In the wet flue gas desulfurization system, the expansion joint of the net flue gas outlet is the key flexible compensation component connecting the outlet of the desulfurization tower and the flue pipe of the chimney inlet. After the clean flue gas is sprayed and washed by the desulfurization tower, the temperature drops to 45-55℃, and it is in a completely saturated state, containing a large amount of acidic condensate. This special working condition puts forward extremely high requirements on the corrosion resistance, sealing performance and drainage performance of the expansion joint at the net flue gas outlet. This paper will systematically explain the professional and technical points of the expansion joint at the outlet of clean flue gas from the characteristics of working conditions, material selection, structural design to installation and maintenance.
1. Special requirements for expansion joints under the working conditions of clean flue gas
1. Low temperature and high humidity corrosion environment
Different from the original flue gas, the temperature of the clean flue gas after spraying and washing by the desulfurization tower is greatly reduced to about 50℃ and is completely saturated。 Wet saturated flue gas will produce condensation and dew condensation on the flue surface. The condensate has low pH value and strong corrosiveness。 Therefore, the selection of the expansion joint of the net flue gas outlet should be different from the original flue gas expansion joint, and it must have the functions of anti-corrosion and anti-leakage。
2. Risk of acidic condensate accumulation
When the non-metallic expansion joint is used, the installation skin will naturally form an annular groove, and a large amount of acid water will accumulate during the operation of the unit。 Acid water will slowly penetrate through the fabric layer of the expansion joint to the pressure plate screw that fixes the skin of the expansion joint, causing the screw to loosen and corrode and break, or the fabric layer to be damaged, and finally flow out from the broken screw hole and the damaged part of the skin, which will seriously appear "small waterfall" phenomenon。 This is the most common failure mode for net flue gas outlet expansion joints.
3. Negative pressure operation requirements
The clean flue is usually in a negative pressure state, and the expansion joint should have good negative pressure sealing performance to prevent external air inhalation from affecting the lifting force of the chimney and the oxygen control of the system.
2. Material selection of expansion joint at net flue gas outlet
1. Skin material requirements
In view of the strong corrosive characteristics of net flue gas, the skin of the expansion joint at the net flue gas outlet must be made of acid-resistant composite material. The recommendation structure is as follows：
| hierarchy | Material | function |
|---|---|---|
| outer layer | fluororubber | Corrosion-resistant, weather-resistant, protects the interior |
| Intermediate layer | Wire-clamped glass fiber | Provides strength and tensile properties |
| inner layer | Tetrafluorine membrane + fluororubber | Contact dielectric layer, resistant to acid, temperature and oxidation |
| Thermal insulation | Aluminum silicate insulation cotton | Reduce external surface temperature to prevent scalding |
| Reinforcement layer | 304 steel wire mesh | Increased overall strength and impact resistance |
Fluororubber (FKM) has excellent acid, oil and high temperature resistance, and its service life can reach 3-5 years in the desulfurization clean flue gas environment. Ordinary silicone rubber will age and crack rapidly in acidic wet flue gas, and its life usually does not exceed 1 year.
2. Metal frame corrosion protection
The metal frame of the expansion joint of the net flue gas outlet must be treated with heavy anti-corrosion treatment:
- Frame material: Q345 carbon steel (economical) or 304 stainless steel (corrosion resistant)
- Anti-corrosion coating: glass flake cement or vinyl ester resin coating, thickness ≥2mm
- Bolt material: 304 or 316L stainless steel to prevent acid corrosion and loosening
3. Guide tube design
The guide tube is the component that is directly washed by smoke inside the expansion joint, and must be made of corrosion-resistant materials. It is recommended that 316L stainless steel or 2205 duplex stainless steel be used for the guide tube of the expansion joint at the net flue gas outlet, and the thickness is ≥3mm. The small end of the guide tube should be facing the direction of flue gas flow, and the large end should be facing the outlet to prevent backward flow of condensate.
Structural design and leak prevention measures
1. Cure the problem of groove water accumulation
Water accumulation in grooves of non-metallic expansion joints is the main cause of leakage of expansion joints at net flue gas outlet。 The radical cure options are as follows:
- Groove filling technology: Fill closed-cell foamed butyl rubber or high-elastic sealant in the groove formed by the skin and the pressure plate to fill the groove flat and prevent acid water from accumulating
- Drainage hole setting: Drainage hole (DN25-DN65) is set at the lowest point of the expansion joint frame to discharge the condensate in time
- Dust-proof net: Install stainless steel dust-proof net (hole diameter 3-5mm) inside the drainage hole to prevent dust blockage
2. Bolt tightening requirements
Because the non-metallic expansion joint pressure plate is 4-6 meters long, the distal bolt will loosen due to skin compression deformation after one-time tightening during installation。 Therefore, when installing a clean flue gas outlet expansion joint, you must:
- Repeated operation by "diagonal tightening and fractional tightening"
- Use torque wrench, tightening torque according to manufacturer's requirements (usually 50-80N·m)
- Tighten bolts once in 1 month and once in 3 months after operation
3. Flange connection seal
For flue gas outlet expansion joints with flanges, when the new expansion joint skin is installed, packing materials such as anti-corrosion washers should be placed between the interface flange and the skin, and then the bolts should be tightened to prevent leakage at the flange and corrosion of the fastening bolts。 All fastening bolts should be tightened repeatedly and repeatedly with an electric wrench. It is strictly prohibited to leak or loosen。
Installation Specifications and Key Control Points
1. Check before installation
- Check that the model, specification and design of the expansion joint are consistent
- Check the skin for damage or scratches
- Verify that the direction of the guide tube is clearly marked
- Inspect Metal Frame Anti-Corrosion Coating Integrity
2. Installation Process Control
- Direction confirmation: The flow direction arrow on the expansion joint must coincide with the flow direction of the net flue gas (from the desulfurization tower to the chimney)
- Welding protection: Protective measures must be taken during welding operation to prevent non-metallic skin from being damaged by welding slag splashing
- Flange Alignment: Ensure that the bolt holes are aligned and the seal gasket is centered when the flange is connected
- Drain pipe installation: Lead drain stub to gutter or acid collection system
3. Acceptance after installation
- Take photo archive after expansion joint installation is complete
- Conduct airtightness test to verify no leakage
- Record the installation date, model number and operator in the running ledger
V. Common faults and preventive measures
| Fault phenomenon | Common causes | Treatment measures |
|---|---|---|
| Skin water leakage | Groove water penetration, skin aging | Filling grooves, replacing skin, adding drainage hole |
| Loose or broken bolts | Not repeatedly tightened, material not corrosion resistant | Replace stainless steel bolts and tighten them regularly |
| Frame corrosion perforation | Damage to anticorrosive coating | Recoating glass flake coating after rust removal |
| Blocked drainage hole | Dust accumulation, too dense dust net | Clean and dredge, and appropriately increase the aperture of dust-proof net |
| Skin bulge | Insulation collapse, overtemperature | Inspect insulation, replace skin |
Suggestions on operation and maintenance management
1. Daily inspection
- Check the appearance of the expansion joint every week for water seepage, bulging and abnormal vibration
- Observe the drainage situation and confirm that the drainage is smooth
- Check bolts for looseness or rust
2. Regular maintenance
- Full tightening of platen bolts quarterly
- Clean drain holes monthly to prevent clogging
- Check the anti-corrosion coating of the frame every six months, and repair it in time if any damage is found
3. Replacement cycle
The service life of non-metallic skin under the condition of clean flue gas is generally 3-5 years. The skin should be replaced as a whole when:
- Large-area aging brittleness of skin
- Repeated leak repair more than 2 times in the same location
- Insulation collapse leads to overtemperature of external surface
sum up
Clean flue gas outlet expansion joint is the key link of flue anti-corrosion in wet desulfurization system. Because its working environment has the characteristics of low temperature and high humidity, strong corrosion and condensate accumulation, the following core principles must be followed in selection and maintenance:
- Material priority: the skin is made of fluororubber composite material, and the metal frame is made of heavy anticorrosive coating
- Structural optimization: Adopt groove filling technology to eliminate hidden dangers of water accumulation, set drainage holes to drain liquid in time
- Installation specification: Tighten bolts repeatedly to ensure sealing, and install strictly according to flow direction marks
- Regular maintenance: Establish inspection ledger, drain, tighten and clean regularly
A clean flue gas outlet expansion joint with reasonable design and standard installation can effectively avoid the phenomenon of "running, dripping and leaking" and ensure the long-term stable operation of desulfurization system. If you are planning a clean flue system or are facing frequent leakage of expansion joints, it is recommended to entrust a professional organization to conduct on-site survey and special design to solve the persistent corrosion leakage problem from the source.