In flue gas treatment systems in coal-fired power plants, iron and steel smelting and chemical industries, the original flue conduit is responsible for transporting unpurified high-temperature and highly corrosive flue gas. The non-metallic expansion joint installed on the original flue, although it seems simple in appearance, plays an irreplaceable role. In-depth understanding of the function of non-metallic expansion joint of original flue is of great significance for ensuring the safe operation of equipment and reducing maintenance cost. This paper will systematically expound the core functions, structural advantages and selection points of the original flue non-metallic expansion joint, and help engineers and technicians to grasp this key component comprehensively.
I. What is the original flue non-metallic expansion joint?
Primary flue refers to the flue section between the boiler or industrial kiln outlet and the desulfurization/dust removal equipment. The flue gas of this section has the characteristics of high temperature (usually 120~180℃, instantaneously reaching above 200℃), large dust content, corrosive gases such as SO₂ and HCl and water vapor. Non-metallic expansion joints, also known as fabric compensators or rubber expansion joints, have flexible bodies composed of multi-layer composite materials, including fluororubber, polytetrafluoroethylene (PTFE), glass fiber cloth and heat insulation layer.
Different from metal expansion joints, non-metal expansion joints can better adapt to the complex working conditions of the original flue by virtue of their unique materials and structures. In order to comprehensively understand the function of non-metallic expansion joint of original flue, it is necessary to analyze it from multiple dimensions such as thermal compensation, anticorrosion and shock absorption.
Second, the core function of the non-metallic expansion joint of the original flue
1. Absorbing multidimensional thermal displacement
The original flue will produce significant thermal expansion during boiler start-stop and load change. When the working temperature of carbon steel flue is 150℃ and the ambient temperature is 20℃, the thermal expansion per 100 meters can reach about 156mm.
The function of the non-metallic expansion joint of the original flue is first reflected in its excellent multi-directional compensation ability:
- Axial compensation: absorbs lengthwise tension or compression of the flue
- Lateral compensation: Absorption of flue displacement in the direction of the vertical axis
- Angular compensation: absorbs angular changes at flue bends
The flexible structure of non-metallic expansion joints allows them to absorb displacements in three dimensions simultaneously, whereas metallic expansion joints usually only absorb primarily axial displacements, a feature that is particularly valuable in space-constrained original flue arrangements.
2. Corrosion resistance and acid and alkali resistance
The original flue gas often contains acidic gases such as sulfur oxide and hydrogen chloride, which easily forms acidic condensate when the temperature fluctuates, causing serious corrosion to metal parts.
The host materials of non-metallic expansion joints (such as fluororubber, PTFE, EPDM) have excellent acid and alkali resistance. Among them, one of the functions of the original flue non-metallic expansion joint is to effectively resist the erosion of corrosive medium in flue gas through the chemical inertness of the material itself, and its service life is much longer than that of ordinary metal expansion joint.
Taking fluororubber coating as an example, it can withstand a variety of corrosive media below 200℃ and pH value of 2~12, and completely adapts to the typical acidic environment of original flue.
3. Isolate vibration and reduce noise
Mechanical vibration and noise are generated by boiler induced draft fan operation, flue gas turbulence, and flue support vibration. If the vibration is transmitted directly to the dust collector or desulfurization tower through the rigid flue, it may cause damage to fine components such as the internal spray layer and demister.
The flexible body of the non-metallic expansion joint has good damping characteristics and can significantly attenuate the vibration transmission. The function of the original flue non-metallic expansion joint is also reflected in: it is equivalent to inserting a "soft joint" in the flue system, which isolates the vibration source from sensitive equipment and reduces the outward radiation of airflow noise.
4. Adapt to the working conditions with high dust content in the medium
The flue gas in the original flue usually contains a large amount of dust (up to 30~50g/Nm³ for coal-fired boilers). The corrugated structure of metal expansion joint is easy to accumulate dust, and may lose its compensation ability due to "ash plugging" after long-term operation.
However, the inner wall of non-metallic expansion joint is usually smooth, and with reasonable guide tube design, dust accumulation can be effectively reduced. In addition, when the non-metallic expansion joint is subjected to a small amount of dust scour, the wear-resistant layer (such as glass fiber cloth) in its multilayer composite structure can play a protective role and extend the overall service life.
5. Make up for installation errors and simplify construction
In practical engineering, flue alignment error and foundation settlement are difficult to avoid completely. Non-metallic expansion joints have certain tolerance capacity, which can make up for installation errors in a certain range and reduce the requirements for civil construction and installation accuracy. This is also the role of the original flue non-metallic expansion joint that is easily overlooked but has significant practical value-it can significantly shorten the construction period and reduce the project cost.
3. Structural composition of non-metallic expansion joint of original flue
In order to gain a deeper understanding of the role of the original flue nonmetallic expansion joint, it is necessary to understand its typical structure:
| Structural layer | Commonly used materials | Main Functions |
|---|---|---|
| Inner layer (dielectric contact layer) | Fluororubber, PTFE | Corrosion resistance, high temperature resistance and leakage resistance |
| Thermal insulation | Glass fiber wool, ceramic fiber | Reduce the temperature of the outer surface and protect the outer layer |
| reinforcing layer | Glass fiber cloth, polyester cloth | Provide strength, withstand pressure |
| Outer layer (protective layer) | Silicone rubber coated cloth, EPDM | Anti-aging, anti-UV, beautiful |
| Metal flanges and plates | Q235B, 304 stainless steel | Connection is fixed to ensure sealing |
Multilayer composite materials work together to enable non-metallic expansion joints to meet multiple requirements such as flexibility, strength, temperature resistance and corrosion protection at the same time.
4. Key points of selection of non-metallic expansion joint of original flue
Correct type selection is the prerequisite to play the role of non-metallic expansion joint of original flue. It is recommended to focus on the following key parameters:
1. Flue gas temperature
- ≤120℃: Ethylene propylene monomer (EPDM) material can be selected
- 120~180℃: fluororubber or silicone rubber composite cloth should be selected
- 180~250℃: Multi-layer insulation structure + high-temperature resistant fluororubber must be adopted
- > 250℃: It is recommended to consider metal expansion joints or special non-metallic solutions
2. Corrosive media
- Ordinary SO₂ flue gas: EPDM or neoprene is sufficient
- High concentration of HCl, HF: PTFE or fluororubber must be selected
- Strong oxidants containing NOx: PTFE is the first choice
3. Pressure Class
The primary flue is usually slightly negative pressure (-3~ -5kPa) or slightly positive pressure (
4. Displacement Amount Requirement
The total displacement is calculated according to the flue length, installation temperature and operating temperature difference, and then the non-metallic expansion joint model with corresponding compensation ability is selected. Common single wave compensation: 30~60mm in axial direction and ±20~ ±40mm in transverse direction.
V. Common Faults and Maintenance Suggestions
Even if the action of the original flue non-metallic expansion joint is reasonably designed, problems may still arise after long-term operation:
| Fault phenomenon | Probable cause | Treatment measures |
|---|---|---|
| Outer layer crack | Aging, Ozone, UV | Apply protective agent regularly and replace it in 5~7 years |
| Flue gas leakage | Damaged inner layer, loose pressure plate bolts | Fastening bolts, small damage can be repaired |
| Bulge/deformation | Overpressure, moisture and evaporation of insulation layer | Check system pressure and replace expansion joint |
| Flange corrosion | Condensate accumulation, improper material | Switch to stainless steel flanges and add drainage holes |
| Integral collapse | Internal guide tube falling off | Shutdown for inspection, re-welding deflector |
Maintenance recommendations:
- Visually inspect the surface of the expansion joint quarterly for any cracks, blisters and burning marks
- Shutdown at least once a year to check the condition of the internal deflector
- After 5 years of operation, professional evaluation shall be arranged to determine whether to replace it
6. Comparison of non-metal and metal expansion joints in original flue
| Comparative Item | Non-metallic expansion joint | Metal expansion joint |
|---|---|---|
| Multi-directional compensation ability | Excellent (axial + transverse + angular) | General (mainly axial) |
| Corrosion resistance | Excellent (material itself corrosion resistant) | Poor (need lining for corrosion protection) |
| High temperature resistance | General (≤250℃, special up to 400℃) | Excellent (up to above 600℃) |
| Ash accumulation problem | Less (smooth inner wall) | More (corrugation is easy to accumulate dust) |
| Cost | medium | Low (common material) ~ high (corrosion-resistant alloy) |
| Installation and maintenance | Convenient (lightweight, on-site repairable) | Complicated |
For typical original flue working conditions (about 150℃, acidic, dusty), the comprehensive performance advantages of non-metallic expansion joints are obvious, which is also the reason why the function of non-metallic expansion joints in original flue is widely recognized.
VII. SUMMARY
The non-metallic expansion joint of the original flue is an indispensable key component for the safe and stable operation of the original flue through many aspects such as heat compensation, anti-corrosion, shock absorption, anti-dust accumulation and installation convenience of the flue gas system. Its core values can be summarized as follows:
- Multidimensional displacement compensation: simultaneously absorbs axial, transverse and angular thermal displacements to adapt to the complex expansion law of the original flue
- Strong corrosion resistance: fluororubber, PTFE and other materials directly resist acid flue gas corrosion without additional anti-corrosion coating
- Remarkable shock and noise reduction: the flexible structure effectively isolates vibration and protects downstream sensitive equipment
- Good anti-dust accumulation effect: the inner wall is smooth and there is no complicated corrugated structure, so as to avoid the failure of "ash plugging"
- Economical and practical: Compared with corrosion-resistant alloy metal expansion joints, the non-metal solution is lower in cost, lighter in weight and more convenient to install
In practical engineering, in order to give full play to the function of non-metallic expansion joint of original flue, it is necessary to select the type correctly according to the parameters of flue gas temperature, corrosive medium, displacement, etc., and establish a regular inspection and maintenance system. Non-metallic expansion joints are often a reliable choice over metallic expansion joints for the original flue operating conditions with acidic corrosive media at temperatures not exceeding 250℃. Through scientific design, standardized installation and maintenance, the non-metallic expansion joint can run stably for 5~8 years, which provides long-term effective guarantee for the flue gas treatment system.