In the wet flue gas desulfurization (FGD) system of coal-fired power plants, the clean flue conduit bears the important task of transporting the desulfurized clean flue gas to the chimney. However, although the net flue gas temperature drops to 45-55℃, it contains strong corrosive media such as residual SO₂, SO₃ and chloride ions, and is in a "wet saturation" state, which poses an extremely high risk of corrosion to pipelines and connecting parts. The rubber-lined expansion joint of wet desulfurization net flue is a special compensation element that emerged under this severe working condition-it is lined with corrosion-resistant rubber layer inside the metal expansion joint body, which not only retains the advantage of strong pressure-bearing capacity of metal structure, but also solves the problem of rapid corrosion failure of ordinary metal expansion joint in acidic wet flue gas environment. This paper systematically expounds the technical characteristics of rubber-lined expansion joint of wet desulfurization flue from the aspects of structure principle, selection key points, installation and maintenance, and provides professional reference for power plant equipment management personnel.
What is Wet Desulfurization Net Flue Liner Expansion Joint?
Wet desulfurization net flue rubber lining expansion joint is a composite structure compensator formed by lining a layer of acid-resistant rubber (such as natural hard rubber, neoprene rubber or butyl rubber) on the inner surface of a metal expansion joint (usually a bellows structure) in contact with flue gas. Its design concept is to organically combine the mechanical strength of metal with the corrosion resistance of rubber according to the corrosion characteristics of wet desulfurization flue gas, so that the expansion joint can resist the erosion of dilute sulfuric acid condensate for a long time while absorbing the thermal displacement of the pipeline.
Compared with ordinary non-metallic expansion joints, the advantages of rubber-lined expansion joints are: higher pressure bearing capacity (up to 0.1-0.5MPa) and stronger anti-scour performance, especially suitable for working conditions with higher dust content or faster airflow speed in flue gas. At the same time, the rubber lining layer also plays an isolation role, preventing the acidic medium from directly contacting the metal matrix, and significantly prolonging the service life of the expansion joint.
Why do you need rubber-lined expansion joints for net flues?
The net flue gas temperature after wet desulfurization is usually lower than the acid dew point (about 70-80℃), and water vapor condenses on the inner wall of flue and the surface of expansion joint to form dilute sulfuric acid solution (pH value can reach 2-3). For ordinary metal expansion joints (even 316L stainless steel), stress corrosion cracking and pitting corrosion are very prone to occur in this high chlorine and acidic environment for a long time. The engineering practice shows that the corrosion penetration of the bellows may occur when the common metal expansion joint is put into operation in the clean flue gas section for about 18-24 months.
Although the non-metallic expansion joint has good corrosion resistance, its skin has limited resistance to high temperature (conventional products ≤200℃), and it is easy to wear under high pressure or high scour conditions. The wet desulfurization clean flue lining expansion joint just fills the gap between the two-the rubber lining provides excellent acid resistance, and the metal structure guarantees sufficient mechanical strength and pressure bearing capacity, making it ideal for clean flue conditions.
Structure and Material of Rubber-lined Expansion Joint
1. Metal substrate
The metal matrix is usually made of Q235B carbon steel or 16Mn low alloy steel, and the structural form can be selected as axial type, transverse type or angular type according to the compensation requirement. The bellows area is the difficulty of rubber lining-due to the uneven surface of the bellows, the rubber lining layer needs a special bonding process to ensure adhesion and uniformity.
2. Rubber lining
The rubber lining layer is the core functional layer of the rubber lining expansion joint of wet desulfurization flue. According to the corrosiveness of the medium, commonly used rubbers include:
- Natural hard rubber: Excellent acid resistance, suitable for acidic media with pH 2-5, relatively economical price.
- Neoprene (CR): Good weather resistance and oil resistance, suitable for smoke containing trace oil mist.
- Butyl Rubber (IIR): Excellent air tightness, outstanding acid and alkali resistance, it is the preferred material for clean flue lining.
The thickness of the rubber lining is generally 3-6mm, and it is firmly combined with the metal matrix by hot vulcanization process to ensure that it does not fall off or blister during use.
3. Protection of metal components
In addition to the rubber lining of corrugated pipe, metal components such as flanges and deflectors of expansion joints also need to be treated with corrosion protection. Deflectors are usually made of 316L stainless steel or 2205 duplex steel to prevent erosion corrosion. On the basis of rubber lining, the flange sealing surface should be equipped with acid-resistant sealing gasket (such as flexible graphite or PTFE) to ensure that there is no leakage at the interface.
Key parameters of model selection
When selecting the expansion joint of wet desulfurization net flue rubber lining, the following working condition parameters should be provided to the manufacturer:
| Parameter category | Specific content | Description |
|---|---|---|
| Diameter dimension | Flue inner diameter/length × width | According to the field measurement |
| Design pressure | Working pressure and fluctuation range | Clean flue is usually slightly positive pressure |
| Design temperature | Maximum continuous operating temperature | Clean flue is generally ≤80℃ |
| Media composition | SO₂, SO₃, Cl⁻¹ concentration and pH | Decide on the choice of rubber material |
| Compensation amount | Axial, transverse and angular displacements | Calculated according to length of pipe section and temperature difference |
| Dust concentration | Flue gas dust content (mg/Nm³) | Decide the deflector material |
Special tip: Rubber-lined expansion joints are not suitable for working conditions with temperatures exceeding 120℃-high temperature will cause the rubber layer to soften and accelerate aging. At this time, non-metallic expansion joints or high-temperature resistant alloy expansion joints should be selected.
Installation and Maintenance Points
Installation Considerations
- Inspection before installation: check the rubber lining layer for bubbles, cracks or mechanical damage; The flatness of flange surface meets the requirements; The rubber layer is detected without pinhole defects by EDM leak detector.
- Flange connection: Acid-resistant sealing gasket (such as EPDM or PTFE gasket) is used, and the bolts are tightened three times in diagonal order. The torque refers to the design value.
- Confirmation of deflector: The direction of deflector must be consistent with the flow direction of flue gas. After the expansion joint is installed, check the gap between deflector and bellows (it should be 10-20mm) to prevent friction damage to the rubber lining layer under hot state.
- Lifting is prohibited: It is strictly prohibited to use the expansion joint as the lifting point or support point of the pipeline to avoid cracking of the rubber lining layer under force.
Maintenance Points
- Regular inspection: check the surface of the expansion joint every quarter for leakage traces, flange bolts for looseness, and bulge on the outer surface of the rubber lining layer.
- Shutdown detection: Every time the furnace is planned to shut down, open the inspection hole to observe the condition of the rubber lining layer, and conduct electric spark leak detection if necessary.
- Rubber lining repair: If local rubber lining is found to be damaged and the area is small (
epilogue
The rubber lining expansion joint of wet desulfurization clean flue is the key component connecting clean flue and chimney in wet desulfurization system. Its composite structure of "metal pressure bearing + rubber anti-corrosion" has achieved a good balance between corrosion resistance, mechanical strength and cost control. Compared with ordinary metal or non-metal expansion joints, the service life of rubber-lined expansion joints can be extended to 5-8 years under clean flue conditions, which effectively reduces the maintenance frequency and operation and maintenance cost. When selecting the model, we should pay attention to the media composition, operating temperature and compensation amount. During installation, we must do a good job in flange seal and deflector alignment. During maintenance, we should insist on regular leak detection and timely repair-all three are indispensable.