In the flue gas treatment system of electric power, metallurgy, chemical industry and other industries, non-metallic expansion joints for high temperature flue gas are the key flexible compensation components connecting boilers, dust collectors, desulfurization towers and chimneys. Different from metal expansion joints, non-metal expansion joints are made of multi-layer composite fabric materials, which can isolate vibration and resist corrosion while absorbing three-way thermal displacement. However, in the face of high-temperature flue gas at 600℃ or even above 1000℃, how to correctly select the type and ensure long-term reliable operation is a realistic problem faced by every engineer and technician. This paper will systematically analyze the technical knowledge of non-metallic expansion joints for high temperature flue gas from structural principle, material selection to installation and application.

1. Structure and working principle of non-metallic expansion joint for high temperature flue gas

Non-metallic expansion energy saving for high temperature flue gas provides larger axial, angular and lateral displacements in a smaller size range, and can compensate multi-directional thermal displacements, which is greatly superior to metal compensators that can only compensate in a single way. Its core structure is composed of flexible compensation part (non-metal skin), connecting part at both ends (flange or pipe), heat insulation material and adjusting part (bolt pull rod).

Core composition hierarchy

Core advantages: Fiber fabric and insulation cotton themselves have the function of sound absorption and shock absorption, which can effectively reduce the noise and vibration of boilers, fans and other systems. At the same time, the main material is fiber fabric, and there is no reverse thrust transmission. The use of non-metallic expansion joints can simplify the design, avoid the use of large brackets, and save materials and labor.

2. Classification by temperature: Material selection determines service life

The core of the selection of non-metallic expansion joint for high temperature flue gas lies in the matching of temperature and material. Different combinations of skin materials need to be selected for different temperature intervals:

Key Material Properties:

• Ceramic fiber cloth: Temperature resistance ≥1260℃, low thermal conductivity, strong thermal shock resistance
• High silicon oxide cloth: Temperature resistance 900℃, silica protective layer is formed when exposed to high temperature
• Alloy foil layer: Inconel 601 is resistant to oxidation temperatures up to 1150℃, insulating corrosive media

According to the data of the National Technical Committee for Standardization of Boilers and Pressure Vessels, the medium temperature of domestic high-temperature non-metallic expansion joints can reach 1200℃, and the long-term working temperature of outer skin is ≤250℃, and it can reach 350℃ in a short time.

3. Core advantages of non-metallic expansion joints for high-temperature flue gas

1. Excellent temperature resistance

The non-metallic expansion joint for high temperature flue gas can withstand the instantaneous high temperature of 1200℃, while the metal compensator is usually ≤1000℃.

2. Thermal shock resistance and rapid cooling and heat resistance

The thermal expansion coefficient of fiber materials is extremely low, and it does not crack under quenching and sudden heat (such as starting and stopping furnace), which is an advantage that metal expansion joints are difficult to match.

3. Multi-directional compensation ability

It can absorb axial + lateral + angular displacements simultaneously, for example, the kiln thermal expansion offset can reach 200mm.

4. Corrosion resistance and sealing

The inorganic fiber layer is resistant to acid dew point corrosion (such as SO₃ in flue gas), and the fluorine glue layer is resistant to leakage. For the desulfurization flue gas environment, fluororubber coated glass fiber cloth can be selected as the sealing layer, which has excellent acid resistance.

5. No reverse thrust

The main material is fiber fabric, and the weak transmission can simplify the design of pipe support and save a lot of materials and labor.

Typical application scenarios and configuration schemes

Application Scenario 1: Flue of coal-fired boiler

Temperature: 350℃ ~650℃ (local superheating zone reaches 900℃)

Recommended Material Combinations:

• Inner layer: high silicon oxide cloth (sealing)
• Middle layer: ceramic fiber cloth +50mm ceramic cotton (heat insulation)
• Outer layer: stainless steel wire mesh (anti-scour)

Key design: Add a guide tube (drainage tube) to prevent smoke from directly washing the skin.

Application scenario 2: fluidized bed boiler bore outlet

The non-metallic expansion joint can effectively compensate the thermal displacement of the outlet flue of the furnace, and the fiber fabric and heat insulation cotton have sound absorption and shock absorption effects, which can effectively reduce the noise and vibration of the boiler system.

Application scenario 3: Flue of desulfurization system

For low temperature and high humidity flue gas of wet desulfurization system, stronger anti-corrosion ability is needed. The typical configuration is: 2 layers of fluororubber + metal mesh +3 layers of thermal insulation cotton +3 layers of PTFE membrane.

In the upgrading project of No.3 furnace smoke duct expansion joint of Huadian Kuqa Power Generation Co., Ltd., the non-metallic expansion joint of the absorption tower adopts the facade flange structure, and is equipped with fluororubber layer and PTFE diaphragm to ensure long-term stable operation in corrosive smoke environment.

Application Scenario 4: Electrolyzer flue gas pipeline

For special working conditions such as electrolytic cells, non-metallic expansion joints for high-temperature flue gas should also have insulation performance, and the insulation value should be greater than 1 megohm.

V. Guide to selection and pit avoidance

1. Temperature Parameter Trap

It is necessary to distinguish between the medium temperature (skin contact surface) and the design temperature (including safety margin). For example: the actual flue gas is 900℃ → the design temperature should be 1100℃ grade skin!

2. Calculation of thermal expansion

Calculation formula:

• $\mathrm{Alpha}$Alpha: Line expansion coefficient of pipeline (12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel)
• $L$L: Length of pipe section between two fixed brackets (mm)
• $\mathrm{Delta}t$Deltat: Difference between operating temperature and installation temperature (℃)

Example: 10m carbon steel pipe, Δ T =500℃ → expansion ≈70mm

3. Protection of negative pressure conditions

When the negative pressure value of the system is> -5000Pa, the steel wire mesh layer must be lined to resist collapse. Aluminum silicate fiber wool is used as the insulation layer material, and rock wool material is not allowed to be replaced.

4. Vibration Scene Processing

The skin and the frame shall be bonded by integral hot press bonding (non-bolt crimping) to prevent fatigue cracking.

VI. Key points of installation and replacement

The installation quality of non-metallic expansion joints for high-temperature flue gas directly affects their service life. The following points should be followed during operation:

1. Preparation before installation

• Check and confirm that the outer diameter size, skin width and technical parameters of the non-metallic expansion joint meet the construction requirements
• Clean up the surrounding flammable objects and prepare fire extinguishing tools

2. Removal of old expansion joints

• Remove the bolt assembly fastening the skin with a wrench or gas cutting equipment
• Remove platen and old skin and store separately
• Reusable pressing plates shall be marked in installation order to ensure that there is no offset between the opening of the pressing plate and the opening of the flange

3. New skin installation

• The new skin is placed flat on the flange surface of the metal frame to prevent wrinkles
• The skin is tightened and then tightened with pressure plates and bolts
• Installation starts from the lower part, make sure that the inner insulation cotton is adequately installed, and spread it out from the lower part

4. Construction precautions

• Prevent the impact of heavy objects or piercing of the soft connecting ring belt by acute angles of metal
• After installation, adjust the nut in the positioning rod according to the compensation amount

sum up

The non-metallic expansion joint for high temperature flue gas has become an indispensable flexible connection component in high temperature flue gas pipeline system due to its unique advantages such as high temperature resistance, thermal shock resistance, multi-directional compensation and no reverse thrust. The correct selection needs to focus on the following points:

1. Temperature matching is the core: the skin material combination of the corresponding grade is selected according to the actual medium temperature, and the safety margin must be reserved in the high temperature section
2. Structural materials should be complete: The typical structure is composed of multi-layer composite materials such as fluororubber/silicone rubber sealing layer, glass fiber heat insulation layer, stainless steel wire mesh reinforcement layer, metal foil/ceramic fiber temperature resistant layer, etc
3. Displacement calculation should be accurate: Calculate the thermal displacement according to the formula Δ L = α × L × Δ T, and the rated compensation amount should be ≥1.2 times the calculated value during model selection
4. Negative pressure protection is indispensable: under negative pressure conditions, steel wire mesh layer must be lined, and aluminum silicate fiber cotton should be used for insulation layer
5. Installation quality is the key: the skin is flat and wrinkle-free, installed from bottom to top, and the bolts are fastened in stages

A correct selection and standard installation can ensure the long-term stable operation of non-metallic expansion joints for high-temperature flue gas under harsh working conditions. It is recommended to entrust a professional technical team to evaluate the scheme in the design stage.