Specialized in manufacturing compensators, expansion joints, baffle doors

In industrial pipeline system, expansion joint is the key component to absorb thermal displacement, isolate vibration and protect the safe operation of equipment. Whether it is the electric power, petrochemical, steel or heating industry, once the expansion joint fails, it will leak and disturb the people, or even cause safety accidents. However, in the face of a dazzling array of metal, non-metal, fabric, rubber and other expansion joints on the market, how to scientifically select, standardize the installation and accurately maintain them? This article will provide you with a complete guide from principles to practical combat.

1. What is the expansion joint? Why can't the system live without it?

Expansion joint, also known as compensator or expansion joint, is a flexible connection device that uses the effective deformation of elastic elements to absorb the displacement, rotation angle or vibration of pipelines or equipment due to thermal expansion and contraction, mechanical vibration, etc. Industrial pipes produce significant thermal elongation when the temperature changes: a length of carbon steel pipe 100 meters long can thermally elongate up to about 450 mm when the temperature rises from 20 °C to 400 °C. Without the expansion joint to absorb this displacement, the thermal stress will bend the pipe, deform the support, and tear the equipment interface.

Therefore, the core functions of expansion joints include: absorbing axial, transverse and angular displacements; Reduce the reverse thrust of the pipeline to the equipment; Isolate mechanical vibration and reduce noise; At the same time, it is convenient to install and disassemble the pipe.

2. Main types and applicable scenarios of expansion joints

According to the differences of materials and structures, expansion joints are mainly divided into the following four categories:

1. Metal expansion joint

Consists of stainless steel bellows, end pipe and guide tube. The advantages are strong pressure capacity (up to several MPa), high temperature resistance (up to more than 600 ℃) and long life. It is suitable for high temperature and high pressure steam pipeline, boiler outlet flue, inlet and outlet of petrochemical high temperature reactor and other working conditions.

2. Non-metallic expansion joint (fabric expansion joint)

Made of multi-layer composite materials (glass fiber, PTFE film, silicone rubber cloth, etc.). It can absorb three-way displacement at the same time, has excellent vibration isolation and noise silencing performance, and is corrosion resistant and economical in price. It is commonly found in low-temperature sulfur-containing flue gas environments such as wet desulfurization system, gas turbine exhaust duct, dust collector inlet and outlet, etc. The temperature resistance generally does not exceed 400℃.

3. Rubber expansion joint (rubber soft joint)

The main body is reinforced by natural rubber or synthetic rubber plus nylon cord. It has good elasticity and excellent vibration reduction effect. It is suitable for inlet and outlet of water pump, air conditioning water system, low pressure water supply and drainage pipeline. Not resistant to high temperatures (usually ≤120℃) and oily media.

4. Sleeve expansion joint

The axial displacement is absorbed by the sliding of the inner and outer cylinders, the structure is simple, the compensation amount is large, but the sealing performance is poor, and it is suitable for low-pressure and large-diameter thermal pipe network.

3. Core parameters of expansion joint selection

The correct selection of expansion joints requires clarification of the following 8 key parameters:

Selection suggestions: preferential selection of metal type for high temperature and high pressure; Moisture-containing sulfur-containing multidirectional displacement is preferably a non-metallic type; For pure vibration damping applications, rubber type is preferred.

4. Installation specifications and common errors of expansion joints

1. Check before installation

• Check whether the model, specification and pressure level are consistent with the design
• Check the bellows surface for mechanical damage and corrosion
• Verify that the direction of the liner cylinder (guide cylinder) is consistent with the flow direction of the medium

2. Installation Critical Requirements

• It is strictly prohibited to adjust the deviation of pipeline installation by stretching or compressing bellows
• After installation, the transport protection rod must be removed (Note: the positioning limit rod cannot be removed)
• Guide brackets shall be provided on both sides of the expansion joint, and the spacing shall be ≤4 times the pipe diameter
• For those with cold tightness requirements, pre-deform according to the design value

3. Common mistakes and their consequences

• Error 1: Transportation tie rod is not removed → the expansion joint cannot compensate for displacement, and the thermal stress of the pipeline causes equipment damage
• Error 2: Missing guide bracket → cylindrical instability of expansion joint and lateral tear of bellows
• Mistake 3: Welding splash damaged bellows during installation → stress corrosion cracks formed, early leakage
• Error 4: The horizontally installed non-metallic expansion joint has no drainage hole → water accumulation in the groove penetrates and leaks

V. Daily maintenance and fault diagnosis of expansion joint

Periodically inspect items

• Appearance inspection: Once a month, check the bellows for corrosion, crack and bulge; Whether the non-metallic skin is aged or damaged
• Fastening inspection: Re-tighten the non-metallic expansion joint pressure plate bolts quarterly (once in 1 month and once in 3 months after initial operation)
• Temperature monitoring: Infrared temperature measurement, if the surface of non-metallic expansion joint abnormally heats up, it indicates that the inner heat insulation layer is damaged
• Leak detection: Check the bottom of the expansion joint with pH test paper for acid water dripping

Typical faults and treatment

VI. Life Prediction and Economic Analysis

The service life of reasonably selected and maintained expansion joints is as follows:

• Metal expansion joint (carbon steel): 3-5 years (corrosive environment) or 8-10 years (clean environment)
• Metal expansion joint (316L/904L): 5-8 years
• Non-metallic expansion joints: 2-4 years (sulfur-containing wet flue gas) or 5-6 years (clean flue gas)
• Rubber expansion joints: 3-5 years

It is recommended to establish an expansion joint ledger to record the installation date, working condition parameters and problems found in each overhaul. When the maintenance cost exceeds 50% of the new purchase price, or the frequency of leakage increases significantly year by year, it should be replaced as a whole.

Call to Action

Are you struggling with problems with frequent leaks, confusion in choosing models, or incorrectly installed expansion joints? Contact our engineering and technical team today for one-on-one expansion joint selection assessment and working condition diagnosis services.

In wet desulfurization systems, flue gas expansion joint water leakage is one of the most common and troublesome equipment failures. Whether it is a non-metal skin expansion joint or a metal expansion joint, once leakage occurs, it can cause ground pollution and pungent acid mist, and in the worst case, it will force the unit to reduce the load or even shut down for treatment。 This paper will systematically explain how to deal with the leakage of flue gas expansion joint from the leakage mechanism, cause analysis to the treatment plan, and help the operation and maintenance personnel to quickly locate the fault and take effective measures.

1. The harm and urgency of water leakage of flue gas expansion joint

Flue gas expansion joint water leakage is not a simple "water seepage" problem. The flue gas temperature after desulfurization usually drops to 45-55℃ and contains a large amount of saturated water vapor and corrosive media such as SO₂, SO₃, Cl⁻¹。 When leakage occurs in the expansion joint:

• Corrosive acid water spillage: Leaked condensate can have a pH as low as 2-3, causing severe corrosion to equipment platforms, steel structures and ground.
• Environmental risks: Pungent acid fog and visible "waterfall" water leakage can easily lead to environmental complaints.
• Equipment interlocking damage: Acid water penetrates into the interlayer of the skin of the expansion joint or the bolt hole, which will accelerate the corrosion failure of the fixture and lead to the expansion of leakage。
• Potential safety hazards: Treatment of leakage points in high-temperature flue areas, there are risks of scald, poisoning and limited space operation。

Therefore, once signs of flue gas expansion joint leakage are found, the cause must be analyzed immediately and targeted measures must be taken.

2. Three root causes of expansion joint leakage

1. Structural design defect: groove water accumulation effect

This is the core cause of non-metallic expansion joint water leakage. When the non-metallic expansion joint is installed with the skin, an annular groove will naturally be formed between the pressure plate and the skin。 When the unit is running, the condensed acid water in the wet flue gas accumulates in this groove and cannot be discharged naturally. Under long-term immersion, acid water causes leakage through the following paths:

• Slowly penetrates the skin fabric layer and corrodes the internal insulating cotton
• Infiltrating through gaps in platen bolts, corroding screws resulting in loosening or breaking
• Acid water flows out from the broken bolt hole or the damaged skin, forming a "small waterfall" phenomenon

Typical performance: Leakage starts after 1-2 years of operation, and leaks again in a short time after replacing the new skin.

2. Improper material selection

In order to reduce the cost, some manufacturers choose non-corrosion-resistant silicone rubber as the skin lining layer. Silicone rubber is rapidly aging and brittle in acidic environment, resulting in acid water contacting the platen bolt directly after the inner layer is damaged。 In addition, the 316L stainless steel expansion joint has an average lifetime of no more than two years in wet flue gas with high Cl⁻¹ concentration。

3. Installation and maintenance defects

• The bolts are not repeatedly tightened: the non-metallic expansion joint pressure plate is 4-6 meters long. After one-time tightening, the distal bolts will loosen due to skin compression deformation。
• Missing or worn deflectors: Causes dusty smoke to directly scour the bellows or skin inner layer.
• Too fast start-stop speed: drastic temperature changes make the expansion joint subject to alternating stress, accelerating fatigue cracking。

3. Quick diagnosis of water leakage types

IV. Rapid repair and radical cure plan

Scheme 1: Temporary leak plugging treatment

For non-emergency flue gas expansion joint water leakage, polymer sealing material can be used for temporary plugging:

1. Clean up dust accumulation and loose anti-corrosion layers around leakage areas
2. Welding repair of corroded and perforated metal framework
3. Use highly elastic tung oil gel or special plugging cement to fill grooves and cracks
4. The surface is sealed with flexible glass flakes for corrosion protection

This protocol can be implemented without shutdown or short shutdown conditions and can be maintained for 6-12 months after treatment.

Option 2: Radical repair-groove filling technique

To fundamentally solve the flue gas expansion joint water leakage, it is necessary to eliminate the groove water accumulation this root source. The mature process scheme is as follows：

1. Shutdown cleaning: remove the dust accumulated around the expansion joint and cut the original damaged skin
2. Skeleton repair: Clean up floating dust of metal frame and weld reinforcement of corroded parts
3. Bottom layer anti-corrosion: brush highly elastic tung oil gel on the bottom of the groove
4. Fill the sealing layer: fill in high-temperature and corrosion-resistant closed-cell foamed butyl rubber filler (compression ratio 7:1), and compact in layers
5. Surface sealing: The groove is completely smoothed with highly elastic tung oil gel, and the thickness is controlled at about 5mm
6. Perimeter strengthening: Flexible glass flakes are used to prevent corrosion at the joints around the expansion joints, with a thickness ≥3mm

Key advantages: After filling the groove, acid water cannot accumulate, which fundamentally eliminates the penetration path, and the filling material is elastic, which does not affect the normal expansion and contraction function of the expansion joint.

V. Preventive maintenance recommendations

1. Regular tightening of bolts: Re-tighten the non-metallic expansion joint pressure plate bolts every time the machine is shut down for maintenance
2. Check water accumulation in grooves: Check the expansion joint for abnormal water seepage through temperature measurement or observation hole during operation
3. Control start-stop rate: avoid sharp temperature change and reduce thermal stress shock of expansion joint
4. Establish a ledger: record the time, location and treatment method of each leak, and provide a basis for subsequent selection

In industrial flue gas treatment system, the selection of expansion joint is important, but the scientific and reasonable expansion joint arrangement of flue gas pipeline is the core to ensure the long-term stable operation of the system. Improper arrangement can lead to premature failure of expansion joints, pipe deformation and even equipment damage. Starting from engineering practice, this paper systematically explains the principles, common misunderstandings and optimization schemes of expansion joint arrangement of flue gas pipeline, so as to help technicians avoid risks from the source.

First, why is the layout of flue gas pipe expansion joint crucial?

Flue gas pipelines are usually connected to boilers, dust collectors, desulfurization towers, induced draft fans, chimneys and other equipment, and the operating temperature ranges from normal temperature to above 600℃. Pipes can produce significant thermal elongation in the hot state, and if the expansion joint is not set in the appropriate position or is arranged in a wrong way, thermal stress can be transmitted through the pipe to the equipment interface, resulting in flange leakage, foundation cracking or equipment shell deformation.

Correct flue gas pipe expansion joint arrangement can: effectively absorb the thermal displacement and vibration of the pipe; Reduce the thrust of the pipe on the fixed bracket and equipment; Preventing weld cracking due to accumulation of thermal stress; At the same time, it is easy to overhaul and replace in sections. Conversely, layout defects are often exposed after months of system operation, but are extremely costly to fix – involving furnace shutdowns, scaffolding erection, and extensive cutting and welding operations. Therefore, it is of significant economic value to master the standard layout method at the design stage.

2. Basic principles of expansion joint arrangement of flue gas pipeline

Regardless of whether metal or non-metal expansion joints are used, the following five principles are generally applicable:

1. Principle of targeted compensation

The expansion joint shall be arranged between the two fixed brackets of the pipe, specifically to absorb the amount of thermal elongation of the pipe section. An expansion joint should not compensate for displacement in multiple directions at the same time, unless a gimbal type construction is adopted.

2. Close to displacement source principle

For equipment inlet and outlet pipes, the expansion joint should be arranged as close to the equipment interface as possible (usually ≤4 times the pipe diameter) to directly absorb the thermal displacement of the equipment body. For example, it is most reasonable to arrange the expansion joint of the flue gas pipe at the inlet and outlet of the induced draft fan at a distance of 1.5-2 meters from the fan housing.

3. Principle of guiding and limiting cooperation

Guide brackets must be provided at both ends of the expansion joint, and the distance between the guide frame and the expansion joint should be controlled within 4 times the pipe diameter. At the same time, a limit bracket is arranged on one side of the expansion joint to prevent excessive lateral swing of the pipeline due to unexpected pressure pulsation.

4. Principle of avoiding blind plate force impact

When arranging the expansion joint at the elbow or blind end position of the pressurized flue gas pipe, the blind plate force generated by internal pressure must be considered. This force can reach several tons or even tens of tons and must be withstood by the main fixing bracket. Do not place the expansion joint directly at the end of a straight pipe section without a fixed bracket.

5. Principle of avoiding high-temperature accumulation zones

The expansion joint of flue gas pipeline of non-metallic expansion joint should be arranged to avoid the direct flushing surface of flue gas, especially not on the flushing side of sharp turn of flue. Install deflectors or insulation liners if necessary.

3. Layout scheme under typical working conditions

Scheme 1: Arrangement of long and straight horizontal flue

For horizontal flue gas pipes exceeding 30 meters in length, a "segmented compensation" strategy should be adopted: a set of fixed brackets every 15-20 meters with an axial-type expansion joint arranged between them. Each expansion joint absorbs the axial thermal elongation of the segment, and adjacent tube segments do not interfere with each other. Note: The expansion joint should be arranged close to the main fixing frame in the fixing bracket, while the guide frame is equally spaced over the pipe section.

Option 2: Vertical flue and equipment connection

On the vertical flue at the inlet of the desulfurization tower or the outlet of the dust collector, the flue gas pipe expansion joint arrangement should take into account the influence of gravity. It is recommended to use an axial type expansion joint with a load-bearing ring and set a spring hanger below it to avoid the expansion joint bearing the flue self-weight. At the same time, the spacing between the guide brackets at both ends of the expansion joint on the vertical pipe should be shortened to less than 3 times the pipe diameter to prevent instability.

Scheme 3: Universal compensation for space-constrained areas

When the flue direction is complex and limited by the building structure, a single expansion joint cannot meet the multi-directional displacement requirements. In this case, a combination arrangement of hinge-type or universal-type expansion joints may be used. For example, a hinge-type expansion joint is arranged on both sides of the horizontal elbow, and it is matched with an intermediate fixing bracket to absorb angular displacement in both directions. This scheme is commonly found in the flue of the inlet and outlet of the denitrification reactor.

Scheme 4: Protective arrangement of high-temperature dusty flue

The flue gas in the tail flue of coal-fired boiler (from the outlet of air preheater to the inlet of dust collector) has high dust content and the temperature is about 150-180℃. When arranging expansion joints of flue gas pipelines in such areas, wear-resistant guide tubes must be installed, and the length of the guide tubes should extend to at least 50mm after the trough of the expansion joints. At the same time, the expansion joint should be arranged at the lower position of the horizontal central axis of the flue section to avoid dust accumulation.

4. Common layout errors and correction methods

V. Key Points of Parameter Calculation in Layout Design

The following key parameters must be obtained before arranging the expansion joint of the flue gas pipe:

• Thermal elongation: Δ L = α × L × Δ T, where α is the linear expansion coefficient of the pipeline (12×10⁻⁶/℃ for steel), L is the length of the pipe between the two fixed frames, and Δ T is the difference between the installation temperature and the working temperature.
• Allowable compensation amount of expansion joint: It should be greater than 1.2 times of the calculated thermal elongation, and the safety margin should be reserved.
• Blind plate force: F = P × A, P is the working pressure (kPa) and A is the effective area of the bellows (m²). For large diameter flues (diameter> 2m), the blind plate force may exceed 30 tons, and a heavy-duty fixing bracket must be designed.
• Guide frame spacing: L_max ≤0.25× (E × I/P_c) ^0.5, where P_c is the critical instability load of the pipeline.

In practical engineering, it is recommended to use professional pipeline stress analysis software (such as CAESAR II) for calibration, especially for pipeline systems involving high temperature, large diameter or complex strike.

VI. Precautions during construction and acceptance

After the layout design is completed, the on-site construction stage still needs to focus on:

• Check whether the actual installation length of the expansion joint is consistent with the design drawing and whether the cold tightness value is correctly marked.
• Check that the clearance of all guide brackets and limit brackets meets the design (usually guide clearance 2-5mm).
• After the expansion joint is installed, remove the transport protection rod (Note: the limit rod with the positioning function cannot be removed).
• During the system pressure test, temporary constraints should be set on the expansion joint area to prevent overpressure deformation.

Call to Action

The reasonable arrangement of flue gas pipe expansion joints is directly related to the continuous production safety and maintenance cost of your plant. If you're planning a new flue system or facing frequent problems with existing pipes, feel free to contact our duct design team today.

During the operation of cement production line, the reasonable formulation of the replacement scheme of expansion joint of tertiary air duct is directly related to the maintenance efficiency of kiln shutdown and the long-term stable operation of equipment. The expansion joint of the tertiary air duct is in the high temperature flue gas environment of 850℃-1100℃ for a long time, and it is subjected to multiple effects of thermal stress, dust erosion and mechanical vibration. Usually, problems such as bellows cracking and fatigue failure occur every 2-3 years。 This paper will systematically elaborate a set of complete and executable three-time duct expansion joint replacement scheme from five dimensions: working condition diagnosis before replacement, preparation before construction, standardized operation process, quality acceptance standard and safety control.

1. Why should we formulate a professional replacement plan?

The expansion joint of the tertiary air duct is located at the key connection between the kiln tail preheater and the decomposition furnace. The replacement operation is usually carried out at high altitude (more than 30 meters above the ground), and involves many risk factors such as high temperature, limited space and hoisting。 If the replacement scheme of the expansion joint of the tertiary air duct is not perfect, it may lead to: excessive replacement period affecting the resumption of production, unqualified welding quality causing secondary leakage, improper pre-stretching of the expansion joint to accelerate damage, and even safety accidents. A scientific scheme should cover the technical points of the whole process, such as dismantling old parts, installing new parts, repairing liners and adjusting supports。

2. Working condition diagnosis and data acquisition before replacement

Before formulating a specific three-time replacement plan for the duct expansion joint, the following diagnostic work must be completed:

1. Failure Cause Analysis: Check the failure mode of the old expansion joint-is it high temperature oxidation of the bellows, intergranular corrosion, or mechanical tension cracking caused by cylinder deformation? For example, the cylinder of the tertiary air duct in a factory is oval due to long-term operation, and the flange of the expansion joint is cracked in the circumferential direction. If the expansion joint is simply replaced without correcting the cylinder, it will fail again in a short time。
2. Field measurement: The length, diameter and support point position of each section of air duct are measured, and the design drawings are corrected in combination with thermal expansion。 Special attention should be paid to the ellipticity of the cylinder before and after the expansion joint-if the deformation is serious, the coil plate repair welding should be performed before replacement to fill the gap (usually 0-200mm)。
3. Bearing Condition Assessment: Check the pipe bearing for sinking, offset, or jamming. Bearing failure will change the stress state of the expansion joint, resulting in insufficient compensation。

3. Preparation before construction: people, machines, materials, methods and environment

A successful tertiary duct expansion joint replacement protocol must include careful preparation:

Standardized replacement operation process

The following are the core operating steps of the tertiary duct expansion joint replacement scheme verified by several cement enterprises:

Step 1: Kiln shutdown, temperature reduction and isolation

Verify that the system is completely out of service and the temperature drops to a safe range. Close the tertiary air sluice plate and install a blind plate to prevent hot air from entering.

Step 2: Remove old expansion joints and accessories

1. Removing the bellows and guide tubes before and after the old expansion joint;
2. Remove the internal castables and refractory bricks (Note: the refractory bricks of the tertiary air duct are usually 114-220mm high alumina bricks or silica moxide bricks, with large thickness and high hardness)；
3. Cut the connecting weld between the old expansion joint and the air duct, and pay attention to protecting the base metal;
4. Hoist the old expansion joint to the designated position on the ground for recovery。

Step 3: Cylindrical correction and support repair

• Carry out coil calibration or partial replacement on the deformed cylinder to ensure that the roundness of the interface of the new expansion joint reaches the standard;
• Adjust and repair damaged bearings to eliminate pipe sinking or offset。

Step 4: New Expansion Joint Installation – Critical Quality Control Points

This is the technical core of the tertiary duct expansion joint replacement scheme:

1. Pre-stretch treatment: The expansion joint is pre-stretched according to the amount of thermal expansion calculated by the design. During installation, the direction of expansion and contraction of bellows must be consistent with the direction of air flow；
2. Alignment welding:
   • The weld seam needs to be welded on both sides, and the weld seam is 2mm higher than the steel plate;
   • The surface is smooth, and pores and trachoma are not allowed;
   • It is strictly prohibited to start an arc on the wave plate of the expansion joint, and the welding splash shall not fall on the wave plate；
3. Gap treatment: If there is a gap between the new expansion joint and the pipe, it should be filled with coil plate repair welding。

Step 5: Liner Refractory Repair

After the expansion joint is replaced, the liners in the front and rear areas of the expansion joint need to be repaired simultaneously:

• It is recommended to adopt the combination scheme of low thermal conductivity mullite brick (114mm) + nano insulation board (80mm), which can reduce the shell temperature by about 55℃；
• The castable needs to be poured in a supporting mold, and the temperature can be raised only after the curing is in place;
• The pouring hole must be sealed after construction to prevent air leakage。

Step 6: Anti-corrosion and Acceptance

• Painting high temperature resistant anti-corrosion paint on pipe surface；
• Check all welds for tightness and conduct airtightness tests if necessary;
• Record the installation data and file it for future reference.

Quality Acceptance Criteria

The qualified tertiary duct expansion joint replacement plan must be matched with clear acceptance indicators:

1. Weld quality: no pores, slag inclusion, unfused, weld height up to standard;
2. Expansion joint state: the pre-stretching amount conforms to the design value, and the bellows is free of twist and scratch;
3. Sealability: There is no hot air leakage around the expansion joint after operation, and the surface temperature is normal;
4. Compensation function: Expansion energy-saving free expansion and contraction after heating, no sticking or abnormal deformation.

6. Typical cases and experience

When a cement enterprise replaced the expansion joint of air duct three times in 2021, the expansion joint cracked again only 6 months after the replacement because the deformed cylinder and support were not corrected synchronously. Subsequently, the process described in this scheme is adopted: first, the ellipticity of the cylinder is corrected, the three sinking supports are repaired, and then the new expansion joint (RA330 material + lining castable) is installed, which has been in stable operation for 26 months so far。 This case proves that the complete tertiary duct expansion joint replacement scheme must include barrel correction and bearing repair in the scope of work, rather than "only replacing the expansion joint itself".

In flue gas discharge system, waste heat recovery pipeline and desulfurization and dust removal device, expansion joint is the key component to ensure the safe operation of pipeline. However, wrong flue expansion joint selection can directly lead to premature equipment failure, air leakage and even system shutdown. In the face of high temperature, corrosion, vibration and other changing working conditions, how to scientifically complete the selection of flue expansion joint? This paper starts from the five core steps, combining the material, structure and installation limitations, and provides a landing selection guide to help enterprises avoid common pitfalls.

1. Clarify the working condition parameters-the data basis of type selection

Any rigorous flue expansion joint selection must start with complete working condition data acquisition. Be sure to collect the following four categories of parameters:

• Temperature parameters: maximum/minimum continuous operating temperature, instantaneous limit temperature (e.g. accident conditions). For example, the temperature of the flue inlet section of coal-fired boiler can reach above 850℃, while the flue after desulfurization is only 50-80℃.
• Pressure parameters: system design pressure, working pressure and possible positive or negative pressure values (significant difference before and after induced draft fan).
• Media composition: the concentration of corrosive substances such as SO₂, NO₂ and Cl⁻¹ in the flue gas, as well as the dust content and particle size distribution.
• Displacement quantity and frequency: axial, transverse and angular displacements caused by thermal expansion of pipelines, and the number of cycles caused by starting and stopping of equipment.

Tip: Ignoring either item may cause the flue expansion joint selection to fail. For example, by providing only temperature without informing media corrosiveness, selected stainless steel bellows may experience stress corrosion cracking within one week in a chlorine-containing environment.

Second, selection of corrugated pipe material-the determinant of life

Material is the most direct influence on durability in flue expansion joint selection. According to the flue gas characteristics, the following matching schemes are recommended:

Note: If the budget is limited but the temperature is high in the selection of flue expansion joint, a double-layer bellows structure can be considered-the inner layer is made of high-temperature resistant alloy and the outer layer is made of ordinary stainless steel, which not only ensures sealing performance but also controls cost.

3. Determine the structural form-the core of compensation ability

The structural form of the expansion joint determines which directions of displacement it can absorb and the amount of compensation. Common types and applicable scenarios are as follows:

1. Single axial type
   It is suitable for straight flues with only axial displacement and long pipe sections. If the displacement of flue expansion joint exceeds 50mm when selecting the type, it is recommended to adopt a structure with pull rod to prevent the internal pressure thrust from damaging the fixed bracket.
2. Complex hinge type
   It is composed of two bellows plus an intermediate pipe and a hinge plate, which can absorb large lateral displacement. Often used at "L" or "Z" flue turns.
3. Pressure balance type
   With its own balanced bellows, it can eliminate the blind plate force caused by internal pressure on the fixed bracket. When the flue internal pressure exceeds 30kPa and the load-carrying capacity of the fixed bracket is limited, this type of structure is the best choice for flue expansion joint selection.
4. Universal hinge type
   It can absorb the angular displacement in any plane, and is suitable for the arrangement with limited space and complicated pipeline direction.

Selection logic: first calculate the combined displacement in three directions, and then compare the allowable compensation values of each type of single group of bellows. Usually, it is recommended that the actual use value should not exceed 80% of the rated value, and the safety margin should be kept.

4. Configuration of flow guide tube and heat insulation layer-protection design cannot be saved

Many users neglect the internal protection when selecting the flue expansion joint, resulting in the corrugated pipe being eroded or burned out in a short time. The following two configurations should be considered mandatory:

• Guide tube (inner bushing)
  Install on the inside of the bellows, guide the flue gas to flow along the center, and avoid the high-speed dusty flue gas directly washing the bellows trough. It is recommended that the thickness of the guide tube is not less than 3mm, and the direction must be consistent with the direction of air flow.
• Thermal insulation
  When the flue gas temperature exceeds 550 ℃, the corrugated metal material will undergo high temperature creep. A ceramic fiber layer with temperature resistance above 1000℃ should be filled between the guide tube and the bellows, and the thickness should be calculated by temperature drop-usually every 10mm thickness can reduce the outer wall temperature by about 80-100℃.

The qualified flue expansion joint selection scheme must be clearly marked with "whether it contains guide tube + heat insulation layer", otherwise it will be easily returned during the drawing review stage.

V. Interface and installation space verification-the last step of landing

After the theoretical calculation is completed, it is necessary to check the actual constraints on the site:

• Flange connection vs welding: Flange connection should be selected for pipe sections that need frequent maintenance, but attention should be paid to the problem of bolt slack at high temperature. It is recommended to use disc spring preloading. Welded connections have better sealing, but are difficult to remove.
• Minimum installation distance: Sufficient length of straight pipe section should be reserved on both sides of the bellows, which is generally required to be 1.5-2 times the diameter of the flue, so as to avoid direct impact of turbulence.
• Fixed bracket position: Flue expansion joint selection must be coordinated with pipe bracket design. The main fixing brackets shall be arranged at both ends of the expansion joint, and the spacing between the guide brackets shall not exceed 4 times the diameter of the pipe.

Common Selection Error Alert

According to maintenance statistics, the following three types of errors cause more than 80% of expansion joints to fail early:

• Only select model according to normal temperature design, the actual operating temperature is higher than 200℃
• Common carbon steel bellows is selected without considering acid dew point corrosion
• Additional lateral displacement caused by equipment foundation settlement is ignored