In wet flue gas desulfurization system, the FGD flue expansion joint is the core flexible component connecting the absorption tower with the inlet and outlet flues. The operating environment of FGD system is extremely harsh-the original flue gas inlet temperature is high, the dust content is large, the corrosion is strong, the net flue gas outlet temperature is below the acid dew point, and the condensate pH value is as low as 2-3. Once the FGD flue expansion joint fails, it will lead to flue gas leakage and aggravated equipment corrosion, and in the worst case, it will lead to environmental protection exceeding the standard or even unplanned shutdown of the unit. This paper will systematically explain the professional technical knowledge of FGD flue expansion joint from the characteristics of working conditions, key points of type selection to maintenance management.
1. Special working condition requirements of FGD flue expansion joint
The wet desulfurization environment in which the FGD flue expansion joint is located has the following notable characteristics:
1. Strong corrosive environment
The flue gas contains acidic components such as SO₂, SO₃, HCl and HF, which is sprayed and washed by the desulfurization tower to form strong acidic condensate, and the pH value can be as low as 2-3. Fluoropolymer fiber felt has extremely high resistance to high concentration acidic condensate in flue of power plant desulfurization process。
2. Temperature alternating
The original flue gas inlet temperature is about 120-180℃, and the net flue gas outlet temperature drops to 45-55℃. The drastic temperature change in the start-stop process puts forward high requirements for the thermal fatigue performance of the expansion joint.
3. High dust content and scour
The flue gas at the inlet of FGD system contains a large amount of dust, which causes continuous erosion wear on the inner wall of the expansion joint.
4. Negative pressure operation risks
The clean flue is usually in a negative pressure state, and the expansion joint should have good sealing performance to prevent external air inhalation from affecting the oxygen control of the system.
2. Types and Selection Points of FGD Flue Expansion Joints
1. Non-metallic expansion joint (preferred)
FGD flue expansion joint is the preferred non-metallic fabric expansion joint in desulfurization wet flue. Non-metallic expansion joints are composed of multilayer composite materials and have the following advantages:
- Strong three-way compensation ability: can absorb axial, lateral and angular displacements simultaneously
- Excellent corrosion resistance: The special combination of fluorine-based hydrocarbon polymers and reinforcements imparts extremely high resistance to corrosive substances at high temperatures, making them ideal for most operating conditions in flue gas desulfurization systems (FGDs)
- No reverse thrust: The main material is fiber fabric, which is friendly to equipment interface
- Vibration isolation and noise reduction: effectively reduce system vibration
Temperature Range: The Garlock Model 8400-HT non-metallic expansion joint can accommodate operating temperatures as low as-75°F/ -60°C and up to 2200°F/1200°C。
2. Rubber expansion joint
In the slurry circulation line of FGD system, rubber expansion joint is a more suitable choice. The rubber expansion joint is made of natural rubber or synthetic rubber and fabric reinforcing layer, and has the following characteristics:
- Abrasion resistance: In high solids content media such as limestone slurry, polyurethane materials have 2-3 times higher wear resistance than natural rubber on the Pico wear index
- Corrosion resistance: EPDM lining with polyester cord reinforcement can withstand strong corrosive slurry with pH value of 1-14
- Good vibration reduction effect: The optimized corrugated structure can reduce the vibration transmission rate of fan by 90%
The study shows that the medium composition, operating temperature and system pressure should be considered in the selection of expansion joints for desulfurization, and the installation should be kept in a natural state, and distortion is strictly prohibited。
3. Limitations of metal expansion joints
For metal expansion joints, almost all of the expansion joints used in the FGD flue downstream of the dust collector are fabricated with reinforced fluoropolymer fiber mats with a thickness of about 4.8 mm, instead of conventional metal bellows。 This is because metal expansion joints face a serious risk of corrosion in wet desulfurization environments.
3. Key design parameters of FGD flue expansion joint
According to the requirements of the technical specification of FGD engineering, the design of FGD flue expansion joint shall meet the following requirements：
| Design elements | Technical Requirements |
|---|---|
| Displacement compensation | Absorbs axial and radial displacement of all connected equipment and flue under all operating and accident conditions |
| Sealability | Can withstand the maximum design positive/negative pressure of the system plus the residual pressure of 1000Pa, and is designed without leakage |
| Anti-corrosion requirements | Anti-corrosion shall be considered for the expansion joint on the cryogenic flue, and anti-corrosion treatment shall be performed on the steel structure part connected to the flue |
| Thermal insulation design | The flue expansion joint shall be insulated, and the manufacturer shall provide the insulation design drawing |
| Drainage system | Expansion joints exposed to wet flue gas and located in the horizontal flue section shall be drained through the expansion joint frame with a drain hole of at least DN150 located on the centerline of the horizontal flue section. Drainage fittings are made of FRP or nickel-based alloy steel |
Special reminder: Fluoropolymer fiber mat has high resistance to high concentration acidic condensate in flue of power plant desulfurization process, but it is easy to fail in alkaline environment with calcium hydroxide。 The media properties must be clearly defined when selecting the type.
4. Installation and replacement of FGD flue expansion joint
1. Connection method requirements
The installation and connection of the FGD flue expansion joint are clearly specified:
- The expansion joint on the flue is connected by welding
- The expansion joint connected with the absorption tower is connected by a flange on the equipment side, and the arrangement can ensure that the expansion joint can be replaced
This design takes into account that the absorption tower side needs more frequent maintenance and overhaul, and the flange connection is easy to disassemble and replace.
2. Expansion joint replacement points
The expansion joint absorbs axial and radial displacement of all connected equipment and flue under all operating and accident conditions。 Note when replacing a new expansion joint:
- Check that the model specifications are consistent with the design
- Verify that the flow direction mark is consistent with the flue gas flow direction
- The bolts are tightened in stages and re-tightened periodically after operation
5. Common faults and prevention of FGD flue expansion joint
1. Expansion joint rupture case
Within 8 months after the commissioning of the desulfurization units of two 660MW units in a power plant, a large displacement occurred from GGH to the original flue of the absorption tower, which cracked the expansion joint at the entrance of the absorption tower, resulting in flue gas leakage。 This case shows that the influence of the overall thermal displacement of the flue must be fully considered when designing the FGD flue expansion joint.
2. Factors affecting service life
The service life of expansion joints of fluoropolymer fiber felt is mainly affected by the following factors：
- Excessive expansion, especially excessive twisting and bending
- Airflow swings generated immediately downstream of the shutter or baffle
- Water-slurry fly ash or scale accumulated at the bottom of the expansion joint
- Smoke leakage at the gasket compacting the fiber cloth due to the presence of torque
3. Routine inspection and maintenance
According to FGD System Operation and Maintenance Specification：
- Flue appearance is intact, heat preservation is intact, and the system is sealed
- There is no smoke or air leakage in all departments, especially in manhole doors and sampling points
- The flue expansion joint is not stressed or cracked
With the advancement of ultra-low emission transformation, a new generation of intelligent expansion joints with leakage monitoring function has begun to be applied to allow real-time warning through humidity sensors。 It is recommended to carry out professional testing every maintenance cycle to ensure that the desulfurization system continues to meet the standard operation.
4. Suggestions for selecting materials
In FGD systems, the two most important factors when choosing expansion joint materials are wear resistance and temperature resistance：
| Operating condition | Recommended Material | Description |
|---|---|---|
| High solids content slurry | Polyurethane | Pico wear index is 2-3 times higher than natural rubber |
| High temperature flue gas (≤300°F) | HNBR | Temperature resistance is comparable to EPDM and abrasion resistance is about 50% higher |
| High temperature sulfur-containing flue gas (1200℃) | Fluorine-based hydrocarbon polymer + ceramic fiber | Garlock 8400-HT Specialty Products |
| General wet smoke | Fluororubber/EPDM | Excellent acid resistance, service life up to 6 years |
VI. SUMMARY
FGD flue expansion joint is a key component in wet flue gas desulfurization system to ensure the safety of pipeline connection, absorb heat displacement and isolate vibration. Correct type selection, standard installation and regular maintenance are important to ensure that Three core elements for its long-term reliable operation:
| Core elements | gist |
|---|---|
| Selection principle | The non-metallic expansion joint (fluorine-based hydrocarbon polymer/ceramic fiber) is selected for wet flue, and the rubber expansion joint (EPDM/polyurethane) is selected for slurry pipeline |
| Design specification | The horizontal flue section shall be provided with DN150 drainage hole (FRP/Nickel-based alloy), and the connection side with the absorption tower shall adopt flange replaceable structure |
| Installation Requirements | The flow direction marks are consistent and kept in a natural state. Twisting is strictly prohibited to prevent welding splash from damaging the skin |
| Maintenance management | Regularly check the stress and tensile crack of the expansion joint, dredge the drainage hole in time, and monitor the surface temperature |
| Life expectancy | Non-metallic expansion joints have a normal service life of up to 6 years in FGD systems |
The selection of FGD flue expansion joint with reasonable design and standard selection can solve the stubborn problem of "running, dripping and leaking" in the desulfurization system from the root cause, and ensure the safe and stable operation of the desulfurization system for a long period. It is recommended that a professional organization should be entrusted to conduct working condition analysis and special selection of expansion joints in the design stage, and the acceptance should be strictly according to the specifications.