Specialized in manufacturing compensators, expansion joints, baffle doors

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.

In the operation and maintenance of boiler flue gas system, how to disassemble and assemble boiler flue gas expansion joint is a practical problem that maintenance personnel often face. Whether it is the aging replacement of non-metallic skin or the overall replacement after corrosion failure of metal expansion joint, the standardized disassembly and assembly process is directly related to construction safety, maintenance quality and service life of equipment after operation. This article will systematically explain how to disassemble and assemble the boiler flue gas expansion joint from the preparation work, disassembly steps, installation points to acceptance criteria, and provide an operable technical guide for on-site maintenance personnel.

Preparation before disassembly and assembly

1. Confirmation of operating conditions

Before disassembling and assembling the boiler flue gas expansion joint, the following conditions must be confirmed:

• System isolation: The boiler has been stopped, the flue has been cut off, and the upstream and downstream equipment has been installed with blind plates or plug-in plates
• Temperature drops to ambient temperature: It is strictly prohibited to work under high temperature to prevent scalding and thermal stress deformation
• Internal gas replacement: The flue gas has been evacuated, the oxygen content has reached the standard (≥19.5%) after testing, and there are no toxic and harmful gas residues
• Work ticket handling: has obtained limited space work ticket, hot fire ticket (if welding is required), high altitude work ticket, etc

2. Preparation of tools and materials

Prepare the corresponding tool according to the expansion joint type:

3. Inspection of new spare parts

The new expansion joint or skin must be unboxed prior to installation:

• Check whether the model, specification and pressure level are consistent with the design
• Check the appearance for transportation damage and deformation
• Verify that the flow direction is clearly identified
• Check whether bolts, gaskets and other accessories are complete

2. Disassembly steps

1. Record the original state

Before disassembly, the following information should be recorded to provide a reference for installation:

• Distance between expansion joint and fixed bracket and guide bracket
• Is there a pre-bias or cold tight value
• Flow direction marking direction on expansion joint body
• Take a photo to preserve the original installation status

2. Remove constraints

• Remove the connecting bolts at both ends of the expansion joint (flange connection) or cut the weld (welded connection)
• For non-metallic expansion joints, use an electric wrench to remove the pressure plate bolts in turn
• Note: If the expansion joint has a transport rod and has not been removed before, it should be removed together

3. Hoisting and disassembly

The key link of how to disassemble and assemble the boiler flue gas expansion joint is the hoisting operation:

• Set a lifting point above the expansion joint, and hang the hand-pulled hoist
• Tie the hoisting belt to the expansion joint frame or lugs, not to the bellows or skin
• Lift slowly until the sling is stressed, and continue lifting after confirming the balance
• After removing the last connection, remove the old expansion joint from the flue

4. Clean up the interface

• Remove old gaskets, rust and debris from pipe flanges
• Polish the welded interface to expose a metallic gloss
• Clean up the dust accumulation at the installation position of the expansion joint

3. Installation steps

1. Frame repair (non-metallic expansion joint)

For non-metallic expansion joints with skin replacement, the metal frame should be repaired before installing the new skin:

• Check the frame for corrosion and deformation
• Polishing and removing rust on the corroded part
• Apply anti-corrosion coating (glass flake cement, thickness ≥2mm)
• Replace damaged platen bolts

2. Expansion joint in place

• Lift new expansion joint to installation position
• Adjust the direction according to the recorded direction mark to ensure that the small end of the guide tube is facing the direction of flue gas flow
• After initial placement, check the levelness with a level ruler, deviation ≤3‰

3. Connection and Fixation

Flanged connection:

• Place new sealing gasket (high temperature resistant asbestos pad or PTFE pad)
• Insert bolts, manually screw nuts
• Evenly tighten 2-3 times in diagonal order, and the final torque is according to the design requirements

Welding connections:

• Fixed by spot welding, corrected position and then fully welded
• When welding, jump welding method is adopted to control the interlayer temperature and prevent overheating deformation
• Clean the weld slag after welding and check the appearance of the weld

4. Non-metallic expansion joint skin installation (if applicable)

For the case of only replacing the skin, the installation steps of how to disassemble and assemble the boiler flue gas expansion joint are as follows:

• Lay new insulating cotton (aluminum silicate fiber cotton) in the frame, the thickness is as original
• Lay the new skin on the frame to ensure even margin all around
• Install platen, insert bolts
• Repeated tightening by "diagonal tightening and fractional tightening", torque 50-80N·m

5. Drain hole setting

For horizontally installed expansion joints, drainage holes (DN25-DN65) shall be provided at the lowest point of the frame, and dust-proof nets shall be installed.

6. Transportation Tie Rod Handling

After installation, the transport protection tie rod must be removed (the positioning limit tie rod must not be removed) so that the expansion joint can be expanded and contracted freely.

Inspection and acceptance after installation

1. Dimensional review

• Check that the expansion joint installation length is consistent with the design
• Verify that the distance between the expansion joint and the adjacent bracket meets the requirements of the drawing

2. Sealability Check

• Perform airtightness test after system restoration
• Apply soapy water or use a leak detector around the expansion joint
• No bubble or alarm signal is qualified

3. Bolt retightening

The pressure plate bolts need to be tightened once one month and three months after the non-metallic expansion joint is put into operation, because the skin compression will cause the bolts to loosen.

4. Data Archiving

Deposit the following data into the device ledger:

• Replacement Date
• New expansion joint models, specifications, manufacturers
• Installation Photos
• Acceptance Record

Safety Precautions

1. Height work protection

• Working height exceeding 2 meters must wear a safety belt and hang it at a reliable anchor point
• Set up a stable work platform or use a lift truck

2. Hoisting safety

• It is strictly prohibited to stand under hanging objects
• The hoisting belt shall not be tied to the bellows or skin
• Check that the lifting point is firm before lifting

3. Limited space operations

• Gas detection before entering flue (oxygen, CO, H₂S)
• Set up special supervision to keep communications open

4. Welding safety

• Clean up surrounding flammable materials
• Equipped with fire extinguishing equipment
• Protect bellows or skin during welding to prevent damage from welding slag splashing

VI. Frequently Asked Problems and Handling

sum up

Mastering how to disassemble and assemble the boiler flue gas expansion joint is the basic skill of boiler flue gas system maintenance. The standardized disassembly and assembly process shall follow the principle of "preparation before disassembly, recording the original state, cleaning the interface, standardized installation and strict acceptance":

• Spare: System isolation, complete tool spare parts, new parts inspection qualified
• Disassembly: record the original state, release the constraint, safely hoist, and clean up the interface
• Installation: frame repair, direction confirmation, even tightening, drainage holes set
• Inspection: Dimensional review, airtightness test, bolt re-tightening, data filing

One standard disassembly and assembly operation can ensure that the new expansion joint reaches the designed service life. If you still have questions about the disassembly process or need professional technical guidance, it is recommended to entrust a qualified maintenance team to implement it to ensure the construction quality and operation safety.

In the wet flue gas desulfurization system, the expansion joint of the net flue gas outlet is the key flexible compensation component connecting the outlet of the desulfurization tower and the flue pipe of the chimney inlet. After the clean flue gas is sprayed and washed by the desulfurization tower, the temperature drops to 45-55℃, and it is in a completely saturated state, containing a large amount of acidic condensate. This special working condition puts forward extremely high requirements on the corrosion resistance, sealing performance and drainage performance of the expansion joint at the net flue gas outlet. This paper will systematically explain the professional and technical points of the expansion joint at the outlet of clean flue gas from the characteristics of working conditions, material selection, structural design to installation and maintenance.

1. Special requirements for expansion joints under the working conditions of clean flue gas

1. Low temperature and high humidity corrosion environment

Different from the original flue gas, the temperature of the clean flue gas after spraying and washing by the desulfurization tower is greatly reduced to about 50℃ and is completely saturated。 Wet saturated flue gas will produce condensation and dew condensation on the flue surface. The condensate has low pH value and strong corrosiveness。 Therefore, the selection of the expansion joint of the net flue gas outlet should be different from the original flue gas expansion joint, and it must have the functions of anti-corrosion and anti-leakage。

2. Risk of acidic condensate accumulation

When the non-metallic expansion joint is used, the installation skin will naturally form an annular groove, and a large amount of acid water will accumulate during the operation of the unit。 Acid water will slowly penetrate through the fabric layer of the expansion joint to the pressure plate screw that fixes the skin of the expansion joint, causing the screw to loosen and corrode and break, or the fabric layer to be damaged, and finally flow out from the broken screw hole and the damaged part of the skin, which will seriously appear "small waterfall" phenomenon。 This is the most common failure mode for net flue gas outlet expansion joints.

3. Negative pressure operation requirements

The clean flue is usually in a negative pressure state, and the expansion joint should have good negative pressure sealing performance to prevent external air inhalation from affecting the lifting force of the chimney and the oxygen control of the system.

2. Material selection of expansion joint at net flue gas outlet

1. Skin material requirements

In view of the strong corrosive characteristics of net flue gas, the skin of the expansion joint at the net flue gas outlet must be made of acid-resistant composite material. The recommendation structure is as follows：

Fluororubber (FKM) has excellent acid, oil and high temperature resistance, and its service life can reach 3-5 years in the desulfurization clean flue gas environment. Ordinary silicone rubber will age and crack rapidly in acidic wet flue gas, and its life usually does not exceed 1 year.

2. Metal frame corrosion protection

The metal frame of the expansion joint of the net flue gas outlet must be treated with heavy anti-corrosion treatment:

• Frame material: Q345 carbon steel (economical) or 304 stainless steel (corrosion resistant)
• Anti-corrosion coating: glass flake cement or vinyl ester resin coating, thickness ≥2mm
• Bolt material: 304 or 316L stainless steel to prevent acid corrosion and loosening

3. Guide tube design

The guide tube is the component that is directly washed by smoke inside the expansion joint, and must be made of corrosion-resistant materials. It is recommended that 316L stainless steel or 2205 duplex stainless steel be used for the guide tube of the expansion joint at the net flue gas outlet, and the thickness is ≥3mm. The small end of the guide tube should be facing the direction of flue gas flow, and the large end should be facing the outlet to prevent backward flow of condensate.

Structural design and leak prevention measures

1. Cure the problem of groove water accumulation

Water accumulation in grooves of non-metallic expansion joints is the main cause of leakage of expansion joints at net flue gas outlet。 The radical cure options are as follows:

• Groove filling technology: Fill closed-cell foamed butyl rubber or high-elastic sealant in the groove formed by the skin and the pressure plate to fill the groove flat and prevent acid water from accumulating
• Drainage hole setting: Drainage hole (DN25-DN65) is set at the lowest point of the expansion joint frame to discharge the condensate in time
• Dust-proof net: Install stainless steel dust-proof net (hole diameter 3-5mm) inside the drainage hole to prevent dust blockage

2. Bolt tightening requirements

Because the non-metallic expansion joint pressure plate is 4-6 meters long, the distal bolt will loosen due to skin compression deformation after one-time tightening during installation。 Therefore, when installing a clean flue gas outlet expansion joint, you must:

• Repeated operation by "diagonal tightening and fractional tightening"
• Use torque wrench, tightening torque according to manufacturer's requirements (usually 50-80N·m)
• Tighten bolts once in 1 month and once in 3 months after operation

3. Flange connection seal

For flue gas outlet expansion joints with flanges, when the new expansion joint skin is installed, packing materials such as anti-corrosion washers should be placed between the interface flange and the skin, and then the bolts should be tightened to prevent leakage at the flange and corrosion of the fastening bolts。 All fastening bolts should be tightened repeatedly and repeatedly with an electric wrench. It is strictly prohibited to leak or loosen。

Installation Specifications and Key Control Points

1. Check before installation

• Check that the model, specification and design of the expansion joint are consistent
• Check the skin for damage or scratches
• Verify that the direction of the guide tube is clearly marked
• Inspect Metal Frame Anti-Corrosion Coating Integrity

2. Installation Process Control

• Direction confirmation: The flow direction arrow on the expansion joint must coincide with the flow direction of the net flue gas (from the desulfurization tower to the chimney)
• Welding protection: Protective measures must be taken during welding operation to prevent non-metallic skin from being damaged by welding slag splashing
• Flange Alignment: Ensure that the bolt holes are aligned and the seal gasket is centered when the flange is connected
• Drain pipe installation: Lead drain stub to gutter or acid collection system

3. Acceptance after installation

• Take photo archive after expansion joint installation is complete
• Conduct airtightness test to verify no leakage
• Record the installation date, model number and operator in the running ledger

V. Common faults and preventive measures

Suggestions on operation and maintenance management

1. Daily inspection

• Check the appearance of the expansion joint every week for water seepage, bulging and abnormal vibration
• Observe the drainage situation and confirm that the drainage is smooth
• Check bolts for looseness or rust

2. Regular maintenance

• Full tightening of platen bolts quarterly
• Clean drain holes monthly to prevent clogging
• Check the anti-corrosion coating of the frame every six months, and repair it in time if any damage is found

3. Replacement cycle

The service life of non-metallic skin under the condition of clean flue gas is generally 3-5 years. The skin should be replaced as a whole when:

• Large-area aging brittleness of skin
• Repeated leak repair more than 2 times in the same location
• Insulation collapse leads to overtemperature of external surface

sum up

Clean flue gas outlet expansion joint is the key link of flue anti-corrosion in wet desulfurization system. Because its working environment has the characteristics of low temperature and high humidity, strong corrosion and condensate accumulation, the following core principles must be followed in selection and maintenance:

• Material priority: the skin is made of fluororubber composite material, and the metal frame is made of heavy anticorrosive coating
• Structural optimization: Adopt groove filling technology to eliminate hidden dangers of water accumulation, set drainage holes to drain liquid in time
• Installation specification: Tighten bolts repeatedly to ensure sealing, and install strictly according to flow direction marks
• Regular maintenance: Establish inspection ledger, drain, tighten and clean regularly

A clean flue gas outlet expansion joint with reasonable design and standard installation can effectively avoid the phenomenon of "running, dripping and leaking" and ensure the long-term stable operation of desulfurization system. If you are planning a clean flue system or are facing frequent leakage of expansion joints, it is recommended to entrust a professional organization to conduct on-site survey and special design to solve the persistent corrosion leakage problem from the source.