1. Harm and Urgency of Boiler Flue Expansion Joint Leakage

In power station boiler and industrial boiler system, flue expansion joint takes on the important functions of absorbing heat displacement, absorbing shock and noise and sealing flue gas. However, long-term exposure to high temperature, corrosive media and alternating stress environment, the expansion joint will inevitably have leakage problems. Whether the leakage treatment of boiler flue expansion joint is timely and appropriate is directly related to the operating efficiency of boiler, environmental protection emission index and the safety of on-site personnel. Flue gas leakage not only leads to the increase of energy consumption of induced draft fans and the aggravation of local flue corrosion, but also may lead to fines for exceeding environmental protection standards. More seriously, high-temperature smoke (up to 150-400℃) leaks to the surrounding areas, resulting in scald risks and fire hazards. Therefore, establishing a set of scientific and rapid leakage treatment scheme is the core skill that boiler operation and maintenance personnel must master. This article will systematically introduce the complete treatment process from leak type diagnosis, temporary plugging to permanent repair.

2. Type and cause diagnosis of boiler flue expansion joint leakage

2.1 Classification by leak location

Before carrying out boiler flue expansion joint leak treatment, it is first necessary to accurately locate the leak location. Common leak sites include:

2.2 Classification by leakage mechanism

Leakage caused by different mechanisms, boiler flue expansion joint leakage treatment methods are also completely different:

• Corrosion leakage: The flue gas contains SO₂, SO₃, Cl⁻¹ and other corrosive media, which forms acidic corrosion under the action of condensate, and the wall thickness of bellows gradually decreases to perforation. It is more common in the expansion joint at the inlet or low temperature section of desulfurization system.
• Fatigue leakage: Under the action of repeated thermal displacement, fine cracks appear at the crest or trough of the bellows, which gradually expand into penetrating cracks. It is common in peak shaving units with frequent start-and-stop.
• Overload leakage: The expansion joint is subjected to displacement beyond the design value due to bracket failure or installation pre-displacement error, resulting in local tearing of the bellows.
• Scour leakage: the guide tube falls off or the guide tube is not set, and the dusty flue gas directly washes the inner wall of the bellows, resulting in abrasion and perforation.

2.3 Methods for rapid diagnosis of leaks

The following methods can be used to quickly locate the leakage point on site:

• Visual inspection: Observe soot traces after shutdown. There is usually white or black soot accumulation around the leakage point
• Soap water leak detection: Spray soap water on the suspected leakage part, and continuous air bubbles can be seen under positive pressure
• Smoke generator: The negative pressure flue can release smoke near the expansion joint, and observe whether the smoke is inhaled
• Infrared thermal imaging: the temperature at the leakage is abnormal, and there are obvious hot or cold spots on the thermal image (positive pressure leakage is a hot spot, and negative pressure leakage of cold air is a cold spot)

3. Temporary treatment measures for boiler flue expansion joint leakage

When a leak is found and the boiler cannot be shut down immediately, the following temporary measures can be taken to control the leak and buy time for the planned shutdown.

3.1 Leak plugging under pressure (positive pressure flue)

For the leakage treatment of boiler flue expansion joint in positive pressure flue, pressure plugging is the most commonly used temporary scheme:

• Small hole leakage (≤3mm): Use tapered wooden plug or metal tapered plug to knock into the leakage hole, and apply high temperature sealant (temperature resistance ≥300℃) on the outside
• Crack leakage: Drill crack stop holes (φ 3-5mm) at both ends of the crack, and then tie them with clamps or steel strips, lined with high temperature resistant gaskets
• Flange leakage: Use flange pressure plugging fixture, inject high temperature sealing injection, or add clamp on the outside

It should be noted that pressure plugging is only used as a temporary emergency measure and should not replace permanent repair.

3.2 Outer Wrap Sealing

For large area leaks that cannot be plugged at a single point:

• High temperature resistant stainless steel sheet (thickness 0.5-1.0mm) is cut into wrapping shape
• Lined with ceramic fiber blanket or high temperature sealing gasket
• Tie and fix on the outside of the expansion joint with stainless steel belt or wire rope
• Apply high-temperature sealant to the interface

This method can significantly reduce the amount of leakage without stopping the furnace, and is suitable for short-term transitions before planned maintenance (generally no more than 1 month).

3.3 Temporary treatment of negative pressure flue

For negative pressure flue, the leakage is manifested by external cold air being sucked in. Although there is no risk of smoke leakage, it will affect the accuracy of smoke monitoring data and increase the load of induced draft fan. Provisional processing may be adopted:

• Apply high temperature resistant aluminum foil tape to the outside of the leak
• Apply high temperature repair mud
• In severe cases, it can be covered with a sealing cloth and vacuum attached

IV. Permanent leakage repair after boiler shutdown

4.1 Local repair welding

For corrosion pits or small-scale cracks, repair welding can be used to repair them. Repair welding in boiler flue expansion joint leakage treatment should strictly follow the following process:

• Clean-up: Thoroughly remove oil, rust and soot around the leakage point and polish to the natural color of metal
• Groove preparation: the crack is polished and grooved, the groove is deep to the bottom of the crack, and the angle is 60-70°
• Welding method: Tungsten Arc Welding (TIG), low current, fast welding
• Welding material selection: Match with base metal, stainless steel expansion joint with the same material welding wire; Transition welding material for dissimilar steel parts
• Post-weld treatment: polishing the weld residual height and performing 100% penetration test

IMPORTANT NOTE: The thickness of corrugated pipe substrate is usually only 1-3mm, which is easy to burn through or cause thermal deformation during repair welding. It is recommended to be operated by welders with thin plate welding qualifications, and the length of single continuous welding does not exceed 10mm, segmental jump welding.

4.2 Local replacement of bellows

When the leakage area is large (greater than 50cm²) or multiple leakage points are concentrated, local repair welding is no longer reliable, and local replacement of bellows should be performed:

• Remove the corrugated pipe from the damaged section, leaving both ends intact
• Prepare a new corrugated pipe section (the wave shape and material should be consistent with the original)
• Butt joint welding was used, and the weld seam was 100% radiographic inspection
• The compensation ability and stiffness of the expansion joint should be re-checked after replacement

4.3 Overall replacement of expansion joints

It is recommended to directly replace the expansion joint in its entirety instead of repairing it locally in the following cases:

• Multiple fatigue cracks in the bellows, approaching the design fatigue life (usually 1000 cycles)
• Improper material selection (if the original 304 is used, the actual need is 316L)
• The guide tube falls off and causes extensive wear of the bellows
• Structural defects of expansion joints (such as wave height and wave pitch not meeting the standard)

When replacing the whole, it is necessary to install in place, butt welding and limit adjustment according to the original design requirements.

V. Liner repair and seal strengthening

5.1 Liner Inspection and Repair

In the treatment of boiler flue expansion joint leakage, lining damage is often the cause of leakage. The repair steps are as follows:

1. Check the guide tube for detachment, deformation and wear
2. Check the heat insulation layer (ceramic fiber blanket) for burn and collapse
3. To repair local damage: fill in ceramic fiber cotton and apply high-temperature cement to the surface
4. If the guide tube falls off, it should be re-welded and fixed, and segment welding should be used to prevent deformation during welding

5.2 Flange Seal Replacement

For flanged connection structure leakage:

• Remove all bolts, clean up old gaskets and corrosion on flange surface
• Check the flatness of the flange surface and smooth the local high points with a grinder
• Select a suitable new gasket: Stainless steel clad graphite pad or high strength graphite composite pad is recommended
• Tighten the bolts 2-3 times in cross-symmetric order to achieve the specified torque
• After tightening, check that the flange clearance is uniform and the deviation is ≤0.3mm

Maintenance strategies to prevent leakage

6.1 Regular inspection system

The establishment of a regular inspection system for expansion joints can greatly reduce the risk of sudden leakage:

• Monthly: Visually check for soot traces and discoloration of the insulation layer
• Quarterly: Infrared thermography examination to compare temperature distribution for abnormalities
• Semi-annually: Internal inspection at shutdown to measure bellows wall thickness (ultrasonic thickness measurement)
• Annually: Check the integrity of deflectors, anchors, lining materials

6.2 Leak warning indicators

Incorporate the following indicators into the boiler operation monitoring system for early warning:

• Induced draft fan current is abnormally increased (flue leakage increases air volume demand)
• Abnormal flue oxygen (high oxygen due to negative pressure leakage and inhaled air)
• Sudden change in surface temperature of expansion joint (abnormal temperature near leakage point)

6.3 Life Management Ledger

Establish a full lifecycle profile for each expansion joint:

• Date of commissioning, design fatigue life
• Time and frequency of each start-and-stop
• Record of previous leaks and repairs
• Analysis of the trend of wall thickness thinning

When the cumulative number of cycles reaches 80% of the design life, it is included in the scheduled replacement list.

VII. Common problems and countermeasures

VIII. Summary

Boiler flue expansion joint leak treatment is a systematic work from rapid diagnosis to standard repair. The key to successfully handling leaks can be summarized as follows: "locate first, classify, plug if urgent, fill if slow, and replace if serious". For the leakage occurring in operation, first of all, the leakage location and mechanism must be accurately judged. Positive pressure flue can be temporarily controlled by pressure plugging, but it must not be relied on for a long time; After the planned furnace shutdown, repair welding, local replacement or overall replacement schemes should be selected according to the damage degree of the bellows, and the lining, guide tube and flange seal should be repaired simultaneously. In the long run, it is more economical and effective to establish a regular inspection system, life management ledger and leak warning indicators than passively handling leaks. For the boiler flue with frequent leakage, it is recommended to fundamentally review whether the expansion joint selection is reasonable, whether the bracket is set correctly, and whether the operating conditions are beyond the design range. Through standardized leakage treatment and perfect preventive measures, the risk of unplanned shutdown caused by expansion joint leakage can be minimized, and the safe, environmental protection and economic operation of boiler system can be ensured.

1. Necessity and difficulty of replacing metal expansion joint in flue of power plant

In power plant boiler and desulfurization and denitrification system, the metal expansion joint has been subjected to the joint action of high temperature flue gas erosion, alternating thermal stress and corrosive medium for a long time, and gradually problems such as bellows cracking, guide tube falling off and seal failure appear. When the leakage exceeds the standard or the structural damage affects the safe operation, it is necessary to face the practical problem of how to replace the metal expansion joint in the flue of power plant. Compared with new installation construction, replacement operation is often limited by various unfavorable factors such as shutdown time window, limited space, deformation of old structure, etc., which puts forward higher requirements for construction scheme and on-site operation.

Preparation before replacement: from furnace shutdown conditions to technical data

2.1 Furnace shutdown conditions and safety isolation

To answer how to replace the metal expansion joint of the flue of the power plant, first of all, the safety premise should be clarified. The flue where the expansion joint to be replaced is located must be randomly shut down, and the following isolation measures must be completed:

• Cool to below 50℃: Forced ventilation or natural cooling to prevent scalding and thermal deformation
• Media cut-off: Close baffle door, plug-in valve, and install blind plate if necessary
• Gas replacement: The flue gas system shall be subjected to air replacement, and the oxygen content and toxic and harmful gases (CO, SO₂, etc.) shall be detected
• Lock and tag: execute energy isolation procedure, prohibit misoperation

2.2 Site Mapping and Condition Assessment of Old Expansion Joints

Due to long-term high-temperature operation, the original installation dimensions may have changed, and it is strictly prohibited to purchase replacement parts directly based on as-built drawings. On-site measurements shall be made:

• Actual spacing of flue flanges or interfaces (3 measuring points each upstream and downstream)
• Axial and transverse offset of bellows
• Connection mode (flanged, direct welded or plug-in)
• Wear and dust accumulation of guide tube

According to the measured data, determine the length, wavenumber and interface size of the replacement parts to ensure "new and old".

2.3 Replacement Plan Preparation and Acceptance Criteria

Before replacement, a construction plan shall be formulated, including at least: dismantling and hoisting methods, welding process requirements, counterpart accuracy standards, non-destructive testing proportions, pressure and leakage test plans. The acceptance criteria are usually DL/T 1121 Technical Specifications for Metal Expansion Joints for Flue Gas Dust Removal of Boilers in Coal-fired Power Plants or GB/T 12777.

3. Demolition operation: How to safely and completely remove the old expansion joint

3.1 Sequence of removal of expansion joint

Demolition is the key first step to answer how to replace the flue metal expansion joint of a power plant. It is recommended to operate in the following order:

1. Cleaning up dust accumulation: Remove dust accumulation at the bottom of wave trough, guide tube and flue to reduce hoisting weight
2. Release Tie Rod/Limit Device: Record factory pre-tension or cold tightening position, loosen nut or remove tie rod
3. Cutting welds or removing flanges: If it is a welded structure, leave 5-10mm margin for cutting along the flue side to avoid damage to the main flue base metal
4. Sectional disassembly: For large-size rectangular expansion joints, one-sided corrugated components can be disassembled first and hoisted out in blocks
5. Groove trimming: Remove residual welds and reprepare grooves according to welding process

3.2 Cutting Method and Flue Protection

It is recommended to use plasma cutting or mechanical cutting, and use gas cutting with caution (large heat influence and easy splash). When cutting, adjacent equipment and cables are shielded from fire, and fire extinguishing equipment is equipped in the flue.

3.3 Hoisting and temporary support

Immediately after the old expansion joint is removed, temporary supports or temporary blind plates shall be set on the disconnected surface of the flue to prevent the pipe from sinking or foreign objects falling into the system. Measure the staggered notch and non-parallelism of two sections simultaneously, as the adjustment basis of the new expansion joint in place.

4. Installation of new expansion joint: countermouth, welding and limit adjustment

4.1 Pipe Matching and Pre-Compression/Pre-Stretching

When installing a new metal expansion joint, it is necessary to determine whether pre-displacement is required according to the design displacement direction and the actual temperature at the time of installation:

• Installation at normal temperature (≤20℃): Pre-stretch or pre-compress according to the cold tightness given by the design institute
• Hot state (> 100℃) emergency repair installation: Calculate the thermal elongation corresponding to the actual shell temperature and adjust the installation length

Allowable deviation of counterpoint: axial deviation ≤3mm, radial misalignment ≤1.5mm (based on the thin-walled side), strong counterpoint is strictly prohibited.

4.2 Welding Process and Weld Quality

Welding shall be performed in accordance with the qualified WPS as assessed. For how to replace the metal expansion joint of the flue of the power plant, it is recommended that:

• Base layer: TIG welding to ensure root penetration and no oxidation on the back
• Filling and cover layer: SMAW or semi-automatic welding, strictly control the interlayer temperature ≤150℃
• Welding sequence: welding in segments symmetrically along the circumference to reduce shrinkage deformation

For different steel welding between austenitic stainless steel expansion joint and carbon steel flue, transition layer welding material (such as E309 series) should be selected, and small parameter and rapid welding process should be adopted to reduce hot crack and dilution rate.

4.3 Resetting of pull rod and limit device

After welding and cooling, reset tie rod nuts according to design requirements:

• Structure for transmitting thrust: The pull rod shall be in a stressed state, and the nut shall be properly tightened
• Structures for transportation protection only: The tie rod should be completely loosened or removed to avoid restricting the normal operation of the expansion joint

For structures with hinges or gimbal rings, confirm that the rotation is flexible and not stuck.

V. Inspection and acceptance: the key link to ensure the quality of replacement

5.1 Non-destructive testing of welds

• Surface test: 100% penetration test (PT) or magnetic particle test (MT) for all pressure-bearing or sealed welds, Class I qualified
• Internal inspection: Ray inspection (RT) for butt welds on a proportional basis, not less than Grade II

5.2 Tightness Test

The tightness test shall be carried out after the metal expansion joint is replaced. Common methods:

• Negative pressure flue: Start the induced draft fan to establish negative pressure, and use a candle or smoke generator to check the weld seam leakage
• Positive pressure flue: under the premise of safety and control, 0.05-0.1MPa compressed air pressure is maintained, and leak detection liquid is applied

5.3 Hot state review and operation monitoring

After the start-up of the unit, infrared thermal imaging and displacement observation of the expansion joint shall be carried out at 30%, 60% and 100% load stages:

• There is no abnormal overtemperature or local overheat on the bellows surface
• The actual displacement direction is consistent with the design
• No abnormal vibration or noise

VI. Frequently Asked Problems and Suggestions

VII. SUMMARY

How to replace the metal expansion joint of the flue of the power plant is not a simple "dismantling the old and installing the new", but a set of systematic engineering including furnace shutdown isolation, surveying and mapping customization, cutting and dismantling, accurate matching, welding control and tightness verification. The key to successful replacement is to guide the manufacture of new parts by measured data, control the deformation by symmetrical welding, and ensure the seal by specification inspection. For power plant maintenance personnel, selecting a construction team with similar coal-fired or gas-fired unit replacement experience, operating in strict accordance with the welding process regulations, and allowing sufficient hot state review time after replacement can significantly reduce the risk of secondary leakage and the probability of non-shutdown of units. Through the standardized replacement operation, not only the sealing and compensation function of the flue system can be restored, but also the whole life cycle of the expansion joint can be prolonged, which provides reliable guarantee for the safe and stable operation of the unit.

In the flue gas treatment system of coal-fired power plant, chemical industry and metallurgy industry, flue expansion joint corrosion is a common problem that affects the safe operation of equipment and the environmental protection standard. Whether it is the cracking of the bellows of the metal expansion joint or the leakage of the skin of the non-metal expansion joint, the corrosion problem not only leads to flue gas leakage and pollutes the environment, but also increases the energy consumption of the fan and shortens the equipment life. So, what about flue expansion joint corrosion? This paper will start with the analysis of corrosion mechanism, systematically explain the corrosion causes, coping strategies and engineering practice cases of different types of expansion joints, and provide professional solutions for technicians.

1. Main types and causes of flue expansion joint corrosion

To answer "What to do about flue expansion joint corrosion", you first need to clarify the type and root cause of corrosion. According to the engineering failure analysis, the corrosion of flue expansion joint is mainly divided into the following categories:

1.1 Dew Point Corrosion

When the flue gas temperature is below the acid dew point, SO₂ and SO₃ in the flue gas will combine with water vapor to form dilute sulfuric acid, which condenses and deposits on the surface of the expansion joint. This condensate is strongly corrosive to carbon steel and ordinary stainless steel。

Typical characteristics: The corrosion is evenly distributed, mostly occurring at the fluid accumulation at the bottom of the expansion joint, showing honeycomb or pit erosion.

1.2 Stress Corrosion Cracking (SCC)

Stress corrosion is the most common cause of metal expansion joint bellows failure. While subjected to pressure and displacement stresses, corrugated pipes are exposed to corrosive media (sulfur-containing flue gas), which can crack under conditions well below the yield strength of the material。

Typical features: The crack is dendritically branched, originating from the inner surface of the bellows and propagating along the crystal. Failure analysis of Inconel 625 bellows in a catalytic cracking unit shows that the NiS eutectic (melting point only 645 ℃) formed at grain boundary is the key factor inducing stress corrosion。

1.3 Intergranular corrosion

Because the grain boundary of the welding heat affected zone or the material itself is chromium depleted, the grain boundary is preferentially eroded under the action of the corrosive medium, resulting in cracking of the bellows along the grain boundary。

1.4 Chemical Corrosion and Liquid Corrosion of Non-Metallic Expansion Joints

In the desulfurization flue, although the main material (fluororubber, PTFE) of the non-metallic expansion joint skin is corrosion-resistant, there are inherent defects in its fixation mode. Grooves will be formed at the connection between the skin and the flange, and condensed acid water will accumulate during operation. The acid water will gradually penetrate through the skin fabric layer and corrode the fixing bolts, resulting in bolt breakage and skin loosening and leakage。

2. Countermeasures of flue expansion joint corrosion

For different types of corrosion, what to do about flue expansion joint corrosion needs to take differentiated technical measures:

2.1 Anti-corrosion measures of metal expansion joints

(1) Material upgrade

Of particular interest is the application of pure titanium expansion joints. The practice of Huadian Luohe Power Plant proves that the 2mm thick TA2 pure titanium expansion joint is adopted in the desulfurization wet flue, and the surface is as clean as new after one year of continuous operation, thus achieving "zero corrosion and zero leakage". Because that density of titanium is low (4.5g/cm³), the weight of titanium is more than 40% lighter than that of stainless steel under the same size, and the total cost is basically the same as that of non-metallic expansion joint。

(2) Structural Optimization

In view of stress corrosion cracking, it is suggested that the expansion joint bellows be upgraded from a single-layer structure to a double-layer structure, and the concept of "single-layer pressure bearing and double-layer design" is adopted. The double-layer structure can cooperate with the interlayer leakage online monitoring system. Once the inner layer leaks, the outer layer plays a buffer and protection role, and at the same time sends out early warning signals to avoid unplanned shutdown caused by sudden failure。

(3) Operation and maintenance measures

• Furnace shutdown protection: During maintenance, hot air is used to purge the inside of the expansion joint to prevent the condensation and deposition of sulfur-containing gas
• Temperature control: Increase the exhaust temperature above the acid dew point to avoid the formation of condensate
• Regular replacement: Replace the bellows regularly according to the design life (usually 3-5 years) to avoid overdue service

2.2 Anti-corrosion measures of non-metallic expansion joints

(1) Skin material optimization

The skin of non-metallic expansion joint should adopt multi-layer composite structure, and the materials of each layer should be accurately selected and matched according to the working conditions:

For the desulfurization clean flue, the skin material should be fluororubber instead of silicone rubber. Fluororubber has extremely high resistance to acid and alkali and high temperature saturated wet smoke。

(2) Structural improvement-radical cure scheme for effusion problem

The root cause of leakage of non-metallic expansion joints is the accumulation of acid water in the grooves at the skin platens。 Solutions include:

• Drainage design: Add a drainage pipe above DN100 at the bottom of the expansion joint to discharge the condensate in time
• Anti-corrosion of flange end face: Laying glass flake anti-corrosion layer and glass fiber reinforced plastic anti-corrosion layer on steel substrate in sequence, and then installing rubber elastic expansion joint, and fill the layers with silica gel seal
• Bolt protection: The fixing bolts are made of 316L stainless steel, coated with anti-loosening glue during installation, and periodically tightened

(3) Installation process control

• When the skin is installed, it should be unfolded from bottom to top, and the pressure plate bolts should be tightened repeatedly for 2-3 times
• Horizontal flue expansion joint, the outermost skin interface shall adopt the "up-pressing-down" overlap mode to prevent rainwater infiltration

3. Overall replacement scheme: thoroughly solve frequent corrosion

For expansion joints with repeated leakage and low repair value, the whole replacement with expansion joints made of high performance materials can be considered.

Project case: The outlet expansion joint of induced draft fan of 2#boiler of a company has been eroded and washed by high-temperature flue gas for a long time, resulting in serious leakage. Through the accurate formulation of special maintenance plans, the removal of old expansion joints, installation of new corrosion-resistant expansion joints, seal inspection and other procedures were completed, and finally one-time debugging was successful. After the replacement, the corrosion loss of the equipment is significantly reduced, and the operating efficiency of the induced draft fan is greatly improved。

4. Suggestions on anti-corrosion selection of different types of flues

V. Summary

Faced with the engineering problem of how to deal with the corrosion of flue expansion joint, the core lies in "prescribing the right medicine"-choosing the appropriate technical scheme according to the corrosion type and working conditions. Summary points are as follows:

1. Identify corrosion types: dew point corrosion requires temperature control and condensation prevention; Stress corrosion requires material upgrading and stress relief; Intergranular corrosion needs to control the welding process and material selection.
2. Material is the key: conventional desulfurization net flue recommends fluororubber non-metallic expansion joint.
3. Anti-corrosion of metal bellows: Nickel-based alloys such as Inconel625 are selected for high-temperature sulfur-containing working conditions; Consider double-layer structure + interlayer leakage monitoring to avoid sudden failure; Prevent dew point corrosion measures during furnace shutdown.
4. Non-metallic expansion joint anti-corrosion: the skin material is preferably fluororubber; A bottom drainage pipe is added on the structure; The pressure plate bolts are periodically tightened.
5. Active maintenance: Establish a regular inspection system for expansion joints, and deal with leakage in time. For the parts with low repair value, decisively replace them with high-performance expansion joints to fundamentally eliminate hidden dangers.

It is recommended that users carry out corrosion risk assessment according to specific working conditions, invite professional and technical personnel to conduct failure analysis when necessary, and formulate targeted anti-corrosion transformation schemes. Only in this way, the corrosion problem of flue expansion joint can be solved from the root cause, and the long-term safe and stable operation of smoke and air system can be ensured.

In industrial flue systems, non-metallic expansion joints (also known as non-metallic compensators) are key components to absorb thermal displacement, isolate vibration and reduce noise。 When the expansion joint reaches its service life or is damaged, the skin (i.e. hemming) needs to be replaced in time. So, how to remove the edging of the flue expansion joint? This paper will systematically explain the removal methods, pre-preparation, operation points and safety precautions of non-metallic expansion joint edging in combination with the general construction specifications of the industry, and provide professional technical guidance for on-site construction personnel.

1. What is flue expansion joint edging?

Before understanding how to remove the edging of flue expansion joint, we need to clarify the definition of edging first. The "edging" of the flue expansion joint usually refers to the skin part of the non-metallic expansion joint, which is the flexible sealing structure of the outermost layer of the expansion joint。

The skin of non-metallic expansion joints is generally made of multi-layer composite materials. Common structures include：

• Outer layer (weather resistant layer): silicone rubber or fluororubber coated fiberglass cloth for sealing and corrosion resistance
• Reinforcement layer: polyester fiber cloth or alkali-free glass fiber cloth to provide tensile strength
• Insulation: Insulation cotton wrapped in fiberglass cloth or stainless steel wire mesh
• Inner layer: Anti-corrosion layer directly in contact with smoke

The skin is secured to the metal frame of the expansion joint by a platen and bolts. When the skin is aged and damaged, it cannot be effectively sealed, and there will be problems such as smoke leakage and water leakage. At this time, the old skin must be removed and replaced with a new skin。

Preparation before demolition

The prerequisite for proper demolition is adequate preparation. Before the flue expansion joint edging removal operation, the following preparatory work must be completed:

2.1 Confirmation of Construction Conditions

• Shutdown isolation: Confirm that the flue system has been shut down to cool down, close the front and rear valves of the expansion joint, and empty the internal media
• Scaffolding erection: For high altitude or inconvenient parts, standard scaffolding should be erected in advance
• Power supply and gas supply: Ensure that the power supply and gas supply required for electric wrench and gas cutting equipment are available on site

2.2 Material and Tool Inspection

The following materials and tools should be counted before construction:

• New skin: confirm that the size, width and technical parameters match the original expansion joint
• Pressing plate and bolts: check the quality of the pressing plate (if the original pressing plate can be reused, it is necessary to confirm that there is no deformation); Prepare matching bolt assemblies
• Insulation cotton and insulation nails: If the internal insulation cotton needs to be replaced, prepare stainless steel wire mesh and insulation nails in advance
• Main tools: electric wrenches, wrenches, gas cutting equipment, fire extinguishers, etc.

2.3 Safety measures

• Fire prevention preparation: clean up the surrounding flammable items and equip fire extinguishing equipment (fire extinguishers, fire sand, etc.)
• Personal protection: construction workers should wear protective gloves, goggles and safety helmets; Working at height must wear seat belts and be supervised by safety officers
• Warning isolation: Warning signs are set up in the construction area to prohibit unrelated personnel from entering

3. Detailed explanation of the removal steps of edging of flue expansion joint

The following is the core operation process of how to remove the edging of the flue expansion joint. It should be noted that the removal method introduced in this paper is mainly suitable for the replacement scenario of non-metallic expansion joint skin.

3.1 Removal of fastening bolt assembly

This is the first step in the demolition work. Use a wrench and gas cutting equipment to cut or unscrew all the bolt assemblies of the original expansion joint fixing skin。

Operation Points:

• Try to remove lightly corroded bolts with a wrench first
• For bolts that are seriously corroded or cannot be twisted, gas cutting can be used to treat them
• Pay attention to controlling the flame direction when cutting to avoid damaging the metal frame

3.2 Removal of platen and old skin

After the bolts are removed, remove the pressure plate and the old skin and store them in the designated position respectively。

Key Notes:

• For platens that can be reused, be sure to mark the installation sequence (such as number or orientation mark)
• Proper marking ensures accurate alignment of the pressing plate openings with the flange openings when installing the new skin again, so as to avoid hole offset
• Centralized storage of old skin and uniform treatment after construction

3.3 Check the internal insulation cotton and metal frame

After removing the old skin, the internal structure of the expansion joint is exposed, and a full inspection is required at this time：

1. Insulation cotton inspection: If the original flue is filled with insulation cotton, judge whether it can be reused. If it can be reused, store it separately and protect it
2. Frame inspection: Inspect deflectors, counter flanges, and metal frames for deformation, corrosion, or damage
3. Repair treatment: For slightly deformed or damaged parts, make corrections; Seriously damaged needs to be replaced

This step is a key prerequisite to ensure that the new skin can be fitted properly and sealed tightly.

3.4 Cleaning the mounting surface

After removing old parts, clean the flange surface of the metal frame with a wire brush or sanding tool to remove residual sealant, rust and debris, and ensure that the surface is smooth and clean。 If the surface is uneven, the new skin cannot fit tightly, which will directly lead to leakage.

IV. Precautions during demolition

4.1 Protecting Reusable Parts

• Reusable components such as pressure plates and bolts should be handled carefully when dismantled to avoid deformation
• The marked pressing plates should be stored in order, and reset according to the marks during installation

4.2 Damage prevention framework

• Pay attention to protecting the metal frame during gas cutting operation, and it is strictly prohibited to cut to the flange surface
• After removing the old bolts, check that the threaded holes are intact

4.3 Safety First

• Pay attention to fire prevention in the whole process, hot fire operation needs to handle hot fire ticket and have special person supervision
• Fasten safety belts when working at height, and fix tools with safety ropes to prevent falling

V. Brief Guide for Installation of New Skin

After removing the old hemming, a new skin needs to be installed as soon as possible to restore system operation. The installation process is briefly described as follows:

1. Install heat insulation cotton: first install the inner skin cotton, distribute and set heat insulation nails, inner layer of heat insulation cotton wrapped with stainless steel wire mesh, then install other layers of heat insulation cotton in turn, and fix it smoothly with heat insulation nails
2. Lay a new skin: The skin must be placed flat on the metal frame flange (to prevent wrinkles). Installation starts at the lower part and unfolds outward in sequence
3. Fasten the platen: After the skin is tightened, press it with the platen and bolts. All compression bolts are tightened with electric wrenches. No missed screwing is strictly prohibited
4. Special requirements: For horizontal flue expansion joint, the upper interface should be pressed down at the outermost skin interface to prevent water seepage

VI. SUMMARY

How to remove the edge of flue expansion joint-this problem is related to the quality and safety of expansion joint replacement project. The core points are summarized as follows:

1. Safety first: shutdown isolation, fire and electricity prevention and high-altitude protection are the prerequisites for demolition operation
2. Orderly removal: first remove the bolts → take the pressure plate → remove the old skin → check the insulation cotton and frame → clean the surface
3. Protection of reuse parts: Mark the reusable plates in order to ensure that the holes are aligned during installation
4. Inspection in place: After removal, deflectors, flanges and frames must be inspected for deformation or damage, and corrected or replaced if necessary

Mastering the correct dismantling method can not only improve the construction efficiency, but also lay a good foundation for the subsequent installation of new skin, ensure the expansion joint to restore good sealing performance, and ensure the long-term stable operation of flue system.

In the equipment management of industrial flue system, maintenance documents are the core basis for guiding on-site operation, ensuring maintenance quality and realizing process traceability. However, the operation and maintenance personnel of many power plants and chemical plants are not clear: What does the flue expansion joint overhaul document include? What should a complete overhaul document package contain to meet the standardized management requirements? This paper will systematically sort out the technical documents, management forms and acceptance data required for the maintenance of flue expansion joints, and provide practical checklist reference for equipment managers and maintenance engineers.

1. Classification framework of maintenance documents of flue expansion joints

To answer "What Does a Flue Expansion Joint Overhaul Document Include", you should first classify the documents from a functional point of view. In general, a complete overhaul document package can be divided into four main categories:

The specific contents of each category of documents will be described in detail below.

Technical specification documents

This is the technical basis for the overhaul work, which must be fully prepared before the overhaul begins.

1. Technical specifications for maintenance of expansion joints

• Contents: Scope of application, reference standards (e.g. GB/T 12777, DL/T 782, etc.), maintenance cycle, maintenance items, quality requirements
• Requirements: It should be an official document prepared or quoted by this unit, and the version is currently valid

2. Equipment drawings and technical data

• Expansion joint body drawing: structure size, material, compensation amount, pressure grade
• Installation position diagram: specific position of the expansion joint in the flue system, connection mode, adjacent bracket arrangement
• Flue system diagram: flue gas flow direction, temperature measuring point, expansion center position
• Original design parameters: design temperature, pressure, displacement, service life

3. Manufacturer Technical Documents

• Product certificate and factory inspection report
• Installation and use instructions (including pre-tension/pre-compression requirements, bolt torque values)
• Spare parts list and recommended replacement period

4. Historical maintenance archives

• Completion report and replacement record of last overhaul
• Problems found in previous inspections and their handling
• Accumulated running time and starting and stopping times of expansion joint

Programme and Plan Documents

This part of the document answers the question of "how to carry out maintenance" and is a direct guidance for field operations.

1. Maintenance construction plan

The main contents include:

• Project Overview: Maintenance Scope, Main Workload, Planned Period
• Organizational measures: Division of labor between the person in charge of maintenance, safety officer and operator
• Technical measures: disassembly sequence, inspection items, installation process requirements
• Safety measures: shutdown isolation, ventilation, gas detection, personal protection, emergency plan
• Quality assurance measures: key quality control points, inspection methods

2. Safety risk analysis and control measures

• JSA/JHA Analysis Table: Analyze the risks of disassembly, hoisting, welding, pressure test, etc. item by item
• Risk Level Assessment: High, Medium and Low Risk Categories
• Control measures: preventive and emergency measures corresponding to each group of risks

3. Maintenance schedule

• Maintenance network diagram or Gantt chart
• Critical nodes: downtime, disassembly completed, inspection completed, reassembly completed, trial operation completed

4. List of spare parts and materials

• Spare parts list: expansion joint body (if replaced), gasket, bolt nut, guide tube
• Material list: welding materials, sealants, anti-corrosion coatings, insulation materials
• Tool list: torque wrench, hand hoist, gas detector, scaffold

IV. Process record files

What is included in the overhaul documents of flue expansion joints-Process record class is the core of overhaul "trace management" and the basis of quality traceability.

1. Inspection records before maintenance

• Appearance inspection record sheet (with photos)
• Record of leak test (if any)
• Raw dimensional measurements (expansion joint installation length, flue flange spacing)

2. Disassembly Process Record

• Record of fastener removal (rust condition, whether to replace)
• Record of damage of old expansion joint (crack, perforation, aging, deformation)
• Inspection record of flange surface (flatness, corrosion depth)

3. Critical process control records

4. Defects and Handling Records

• Defect ledger: defect location, type (crack, corrosion, deformation, etc.), size, photo number
• Processing records: processing methods (grinding, repair welding, replacement, repainting, etc.), processing personnel, acceptance results

5. Maintenance log

• Daily homework content and completion progress
• Problems encountered and solutions
• Work plan for the next day

V. Acceptance and delivery documents

After the overhaul is completed, a series of confirmation documents need to be formed, marking the closed loop of the overhaul work.

1. Quality inspection/acceptance records

• Sub-item acceptance records: flange cleaning acceptance, expansion joint installation acceptance, bolt tightening acceptance
• Test Report:
  • Airtightness test report (test pressure, holding time, pressure drop value)
  • Where welding is involved: NDT report (PT, MT report)
• Level 3 acceptance signature form: team self-inspection → professional re-inspection → department final inspection

2. Completion report

Main content:

• Overview of Maintenance: Comparison between Planned and Actual Duration
• Main work content: List of replacement parts, handling of major defects
• Quality Evaluation: Acceptance Conclusions
• Remaining Issues and Suggestions: Note if there are any unaddressed issues
• List of attachments: Index of all process records

3. Trial operation records

• Inspection record of expansion joint condition after system start-up (with leakage or abnormal deformation)
• Displacement inspection record under hot condition (comparison of cold and hot dimensions)
• Operation Parameter Record (Temperature, Pressure)

4. Document Filing Checklist

What is included in a complete flue expansion joint overhaul document should ultimately be summarized in the form of a filed checklist:

• Maintenance construction plan (signed version)
• Safety Risk Analysis Table
• Drawings and technical data (stamped with "verified on site")
• Certificate of Spare Parts and Material Certificate
• Pre-maintenance inspection records and photos
• Process Control Record (Process Card)
• Defect Handling Ledger
• Maintenance log
• Quality Acceptance Document and Test Report
• COMPLETION REPORT
• Trial operation record
• Electronic version of scanned copy CD/folder

VI. Differences in documents of different types of expansion joints

What is included in the flue expansion joint overhaul documentation will vary slightly depending on the type of expansion joint:

Common Problems and Improvement Suggestions of Document Management

In actual maintenance, the following problems often occur in document management:

VIII. Summary

What does the maintenance document of flue expansion joint include? A set of standardized and complete maintenance document package should cover the whole process of "technical basis → operation instruction → process record → acceptance and delivery", which can be summarized as follows:

1. Technical specification category: maintenance procedures, equipment drawings, manufacturer's documents, historical files-answer "what standard to repair"
2. Plan category: construction plan, risk analysis, schedule, spare parts list-answer "how to repair it, who will repair it, what to repair it with"
3. Process record category: inspection record, process control record, defect ledger, maintenance log-answer "what was repaired, what was found, and how to deal with it"
4. Acceptance and delivery category: quality acceptance document, test report, completion report, trial operation record-answer "How is the repair and whether it is qualified"

Summary of core points:

• Documents are not the formalism of "finishing and replenishing", but the process control tool of overhaul quality. Each record serves as the basis for future failure analysis and life assessment.
• It is recommended to use the "package" management mode, where all the above documents are packed into a dedicated file box (or electronic folder) with a list of documents attached to the cover.
• For outsourced maintenance projects, Party A shall incorporate the "completeness and standardization of maintenance document package" into the contractor's assessment index.
• After the overhaul, the file package shall be filed within 15 working days, and the storage period shall not be less than two overhaul cycles (usually 8~10 years).

Mastering "what is included in the maintenance documents of flue expansion joints" is not only the basic skill of equipment managers, but also the basis for realizing preventive maintenance and whole life cycle management. A standardized and detailed maintenance document package can not only guide on-site operation, ensure maintenance quality, but also provide valuable analysis basis in case of failure. It is an indispensable link in the equipment management system of power plants and chemical plants.