stressed, the skin has no corner stress concentration, the sealing reliability is higher, and the circulation area is larger under the same circumference.
- Rectangular expansion joints are considered only in locations where the flue transitions from circular to rectangular (such as the reduced diameter section of the fan inlet and outlet).
- The manufacturing cost of circular expansion joints is usually 10% ~15% lower than that of rectangular expansion joints of the same caliber because the flanges can be rolled rather than welded.
Therefore, unless the field is limited, the circular flue non-metallic fabric expansion joint is the best matching scheme for circular flue.
VIII. Summary
Circular flue non-metallic fabric expansion joint has become the preferred solution for flexible connection of circular flue gas pipes due to its large displacement compensation ability, excellent vibration and sound insulation effect and excellent corrosion resistance. The four core parameters of temperature, medium corrosivity, displacement and pressure grade should be defined in the correct selection; During installation, the four key processes of cold pre-compression, guide tube centering, bolt diagonal tightening and concentricity correction must be strictly controlled; After operation, a hierarchical maintenance system of monthly inspection, quarterly temperature measurement and annual internal inspection will be established.
By systematically mastering the above technical points, engineers and technicians can stabilize the actual service life of non-metallic fabric expansion joint of circular flue at 6~8 years, and control the leakage rate below 0.5%, which provides reliable guarantee for the long-term safe operation of desulfurization, denitrification and boiler smoke exhaust system. When uncertain working conditions are encountered, it is recommended to provide complete flue layout drawings and operating parameters to professional manufacturers for customized design.