Specialized in manufacturing compensators, expansion joints, baffle doors

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.

In the flue gas pipeline system, the plugging and repair of flue gas expansion joint is the most common urgent task faced by operation and maintenance personnel. Whether it is non-metal skin seepage, metal bellows crack, or flange bolt leakage, if it can't be treated effectively in time, it will lead to aggravated corrosion, exceeding environmental protection standards or even unplanned shutdown. This paper systematically sorts out three mainstream methods of plugging and repairing flue gas expansion joints-temporary plugging, local repair and overall replacement, and compares them from three dimensions: applicable scenarios, operation steps and effect cycle, so as to help technicians make quick decisions and make accurate construction.

1. Leak diagnosis: the first step before plugging the leak

The location, type and severity of the leak must be accurately determined before any flue gas expansion joint plug and repair operation is performed.

1. Leak Location Identification

• Water seepage from skin surface: Non-metallic expansion joints are the most common, manifesting as moist patches or dripping
• Leakage at pressure plate bolts: caused by loose or corrosion fracture of bolts
• Cracks in metal bellows: usually located at crests or troughs, accompanied by a whistling sound
• Flange end face leakage: aged gasket or insufficient bolt preload
• Weld cracking: metal expansion joint frame weld or pipe connection weld

2. Leak Severity Classification

3. Downtime and Online Decision Making

• Shutdown: Local repair or whole replacement is preferred for the most reliable effect
• Unable to shutdown: On-line plugging technology is used, but safety risks are assessed

2. Method 1: Temporary blocking (emergency treatment without stopping)

Suitable for temporary flue gas expansion joint plug repair during primary leakage or waiting for spare parts.

Applicable scenarios

• Small area water seepage of non-metallic skin
• Fine crack in metal bellows
• Slight leakage of flange surface

Material Preparation

• High temperature plugging cement (temperature resistance ≥200℃)
• Stainless steel cable ties or clamps
• Acid-resistant rubber sheet or PTFE film
• Special leak plugging fixture (prefabricated)

Operation Procedures

1. Surface cleanup: Use a wire brush or sandpaper to remove dust, rust, and old sealant from around the leakage area
2. Pre-treatment: For water seepage, dry the surface with compressed air first
3. Construction of leak-plugging cement: After kneading the leak-plugging cement evenly, press it firmly at the leakage point to form a covering layer with a thickness of 5-10mm
4. Reinforcement: For the parts with high pressure or high vibration, wrap stainless steel cable ties or install leak plugging fixtures around the outer layer of the cement
5. Curing: Stand to cure according to the product instructions (usually 30-60 minutes)
6. Inspection: After curing, check whether there is still any leakage, and apply a layer if necessary

Effects and Cycles

• Temporary occlusion can be maintained for 1-6 months
• Suitable for transitional treatment, not as a permanent option

3. Method 2: Local repair (shutdown treatment)

Suitable for secondary leakage, it is the most cost-effective plugging and repair scheme of flue gas expansion joint.

Applicable scenarios

• Local damage of non-metallic skin (area ≤0.5m²)
• Single wave crack of metal bellows
• Platen Bolt Corrosion Replacement

Material Preparation

• Skin patch of the same material (non-metal) or bellows patch of the same specification (metal)
• High temperature sealant (silicone sealant or fluororubber glue)
• Stainless steel bolts and nuts
• sanding tool

Operation steps (take non-metallic skin repair as an example)

1. Shutdown Cooling: Confirm that the system has been shut down and the temperature has dropped to ambient temperature
2. Area isolation: Blind plates or baffles are set before and after the expansion joint to prevent smoke from returning
3. Clean up the damaged area: Cut the damaged skin with a knife to form a regular rectangular opening (be careful not to damage the internal insulation layer)
4. Base surface grinding: Use grinding wheel to grind the metal frame around the repair area to expose the metallic luster
5. Prefabricated patch: cut the skin patch of the same material according to the opening size enlarged by 50mm
6. Glue application: Evenly apply high temperature sealant to the back of the patch and the frame attachment surface
7. Fit and compress: Cover the repair piece on the opening, and fix it with pressing strips and bolts. The spacing between bolts is ≤100mm
8. Curing: Stand curing for 24 hours
9. Airtightness test: Airtightness check after system restoration

Effects and Cycles

• 50%-70% of the original design life can be restored after local repair
• Suitable for local skin damage, not suitable for large area aging

4. Method 3: Overall replacement (complete repair)

It is suitable for tertiary leakage, large-area aging or repeated failure, and is the most thorough plugging and repair scheme of flue gas expansion joint.

Applicable scenarios

• Large-area aging and brittle cracking of non-metallic skin (aging area ≥50%)
• Multiple cracks or severe corrosion of metal bellows
• More than 2 repairs in one year at the same location

Operation Procedures (Take Non-Metal Expansion Joint Replacement as an Example)

1. System isolation: after shutdown, cut off flue and install blind plate
2. Old expansion joint removal: remove the pressure plate bolts and remove the old skin; Check frame for corrosion
3. Frame repair: polishing and removing rust from the corroded frame, and welding and reinforcing if necessary; Re-apply anti-corrosion coating (glass flake cement, thickness ≥2mm)
4. Infill insulation: new aluminum silicate fiber cotton is laid in the frame in the original design thickness
5. Installation of new skin: Lay the new skin on the frame to ensure even allowance all around
6. Fixation of pressure plate: Install pressure plate and bolts, operate according to the principle of "diagonal tightening and fractional tightening", with a torque of 50-80N·m
7. Drain hole setting: Drill drain hole at lowest point for horizontal installation (DN50-DN100)
8. Re-tightening: Re-tighten the bolts once in 1 month and once in 3 months after operation

Effects and Cycles

• All-new performance can be restored after overall replacement, with a service life of 3-5 years (non-metal) or 5-8 years (metal)
• Is the most fundamental measure to solve repeated leakage

5. Comparison and selection suggestions of three methods

Suggestions for selection:

• Select temporary plugging for emergency production guarantee
• Local repair for planned overhaul
• Reach the replacement cycle or repeat failure, select whole replacement

VI. Preventive measures after leak plugging and repair

After completing the plugging and repair of the flue gas expansion joint, the following measures can effectively prolong the next failure interval:

1. Establish a patrol inspection system: check the repair area every week for re-leakage
2. Control start-stop rate: avoid sharp temperature change and reduce thermal stress shock of expansion joint
3. Tighten bolts periodically: Tighten the non-metallic expansion joint pressure plate bolts once a quarter
4. Clean drainage holes: Check drainage holes for blockage every month to ensure timely drainage of condensate
5. Ledger record: Record the time, location, method and effect of each repair to provide the basis for subsequent replacement

In industrial pipeline system, the design of pipeline flue gas expansion joint is the key technical link to ensure the safe operation of thermal pipeline network. Improper design can lead to premature failure of expansion joints, pipe deformation and even equipment damage. A scientific and reasonable design of pipeline flue gas expansion joint needs to comprehensively consider multiple factors such as temperature, pressure, displacement, media corrosion and pipeline arrangement. This paper will systematically explain the technical key points of pipeline flue gas expansion joint design from the design process, parameter calculation to drawing output.

1. Basic flow of pipeline flue gas expansion joint design

A complete pipeline flue gas expansion joint design typically follows the following steps:

1. Collect basic data: pipe direction, pipe diameter, material, wall thickness, working temperature, installation temperature, working pressure, media composition
2. Calculate thermal displacement: Calculate thermal elongation in each direction according to length and temperature difference of pipe section
3. Determine the position of the fixed bracket: divide the pipeline into several independent compensation pipe sections
4. Selection of Expansion Joints: Determine the type and specification according to the amount of displacement, space constraints and corrosive environment
5. Calculate reaction force and blind plate force: Check the bearing capacity of fixed brackets and equipment
6. Draw the layout drawing: mark the expansion joint position, bracket type and installation requirements
7. Prepare technical specifications: specify materials, performance parameters and acceptance criteria

Each step directly affects the final quality of the pipe flue gas expansion joint design and cannot be simplified or skipped.

2. Calculation of core parameters of pipeline flue gas expansion joint design

1. Calculation of thermal displacement

Thermal displacement is the most basic parameter in the design of pipeline flue gas expansion joint. The axial thermal elongation is calculated as follows:

Δ L = α × L × Δ T

Among them:

• Δ L: thermal elongation (mm)
• α: Line expansion coefficient of pipeline (12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel)
• L: length of pipe section between two fixed brackets (mm)
• Δ T: Difference between operating temperature and installation temperature (℃)

Example: A 20-meter-long carbon steel flue gas pipeline with an installation temperature of 20℃ and an operating temperature of 400℃, then:
Δ L =12×10⁻⁶ ×20000×380=91.2 mm

For the lateral displacement, which usually occurs at the turn of L-shaped or Z-shaped pipe segments, the calculation is complicated and needs to be analyzed using the elastic center method or using professional software (CAESAR II).

2. Selection of compensation amount of expansion joint

The rated compensation amount of the selected expansion joint should be greater than 1.2 times of the calculated thermal displacement, with a safety margin reserved. For example, in the above example, the axial displacement is 91.2mm, and an axial expansion joint with a rated compensation amount ≥110mm should be selected.

3. Blind plate force calculation

In a pressure pipe, the internal pressure creates a huge blind plate force on the expansion joint bellows, which acts directly on the fixed bracket. The formula is:

F = P × A

Among them:

• F: blind plate force (N)
• P: working pressure (Pa)
• A: Effective area of bellows (m²)

Example: The working pressure of the flue gas pipe is 5000Pa, and the effective area of the bellows is 0.5m². The blind plate force is:
F =5000×0.5=2500 N ( About 255 kg force)

For large diameter flue (diameter> 2m), the blind plate force can reach several tons or even tens of tons, which must be designed with heavy fixed brackets.

4. Expansion joint fatigue life check

The fatigue life of the expansion joint is inversely proportional to the working displacement. According to EJMA standards, the design cycle life of expansion joints is typically 1000 times (corresponding to the allowable displacement). If there are frequent starts and stops in actual operation (such as peak shaving units), a higher cycle life (such as 10,000 times) should be required. At this time, multi-wave structure should be selected or the bellows size should be enlarged.

3. Selection principle of pipeline flue gas expansion joint design

1. Select according to temperature

2. Select type according to medium corrosivity

• Dry smoke, low corrosion: 304 stainless steel or silicone rubber non-metallic
• Sulfur-containing wet flue gas: fluororubber non-metallic or 316L stainless steel (note Cl⁻Restriction)
• High Cl⁻wet flue gas (after desulfurization): pure titanium (TA2) or 904L super austenitic stainless steel

3. Select according to displacement direction

• Mainly axial displacement: axial expansion joint
• Mainly transverse displacement: compound universal type or non-metallic expansion joint
• Angular displacement: hinged expansion joint
• All three-way displacements are large: non-metallic expansion joint or compound universal hinge type

4. Bracket configuration in pipeline flue gas expansion joint design

Scientific and reasonable bracket configuration is an integral part of pipeline flue gas expansion joint design. Here are the mandatory stent setup rules:

1. Fixed bracket

Main fixing brackets must be provided at both ends of each compensation pipe section. The primary fixation bracket must be designed to be strong enough to withstand the vector sum of the following forces:

• Elastic reaction force generated by expansion joint
• Blind plate force
• Pipe frictional resistance
• Wind load and earthquake load

2. Guide bracket

Both sides of the expansion joint must be provided with guide brackets, whose function is to prevent transverse instability of the pipeline. The distance between the guide bracket and the expansion joint shall meet the following:

• First guide bracket: ≤4 times the tube diameter from the expansion joint
• Second guide bracket: ≤14 times the tube diameter from the first guide bracket

3. Limit bracket

Limit brackets shall be provided in the following cases:

• Preventing expansion joints from bearing lateral displacements beyond design values
• Limit the amount of displacement of a pipe in a specific direction

5. Output of design drawings and technical documents

A complete design drawing of pipeline flue gas expansion joint shall contain the following contents:

1. Pipe path diagram: Mark pipe diameter, length, and medium flow direction
2. Expansion joint position: Clearly mark the number, type and coordinates of each expansion joint
3. Bracket layout: Mark the position and model of fixed bracket, guide bracket and limit bracket
4. Installation requirements: Indicate the cold tightening amount, pre-offset amount, and removal instructions for transport tie rod
5. Material list: List the model, quantity, material and implementation standard of the expansion joint
6. Technical specification: including design pressure, design temperature, fatigue life, air tightness requirements, etc

6. Common Design Errors and Avoidance Methods

Call to Action

The design of pipeline flue gas expansion joint is a very professional work, which is directly related to the safety and operating cost of your pipeline system. If you're planning a new flue system or your existing pipes are experiencing frequent expansion joint failures, feel free to contact our design team today.