In wet flue gas desulfurization system, the expansion joint of desulfurization flue is the core flexible component connecting the absorption tower with the inlet and outlet flue. After desulfurization, the flue gas temperature drops to 45-55℃, which is completely saturated. The pH value of the condensate is as low as 2-3, and the concentration of Cl⁻¹ is as high as 20,000-60000ppm, which causes extremely serious corrosion to the expansion joint. Traditional non-metallic expansion joints often have perforation leakage in one to two years in this environment, which becomes the hardest hit area of desulfurization system. This paper will systematically explain the professional technical knowledge of desulfurization flue expansion joint from corrosion mechanism, material selection to zero leakage transformation.
1. Working condition characteristics and corrosion mechanism of expansion joint of desulfurization flue
1. Harsh operating environment
The expansion joint of the desulfurization flue is mainly installed at the original flue gas inlet and the net flue gas outlet of the absorption tower. The inlet temperature of the original flue gas is about 120-180 DEG C, and it contains high concentration of SO₂ and dust; The net flue gas outlet temperature drops to 45-55℃ and is in a completely saturated state. Wet saturated flue gas will produce condensation and condensation on the surface of flue, and the condensate is highly corrosive.
According to the actual operating data of coal-fired power plants, the corrosive environment in the desulfurization flue is extremely harsh:
- Cl-concentration: up to 20000-60000ppm (inlet)
- Condensate pH: as low as 2-4
- Temperature: 45-80℃ (below acid dew point)
2. Root cause analysis of corrosion and leakage
The most common failure mode of desulfurization flue expansion joints is acid permeation leakage. The traditional non-metallic expansion joint consists of a skeleton and a skin. When the skin is installed, an annular groove will be formed naturally, and a large amount of acid water will be accumulated 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 skin, resulting in a serious "small waterfall" phenomenon.
In addition, there is a special phenomenon of "starting leakage" in the practical application of the expansion joint of desulfurization flue: the expansion joint will be displaced and deformed after heating, and the bolts need to be tightened repeatedly. However, every tightening will compress the glass flakes at the end face of the flange, resulting in corrosion of the channel, forming a vicious circle of "the tighter the leakage, the tighter the leakage".
2. Innovation and transformation scheme of expansion joint of desulfurization flue
1. Fluororubber non-metallic expansion joints: an economical option
For the working conditions with moderate temperature (
Structure configuration:
| hierarchy | Materials | function |
|---|---|---|
| outer layer | Fluorine rubber (FKM) | Acid and temperature resistant |
| Reinforcement layer | Wire-clamped fiberglass cloth | Strength enhancement |
| Sealing layer | Tetrafluorine membrane + fluorine glue | Dielectric isolation |
| Thermal insulation | Aluminum silicate insulation cotton | Lowering the external surface temperature |
| protective layer | 304 stainless steel wire mesh | Scour-proof, supportive |
2. Groove filling technology: solving leaks from the root cause
For already installed expansion joints, the groove filling technique can be used for retrofitting. In this technology, high-elasticity glue is applied to both sides of the groove of the expansion joint, filled with high-temperature and corrosion-resistant polyurethane filler (compression ratio 7:1), compacted in layers, and the uppermost layer is flattened with high-elasticity acid-resistant glue. This scheme makes the acid water in the flue not contact the skin of the expansion joint, and prevents leakage from the root source.
3. Drainage design of expansion joint of desulfurization flue
1. Installation of drainage hole
Horizontally mounted desulfurization flue expansion joints must be provided with drainage holes at the lowest point of the frame. Technical requirements for drainage holes include:
- Minimum aperture: DN50-DN100
- Material: FRP (FRP) or nickel-based alloy steel (acid corrosion resistant)
- 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
2. Automatic drainage device
The traditional drainage method relies on manual operation, and there are problems such as untimely operation and negative pressure leakage caused by normally open valve. Patent CN1804465A discloses an adaptive drainage device: accumulated water enters a drainage pipe through a low-level discharge port, a float in an automatic drainage valve rises and falls with a liquid level, automatically starts drainage when the liquid level reaches a set value, and automatically closes after drainage is completed to prevent smoke leakage.
4. Key points of installation and maintenance
1. Installation specifications
The installation of the expansion joint of the desulfurization flue should follow the following points:
- Installation direction confirmation: the flow direction arrow is consistent with the direction of the medium, and the small end of the guide tube faces the incoming flow
- Bolt tightening: adopt the method of "diagonal tightening 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 splashing welding slag
- Drain pipe connection: Drain pipe leads to desulfurization tower trench or collection system
2. Operation and maintenance
| Check Items | Frequency | Contents |
|---|---|---|
| Appearance inspection | Weekly | Whether there is water leakage, air leakage, bulge, loose bolt |
| Drainage operation | per shift | Open the drain valve for 1-2 minutes and observe the properties of the discharged liquid |
| bolt re-tightening | Quarterly | Full tightening of platen bolts |
| Drainage hole dredging | Monthly | Clean up the dust net to prevent clogging |
| Replacement cycle | 3-5 years | Replace non-metallic skin when it reaches aging standard |
3. Diagnostic value of draining fluids
The internal state of the expansion joint can be judged by observing the liquid properties during drainage:
- Clear, colorless: normal condensate
- Yellow or reddish brown: Frame starts to corrode, anti-corrosion coating needs to be checked
- Cloudy or particle-containing: Damaged insulation, replace during scheduled maintenance
- Abnormal increase of discharge volume: poor upstream flue insulation, check flue insulation
V. Suggestions for selection decision-making
The selection of desulfurization flue expansion joint should be determined according to specific working conditions:
| Operating condition | Recommended Protocol | Life Expectancy |
|---|---|---|
| Original flue gas inlet (120-180℃, dusty) | Non-metallic expansion joint of fluorine rubber + dust bag | 3-5 years |
| Clean flue gas outlet (45-55℃, strong acid) | Pure titanium TA2 expansion joint | Over 10 years |
| Renovation of existing projects | Groove filling technology + fluororubber skin | 3-5 years |
| Economical choice | Fluororubber non-metallic expansion joint + drainage hole | 3-5 years |
sum up
Desulfurization flue expansion joint is the key equipment to resist acid corrosion in wet desulfurization system. The leakage of traditional non-metallic expansion joints caused by water accumulation in grooves is an "old problem" in the industry. This paper summarizes three mature technology paths:
- Fluororubber non-metallic expansion joint + groove filling: By filling grooves and upgrading skin material, acid penetration is prevented, suitable for economical needs
- Intelligent drainage system: Automatic drainage valve realizes "automatic drainage with water accumulation and automatic closing without water accumulation". With anti-corrosion skin and regular drainage, it can effectively prolong the life
Regardless of the scheme, horizontally mounted expansion joints must be provided with drainage holes (DN50-100, FRP/Nickel-based alloy), drainage per shift and quarterly tightening of bolts are essential maintenance measures. A desulfurization flue expansion joint with reasonable design and suitable material can fundamentally solve the vicious circle of "just after replacement and then leakage" and ensure the long-term stable operation of desulfurization system.