In the flue gas system of coal-fired power station, the flue expansion joint of power plant is the key flexible compensation component connecting boiler, dust collector, desulfurization tower, induced draft fan and chimney. The flue gas working conditions of power plants are complex and changeable-from the ultra-high temperature flue gas at the boiler outlet (which can reach above 600℃) to the low temperature and high humidity corrosive flue gas behind the desulfurization tower, the requirements for expansion joints are very different in different sections. Once the flue expansion joint of the power plant is improperly selected or lack of maintenance, it will lead to air leakage and increased energy consumption at least, and unplanned shutdown of the unit at worst. This paper will systematically explain the professional technical knowledge of flue expansion joint in power plant from working condition division, material selection to operation and maintenance.
1. Working condition division of flue expansion joint in power plant
The selection of flue expansion joints in power plants must be "treated in zones" according to the flue gas characteristics of the flue section where they are located. The flue gas flow of thermal power units can be divided into the following typical sections:
1. High temperature original flue section (boiler outlet to dust collector)
The flue gas temperature in this section is high (300-600℃) and the dust content is large. The main challenges are high temperature resistance and erosion resistance. As shown in Figure 1, the expansion joint in this area is subject to severe thermal expansion and high-temperature flue gas erosion.
Selection suggestion: metal bellows expansion joint, bellows material choose 321 or 310S stainless steel, guide tube thickened wear-resistant design.
2. Low temperature wet flue section (desulfurization tower outlet to chimney)
After the ultra-low emission transformation, the exhaust temperature of some units after desulfurization has been as low as 45℃。 The flue gas in this section is completely saturated, the pH value of the condensate is as low as 2-3, and the concentration of Cl⁻¹ can reach 20,000-60,000 ppm, which belongs to a strong corrosive environment, and requires extremely high anti-corrosion performance of the expansion joint.
Selection recommendation: preferentially choose fluororubber non-metallic expansion joint, or upgrade to pure titanium TA2 expansion joint to achieve "zero corrosion and zero leakage"。
3. Inlet and outlet section of induced draft fan
This section has large vibration and is sensitive to thrust, so the expansion joint should have both vibration isolation and low reaction force characteristics.
Selection suggestion: Non-metallic fabric expansion joint, using the damping characteristics of fiber fabric to effectively absorb fan vibration.
2. Main types of flue expansion joints in power plants
1. Rectangular fillet expansion joint (CE standard)
Rectangular rounded expansion joint is a flexible duct designed with reference to US CE standard and international general products。 It adopts rectangular cross-section structure with large circular arc transition at edge and corner, and can be made into single vector thermal displacement monomer or three-dimensional compensation expansion joint.
Core Strengths:
- The stress level in the corner section is reduced, and the risk of deformation dead angle and local high stress failure is eliminated
- Reduce the thrust of pipe system and improve the thermal compensation capability under the condition that the whole system is not instable
- Arc steel plates at corners are welded and strengthened to enhance structural stiffness
Through practical engineering verification, the structure design has been highly recognized by domestic design institutes and owners, and the selection rate and application scope in flue gas conveyance system are continuously expanded。
2. Non-metallic fabric expansion joint
Rectangular non-metallic expansion joint is the most widely used type of flue duct in power plants。 It adopts fiber fabric main material, has that characteristics of thermal expansion compensation, noise and vibration isolation, the working temperature covers-20 DEG C to 1000 DEG C, corrosion resistance and excellent sealing performance。
According to DL/T 5121-2020 "Technical Specifications for Design of Flue, Wind and Pulverized Coal Pipeline in Thermal Power Plant", the expansion joint of SCR flue gas system should be preferred for non-metallic expansion joint with inner baffle and high temperature resistance because it is necessary to bear axial, radial and angular displacement and effectively absorb vibration。
3. Pure titanium expansion joint
In view of the strong corrosive environment of desulfurization wet flue, Huadian Luohe Company innovatively adopts pure titanium TA2 expansion joint, which is only 2mm thick and designed as a split single corrugated structure。 After one year of continuous operation, the inspection shows that the inner and outer surfaces of titanium expansion joints are as clean as new, and the welds are intact, achieving "zero corrosion and zero leakage"。
The price of titanium has dropped from hundreds of thousands of yuan/ton to more than 100,000 yuan/ton, which is basically the same as that of nickel-based stainless steel. Because of its low density and material saving of more than 40%, the comprehensive cost is the same as that of non-metallic expansion joints。
III. Installation specification and anti-corrosion design
1. Pre-bias installation of high-temperature flue
For high-temperature flue such as SCR flue gas system, due to the high flue gas temperature, the lateral displacement and horizontal displacement of the flue are large, and the metal expansion joint generally cannot meet the requirements of safe use. Therefore, non-metal expansion joint is used to compensate the thermal displacement。
In order to ensure the uniformity of flue gas flow field, the expansion joint should be 100% pre-biased during installation, that is, the flue on both sides of the expansion joint should be misaligned during installation, and the flue axes on both sides should be on a straight line during operation。
2. Anti-corrosion points of non-metallic expansion joints
In the application of traditional non-metallic expansion joints in desulfurization wet flue, there is a combination structure of "flange-scale-strip-skin-pressing plate". When the pressing plate bolts are installed, the glass flakes at the end face of the flange will be compressed and corroded channels will be formed。
Improvements:
- Transition of titanium-steel composite plate: welding steel plate on flue side and titanium plate on expansion joint side to realize the welding principle of "steel to steel and titanium to titanium"
- 100% coloring inspection of weld seam
- 2mm glass flake +2mmFRP reinforcement and anticorrosion at the interface position
3. Stent configuration requirements
Both ends of the expansion joint must be reasonably provided with guiding and fixing supports. According to DL/T 5121-2020 and other specifications, after the expansion joint is installed, all the transportation protective rods of the original belt should be relaxed, and the relaxation distance should be greater than the designed maximum compensation amount of the compensator.
Operation and maintenance and typical faults
1. Regular inspection and fastening
The skin of the non-metallic expansion joint is fixed by a platen bolt. Due to the large length of the platen (4-6 meters), the distal bolt will loosen due to the compression deformation of the skin after one-time tightening. It needs to be tightened once in one month and three months after it is put into operation, and then inspected every quarter.
2. Typical Failure Cases
Furnace shutdown caused by cracking of expansion joint drum: In a power plant, due to poor manufacturing quality of air duct expansion joint and construction error of fixed bracket (which caused the expansion joint to change from design pressure to tension), the expansion joint drum opened, the primary air pressure decreased, and the boiler was shut down for fire extinguishing。 Precautionary measures include: comprehensive inspection of air duct expansion joint, maintaining normal wind pressure during operation, and decisive manual emergency shutdown of boiler when extinguishing fire。
Skin damage and smoke leakage lead to shutdown: The skin of horizontal flue expansion joint of a gas turbine power plant is designed as a single layer, which is weathered and perforated after running for a period of time, and the perforation is enlarged by high temperature flue gas, which finally leads to the disassembly and shutdown of the unit。 Lesson: The skin should adopt multi-layer composite structure, and the single-layer design has poor force capacity and is easy to age.
3. Replacement cycle management
The replacement cycle of flue expansion joints in power plants varies depending on the working conditions. The expansion joint at the outlet of induced draft fan and other parts is eroded and washed by high temperature smoke for a long time, which will lead to smoke leakage and serious corrosion, so it needs to be replaced regularly。
Replacement points: shutdown isolation, standardized dismantling of old parts, precise positioning and installation of new expansion joints, sealing performance testing。
V. Suggestions for selection decision-making
The selection of flue expansion joints in power plants should follow the following principles:
sum up
The flue expansion joint of power plant is the key component to ensure the safe operation of flue gas system of thermal power unit. The correct selection must be "treated in zones"-high-temperature resistant metal expansion joints are selected in the high-temperature section, and fluororubber non-metal or pure titanium expansion joints are arranged in the wet flue section. Rectangular fillet expansion joints reduce the risk of corner stress by virtue of large circular arc transition structure, and have become the preferred scheme for smoke ducts in power plants。 Attention should be paid to the 100% pre-bias setting of high-temperature flue during installationAnd anti-corrosion detail control. For operation and maintenance, a periodic inspection, bolt tightening and replacement cycle management system should be established to avoid unplanned shutdown events caused by expansion joint failure from the source。