In the flue gas discharge system of power plants, chemical plants, iron and steel smelting and other industrial fields, the flue interface often bears multiple tests such as high temperature, corrosive gas, fan vibration and pipeline thermal expansion. If the flue interface is rigid connection, it can lead to weld cracking at least, and equipment damage or flue gas leakage accident at worst. At this time, the function of flue interface expansion joint is particularly critical. This paper will systematically analyze the irreplaceable value of flue interface expansion joint in industrial pipeline from four core dimensions: displacement compensation, vibration and noise reduction, sealing and anticorrosion, and protecting equipment.
1. Absorb heat displacement and prevent pipeline deformation
Flue systems tend to have temperatures as high as 150℃ to 600℃ or even higher in working conditions. The metal flue produces significant thermal expansion at high temperatures, and the carbon steel flue elongates by about 12 mm per 100 °C of temperature rise per 100 meters. If a hard connection is used at the flue interface, the thermal stress cannot be released, which can lead to pipe bending, bracket damage, or weld cracking.
The function of flue interface expansion joint is first embodied in absorbing axial, transverse and angular thermal displacement. The bellows or flexible fabric layer inside the expansion joint, through elastic deformation, converts thermal displacement into flexible compensation, thereby eliminating thermal stress. For example, at the outlet of the boiler flue, non-metallic expansion joints are usually installed to withstand three-way displacement, ensuring that the flue will not fail due to thermal fatigue even under frequent start-and-stop conditions.
2. Reduce vibration and noise to protect upstream and downstream equipment
Flue systems are often connected with rotating or vibrating equipment such as induced draft fans, dust collectors, desulfurization towers, etc. The frequency of the blades of the fan, the pulsation of the airflow, and the mechanical imbalance all produce constant vibration. If the flue interface is a rigid connection, the vibration will be transmitted along the flue to the chimney, desulfurization tower and even the boiler body, accelerating structural fatigue.
The flexible structure of the expansion joint can effectively cut off the vibration propagation path. For metal expansion joints, the elasticity of the bellows can absorb medium and high frequency vibrations; For non-metallic expansion joints, the fabric composite layer itself has damping characteristics, and the vibration damping effect is more significant. In practical engineering, after the expansion joint is installed at the inlet and outlet of induced draft fan, the vibration amplitude of equipment foundation can usually be reduced by more than 50%. Therefore, the function of flue interface expansion joint is not only limited to thermal compensation, but also the key means of vibration isolation.
3. Realize reliable sealing and prevent smoke leakage
Flue gas often contains harmful components such as sulfur dioxide, nitrogen oxides, dust particles, etc. Once the leakage occurs at the flue interface, it will not only cause heat loss, but also bring the risk of environmental protection exceeding the standard and potential safety accidents. Traditional rigid interfaces are filled with asbestos pads or ceramic fibers, and the seal fails quickly with temperature cycling and flue displacement.
The expansion joint achieves dynamic sealing through a monolithic structure. The metal bellows expansion joint is welded or formed integrally during manufacture, and there is no leakage point; Non-metallic expansion joints are made of multi-layer composite fabrics (such as silicone cloth, fluorine adhesive cloth and glass fiber cloth superimposed) to maintain airtightness while withstanding displacement. For clean flue after wet desulfurization, non-metallic expansion joints become standard with their superior sealing and acid resistance due to the low flue gas temperature (~50°C) and high corrosiveness.
It can be said that in the role of flue interface expansion joint, the sealing function is directly related to environmental compliance and equipment safety.
4. Resist corrosive media and prolong the life of flue
The composition of flue gas is complex, especially in coal-fired power plants or waste incineration plants. The flue gas contains chloride ions, sulfuric acid, hydrofluoric acid and other corrosive substances. When the flue gas temperature is lower than the acid dew point, condensed acid liquid will form on the inner wall of the flue, which will cause rapid corrosion to the carbon steel flue and the common metal expansion joint.
To solve this problem, the expansion joint can be solved by material selection. There are three common solutions:
- Non-metal expansion joint: Using fluorine adhesive tape or PTFE film as the dielectric contact layer, it has excellent acid and alkali resistance.
- High nickel alloy expansion joint: such as Inconel 625 or C-276 alloy, suitable for high temperature and high corrosion conditions.
- Lined with anti-corrosion coating: Spray glass flakes or enamel on the inside of the metal expansion joint.
Through reasonable selection, the expansion joint not only resists corrosion itself, but also reduces condensate collection through a segmented structure, thus protecting the overall life of the flue interface area.
5. Simplify installation and maintenance and reduce system stress
Rigid flue interface requires extremely high installation accuracy, and additional stress may occur if the nozzle alignment error exceeds 2 mm. The expansion joint has certain installation tolerance capacity, which can compensate the manufacturing and construction errors when the flue is matched.
Additionally, the expansion joint makes it easy to remove and maintain. For example, when overhauling the induced draft fan or desulfurization spray layer, simply remove the connecting flange of the expansion joint to obtain the working space without cutting the flue. From the perspective of the whole life cycle, although the expansion joint itself needs to be replaced regularly (non-metal expansion joints usually have a life span of 3-5 years, and metal expansion joints have a life span of 8-10 years), it effectively avoids the structural damage of the flue body, and the comprehensive maintenance cost is lower.
6. Classification of expansion joints and differences in their functions
It should be pointed out that the role of expansion joints with different structures in the flue is different:
| Type | Core role | Typical application scenarios |
|---|---|---|
| Non-metallic expansion joint | Multidirectional compensation, vibration reduction, anti-corrosion, sealing | Desulfurization flue, boiler outlet, rectangular flue |
| Metal axial expansion joint | Axial thermal displacement absorption | Straight pipe section, flue with relatively uniform temperature |
| Large metal tie rod transverse type | Lateral displacement compensation, withstanding internal pressure thrust | Flue elbow, equipment inlet and outlet |
| rubber expansion joint | Vibration damping and sound insulation | Fan inlet and outlet, low pressure flue |
Therefore, after understanding the function of flue interface expansion joint, it should be selected in combination with specific working conditions, so as to maximize the efficiency.
conclusion
To sum up, the functions of flue interface expansion joints can be summarized into four aspects: absorbing thermal displacement to prevent pipeline deformation, protecting equipment with vibration and noise reduction, dynamic sealing to prevent leakage, and resisting corrosion to prolong system life. In the industrial flue environment with high temperature, corrosion and vibration, the expansion joint is by no means an optional accessory, but a core component to ensure the safe, environmental protection and stable operation of the whole flue gas system. Proper selection and regular maintenance of expansion joints at flue interfaces not only avoids unplanned downtime and environmental fines, but also significantly reduces life-cycle operation and maintenance costs. It is hoped that this paper can provide a clear and practical technical reference for engineers engaged in the design, operation and maintenance of flue system.