In dry desulfurization system, flue gas dry desulfurization expansion joint is the key component to connect flue and desulfurization tower, absorb heat displacement and ensure the integrity of system seal. Different from wet desulfurization, the flue gas medium of dry desulfurization has the characteristics of higher temperature, large dust content, relatively low corrosiveness, but obvious erosion wear. Once the flue gas dry desulfurization expansion joint is improperly selected or installed, it will lead to air leakage and increased energy consumption, or system shutdown and environmental protection risks in the worst case. This paper will systematically explain the core technical points of flue gas dry desulfurization expansion joint from the characteristics of working conditions, key points of type selection to installation and maintenance.
1. Special requirements for expansion joints under dry desulfurization conditions
In dry desulfurization process (such as CFB circulating fluidized bed method, SDA rotary spray method, NID method), the flue gas temperature is usually controlled between 120-180℃, and the desulfurizer is alkaline dry powder such as slashed lime. Compared to the acidic wet flue gas environment of wet desulfurization, the flue gas dry desulfurization expansion joint faces different challenges:
- High temperature dust-containing scour: The flue gas flow rate in the dry desulfurization tower is high, which carries a large amount of desulfurizer particles and circulating ash, which causes continuous scour wear on the inner wall of the expansion joint
- Frequent temperature fluctuation: When the boiler load changes, the flue gas temperature may fluctuate by tens of degrees in a short time, and the expansion joint needs to bear alternating thermal stress
- Alkaline environmental corrosion: Desulfurizer dust is alkaline, although the corrosiveness is weaker than acidic medium, but long-term contact will still have certain influence on metal components
- Negative pressure operation risk: Dry desulfurization system is usually operated under negative pressure, and the expansion joint needs good negative pressure sealing performance
Based on the above characteristics, the flue gas dry desulfurization expansion joint has its unique requirements in material selection, structure design and installation technology.
2. Main types of flue gas dry desulfurization expansion joints
1. Non-metallic fabric expansion joint
This is the most commonly used type in dry desulfurization systems and is made of multi-layer composite materials (silicone rubber cloth, fiberglass cloth, PTFE film, etc.)。 Its advantages are:
- Can simultaneously absorb axial, lateral and angular displacement
- The effect of vibration isolation and noise reduction is good, and the vibration transmitted by the fan is reduced
- Light weight, convenient installation and low maintenance cost
- Economical price, suitable for large diameter flue
For dry desulfurization, silicone rubber tape is recommended for the outer skin of non-metallic expansion joints. Because silicone rubber has excellent thermal stability, temperature resistance is better than other rubber types except fluorine rubber, and the price is moderate。 It is worth noting that the content of acidic substances such as SO₂ and SO₃ in dry desulfurization flue gas is low, SO the acid resistance requirements of the skin are relatively loose, and the silicone rubber composite skin can meet the requirements。
2. Metal expansion joint
Consists of stainless steel bellows, end pipe and guide tube. It is suitable for flue in high temperature section (such as the front section of desulfurization tower inlet), and the temperature resistance can reach above 600℃. However, in the dust-containing environment of dry desulfurization, metal bellows are susceptible to dust erosion and wear, and their ability to compensate multi-directional displacement is limited, so they are less used than non-metal types.
Selection suggestions: Non-metallic expansion joints are preferred for dry desulfurization system, which has higher comprehensive cost performance; Metal expansion joints are considered only in special sections where the flue gas temperature exceeds 400°C.
3. Selection of core parameters of flue gas dry desulfurization expansion joint
Correct selection of flue gas dry desulfurization expansion joint requires clarification of the following parameters:
| parameter | Description | Typical values of dry desulfurization |
|---|---|---|
| Design temperature | Maximum/Minimum Operating Temperature | 120-180℃, instantaneous ≤250℃ |
| Media composition | Flue gas, desulfurizer dust | CaO, Ca (OH) ₂ dust with low SO₂ concentration |
| Dust concentration | Inlet dust content | 30-50 g/Nm³ |
| Pressure range | System positive/negative pressure | Negative pressure-5000Pa to positive pressure 3000Pa |
| displacement amount | Three-way thermal displacement | Calculated from pipe layout |
| Cycle life | Expected number of fatigue | ≥1000 times |
Key points of selection:
- Skin material: silicone rubber composite skin is recommended for dry desulfurization, which has stable performance when the working temperature is ≤250℃
- Frame anti-corrosion: Although dry flue gas is less corrosive, there is still a risk of dew point corrosion, and the frame should be treated with anti-corrosion treatment
- Deflector design: Wear-resistant deflector must be set to prevent high-concentration dust from directly washing the skin and bellows
- Dust-proof structure: Dry desulfurization has high dust content, and dust-proof bags or baffles should be set inside the expansion joint to prevent skin swelling and bursting caused by dust accumulation in the cavity
Installation Specifications and Key Precautions
1. Installation direction confirmation
Before installation, check the flow direction arrow on the expansion joint to ensure that the direction of the guide tube is consistent with the direction of the medium flow。 This is the most common installation mistake – reverse orientation can cause smoke to wash directly through the skin, accelerating failure.
2. Welding protection
During welding operations, measures must be taken to prevent weld slag from splashing onto the surface of the non-metallic skin. At the same time, hard objects should be avoided to stab or scratch the skin layer。
3. Bolt tightening requirements
The skin of the non-metallic expansion joint is secured to the frame by a platen and bolts. Because the platen length can reach 4-6 meters, the distal bolt will loosen due to skin compression deformation after one-time tightening. Therefore, all bolts need to be tightened repeatedly during installation。 It is recommended to use an electric wrench and operate according to the principle of "diagonal tightening, fractional tightening".
4. Removal of transport tie rod
After installation, the auxiliary positioning tie rod for transport protection must be removed. If it is not removed, the expansion joint will lose its ability to compensate, and the thermal stress of the pipe will not be released, which may lead to equipment damage.
5. On-site assembly requirements
When the size of the expansion joint exceeds 4 meters, it is usually transported by parts and assembled on site. During on-site assembly, temporary fixed angle steel and plane support rods should be set to prevent deformation during transportation and installation。
V. Common faults and preventive measures
1. Powder accumulation in the cavity leads to swelling and
Dry desulfurization flue gas has high dust content. If there is no dust-proof structure inside the expansion joint, the dust will gradually accumulate in the expansion joint cavity. When system pressure fluctuates, accumulated dust may cause skin to burst。
Precautionary measures: choose expansion joints with dust-proof bags or two-way baffle structure; Regularly inspect and clean up dust accumulation.
2. Early leakage of skin
In order to save costs, some manufacturers use inferior silicone rubber sheets or reduce the number of skin layers, resulting in leakage after several months of operation。
Precautionary measures: Select qualified products that meet the standards, and the skin should be made of multi-layer composite materials (silicone rubber cloth + glass fiber cloth + polytetrafluoroethylene membrane) to ensure sealing and durability.
3. Screw corrosion and loosening
Acid water accumulates in the skin groove and penetrates into the fixing screw, causing corrosion and fracture。
Precautionary measures: Although dry desulfurization is weak in acid, it is still recommended to set a hydrophobic hole (≤ DN100) at the bottom of the expansion joint in the horizontal section to discharge the condensate in time； Check and tighten bolts regularly.
VI. Maintenance and overhaul points
- Regular inspection: check the skin surface every month for damage, aging and bulging; Check bolts for looseness or corrosion
- Fastening inspection: Full bolt tightening shall be carried out once in 1 month and once in 3 months after commissioning, and then quarterly inspection shall be carried out
- Temperature monitoring: Infrared temperature measurement, if the skin surface abnormally heats up (over 50℃), it indicates that the internal heat insulation layer may be damaged
- Replacement cycle: The service life of non-metallic expansion joint skin is generally 3-5 years under dry desulfurization conditions. When there is obvious aging brittleness or leakage, it should be replaced in time
Call to Action
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