In the wet flue gas desulfurization system, the flue anti-corrosion expansion joint is the core flexible component connecting the inlet and outlet of desulfurization tower with the flue. After the clean flue gas is washed by the desulfurization tower, the temperature drops to 45-55℃, and it is completely saturated. The pH value of the condensate is as low as 2-3, which causes extremely serious corrosion to the expansion joint. Ordinary expansion joints often have perforation leakage in one to two years in this environment, while flue anti-corrosion expansion joints can run stably for a long time in strong corrosion environment through special materials and structure design. This paper will systematically explain the professional technical knowledge of flue anti-corrosion expansion joint from material selection, structure design to installation and maintenance.
First, why do you need to specially design anti-corrosion expansion joints?
In the wet desulfurization system, the working conditions of flue anti-corrosion expansion joint are extremely harsh:
- Strong acidic environment: The flue gas contains SO₂, SO₃, HCl and a large amount of water vapor, and the pH value of the condensate can be as low as 2-3
- Chloride ion concentration: Cl⁻¹ concentration in desulfurization slurry can reach 20000-60000ppm, which is very easy to induce pitting and stress corrosion cracking
- Temperature alternating: frequent fluctuation from normal temperature to 80℃, thermal stress accelerates material aging
- Negative pressure operation: Some flues are under negative pressure, which requires higher sealing performance of expansion joints
Ordinary non-metallic expansion joints adopt silicone rubber skin, which has insufficient acid resistance, and will rapidly age and crack in acidic wet flue gas environment. In addition, when the non-metallic expansion joint is installed, an annular groove will naturally be formed between the skin and the pressure plate, and a large amount of acid water will accumulate during operation. The acid water slowly penetrates into the fixing screw through the fabric layer of the expansion joint, causing the screw to loosen and corrode and break. Finally, the acid liquid flows out from the broken screw hole and the damaged part of the skin, resulting in a serious "small waterfall" phenomenon.
2. Material selection of flue anticorrosion expansion joint
1. Skin material: fluororubber is the first choice
The skin material selection of flue anti-corrosion expansion joint is the core of anti-corrosion performance. Obvious differences can be seen comparing commonly used materials:
| Material Type | Acid resistance | Applicable working conditions | Life expectancy |
|---|---|---|---|
| Silicone rubber | Poor, not acid-resistant | drying flue gas | Number of months – 1 year |
| Ethylene propylene monomer (EPDM) | normal | General corrosive environment | 1-2 years |
| Fluorine rubber (FKM) | Excellent | Desulfurization wet flue gas | 3-5 years |
The fluororubber skin has the following key characteristics:
- Fluoride content ≥66%, with excellent resistance to acid and alkali attack
- The sealing layer achieves a seamless structure through a hot-press vulcanization process to prevent media leakage
- Mechanical properties: tensile strength 10-20MPa, elongation at break 150-350%, tear resistance 3-4KN/m
For projects with high requirements, the chimney expansion joint of a 2×1000MW expansion project in Ningbo is designed with fluorine rubber ring, which requires that the fluorine content of fluorine rubber is not less than 66%, and the flange frame is made of titanium steel composite plate (TA2 titanium plate +10mm carbon steel substrate). This configuration achieves a perfect combination of corrosion resistance and structural strength.
2. Multilayer composite structure of skin
The typical flue anti-corrosion expansion joint skin adopts a multi-layer composite structure, each performing its own duties:
| hierarchy | Materials | function |
|---|---|---|
| Dielectric contact layer | fluororubber | Direct contact with corrosive media, acid and temperature resistant |
| Sealing layer | Fluororubber/PTFE | Ensure airtightness and prevent leakage |
| Reinforcement layer | Alkali-free fiberglass cloth/stainless steel wire fiberglass cloth | Improve tensile strength and wear resistance |
| Thermal insulation | Aluminum silicate fiber cotton | Lowering the external surface temperature |
| Outer shielding | Silicone rubber coated fiberglass cloth | Protecting internal structure |
3. Metal frame corrosion protection
The metal frame of the flue anti-corrosion expansion joint also needs to be strictly anti-corrosion:
| Working condition | Frame Material | Anticorrosive coating |
|---|---|---|
| General corrosion | Carbon steel | Glass flake cement ≥2mm |
| Strong corrosion | 304 stainless steel | Pickling passivation |
| Strong corrosion (Cl⁻high) | 316L stainless steel | Pickling passivation |
| Extreme corrosion | Titanium steel composite plate (TA2) | - |
In engineering practice, in the anti-corrosion project of GGH flue of Zhuhai Power Plant, the rust removal treatment of newly installed steel plate reaches the Sa2.5 standard, and the anti-corrosion area of flue with glass flakes is about 100 square meters.
3. Structural design and anti-corrosion key points
1. Cure the problem of groove water accumulation
The grooves formed when the skin of the non-metallic expansion joint is installed are the main reason for the leakage of the flue anti-corrosion expansion joint. To thoroughly solve the corrosion problem, it is necessary to solve the water accumulation in the groove.
Radical Cure-Groove Filling Technique:
- Shutdown to clean up dust around expansion joint and remove damaged skin
- Clean up the floating ash of the metal skeleton and repair the corroded parts by welding
- Install the fluororubber expansion joint skin and repeatedly tighten the fixing screws
- Apply high elastic glue on both sides of the groove of the expansion joint
- Fill with high temperature and corrosion resistant polyurethane filler (compression ratio 7:1), and compact in layers
- Seal the adhesive layer with each layer of filler until it is filled
- The top layer is flattened with high elastic acid-resistant glue, and the thickness is about 3mm
This scheme keeps the acid water in the flue out of contact with the skin of the expansion joint, and eliminates the leakage from the root source.
2. Drainage port setting
Horizontally installed flue anti-corrosion expansion joints must be provided with drainage openings at the lowest point of the frame:
- Aperture: DN50-DN100
- Material: Stainless steel 316L or alloy steel
- Function: To discharge condensate acid liquid in time to prevent the expansion of heat insulation cotton and stress corrosion of skin caused by water accumulation
3. Guide tube design
Deflectors (baffles) are used to protect the skin/bellows from direct flushing by smoke:
- The material of the guide tube is not lower than that of the bellows
- The small end faces the direction of flue gas flow
- Wear resistance needs to be considered
4. Corner Optimization of Rectangular Expansion Joint
For the flue duct with rectangular cross section, the flue anti-corrosion expansion joint can adopt rectangular fillet structure. The structure adopts a rectangular cross section with large circular arc transition at the corners, which can effectively reduce the stress level at the corners, eliminate the risk of deformation dead angle and local high stress damage, and prolong the service life of products.
Installation Specifications and Key Control Points
1. Preparation before installation
- Check that the model, specification and design are consistent
- Check the skin for damage or scratches
- Confirm Flow Direction Identification (Small End Facing Incoming Flow)
- Inspect Metal Frame Anti-Corrosion Coating Integrity
- Clean flue flange surface to remove old gasket residue
2. Installation Process Control
According to the installation experience of non-metallic expansion joints of smoke and air ducts:
| Key points of control | Specific requirements |
|---|---|
| Direction confirmation | The marking direction of the wear-resistant bushing is consistent with the flow direction of the medium |
| Skin laying | Install from the lower part, and the skin is flat against the flange surface to prevent wrinkles |
| Bolt tightening | Adopt "diagonal fastening and fractional tightening", and the pressure plate is 4-6 meters long and needs to be tightened repeatedly |
| Welding protection | It is strictly prohibited to damage the skin by welding slag splashing |
| Temporary support removal | Temporary support added from the factory for large diameter expansion joints, remove as soon as possible after installation |
3. Transportation/Limit Rod Handling
- Set the transport fixing rod. After the installation is complete, adjust the installation position of the inner nut marked with yellow (the position where the one-side nut is loose more than 1/2 of the axial compensation amount)
- The fixed tie rod is not allowed to be removed to ensure the normal operation of the compensator
Operation, maintenance and replacement cycle
1. Daily inspection
- Check the appearance of the expansion joint weekly for any smoke leakage, bulging and loose bolts
- Observe whether the drainage hole is unobstructed
- Open drain valve regularly to drain condensate
2. Regular maintenance
- Fully tighten the pressure plate bolts every quarter (the non-metallic expansion joint needs to be tightened once in 1 month and 3 months after replacement and operation)
- Clean up the dust in the expansion joint for each overhaul
- Check Frame Anti-Corrosion Coating Integrity
3. Replacement cycle judgment
The normal service life of flue anti-corrosion expansion joint is 3-5 years. It should be replaced when:
- Large-area aging brittleness of skin
- Repeated leak repair more than 2 times in the same location
- Metal frame corrosion thinning more than 30% of design thickness
VI. Common Faults and Treatment Measures
| Fault phenomenon | Common causes | Treatment measures |
|---|---|---|
| Skin seepage | Water accumulation in grooves, material not acid resistant | Groove fill, skin upgraded to fluororubber |
| Bolt loosening | Not repeatedly tightened, vibrated | Regular re-tightening, adding spring washers |
| Frame corrosion | Damage to anticorrosive coating | Recoating glass flake coating after rust removal |
| Blocked drainage hole | Dust accumulation | Regular cleaning and dredging |
| Skin bulge | Insulation collapse | Replace insulating cotton |
sum up
Flue anti-corrosion expansion joint is the key equipment to resist acid corrosion in wet desulfurization system. Proper type selection, standardized installation and regular maintenance are the three core elements to ensure its long-term reliable operation:
- Material priority: the skin must be made of fluororubber (FKM), with fluorine content ≥66%; Carbon steel + glass flake coating, 316L stainless steel or titanium steel composite plates are selected for metal frames according to the degree of corrosion
- Structural optimization: horizontal installation must be provided with drainage holes (DN50-100), and the bottom is hydrophobic to avoid water accumulation; The groove filling technology is adopted to fill the groove between the skin and the pressure plate to prevent leakage from the root source; It is recommended that the guide tube and rectangular fillet structure reduce the stress concentration
- Installation specifications: consistent flow direction marks, no wrinkles on the skin, multiple tightening of bolts, and repeated tightening in the initial stage of operation
- Regular maintenance: Establish inspection ledger, drain water regularly, tighten bolts, inspect anti-corrosion coating
A flue anti-corrosion expansion joint with reasonable design and standard installation can achieve leakage-free operation for 3-5 years in strong acidic wet flue gas environment. It is recommended to consult our company by telephone in the design stage, carry out working condition analysis and material selection, and ensure the long-term safe operation of the equipment from the source.