In the flue gas treatment system of coal-fired power plant, the non-metallic expansion joint of the flue system of power plant is the key flexible compensation component connecting boiler, desulfurization tower, dust collector and chimney. Different from metal expansion joints, non-metal expansion joints are made of flexible materials such as fiber fabrics, which have unique advantages such as strong multi-directional compensation ability, no reverse thrust, vibration isolation and noise reduction, and corrosion resistance. With the advancement of ultra-low emission transformation, the exhaust temperature of some units after desulfurization has been as low as 45℃, and the corrosiveness of wet flue gas to flue system has been further strengthened, and the flue expansion joint has become the hardest hit area of corrosion and leakage. This paper will systematically explain the technical knowledge of non-metallic expansion joint in flue system of power plant from structural characteristics, key points of type selection to engineering transformation.
1. Structure and characteristics of non-metallic expansion joint in flue system of power plant
The core working body of the non-metallic expansion joint in the flue system of power plant is the non-metallic loop belt composed of fiber fabric composite materials. Its typical structure consists of non-metal skin, metal frame, heat insulation filler layer, flow guide tube, pressure plate and fasteners.
Core structure hierarchy
| Structural hierarchy | Materials | function |
|---|---|---|
| Dielectric contact layer (inner layer) | Fluorine rubber cloth/silicone rubber cloth | Direct contact with flue gas, high temperature and corrosion resistance |
| Sealing layer | PTFE membrane/PTFE cloth | Ensure airtightness and prevent media leakage |
| Reinforcement layer | Alkali-free fiberglass cloth/wire-clamped fiberglass cloth | Improve tensile strength and wear resistance |
| Thermal insulation | Aluminum silicate insulation cotton | Lower external surface temperature and reduce heat loss |
| Outer protective layer (outer layer) | Fluorine or silicone rubber coated cloth | Protect the external environment, weather-resistant and waterproof |
Unique advantages of non-metallic expansion joints
Compared to metal expansion joints, non-metallic expansion joints for power plant flue systems have the following significant advantages:
| peculiarity | Specific Description |
|---|---|
| Multidirectional compensation | It can compensate axial, transverse and angular displacements simultaneously, which is far better than the metal expansion joint which can only compensate by single type |
| Large amount of compensation | The compensation function is much larger than that of the metal expansion joint under the same shape and size condition |
| No reverse thrust | The main body is made of fiber fabric, which can simplify the design of pipe support by weak transmission |
| Noise and vibration isolation | Fiber fabric and thermal insulation cotton have sound absorption and vibration isolation functions to reduce system noise and vibration |
| Strong corrosion resistance | Fluorine rubber, silicone rubber and other materials have natural resistance to acid, alkali and smoke |
| Easy maintenance | If the metal structure is not damaged, only the non-metal skin part can be replaced |
| Lower cost | Under the same conditions, the price of the product is lower than that of the metal expansion joint |
Application Scenario and Selection of Flue System in Power Plant
The application range of non-metallic expansion joints for flue systems in power plants covers several key parts of power plants:
| Application Scenario | Specific location | Characteristics of working conditions | Type selection recommendations |
|---|---|---|---|
| Boiler smoke duct | Boiler outlet, air preheater inlet and outlet | High temperature 100-400℃, dusty | Medium temperature non-metallic expansion joint |
| Desulfurization raw flue | Absorption tower inlet flue | 120-180℃ with SO₂ corrosion | Corrosion-resistant, fluororubber skin |
| Clean flue for desulfurization | Absorption tower outlet to chimney | 45-55℃, strong acid condensate | High corrosion resistance, fluororubber + PTFE |
| Inlet and outlet of induced draft fan | Fan connecting duct | Large vibration, sensitive to thrust | Vibration isolation type non-metallic expansion joint |
| Dust collector inlet and outlet | Electrostatic precipitator/bag dust removal | High dust content | Wear-resistant type, with guide tube |
Range of Performance Parameters
Typical performance parameters of non-metallic expansion joints for flue systems in power plants according to industry product standards are as follows:
| parameter | scope |
|---|---|
| Operating temperature | Standard type ≤450℃, high temperature customized type 800-1100℃ |
| Bear Stress | ≤40000Pa |
| Three dimensional displacement compensation | Up to 30%-40% of effective length |
| Outer skin temperature | ≤250℃ for a long time, ≤50℃ after installation (with heat insulation filler) |
Calculation method of thermal expansion
According to DL/T 5121-2020 and other design regulations, the selection of non-metallic expansion joint in flue system of power plant needs to accurately calculate the thermal displacement of pipe. The calculation formula of thermal elongation is:
Δ L = α × L × Δ T
Among them:
- α: Line expansion coefficient of pipeline (carbon steel 12×10⁻⁶/℃, stainless steel 16×10⁻⁶/℃)
- L: length of pipe section between two fixed brackets (mm)
- Δ T: Difference between operating temperature and installation temperature (℃)
3. Corrosion Problem and Improvement Scheme of Expansion Joint in Wet Flue of Desulfurization
Non-metallic expansion joints of power plant flue systems face severe corrosion challenges in desulfurization wet flue applications. There are three main causes of corrosion in desulfurization wet flue: chemical corrosion caused by SO₂ and SO₃; Electrochemical corrosion caused by Cl⁻¹ and F⁻¹; Mechanical corrosion caused by fluid scouring.
Limitations of traditional schemes
After the wet flue expansion joint of some power plants was put into operation, several sets of skins and frames were replaced successively, and the frame pressure plate was upgraded to 316L material, but the water leakage problem was not effectively solved. The nickel-based metal expansion joints are mainly 316L, 2205 and 2507. The survey shows that the average life of 316L material expansion joint in the actual use of desulfurization wet flue is no more than two years; 2205, 2507 metal expansion joints have more good performance.
Upgrading scheme of fluororubber and composite materials
In view of the strong corrosive environment of desulfurization wet flue, higher grade composite materials should be selected for the skin material of non-metallic expansion joint of flue system of power plant:
- Fluororubber (FPM): Excellent acid and alkali resistance and high temperature resistance, suitable for sulfur-containing humid flue gas environment
- PTFE composite layer: PTFE membrane/PTFE cloth as sealing layer, providing additional chemically inert barrier
- Multi-layer reinforced structure: Add reinforcing layers such as steel wire glass fiber cloth to improve the overall strength and durability of the skin
- Dust-proof bags and two-way baffles: For the original flue with high dust content, dust-proof bags and two-way baffles are set to prevent dust accumulation
Key points of installation and structural improvement
- Drainage design: Horizontally mounted wet flue expansion joints must be provided with drainage holes (at least DN150) to avoid acidic condensate accumulation and corrosion
- Bolt tightening: The skin of the non-metallic expansion joint is made of soft fabric material, and the fixing bolt should be tightened repeatedly, once in 1 month and once in 3 months after operation
- Anti-corrosion coating: Heavy anti-corrosion coating (such as glass flake cement, thickness ≥2mm) is required for metal frames
- Groove filling: The groove formed by the skin and the pressure plate is filled to prevent acid water from accumulating and penetrating
IV. Installation specifications and maintenance points
The installation quality of non-metallic expansion joints in flue systems of power plants directly affects their service life and the following specifications shall be followed:
Installation deviation requirements
| Installation Project | Allowable deviation |
|---|---|
| Coaxiality | ±5mm |
| Torsion angle | ≤10° |
| Offset angle | ±0.1° |
| Installation length | ±5mm |
Key Installation Points
- Direction confirmation: The marking direction of the wear-resistant bushing must be consistent with the flow direction of the pipeline medium, so as to avoid the medium directly washing the fabric layer
- Bolt tightening: The flange bolts shall be tightened gradually and evenly under pressure, and the tightness of all bolts shall be as consistent as possible
- Welding protection: If there is welding work near the connection of the pipe, cover the rubber or fabric surface to prevent welding slag damage
- Treatment of transportation rod: After installation, the nut in the positioning rod shall be adjusted according to the compensation amount so that the expansion joint can expand and contract freely
Operation and maintenance
- Periodically inspect the loop for wear, aging or damage
- Check for good sealing performance
- Clean up dust and debris accumulated on the surface in time
- Small holes can be repaired with special glue, and the skin needs to be replaced if serious damage is found
- Full tightening of platen bolts quarterly
V. Summary
The non-metallic expansion joint of flue system of power plant is the key component to ensure the safe operation of flue gas system of thermal power unit. The core points can be summarized as follows:
| gist | Core Content |
|---|---|
| Structural cognition | The non-metallic expansion joint is composed of multi-layer composite structure such as skin, frame, heat insulation layer and guide tube |
| Core Strengths | Multi-directional compensation, no reverse thrust, noise and vibration isolation, corrosion resistance, easy maintenance |
| Selection principle | Select silicone rubber (≤200℃) /fluororubber (≤400℃) /ceramic fiber (high temperature) according to flue gas temperature, and give priority to anti-corrosion in wet flue |
| Corrosion Response | Fluororubber + PTFE composite, drainage hole design, groove filling, anti-corrosion coating |
| Installation Specifications | Consistent flow direction marking, multiple bolt tightening, protecting skin from weld slag damage |
| Criterion basis | JB/T 12235-2015, Installation deviation: Coaxiality ≤ ±5mm, Torsion angle ≤10° |
The non-metallic expansion joint of flue system of power plant with reasonable design and standard installation can operate stably for a long time under harsh working conditions such as high temperature and corrosion. For the strong corrosive environment of wet desulfurization system, it is recommended to give priority to fluororubber composite skin and perfect drainage and groove filling design to solve the leakage problem from the root cause.