In the wet flue gas desulfurization (WFGD) system, the clean flue conveys the saturated wet flue gas after desulfurization treatment. Because the flue gas temperature is usually lower than the acid dew point, a large amount of condensed water will be produced, and a small amount of slurry is still entrained in the flue gas after the demister, so the internal environment of the clean flue is extremely humid. Under this condition, the waterproofing of the flue expansion joint of desulfurization net flue becomes a key problem that affects the life of equipment and the reliability of system operation. This paper will systematically analyze the reasons, hazards and waterproof design points of water intake in the expansion joint of the clean flue.
1. Why does the net flue expansion joint need to be waterproof
The importance of desulfurization net flue expansion joint waterproofing stems from the special working conditions of the net flue. The net flue gas temperature after wet desulfurization is generally 45~55℃, which is saturated wet flue gas. When the temperature drops further, the water vapor in the flue gas will condense into liquid water, which flows down the inner wall of the flue. At the same time, the microscopic slurry droplets that the demister fails to capture will also be carried to the clean flue with the flue gas.
Unlike ordinary flues, the condensate in the clean flue has the following characteristics:
- Strong acidity: the pH value is usually 1.5~3.5, and contains corrosive components such as H₂SO₄ and HCl
- High salt content: contains chloride ion, fluoride ion and sulfate, strong corrosion
- Persistence: Condensate continues to be generated and flowed as long as the system is running
This condensate and slurry can cause a series of serious problems if it seeps inside the expansion joint or accumulates at the bottom of the expansion joint for a long time.
2. Main harm of incoming water to expansion joint
2.1 Corrosion failure of non-metallic expansion joints
Although the inner layer of non-metallic fabric expansion joint is made of corrosion-resistant materials such as fluororubber or PTFE, under long-term immersion or high-pressure seepage, it may still appear:
- Bulging delamination of isolation layer: condensate penetrates between layers through tiny defects, resulting in separation of layers
- Reinforcement layer corrosion: the strength of glass fiber cloth decreases after long-term soaking in acidic environment
- Thermal insulation cotton powder: ceramic fiber or aluminum silicate cotton after moisture loss of thermal insulation performance, even collapse
2.2 Local corrosion of metal expansion joints
Desulfurization Net Flue Expansion Joint Waterproofing is equally important for metal expansion joints. Metal bellows expansion joints face in a clean flue environment:
- Pitting: Chloride ions form pitting pits on the surface of stainless steel, which rapidly develop into perforations
- Crevice corrosion: Condensate is easy to accumulate at the trough of bellows, forming oxygen concentration battery, which accelerates corrosion
- Stress corrosion cracking: Under the combined action of tensile stress and corrosive medium, cracks are generated at the wave crest of bellows
Practical cases show that the lifetime of metal expansion joints without waterproof design is usually less than 2~3 years in clean flue.
2.3 Seal Failure and Environmental Risks
Condensate leakage is the most direct hazard to the flue expansion joint. When the bottom of the expansion joint is perforated or leaked at the joint, acidic liquid will drip from the flue into the ground or into the insulation layer, causing:
- Environmental complaints and penalties
- Steel structure corrosion
- Forced to shut down for emergency repair, affecting power generation or production
3. Common paths of water inlet from expansion joints
To achieve effective waterproofing, you first need to understand the way the condensate enters the expansion joint:
| Inlet path | site of occurrence | Typical performance |
|---|---|---|
| flue inner wall flow | Inner bottom of expansion joint | Long-term fluid soaking skin or bellows |
| Guide tube leakage | Connection between guide tube and end tube | Weld cracking or corrosion perforation |
| Flange sealing surface | Between pressure plate and flange | Loose bolts or gasket failure |
| Interskin permeation | Fabric layer edge | Capillary action leads to fluid inhalation |
| External rainwater | Expansion joint lateral | Water enters the insulation layer, and the outer protective layer is damaged |
4. Key points of waterproof design
4.1 Structural waterproofing design
The waterproofing of the expansion joint of desulfurization net flue should start with the structural design:
Set up bottom hydrophobic device:
- Open a drainage hole at the lowest point of the expansion joint, and connect the DN25~ DN50 drainage pipe
- Water seals or valves are provided in the drainage pipe to prevent smoke leakage
- The location of the hydrophobic hole should avoid the structural stress concentration area
Waterproof structure of guide tube:
- The free end of the guide tube shall extend downstream and the extension length shall be ≥100mm
- A water retaining ring shall be set on the outside of the guide tube to guide the condensate to the drainage opening
- Keep a gap of 15~20mm between the guide tube and the inner wall of the expansion joint to avoid liquid accumulation
Protection against bottom accumulation:
- The inner layer of the bottom of the non-metallic expansion joint shall be thickened or PTFE anti-corrosion liner shall be added
- Drainage holes are opened in the trough at the bottom of the metal expansion joint (6~8mm in diameter, equidistant arrangement)
4.2 Material Waterproofing Selection
| Component | Waterproofing Requirements | RECOMMENDED MATERIAL |
|---|---|---|
| Inner isolation film | Acid resistance, low permeability | Fluorine rubber (FKM), PTFE coated fiberglass cloth |
| Sealing gasket | Acid resistance and good elasticity | Expanded polytetrafluoroethylene (ePTFE) |
| guide tube | Acid and wear resistance | 316L stainless steel, 2205 duplex |
| bolt | Anti-corrosion | 316L stainless steel, Dacromet treatment |
4.3 External waterproofing measures
For net flue expansion joints installed outdoors, external waterproofing is equally important:
- Metal shell on the periphery of insulation layer: 0.5~0.7mm galvanized iron sheet or aluminum sheet with seam facing down
- Shell overlapping method: adopt overlapping along the water (the upper piece presses the lower piece) to prevent rainwater from penetrating
- Top Rain Cover: A rain cover shall be installed at the top of the horizontal flue expansion joint
V. Waterproof control during installation and construction
5.1 Key points of waterproofing during installation
- Installation direction of expansion joint: the bottom drainage hole must be located at the lowest point, and be corrected with a level ruler before installation
- Flange sealing: Apply acid-resistant sealant and tighten bolts diagonally in three times to design torque
- Guide tube centering: Ensure that the guide tube is concentric with the expansion joint to avoid liquid accumulation caused by eccentricity
- Smooth drainage pipeline: Water injection test after installation to confirm smooth drainage
5.2 Common Installation Errors and Corrections
| Wrong practice | consequence | Correct practice |
|---|---|---|
| Hydrophobic pores facing up or offset | Accumulated fluid cannot be drained | Find the lowest point and mark it when installing |
| The fixed end of that guide tube is downstream | Backflow of condensate into expansion joint | Fixed end upstream, free end downstream |
| Flange bolts not evenly tightened | Local leakage | Fasten three times in diagonal order |
| Outer shell facing seam | Rainwater infiltration into insulation layer | Seal by overlapping or gluing along the water |
VI. Waterproof inspection during operation and maintenance
6.1 Daily inspection items
- Hydrophobic status: Check monthly for water outflow from the drainage port. Long-term lack of drainage may be blockage, which needs to be dredged in time
- External surface temperature: Scan with infrared thermometer, local low temperature point indicates internal water intake and moisture
- Appearance inspection: whether the skin is bulging or delaminated; Whether the outer shell is corroded or damaged
6.2 Annual Shutdown Inspection
- Internal inspection: Enter the flue and check the inner wall of the expansion joint to confirm that there is no fluid accumulation, corrosion or perforation
- Guide tube inspection: check for detachment, corrosion perforation, weld cracking
- Drainage hole dredging: Clean the drainage hole and pipeline with thin steel wire
6.3 Troubleshooting Plan
| Fault phenomenon | Treatment measures |
|---|---|
| Expansion joint bottom leakage | Clean the surface after shutdown and temporarily seal it with acid-resistant repair glue; Planned overhaul and replacement |
| Hydrophobic pore blockage | Use special dredging tools to dredge from the outside to avoid piercing the inner layer |
| Skin bulge (water inflow) | Put a small hole at the lowest point to drain water, and repair it after drying; Overall replacement in severe cases |
| Inlet of insulation layer | Remove outer shell, replace moisture insulation cotton, and reseal |
VII. SUMMARY
The waterproofing of flue expansion joint of desulfurization net flue is the key link for long-term stable operation of wet desulfurization system. The core conclusions are as follows:
- Clear hazards: Condensate soaking and leakage are the primary causes of the failure of the clean flue expansion joint, resulting in delamination of the non-metallic expansion joint, failure of insulation cotton, and pitting and stress corrosion cracking of the metallic expansion joint
- Source control: The influent water mainly comes from the flow of the inner wall of the flue, leakage of the guide tube, failure of the flange sealing surface and external rainwater
- Design points: Bottom water phobic device, water retaining ring of guide tube, acid-resistant material, and external waterproof shell must be set
- Installation Key: Ensure that the drainage hole is at the lowest point, the guide tube is correctly oriented, and the flange seal is reliable
- Operation and maintenance inspection: Regular inspection of hydrophobic status and external surface temperature, annual shutdown for internal inspection
Through systematic waterproof design and standardized construction, the leakage failure rate of clean flue expansion joint can be reduced by more than 80%, and the service life can be extended from 2~3 years to 6~8 years, which effectively ensures the environmental protection standard and operation economy of desulfurization system.