Specialized in manufacturing compensators, expansion joints, baffle doors

In the industrial flue gas pipeline system, how to keep the flue expansion joint warm is the key technical problem to ensure the safe operation of equipment, reduce the heat loss and prevent personnel from burning. The expansion joint should not only absorb thermal displacement and isolate vibration, but also keep the external surface temperature within a safe range under high temperature conditions (usually requires ≤60℃). Improper insulation design can lead to increased heat loss and energy waste, or overtemperature aging of expansion joint, burning of outer skin and even safety accidents. This paper will systematically explain the professional technical knowledge of how to insulate flue expansion joint from insulation structure, material selection to construction steps.

1. Why does the flue expansion joint need to be insulated?

How to insulate the flue expansion joint First of all, we should understand the necessity of insulation. The thermal insulation of expansion joints is mainly due to the following four considerations:

1. Prevent over-temperature aging

The outer skin of the expansion joint is usually made of non-metallic composite materials (fluororubber, silicone rubber, ceramic fiber cloth, etc.), which will accelerate aging and brittleness when exposed to high temperature for a long time. According to industry experience, the problem of overtemperature of expansion joint not only affects its life, but also affects the operation safety of boiler。 Effective heat preservation can reduce the working temperature of the skin and prolong the service life.

2. Reduce heat loss

High temperature flue heat loss will reduce the system thermal efficiency and increase energy consumption. Standardized insulation can significantly reduce heat loss.

3. Anti-scald protection

According to safety specifications, the surface temperature of equipment accessible to personnel shall not exceed 60°C. Insulation of expansion joint is an important measure to ensure the safety of maintenance personnel.

4. Prevent embrittlement in winter

Under the low temperature environment in winter, non-metallic skin materials are easy to harden and brittle. Low temperature resistant modified materials such as low temperature fluororubber can be adapted to environment ≤ -30℃Cooperate with heat preservation measures to effectively prevent embrittlement.

Second, heat insulation structure design of flue expansion joint

The core of how to insulate the flue expansion joint lies in the reasonable design of the insulation structure. According to the latest patented technology, the insulation structure of the expansion joint mainly includes two parts: the inner insulation layer and the outer skin。

1. Inner insulation layer structure

The inner insulation layer is the core of expansion joint insulation, and the structural design of multi-layer insulation cotton + steel mesh barrier is usually adopted:

Key technical points:

• The adjacent insulation cotton layers are separated by steel wire mesh to prevent the insulation layer from settling
• The heat insulation cotton shall be wrapped and fixed with stainless steel wire mesh, and the heat insulation nails shall be distributed to be fixed smoothly
• The innermost layer is laid with ceramic fiber cloth and fine mesh steel wire mesh to enhance the overall strength

2. Thermal insulation extension of flange part

The metal frames (flanges) at both ends of the expansion joints also need to be insulated. The patent technology proposes that that inner side of each folding plate-shaped flange is fixed with an inner lining folding plate, the inner lining folding plate is filled with heat preservation cotton, and an expansion gap is left between the two inner lining folding plates。

The advantages of this design are:

• The insulation layer is continuous and uninterrupted to prevent heat from escaping from the flange
• The inclined opening of the inner lining folding plate is consistent with the flow direction of smoke gas, so as to reduce the flow of smoke gas into the gap
• The expansion gap meets the thermal displacement requirements

3. Guide for selection of insulation materials

The key link of how to preserve the flue expansion joint is the material selection. Select the appropriate insulation material according to the working condition temperature:

1. Main insulation material

2. Multilayer composite structure of outer skin

A typical outer skin structure (from the inside out) comprises：

3. Supplementary heat insulation in low temperature working conditions

For expansion joints running in winter, anti-embrittlement measures should be considered:

• Avoid using non-metallic skin materials with low molecular weight and easy freezing
• Use low temperature resistant modified materials such as low temperature fluorine rubber to adapt to the environment ≤ -30℃
• Add anti-brittle coating, such as silicon-based protective agent or thermal insulation film, to reduce temperature stress and moisture erosion

Four, flue expansion joint heat insulation construction steps

The construction of how to insulate the flue expansion joint should follow the standard process:

Step 1: Construction Preparation

1. Check that the outer diameter size, skin width and technical parameters of the expansion joint meet the requirements
2. Check the quantity and quality of insulation materials (insulation cotton, wire mesh, skin, bolt assembly)
3. Clean up the surrounding flammable objects and prepare fire extinguishing tools

Step 2: Installation of the inner insulation layer

How to insulate the flue expansion joint, the inner insulation layer is the most critical link:

1. Laying the second wire mesh: fixing the second wire mesh between the left flange and the right flange as the base frame of the insulation layer
2. Laying bottom layer protection: the steel wire mesh is concave to form the installation area. First, a layer of ceramic fiber cloth is laid, and then a layer of fine mesh steel wire mesh is laid
3. Laying insulation cotton layer by layer: Laying insulation cotton layer and first steel wire mesh layer layer by layer, adjacent insulation cotton layers are separated by steel wire mesh
4. Insulation cotton fixation: Fix flat with insulation nails, and insulation cotton should be wrapped with stainless steel wire mesh
5. Outer wrap: Ceramic fiber cloth is laid on the outer part of the outermost insulation cotton, and ceramic fiber cloth is sewn on the edge

Thickness requirements: The total thickness of insulation cotton is determined according to the design temperature. Generally, low-temperature flue (400 °C) at 200-250 mm.

Step 3: Outer skin laying

1. Laying sequence: one layer of steel wire mesh, two layers of ceramic fiber cloth, one layer of PTFE cloth, one layer of ceramic fiber cloth and one layer of PTFE membrane are laid sequentially from the inside to the outside
2. PTFE membrane welding: Weld the PTFE membrane to the previous layer of ceramic fiber cloth to ensure sealing
3. Hemming and stitching: Finally, the outer side is hemmed and stitched with ceramic fiber cloth

Step 4: Skin fixation

1. Flat skin: Flat skin on the flange surface of metal frame to prevent wrinkles
2. Install from bottom to top: Install from the bottom, make sure that the inner insulation cotton has been installed sufficiently, and spread out from the bottom point to the outside in sequence
3. Tightening and compressing: After the skin is tightened, it is compressed with pressure plates and bolts
4. Bolt tightening: All compression bolts are tightened with an electric wrench, and no missing screwing is strictly prohibited

V. Installation precautions

How to insulate the flue expansion joint? The following matters should be paid attention to in the construction:

1. Prevent dust accumulation and blockage

For the non-metallic expansion joint of horizontal flue, the design of anti-plugging structure should be considered to prevent ash from entering the expansion joint and affecting the heat insulation effect。

2. Interface waterproof

For the expansion joint skin of horizontal flue, the upper interface should be pressed down at the interface of the outermost skin to prevent water seepage at the interface。

3. Welding protection

If welding is required during thermal insulation construction, protective measures must be taken to prevent welding slag splashing from damaging the skin.

4. Drainage design

According to the technical requirements, the expansion joint in contact with wet flue gas and located in the horizontal flue section shall be provided with a drainage hole, the drainage hole shall be at least DN150, and the drainage fitting shall be made of FRP or nickel-based alloy steel material。 Drainage design should be carried out in coordination with heat insulation construction, and drainage channels should be reserved.

5. Insulation thickness verification

After the installation of the insulation layer, the surface temperature shall be measured to confirm that the external surface temperature is ≤60℃ (or the design requirement value).

VI. Maintenance and Inspection

After the heat insulation construction is completed, how to insulate the flue expansion joint needs to be included in daily maintenance:

1. Regular inspection: check the outer skin for damage, aging and cracking
2. Inspection of insulation layer: Local overheating indicates collapse or missing insulation cotton
3. Winter inspection: Check the skin for embrittlement cracks during winter operation, and repair it in time
4. Drainage inspection: Check whether the drainage hole is unobstructed to prevent the accumulation of acidic condensate [ci^tation:2]

sum up

How to keep the flue expansion joint warm is the key technology to ensure the long-term safe operation of the expansion joint. The core points can be summarized as follows:

A flue expansion joint with reasonable heat insulation design and in-place construction specification can effectively reduce external surface temperature, prevent over-temperature aging, reduce heat loss, and realize long-term safe and stable operation under harsh working conditions such as high temperature and corrosion.

In the engineering construction and maintenance of boiler flue gas system, the acceptance of boiler flue expansion joint is the last barrier to ensure the safe operation of equipment and eliminate hidden dangers in operation. After the expansion joint is installed, if the acceptance is not strict and the standard is unclear, it will lead to air and smoke leakage and energy consumption after operation, and in the worst case, it will lead to expansion joint failure, unplanned furnace shutdown or even safety accidents. So, what exactly should be checked for boiler flue expansion joint acceptance? What is the standard basis? This paper will systematically explain the professional and technical points of acceptance of boiler flue expansion joint from acceptance basis, inspection items to test methods.

1. Standard basis for acceptance of boiler flue expansion joint

The acceptance of boiler flue expansion joint shall strictly follow the following national standards and industry norms:

According to the provisions of Article 10.2.1 of GB 50517-2010, the expansion joint shall participate in the system test: the expansion joint of the self-restraining device shall participate in the system test; Externally constrained expansion joints shall participate in system tests; All expansion joints shall be subjected to system test during leakage test。

In the waste heat power generation project of cement factory, GB51005-2014 Code for Construction and Quality Acceptance of Waste Heat Power Generation Project of Cement Factory clearly lists "Installation of Expansion Joints of Smoke and Air Ducts" as an independent acceptance chapter, covering the installation quality inspection of expansion joints of smoke and air ducts。

2. Installation quality acceptance: full inspection from appearance to size

1. Appearance inspection

Appearance inspection for acceptance of boiler flue expansion joint shall include the following:

2. Dimensional inspection

The accuracy of the mounting dimensions directly affects the compensation ability of the expansion joint:

• Installation length: Check whether the distance from the flange surface to the flange surface of the expansion joint meets the design value, and the deviation shall not exceed ±5mm
• Coaxiality: The expansion joint shall be concentric with the pipe with a deviation ≤3mm
• Levelness: horizontally mounted expansion joint, level deviation ≤3‰
• Perpendicularity: Expansion joint installed vertically, perpendicularity deviation ≤3‰

3. Direction confirmation

This is the "one-vote veto item" in the acceptance of boiler flue expansion joint:

• Flow direction identification: The flow direction arrow on the expansion joint must coincide with the flue gas flow direction
• Direction of guide tube: The small end of guide tube must be facing the direction of flue gas flow
• Inner sleeve position: the end of the inner sleeve with weld, which should be located at the inflow end of the medium on the horizontal pipeline; It should be placed on the upper part of the vertical pipe

Special warning: A 410t/h boiler ignition air duct expansion joint fault case shows that the installation misalignment amount of 80mm is not reserved according to the design requirements during installation, and the expansion joint bellows is seriously deformed during operation, and the internal pouring material is damaged and fallen off, which finally leads to the expansion joint damage。

3. Inspection of transportation and limiting device

In the acceptance of boiler flue expansion joint, the treatment of transportation protection device is the most easily overlooked link.

1. Removal of transport tie rod

According to the provisions of Article 7.0.3 of GB 50753-2012, after the installation of the compensator, all the protective screws of the original belt shall be loosened, and the loosening distance shall be greater than the designed maximum compensation amount of the compensator.

Acceptance inspection points:

• Whether the transport protection tie rod of the metal expansion joint has been completely removed
• Whether the positioning limit pull rod is retained (not removed)
• Whether the release nut has been adjusted to the pre-bias position required by the design

2. Limit device adjustment

• Whether the limit tie rod nut has been adjusted in place according to the design drawing
• Whether the limit clearance meets the design requirements (generally about 1.2 times the axial compensation amount)
• Is the spring hanger reset (if any)

3. Temporary support removal

Transportation temporary supports added to large-diameter expansion joints when they leave the factory shall be completely removed after installation.

4. Sealability inspection and pressure test

1. Sealability Check

The tightness inspection of boiler flue expansion joint acceptance is the key link to verify the installation quality:

• Inspection method: Soap water application method or leak detector can be used
• Inspection site: Flange connection surface, weld, skin joint, pressure plate bolt
• Acceptance criteria: No bubble generation or no alarm from the detector is qualified

According to GB 50517-2010, all expansion joints shall participate in system test during leak test。

2. Pressure test precautions

When the expansion joint participates in the system pressure test, attention should be paid to:

• Confirm that the limit device of the expansion joint is set correctly before pressure test
• The expansion joint of the self-restraining device can directly participate in the system test
• The water used for hydraulic test shall be clean and non-corrosive, and the chloride ion content shall not exceed 25ppm

3. Airtightness test

For flue gas systems, air tightness tests should also be performed:

• The test pressure is 1.1 times the design pressure
• Hold time ≥30 minutes
• Pressure not reduced to qualified

V. Special acceptance items of non-metallic expansion joints

Acceptance of boiler flue expansion joints for non-metallic expansion joints requires additional attention to the following items:

1. Bolt tightening inspection

The skin of the non-metallic expansion joint is fixed by a platen bolt. Due to the large length of the platen (up to 4-6 meters), the distal bolt will loosen due to the compression deformation of the skin after one-time tightening.

Acceptance points:

• Check that all bolts have been tightened as per "diagonal tightening, fractional tightening"
• Use a hand hammer to gently tap the pressure plate, the sound should be crisp and consistent, without loosening
• Whether the bolt torque meets the design requirements (generally 50-80N·m)

2. Drain hole inspection

For horizontally installed boiler flue expansion joints (especially wet flue sections), drainage holes must be provided:

• Check Location: Whether the drain hole is located at the lowest point of the frame
• Pore size: ≥ DN50 (recommended DN50-DN100)
• Smoothness: Whether the drainage pipe is unobstructed and the dust net is intact
• Material: Whether drainage fittings are made of corrosion-resistant materials (FRP or stainless steel)

3. Skin lamination examination

The skin of the non-metallic expansion joint is a multi-layer composite structure, and the following shall be confirmed during acceptance:

• Whether the number of skin layers meets the design requirements
• Whether the layers are tightly attached and without delamination
• No wrinkles or scratches on the skin surface

VI. Acceptance of quality certification documents

The acceptance of boiler flue expansion joint should not only check the physical objects, but also check the documents. Complete quality certification documentation.

Common Acceptance Problems and Treatment

8. Work after acceptance

After the boiler flue expansion joint passes the acceptance, the following work shall be completed:

1. Establish equipment ledger: record the model, specification, installation date and acceptance person of the expansion joint
2. Photo archive: Take photos of the expansion joint after installation (including nameplate and flow direction identification)
3. Formulate maintenance plan: specify the first tightening time (1 month, 3 months after commissioning) and inspection cycle
4. Handover to Operation Department: Provide acceptance report and maintenance instructions

sum up

The acceptance of boiler flue expansion joint is the key link to ensure the safe operation of flue gas system. The standard acceptance shall follow the five-step method of "checking the appearance, nuclear size, checking the direction, removing the tie rod and testing the seal":

Same Ensure that the quality certification documents are complete, establish the equipment ledger, and formulate the maintenance plan. A standardized acceptance can prevent the early failure of the expansion joint caused by installation quality problems from the source, and ensure the long-term safe and stable operation of the boiler flue gas system.

In the design of industrial flue gas pipeline system, the reasonable arrangement of flue expansion joint directly determines whether the pipeline can operate safely and whether the expansion joint can play the normal compensation function. Improper arrangement can lead to premature failure of expansion joints, pipe deformation and even equipment damage. The expansion joint is not randomly installed in any position of the pipe. It needs to work together with the fixing bracket, guide bracket and limit bracket to form a complete compensation system. This article will systematically explain the technical key points of flue expansion joint arrangement from the arrangement principle, bracket configuration to typical examples.

1. Basic principles of flue expansion joint arrangement

The core objective of flue expansion joint arrangement is to absorb the axial and radial displacement of all connected equipment and flue with the most reasonable number and position of expansion joints, and at the same time control the thrust and torque of pipe to equipment within the safe range。

1. Principle of segmented compensation

The long straight pipeline is divided into several independent compensation pipe sections, and fixed brackets are arranged at both ends of each pipe section, and an expansion joint is arranged in the middle. Each expansion joint is only responsible for absorbing the amount of thermal elongation of this pipe section. The principle of dividing the length of the pipe section is: the upper limit of the compensation amount of each expansion joint determines the maximum length of the pipe section. Taking carbon steel pipe as an example, when the design temperature is 400℃, the thermal elongation of the pipe per 10 meters is about 45-50mm, and the length of the pipe section can be determined accordingly.

2. Principle of proximity to fixed bracket

The expansion joint should be arranged as close to the fixed bracket as possible. According to the engineering experience, when arranging the flue expansion joint, ensure that the distance from the expansion joint to the fixed bracket should be controlled within 4 times the pipe diameter (4D)。 The purpose of this requirement is to prevent transverse instability of the pipeline and to ensure that the expansion joint only bears axial displacement and does not bear additional bending moments.

3. Guide bracket matching principles

Both ends of the expansion joint must be provided with guide brackets, and the distance between the guide brackets and the expansion joint is strictly regulated. The first guide bracket shall be installed at a distance of 4D from the expansion joint, and the distance between the second guide bracket and the first guide bracket shall be 14D。 The function of the guide bracket is to ensure that the pipe expands and contracts along the axis direction and prevents lateral swing.

2. Arrangement requirements of fixed brackets

The fixing bracket is the most important component in the flue expansion joint arrangement. Its function is to limit pipe displacement and divide pipe sections into independent compensation units. The fixed bracket must be able to withstand the vector sum of the following loads:

The research published in Pressure Vessel, Issue 2, 2024 clearly points out that the selection and rational arrangement of pipeline supports are the key to ensuring the safe and effective service of expansion joints。 In high temperature flue gas pipeline, if the strength of the fixed bracket is insufficient, the blind plate force will lead to the failure of the bracket and the instability of the expansion joint.

Third, the spacing of the guide bracket and the golden combination

The arrangement spacing of the guide brackets directly affects the working state of the expansion joint. The correct flue expansion joint arrangement should follow the following "golden combination":

Guide bracket spacing formula

According to the industrial pipeline construction specifications, the bracket configuration of flue expansion joint arrangement shall follow the combination mode of "fixed bracket-expansion joint-first guide-second guide-sliding bracket":

1. Fixed bracket: end point of pipe section, bearing full load
2. Expansion joint: mounted against fixed bracket
3. First guide bracket: ≤4D from expansion joint
4. Second guide bracket: distance from first guide bracket =14D
5. Sliding brackets: The spacing is determined according to specification, allowing the pipe to slide freely

This "4D-14D" rule is derived from the engineering practice of diesel generator smoke exhaust pipe, and also applies to general industrial flue。

4. Difference between horizontal flue and vertical flue

1. Horizontal flue expansion joint arrangement

Horizontal flues are the most common scenario for flue expansion joint arrangements. In addition to following the above stent rules, note that:

• Drainage slope: When installing horizontal pipelines, the drainage slope should be ≥3‰ to ensure that the condensate can flow to the drainage point
• Drainage hole setting: Expansion joint contacting wet flue gas and located in horizontal flue section, drainage hole is at least DN150, located on the center line of horizontal flue section
• Material of drainage fittings: Drainage pipes should be made of corrosion-resistant materials such as FRP or nickel-based alloy steel

2. Vertical flue expansion joint arrangement

The expansion joint arrangement of a vertical flue is more complicated than that of a horizontal pipe because gravity factors can significantly affect the pipe stress:

• Guide brackets are required to prevent pipeline instability
• Load-bearing device: Consider the self-weight of the pipeline bearing by the expansion joint, and set a spring hanger if necessary
• Direction of inner sleeve: One end of the bellows expansion joint with welding seam, which should be placed on the upper part of the vertical pipe

3. Special arrangement of high-temperature flue

For high temperature flue such as SCR flue gas system, there are special requirements for flue expansion joint arrangement. According to DL/T 5121-2020 "Technical Regulations for Design of Flue, Wind and Pulverized Coal Pipeline in Thermal Power Plant", due to the high flue gas temperature, the lateral displacement and horizontal displacement of the flue are large, and the flue is not long enough, the metal expansion joint generally cannot meet the requirements of safe use, so the non-metal expansion joint is preferred。

Pre-offset setting: In order to ensure the uniformity of flue gas flow field, the expansion joint should be 100% pre-offset during installation, that is, the flue on both sides of the expansion joint is misaligned during installation, and the axis of the flue on both sides is on a straight line during operation。

V. Typical Arrangement Examples and Bracket Spacing Reference

Example 1: Smoke Exhaust Pipe for Diesel Generator

A 30-meter-long metal smoke pipe has a temperature rise of 520℃ and an expansion amount of 176mm. The correct arrangement scheme of flue expansion joint is as follows: a fixed bracket is arranged near the equipment side, the expansion joint is installed close to the fixed bracket, the first guide bracket is located at 4D, the second guide bracket is located at 14D, and the other positions are arranged with sliding brackets according to specifications。

Example 2: SCR Denitrification System Flue

The flue at the inlet and outlet of SCR reactor is subjected to three-way displacement, and non-metallic expansion joint is adopted. Special attention should be paid to the arrangement: fixing brackets with sufficient strength are set on both sides of the expansion joint; The spacing of guide brackets is according to the 4D principle; 100% pre-bias at installation。

Example 3: FRP Flue Gas Pipe

According to SH/T 3522-2017, the expansion joint arrangement of FRP flue gas pipe also follows the principle of distance from fixed bracket ≤4D. Set drainage slope ≥3‰ when installing horizontal pipeline。

6. Key control points in construction and installation

1. Pre-stretching/Pre-compression

Before installation, the expansion joint shall be pre-stretched or compressed according to the design documents, and the force shall be uniform。 The purpose of pre-stretching is to give the bellows an initial deformation in the cold state, so that the bellows are in optimal working condition in the hot state.

2. Temporary restraint device

When installing the bellows expansion joint, a temporary restraining device shall be installed, which shall be removed after the pipeline is installed and fixed。 After installation, the transport protection tie rod must be removed immediately (the positioning tie rod must not be removed) so that the expansion joint can be expanded and contracted freely.

3. Coaxiality Control

During installation, the bellows expansion joint shall be concentric with the pipe and shall not be skewed, and circumferential torsion of the expansion joint caused by installation shall be avoided。 The installation error of the pipeline shall not be adjusted by means of making the pipeline deform or compensation for the expansion joint.

4. Welding protection

During installation, welding slag is not allowed to splash on the surface of the bellows, temporary supports are not allowed to be welded on the wave nodes, and hoisting fixtures such as wire ropes are not allowed to be directly tied on the wave nodes, so as to avoid mechanical damage to the wave nodes。

VII. Common mistakes and correction of layout design

sum up

The arrangement of flue expansion joint is a systematic project. The correct arrangement should be closely integrated with the bracket system, and follow the cooperative arrangement principle of "fixed bracket-expansion joint-guide bracket":

• Section determination of fixed bracket: the pipeline is divided into several independent compensation pipe sections, and the fixed bracket shall bear blind plate force
• The expansion joint is close to the fixing frame: the distance is ≤4D, and the high-temperature flue should be 100% pre-biased
• The guide brackets are sequentially arranged: a first guide distance expansion joint 4D, a second guide distance first 14D
• Drainage design should not be neglected: DN150 drainage hole for horizontal wet flue expansion joint, material FRP or nickel-based alloy steel
• Welding specifications must be followed: Splash of welding slag is strictly prohibited, and the transport tie rod is removed after installation

The selection and reasonable arrangement of the support is the key to ensure the safety and effective service of the expansion joint。 A reasonably designed expansion joint arrangement scheme can avoid the failure problem from the source and ensure the long-term safe operation of the flue system.

In industrial flue gas treatment systems, flue expansion joints (also customarily called compensators or expansion joints) are the key flexible compensating components connecting boilers, dust collectors, desulfurization towers and chimneys。 Because the flue gas pipeline is under the bad working conditions of high temperature, corrosion and dust for a long time, if the stress caused by thermal expansion and contraction is not absorbed, it will lead to pipeline deformation and weld cracking, and in the worst case, it will lead to equipment damage or even safety accidents. This paper will systematically explain the core technical knowledge of flue expansion joint from structural principle, key points of type selection to installation and maintenance.

1. What is the flue expansion joint?

Flue expansion joint is a flexible connection device that takes advantage of the effective deformation of elastic elements to absorb the displacement (axial, transverse, angular) of the pipe due to temperature change and mechanical vibration。 Its core functions include: compensating the dimensional changes caused by thermal expansion and contraction, isolating equipment vibration, reducing the thrust of pipes to equipment, and at the same time facilitating the installation and disassembly of pipes.

According to the difference of structure and material, flue expansion joints are mainly divided into two categories: metal expansion joints and non-metal expansion joints.

1. Metal expansion joint

The metal expansion joint consists of stainless steel bellows, end pipe and guide tube. Its advantage are strong pressure bearing capacity, high temperature resistance (up to 600℃ or above), and it is suitable for flue in high temperature section, such as the area from the boiler outlet to the economizer and air preheater。 Common types of metal expansion joints include:

2. Non-metallic expansion joint

Non-metallic expansion joints are made of multi-layer composite materials (fluororubber/silicone rubber coated fiberglass cloth, PTFE membrane, fiberglass cloth, etc.)。 Its advantages are that it can simultaneously absorb three-way displacement, has good vibration isolation and noise reduction effect, excellent corrosion resistance, no reverse thrust transmission, and can simplify the design of pipeline support。 It is suitable for low temperature sulfur-containing flue gas environment such as wet desulfurization system, gas turbine exhaust passage, dust collector inlet and outlet。

For the smoke duct with rectangular section, the rectangular fillet expansion joint is a better choice. It adopts a rectangular cross-section structure with large circular arc transition at the edge corners, which can effectively reduce the stress level at the edge corners, eradicate the risk of deformation dead corners and local high stress damage, and prolong the service life of products。

Second, the main points of the selection of flue expansion joint

1. Select according to working conditions

2. Selection core parameters

The latest national standard for flue expansion joints is GB/T 12777-2019, which was officially implemented in December 2019, replacing the old GB/T 12777-2008。 The following parameters should be focused on when selecting the model:

• Nominal diameter (DN): Consistent with pipe inner diameter
• Design temperature: Not lower than maximum operating temperature +50℃
• Design pressure: Not less than maximum working pressure
• Compensation amount: determined according to thermal displacement calculation (rated compensation amount ≥1.2× calculated thermal displacement)
• Material: The material of the guide tube is not lower than the material of the bellows

Calculation formula of thermal displacement: Δ L = α × L × Δ T

Where α is the line expansion coefficient of the pipeline (carbon steel 12×10⁻⁶/℃, stainless steel 16×10⁻⁶/℃)。

3. Material selection specification

Special reminder: The fault analysis of the expansion joint of the ignition air duct of a 410t/h boiler shows that the metallic properties of the part in direct contact with the flue gas should not be lower than 0Cr25Ni20Si2 (310S). However, the factory actually uses ordinary stainless steel, which leads to the rapid failure of the expansion joint under high temperature。 High-temperature sulfur-containing environmental materials shall not be downgraded.

3. Installation specification of flue expansion joint

The installation quality of flue expansion joint directly affects its service life. The following specifications should be followed during installation：

1. Preparation before installation

• Check whether the model, specification and pressure level are consistent with the design
• Check bellows/skin surface for mechanical damage
• Verify that the direction of the liner cylinder (guide cylinder) coincides with the flow direction of the medium (the small end faces the incoming flow)

2. Critical controls during installation

3. Special requirements for non-metallic expansion joints

• Installation starts from the lower part, and the skin is placed flat on the flange surface of the metal frame to prevent wrinkles
• The marking direction of the wear-resistant bushing must be consistent with the flow direction of the pipe medium
• Tighten the bolts by "diagonal tightening and fractional tightening"

4. Common faults and treatment

1. Leak Cause Analysis

The most common cause of leakage in non-metallic flue expansion joints is water accumulation in grooves。 When the skin is installed, an annular groove is naturally formed between the pressure plate and the skin, and a large amount of acid water is accumulated during the operation of the unit. Acid water penetrates the fabric layer to reach the position of the fixing screw, causing the screw to loosen and corrode and break. Finally, the acid liquid flows out from the broken screw hole and the damaged part of the skin, and seriously appears the "small waterfall" phenomenon。

Other causes of leakage include: the skin material is not corrosion-resistant (silicone rubber is not acid-resistant, so fluororubber should be used instead)The bolts are not tightened repeatedly and loosened due to vibrationFlue overall displacement pulling and cracking expansion joint。

2. Treatment Methods

V. Operation and maintenance suggestions

1. Daily inspection: check the appearance of the expansion joint every week for smoke leakage, bulging and loose bolts
2. Regular tightening: Full tightening of non-metallic expansion joint platen bolts on a quarterly basis
3. Temperature monitoring: Infrared temperature measurement, if the surface of non-metallic expansion joint abnormally heats up, it indicates that the inner heat insulation layer is damaged
4. Drainage inspection: Horizontally installed expansion joints should be provided with drainage holes (above DN50) at the lowest point of the frame, and should be cleaned every month to prevent blockage
5. Replacement cycle: Non-metallic skin is usually replaced once every 3-5 years; If the metal frame is severely corroded, it should be replaced as a whole

sum up

Flue expansion joint is the key component to ensure the safe operation of flue gas system. The correct type selection shall be based on the flue gas temperature, corrosiveness, displacement direction and other factors to determine the type and material. The material shall not be degraded under high temperature conditions (321, 310S or nickel-based alloy shall be selected)Wet flue gas environment shall be provided with drainage holes and use fluororubber skin。 During installation, the flow direction sign shall be strictly followed. The non-metallic expansion joint shall be repeatedly tightened with bolts and the transportation rod shall be removed in time。 Regular maintenance, including inspections, tightening and replacement, can effectively extend the life of the equipment. It is suggested to select and calculate according to the latest national standards in the design stage to ensure the safety of the system from the source.

In industrial flue gas treatment system, the choice of material of flue gas inlet and outlet expansion joint is directly related to the service life of equipment and the safe operation of the system. Whether it is the desulfurization system of coal-fired power plant, the energy recovery system of catalytic cracking unit, or the flue gas pipeline in chemical production, the working conditions and environments of expansion joints are different-high temperature, corrosion, dust, vibration and other factors are intertwined, which puts forward differentiated requirements for materials. Correct selection of flue gas inlet and outlet expansion joint material is the key prerequisite to ensure the long-term stable operation of equipment. This paper will systematically analyze the performance characteristics, applicable scenarios and selection principles of various materials, and provide professional reference for engineers and technicians.

1. Classification and performance comparison of expansion joint materials

The materials of flue gas inlet and outlet expansion joints are mainly divided into metal materials and non-metal materials. Different materials have their own advantages and disadvantages in temperature resistance, corrosion resistance and compensation ability.

1. Metal bellows material

The core component of metal expansion joint is corrugated pipe, and its material choice directly affects the temperature resistance and corrosion resistance.

According to JB/T 6171-2013 "Multi-layer metal bellows expansion joint" and GB/T 12777-2019 national standard, the design and manufacture of metal expansion joint shall follow the corresponding specifications. For high temperature conditions, nickel-based alloys such as 321 or INCONEL are recommended materials for bellows; For media containing sulfur dioxide, SUS316, 316L have good corrosion resistance.

2. Non-metallic skin material

Non-metallic expansion joints are composed of multiple layers of composite materials, and different levels of materials perform their respective duties.

2. Guide for selection of zones according to working conditions

The flue gas characteristics of different sections are significantly different, and the material selection of flue gas inlet and outlet expansion joints must be "one area, one policy".

1. High temperature original flue (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.

Material recommendation:

• Metal expansion joint: 321 or 310S stainless steel for bellows
• Guide tube: same material or more wear-resistant material, thickness ≥3mm
• Note: Intergranular corrosion may occur in the normal 304 stainless steel in this temperature range, so it is not suitable for use

In the actual project case, the expansion joint of the flue gas inlet and outlet of the air preheater of a power plant is upgraded with 304 stainless steel metal expansion joint. For higher temperature catalytic cracking flue gas, the bellows need to use nickel-based alloys such as INCONEL600 and 625.

2. Low temperature wet flue (inlet and outlet of desulfurization tower to chimney)

This section is the most corroded area with low flue gas temperature (45-80℃), saturated humidity, strong acidity (pH 2-4) and high Cl⁻¹ concentration.

Material recommendation:

• Non-metal expansion joint: the skin adopts fluororubber + PTFE multilayer composite structure
• Metal frame: make glass flake heavy anti-corrosion coating (thickness ≥2mm)

Typical case: The desulfurization wet flue expansion joint of Huadian Luohe Power Generation Co., Ltd. originally adopted a non-metallic structure. Two sets of skins and one set of frames were replaced successively, and the pressure plate was upgraded to 316L material, all of which failed to effectively solve the water leakage problem. In 2016, pure titanium TA2 expansion joint (thickness 2mm, designed as a split single corrugated structure) was innovatively adopted, which achieved "zero leakage" after continuous operation for one year. During maintenance, the inner surface was as clean as new, and the weld was intact.

This case fully shows that in the high Cl⁻¹ environment of wet desulfurization, the average life of 316L does not exceed two years, while titanium can achieve "zero corrosion and zero leakage".

3. Medium temperature flue (dust collector outlet to desulfurization tower inlet)

The flue gas temperature in this section is about 120-180℃, and the dust content is reduced, but it still contains corrosive components such as SO₂.

Material recommendation:

• Non-metallic expansion joints: fluorine or silicone rubber skin
• Metal expansion joint: 316L stainless steel

The metal expansion joint at the outlet of induced draft fan of a power plant is upgraded with 316L stainless steel. In chemical flue gas pipelines, material 321 (equivalent to 0Cr18Ni10Ti) is also a common choice.

4. Catalytic cracking flue gas (energy recovery system)

The flue inlet and outlet of the flue gas turbine in the catalytic cracking unit has high temperature, large pipe diameter and sensitive force, so the material requirements are extremely strict.

Material recommendation:

• High temperature segment: INCONEL600, 625 or INCOLOY800, 825 nickel-based alloy
• Medium temperature section: SUS321 stainless steel
• Bellows material is selected according to the working temperature: 321/INCONEL is selected for 450-600℃, and INCOLOY is selected for> 600℃

III. Material configuration of typical application scenarios

The following table summarizes the recommended configuration of flue gas inlet and outlet expansion joint materials under different working conditions:

Selection decision-making process

The correct material selection of flue gas inlet and outlet expansion joint should follow the following steps:

Step 1: Clarify the working condition parameters

• Maximum/Minimum Operating Temperature
• Media composition (SO₂, SO₃, Cl⁻¹, dust concentration)
• Pressure range (positive/negative)
• Three-way displacement
• Start-stop frequency (affects fatigue life)

Step 2: Primary Type by Temperature

• > 400 °C: metal expansion joint (321/310S/nickel-based alloy)
• 120-400℃: metal or non-metal can be used

Step 3: Precisely determine the material according to corrosiveness

• Dry smoke, low corrosion: 304/316L or silicone rubber
• Sulfur-containing wet flue gas: fluororubber non-metallic or 316L (note Cl⁻Restriction)
• High Cl⁻wet flue gas (after desulfurization): pure titanium TA2 or 2205 duplex steel

Step 4: Comprehensive economic assessment

• Non-metallic expansion joints have low initial cost, but short replacement cycle (3-5 years)
• The initial cost of titanium expansion joint is the same as that of non-metal (due to the low density and thin thickness of titanium), but the service life can reach more than 10 years, with "zero maintenance" cost

V. Material failure cases and lessons

Case 1: Ordinary stainless steel used in high-temperature sulfur-containing environment

The technical agreement requires that the material of the ignition air duct expansion joint of a 410t/h boiler is not less than 0Cr25Ni20Si2 (310S), but the actual use of ordinary stainless steel leads to rapid corrosion and damage of the expansion joint in high-temperature flue gas. Lesson: High-temperature stainless steel must be used in high-temperature sulfur-containing environment, and the material should not be degraded.

Case 2: 316L for desulfurization wet flue

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. Lesson: 2205 duplex steel or pure titanium should be used in high Cl⁻wet flue gas environment.

Case 3: Repeated water leakage of non-metallic expansion joint

The non-metallic expansion joint of Huadian Luohe Company has successively replaced two sets of skins and one set of frames, but the water leakage problem has not been solved. Finally, the titanium expansion joint has been completely cured. Lesson: Repeated failure should be considered for material upgrading, not repeated replacement.

sum up

The material selection of flue gas inlet and outlet expansion joint is a systematic project, which must be comprehensively decided according to flue gas temperature, medium corrosiveness, displacement requirements and other working conditions parameters:

• High temperature drying section (> 400℃): 321, 310S stainless steel or INCONEL nickel-based alloy is selected
• Intermediate temperature sulfur section (120-400℃): 316L stainless steel or fluororubber non-metallic expansion joint
• Low temperature wet flue (
• Catalytic cracking high temperature section: Nickel-based alloy such as INCONEL600/625

Correct material selection can not only prolong the service life of the equipment, reduce the maintenance cost, but also ensure the long-term safe operation of the flue gas system. It is suggested to entrust professional institutions to conduct working condition analysis and material selection evaluation in the design stage, so as to avoid the vicious circle of "improper material selection and repeated failure" from the source.