Specialized in manufacturing compensators, expansion joints, baffle doors

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.

In the operation and maintenance of flue gas pipeline system, how to deal with the gap of flue gas expansion joint is a practical problem that technicians often face. Whether it is the platen joint of non-metallic expansion joint, the flange joint surface of metal expansion joint, or the skin overlap joint, improper treatment of any gap can lead to air leakage, water leakage, accelerated corrosion and even system shutdown. This paper will systematically explain how to deal with the gap of flue gas expansion joint from the gap type diagnosis, sealing scheme selection to construction steps, and provide an operable technical guide for on-site maintenance personnel.

1. Type and risk level of expansion joint gap

The type, location and leak severity of the flue expansion joint gap must be accurately judged before any treatment is performed.

1. By location

2. Classification by leak severity

2. Scenario 1: Joint treatment of non-metallic expansion joint pressure plate

Platen joints are the most common leakage points for non-metallic expansion joints. How to deal with the gap of flue gas expansion joint The core of this scenario is to eliminate the gap between the platen and the skin.

Causes of problems

• The pressure plate bolt is not tightened repeatedly according to the specification, and the bolt is loose after the skin is compressed
• No sealing gasket between skin and platen or ageing of sealant
• Platen deformation or insufficient cutting size of skin

Material Preparation

• High temperature resistant sealant (silicone type, temperature resistance ≥250℃)
• Fluorinated rubber sealing strip (thickness 3-5mm, width matched with pressure plate)
• Electric wrench, torque wrench
• Stainless steel spacer (for clearance compensation)

Operation Procedures

1. Remove old seals: Loosen platen bolts, remove platen, and remove old sealant and aged strip
2. Clean the base surface: Use a wire brush or sandpaper to clean the skin surface and the contact surface of the pressure plate to remove dust and oil
3. Place new sealing strip: Continuously lay fluororubber sealing strip on the skin along the pressing plate position, and miter joint at 45°
4. Apply sealant: Evenly apply high temperature resistant sealant on the surface of the sealing strip and the contact surface of the pressure plate
5. Install platen: Place platen, insert bolts, manually screw in nuts
6. Fractional tightening: adopt the method of "diagonal tightening and fractional tightening", the first torque is 30N·m, the second time is 50N·m, and the third time is 80N·m
7. Check the seal: check for leaks after restoring the system

Precautionary measures

• Tighten bolts once in 1 month and once in 3 months after operation
• Check the sealing status of pressure plate joints every time the machine is shut down for maintenance

3. Scenario 2: Skin overlap seam treatment

The skin of large non-metallic expansion joints (width> 2 meters) is usually made of multiple pieces, and overlap joints are the weak links.

Causes of problems

• Insufficient overlap width (
• The overlap is not glued or the adhesive is aging
• Heat shrinkage of the skin causes the lap to pull apart

Material Preparation

• Skin patch of the same material (width 150-200mm)
• Fluororubber glue (two components)
• Rolling tool

Operation Procedures

1. Clean the lap area: Use solvent (acetone or alcohol) to clean the 50mm area on both sides of the lap joint
2. Apply glue: Evenly apply fluororubber glue on both sides of the overlap seam, and let it dry until it is not stick to your hands (about 10-15 minutes)
3. Fit: Press the skin overlap and exhaust the air from center to edge with a rolling tool
4. Reinforcement covering: Cover a layer of reinforcing sheet of the same material with a width of 100mm above the overlap seam, and apply the same glue to fit
5. Curing: 24 hours at room temperature
6. Inspection: Airtightness inspection after curing

Precautions

• Construction ambient temperature should be ≥10℃, humidity ≤85%
• The overlapping direction should be consistent with the flow direction of smoke gas (the rear piece presses the front piece to prevent smoke gas from washing the opening)

4. Scenario 3: Treatment of gap on flange joint surface

The gap treatment of flange connection is a scenario with high technical requirements in how to deal with the gap of flue gas expansion joint.

Causes of problems

• Uneven flange surface or corrosion pits
• Sealing gasket aging, compression permanent deformation
• Uneven or insufficient bolt preload
• Flange deflection caused by thermal displacement of pipe

Material Preparation

• High temperature resistant gasket (flexible graphite composite gasket or PTFE coated gasket)
• Stainless steel bolts and washers
• torque wrench
• Flange separator (if necessary)

Operation Procedures

1. Remove the old gasket: Loosen the flange bolt, use the flange separator to push the flange open, and remove the old gasket
2. Clean the flange surface: Use an angle grinder with a hundred impeller to polish the flange sealing surface to remove the residue and rust of the old gasket
3. Check flatness: Use a straight ruler to check flatness of flange surface. If the deviation is> 0.5mm, it should be repaired or replaced
4. Place new gasket: Place new sealing gasket centered on the flange face, ensuring that the bolt holes are aligned
5. Fastening bolts: Evenly tighten 3 times in diagonal order, and the final torque is as designed (usually 80-120N·m)
6. Hot re-tightening: After the system heats up to the working temperature, re-tighten the bolts once again

Suggestions for gasket selection

V. Scenario 4: Treatment of welding gap of metal corrugated pipe

Cracks or pinholes appear in the weld of metal expansion joint bellows, which is a serious fault.

Causes of problems

• Weld defects (unfused, pores)
• Stress corrosion cracking
• Fatigue crack

Material Preparation

• Angle grinder
• Welding wire of the same material (e.g. ER316L, ER2209)
• Argon arc welding machine
• Permeation detection agent

Operation Procedures

1. Crack Localization: Determine the precise location and length of cracks by penetration detection
2. Grinding and clearing: Use an angle grinder to grind a V-shaped groove along the crack and penetrate the crack deeply
3. Welding repair: Adopt argon arc welding, low current, fast welding, control the interlayer temperature ≤150℃
4. Grinding and dressing: Grinding the weld seam flush with the base metal
5. Permeation re-inspection: perform permeation inspection again to confirm that there are no defects
6. Passivation treatment: the weld area is passivated by pickling

IMPORTANT

• Stress corrosion cracks are often more than one, so the detection range should be expanded
• The corrugated pipe with multiple cracks or repeated cracks should be replaced as a whole and should not be repaired repeatedly

VI. Emergency temporary blocking (handling without stopping)

In an emergency situation where the machine cannot be stopped, the following emergency plan can be used to deal with the leakage of the flue gas expansion joint gap.

Material Preparation

• High temperature resistant leak plugging glue stick (instant curing type)
• Stainless steel belt or throat hoop
• Acid-resistant rubber sheet (thickness 3-5mm)

Operation Procedures

1. Quick Surface Cleaning: Use a wire brush to remove floating dust and rust around gaps
2. Glue stick sealing: After kneading and softening the leak-plugging glue stick, press it firmly on the gap
3. Rubber mat covering: Cover acid-resistant rubber sheet over the cement
4. Clamp fastening: Clamp the rubber plate to the expansion joint with stainless steel belt or throat hoop

Precautions

• Only suitable for low pressure (≤5kPa) small leakage
• Temporary measures are valid for 1-4 weeks
• Must be permanently repaired at the latest scheduled downtime

VII. Preventive maintenance suggestions

After the clearance treatment of flue gas expansion joint is completed, the following measures can be taken to effectively prevent re-leakage:

1. Regular tightening of the bolts: Tighten the non-metallic expansion joint once in 1 month and once in 3 months after it is put into operation, and then check it quarterly
2. Patrol inspection: Visually inspect the gaps every week for discoloration and leakage traces
3. Aging inspection of sealant: check whether the sealant is cracked or powdered every year, and reapply it in time
4. Gasket Replacement Cycle: Flange gaskets are recommended to be replaced every 2-3 years
5. Ledger record: Record the time, location, method and effect of each gap treatment

sum up

How to deal with the gap of flue gas expansion joint needs to choose the most suitable scheme according to the type, location and leakage severity of the gap:

The core of gap treatment lies in "thorough clearing, material matching, uniform fastening and regular review". One standard gap treatment ensures that the expansion joint restores its sealing performance. If you are facing complex gap leakage problems, it is recommended to entrust a professional maintenance team to deal with it to ensure the construction quality and system safety.

In wet desulfurization system and chemical flue gas pipeline, how to repair the corrosion of flue gas expansion joint is the most common thorny problem encountered by operation and maintenance personnel. Whether it is the pitting and perforation of metal bellows or the corrosion and damage of non-metal expansion joint frame, if it can't be treated in time and effectively, it will lead to leakage expansion, system shutdown and even safety accidents. This paper will systematically explain how to repair the corrosion of flue gas expansion joint from corrosion type diagnosis, comparison of repair schemes to practical steps, and provide an operable technical guide for field technicians.

1. Corrosion type diagnosis: the first step before repair

The type, location and severity of flue gas expansion joint corrosion must be accurately judged before any repair operation is performed.

1. Corrosion types of metal bellows

2. Non-metallic expansion joint corrosion

The "corrosion" of non-metallic expansion joints is mainly manifested as:

• Aging brittleness of skin fabric layer
• Swelling or degradation of fluororubber/silicone rubber layer
• Metal frame corrosion perforation

3. Corrosion Severity Classification

II. Scheme 1: Pitting corrosion repair of metal bellows (unperforated)

It is suitable for corrosion treatment of flue gas expansion joint with corrosion grade one.

Material Preparation

• Stainless steel acid pickling passivation paste
• Passivation test solution
• Stainless steel repair agent (polymer alloy type)
• Grinding tool (angle grinder + hundred impeller)

Operation Procedures

1. Surface Cleaning: Use an angle grinder to sand corroded areas to remove corrosion products and discolored oxide layers, exposing metallic luster
2. Pickling passivation: Apply pickling passivation paste, leave for 15-30 minutes, rinse with clean water
3. Passivation test: Add passivation test solution dropwise and observe the color change (blue means that the passivation film is intact, red needs to be reprocessed)
4. Repair agent construction: Add curing agent to stainless steel repair agent, stir evenly and apply to corroded area with a thickness of 1-2mm
5. Curing maintenance: Stand and cure according to product instructions (usually 24 hours)

Effects and Cycles

• Can extend service life by 1-2 years
• Suitable for pitting depth

3. Scheme 2: Perforation and plugging of metal bellows (already penetrated)

It is suitable for corrosion repair of flue gas expansion joint with corrosion grade two.

Material Preparation

• High temperature plugging cement (temperature resistance ≥300℃)
• Stainless steel patch (same material)
• Stainless steel cable ties or clamps
• High temperature resistant sealant

Operation Procedures

1. Hole reaming cleaning: Use a round file to enlarge the perforated area to a regular circular shape, and clean the corrosion around the hole
2. Base surface treatment: Polish the perimeter of the hole in the range of 20mm to expose the metallic luster
3. Plugging cement construction: After kneading the cement evenly, fill the perforations from the inside to the outside to form a mushroom head shape
4. Patch covering: Apply high temperature sealant to the back of the stainless steel patch and cover the perforation
5. Cable tie fixing: Fasten the patch to the bellows with stainless steel cable ties
6. Cure Check: Check the tightness with soapy water after curing

Applicable restrictions

• Perforation diameter ≤5mm
• Working pressure ≤10kPa
• Suitable for temporary repair, should be replaced during planned maintenance

4. Scheme 3: Corrosion repair of metal frame of non-metallic expansion joint

Metal frame corrosion of non-metallic expansion joints is a common problem of flue gas expansion joint corrosion, and frame corrosion will directly affect skin fixation and overall sealability.

Material Preparation

• Angle grinder, wire brush
• Epoxy primer
• Glass flake cement (vinyl ester resin)
• Glass fiber cloth
• Stainless steel reinforcement plate (if necessary)

Operation Procedures

1. Rust removal: Treat the corroded area of the frame to Sa2.5 by angle grinder or sandblasting
2. Primer painting: Apply one layer of epoxy resin primer, dry film thickness ≥50 μ m
3. Cement construction: scrape glass flake cement, single thickness 1-1.5mm, total construction 2-3 layers, total thickness ≥2mm
4. Glass fiber reinforcement: Laying glass fiber cloth between cement layers to enhance crack resistance
5. Local reinforcement: For parts with severe corrosion thinning, stainless steel reinforcing plate is welded on the back
6. Curing: Curing at room temperature for 24-48 hours

Effects and Cycles

• Can be continued for 2-3 years after recoating
• Suitable for local corrosion of the frame, not for overall severe corrosion

V. Emergency temporary repair (non-stop processing)

In an emergency situation where the machine cannot be stopped, the following emergency schemes can be used to deal with flue gas expansion joint corrosion leakage.

Material Preparation

• High temperature resistant leak plugging glue stick (instant curing type)
• Stainless steel belt or throat hoop
• Acid-resistant rubber sheet

Operation Procedures

1. Quick Surface Cleaning: Use a wire brush to remove floating rust around leakage points
2. Glue stick sealing: After kneading and softening the leak-plugging glue stick, press it firmly at the leakage point
3. Rubber mat covering: Cover acid-resistant rubber sheet over the cement
4. Clamp fastening: Clamp the rubber plate to the expansion joint with stainless steel belt or throat hoop

Precautions

• Only suitable for low pressure (≤5kPa) small leakage
• Temporary measures are valid for 1-4 weeks
• Must be permanently repaired at the latest scheduled downtime

VI. Preventive Measures and Maintenance Suggestions

After the corrosion repair of flue gas expansion joint is completed, the following measures can effectively delay the re-corrosion:

1. Improvement of operating environment

• Control flue gas temperature not lower than acid dew point (usually> 80℃) to reduce condensate generation
• Purge the flue with nitrogen in time after shutdown to replace the wet flue
• Periodic neutralization with alkali wash to remove deposited acidic substance

2. Material upgrade

• Repeatedly corroded parts, upgrade the material when replaced next time:
  • 304 → 316L → 2205 → Titanium
  • Ordinary silicone rubber → fluorine rubber

3. Structural Optimization

• The horizontal expansion joint is provided with drainage holes to discharge the condensate in time
• Install a guide tube to prevent smoke from directly washing the bellows
• Improve heat preservation and reduce local condensation

4. Regular inspections

• Check the surface of the expansion joint monthly for signs of corrosion
• Check the acidity of the liquid discharged from the drain hole quarterly with a pH test strip
• Wall thickness sampling (metal expansion joint) is conducted once a year

sum up

How to repair the corrosion of flue gas expansion joint should choose the most suitable scheme according to the corrosion type, severity and working conditions:

• Unperforated pitting: sanding + pickling passivation + polymer repair agent, can extend the life by 1-2 years
• Small perforation leakage: plugging cement + patch + cable tie fixation, suitable for temporary emergency response
• Frame corrosion: Rust removal + glass flake heavy anti-corrosion coating to restore anti-corrosion ability
• Serious corrosion: the whole material should be replaced and upgraded, and repeated repairs should not be made

Repair is only a cure for the symptoms, and the root cause lies in:

1. Improve the operating environment and reduce condensate generation
2. Upgraded corrosion-resistant materials to solve problems at source
3. Standardizing drainage design to eliminate accumulation conditions
4. Establish a regular inspection system for early detection and early treatment

If you are facing the trouble of corrosion leakage of flue gas expansion joint, it is recommended to choose an appropriate repair plan according to the severity of corrosion, and plan to shut down and replace it in the near future. For the parts with repeated corrosion, professional organizations should be entrusted to carry out material upgrading design to thoroughly solve the problems.