Specialized in manufacturing compensators, expansion joints, baffle doors

In the flue system of power plant, metallurgy, chemical industry and other industries, the expansion joint skin, as the core component of non-metallic compensator, has been in the high temperature flue gas environment for a long time. Its fire-proof performance is directly related to equipment safety and personal safety. So, is the flue expansion joint skin fireproof? The answer is yes-the flue expansion joint skin produced by regular manufacturers has excellent flame retardant performance, and some products can reach Class A fire protection standard. This paper will systematically analyze the fire resistance grade, flame retardant mechanism and selection points of skin materials.

1. Flame retardant properties of skin materials

Is the flue expansion joint skin fireproof depends on the composition of its composite material. The skin of non-metallic expansion joint is mainly made of multi-layer soft composite materials, which has many advantages such as wide temperature resistance range, high pressure resistance, strong corrosion resistance, good flame retardancy, sound absorption and shock absorption, good flexibility, etc。

1.1 Fire resistance of base fabric material

The main substrate of the skin is glass fiber cloth, and glass fiber itself is a non-combustible material. The fiberglass cloth maintains good fire resistance after being coated with silicone rubber or fluororubber. The flame retardant grade of silicone cloth material can reach fire resistance grade A, which meets the requirements of GB8624-2006 and German DIN4102 A1。 Glass fiber cloth is used as a base cloth to produce silicone gel cloth by coating or calendering. It is a high-performance and versatile composite material that can be used for a long time between low temperature-70℃ and high temperature 230℃。

1.2 Combustion performance of silicone rubber coating

The fiberglass cloth coated with high temperature curing silicone rubber has the following behaviors after combustion: smokeless, odorless, quick extinguishing, ash whitening, and longer service life。 This feature makes the skin not burn continuously when encountered with open flame, and has self-extinguishing ability.

1.3 Fire protection requirements in actual procurement

Judging from the actual procurement technical specifications, the power plant has clear requirements for the fire resistance performance of the skin. It is clearly stipulated in the procurement technical conditions of a large power group that the fire retardant grade of the expansion joint skin should reach UL94-V0 (the highest grade)。 This grade requires the material to self-extinguish within 10 seconds of leaving the fire in a vertical combustion test without combustion drips.

2. Multi-layer structure and fire protection design of skin

The answer to whether the flue expansion joint skin is fireproof also depends on the design of its multilayer composite structure. A typical skin consists of the following functional layers:

Taking the non-metallic expansion joint of the rear flue of a power plant as an example, its skin adopts 8-layer composite structure, which clearly requires flame retardant, high temperature resistance of 200℃, acid corrosion resistance and good seal。 The fireproof and flame retardant grade of zirconium-containing aluminum silicate needle blanket also reaches UL94-V0 standard。

2.1 Special configuration for high temperature operating conditions

For high-temperature flues with temperatures exceeding 400℃, silicone rubber coating alone is no longer enough to guarantee long-term fire safety. At this time, it is necessary to set a heat insulation layer inside the expansion joint, and use high-temperature composite materials such as fluororubber cloth, polytetrafluoroethylene membrane, glass fiber cloth and ceramic fiber cloth in combination to achieve high-temperature resistance, aging prevention and heat insulation。 The ceramic fiber material can withstand high temperatures above 800℃ and is completely non-flammable。

3. Comparison of fire resistance performance of different types of skins

When answering whether the skin of the flue expansion joint is fireproof, you need to distinguish between different materials:

3.1 Silicone rubber skin

• The long-term working temperature is ≤250℃, and it can reach 350℃ in a short time
• Flame retardant grade up to Class A, self-extinguishing off fire
• Good economy, suitable for conventional flue

3.2 Fluororubber Skin

• Temperature resistance 200~300℃, better corrosion resistance than silicone rubber
• Also have excellent flame retardant properties
• Especially suitable for corrosive environment such as desulfurization system

3.3 Composite high temperature resistant skin

• Contains ceramic fiber layer, and the fire resistance temperature can reach 800~1200℃
• Optimum fireproof performance, suitable for high temperature sections such as boiler outlet
• When the flue gas temperature is 1000℃, the outer skin can still work normally

4. Practical significance of skin fire prevention

Whether the skin of flue expansion joint is fireproof is not only a problem of product performance, but also related to:

4.1 Preventing the spread of fire

In the case of a fire in the flue system of power plants and chemical plants (such as carbon deposit combustion and combustible gas deflagration), the non-flammable or flame-retardant skin can effectively prevent the fire from spreading along the flue and buy time for emergency response.

4.2 Eliminate the risk of combustion drip

The UL94-V0 rating requires the material to burn without dripping. Ordinary plastic materials will drip molten matter when burning, which may ignite the equipment below or cause people to burn. However, the silicone rubber/fluororubber composite material is carbonized and does not drip during combustion, which is more safe。

4.3 Meet fire acceptance requirements

The fire protection acceptance of industrial buildings has clear provisions on the fire protection grade of pipeline insulation and sealing materials. The selection of flame-retardant skin is a necessary condition for enterprises to pass fire inspection.

V. Suggestions on selection and use

In order to ensure that the flue expansion joint skin is fireproof to get a positive answer, attention should be paid to the following in the selection and use:

5.1 Confirmation points when purchasing

• Verify that the temperature requirements of the operating condition are met
• View product structure layers and material description

5.2 Installation Precautions

• The skin surface coating shall not be damaged during installation
• When welding the end pipe, cover the skin with asbestos cloth to prevent burning by welding slag
• Ensure that the guide tube is installed correctly to avoid high-temperature smoke directly washing the inner layer of the skin

5.3 Operation and Maintenance

• Check the skin surface regularly for any signs of carbonization, cracking and burning
• If the flame retardant coating is found to be aging and falling off, it should be replaced in time
• Over-temperature operation will accelerate material aging, smoke temperature should be strictly controlled

VI. SUMMARY

Is the skin of flue expansion joint fireproof-regular products have excellent flame retardant properties. The core conclusions are as follows:

• Material is the key: silicone rubber/fluororubber coated glass fiber cloth as the base material, glass fiber is non-flammable, the coating layer is self-extinguishing off fire
• High temperature requires composite structure: working conditions above 400℃ need to be equipped with non-combustible insulation layer such as ceramic fiber, and the fire resistance temperature can reach 800-1200℃
• Required inspection report for procurement: Suppliers shall be required to provide fire rating inspection report during model selection to ensure that products meet fire protection specifications

Therefore, as long as you choose the products produced by regular manufacturers and meet the relevant fire protection standards, the flue expansion joint skin has reliable fire protection performance. However, it should be noted that long-term overtemperature operation will accelerate the aging of materials, resulting in the decline of flame retardant performance. In daily operation, the flue gas temperature should be strictly controlled within the allowable range of the skin, and the aging products should be regularly inspected and replaced.

In the installation or maintenance of flue system, the welding of expansion joint is the key process to determine the sealing performance and service life. Unqualified welding quality will lead to leakage, deformation and even early failure of the expansion joint. However, many on-site construction workers have misunderstandings about how to weld the expansion joint of the flue-the expansion joint is welded equally with the ordinary flue pipe, which causes serious consequences. This paper will systematically explain the standard welding method of flue expansion joint from four aspects: welding preparation, process parameters, operation steps and quality inspection.

I. Preparation before welding

Before discussing how to weld the expansion joint of the flue, an important principle must be clarified first: it is strictly prohibited to pass welding current through the flexible components of the expansion joint. Whether it's a metal bellows or a non-metal skin, the welding current can cause irreversible damage to it.

1.1 Ground Wire Location Selection

• The ground wire must be clamped on the flue body on the same side as the part to be welded and must not be connected to the other side across the expansion joint.
• For metal bellows expansion joints, the grounding wire cannot be clamped on the bellows crest, otherwise the arc will ablate the thin wall of the bellows.

1.2 Protection of flexible elements

• Non-metallic expansion joint: When welding end pipe or flange, the skin surface must be completely covered with asbestos cloth or fire blanket to prevent welding slag from splashing and burning the fabric layer.
• Metal expansion joint: Cover the bellows trough with thick cardboard or rubber sheet to prevent welding slag from embedding in the corrugated gap.

1.3 Group-to-Size Review

• Before welding, measure whether the actual length of the expansion joint is consistent with the designed cold length (non-metallic expansion joint needs to be pre-compressed by 5% ~8%).
• Check the concentricity of the pipes (or equipment interfaces) at both ends; the deviation shall be ≤3mm.

2. Welding process of metal expansion joint

There are usually two ways to connect the expansion joint of metal bellows with the flue: flange connection and direct welding. How to weld the expansion joint of the flue is mainly aimed at direct welding.

2.1 End pipe welding

Both ends of the metal expansion joint come with end pipes (short joints), and only butt welding of the end pipe and the flue pipe is necessary during construction.

Welding process parameters (taking carbon steel flue, thickness 8mm as an example):

• Welding method: Manual arc welding (SMAW) or CO₂ gas shielded welding (GMAW)
• Electrode/Wire Model: E5016 (J506) or ER50-6
• Electrode diameter: φ 3.2mm (base) → φ 4.0mm (filling cover)
• Welding current: 110~130A (φ 3.2), 160~190A (φ 4.0)
• Interlayer temperature: ≤150℃

2.2 Groove Form

• When the wall thickness of the flue is ≤6mm, the groove may not be opened, but the gap of 2~3mm should be ensured.
• When the wall thickness is> 6mm, a V-shaped groove should be opened with an angle of 60° ~70° and a blunt edge of 1~2mm.

Key point: The root of the weld must be welded through, but the internal guide tube must not be burned through. There is usually only 10~15mm gap between the guide tube and the inner wall of the bellows, and excessive welding current will break down the end tube and damage the guide tube.

2.3 Welding sequence and deformation control

• Symmetric segment welding is adopted: for circular flue, segment according to clock point (12 o'clock → 6 o'clock → 3 o'clock → 9 o'clock), each segment is 50~100mm long, and welding is applied alternately.
• For rectangular flue, welding should be applied from the midpoint of the long side to both ends, then the short side, and finally the corner.
• After each weld, tap the weld and the heat affected zone with a wooden hammer to release the welding stress.

2.4 Prohibited Matters

• No welding (including spot welding, repair welding) shall be performed on the bellows. The wall thickness of the bellows is only 0.8~2.0mm, and the welding will immediately burn through or cause stress cracking.
• When welding the end pipe to the flue, do not clamp the ground wire to the bellows or the opposite flue.

3. "Welding" related procedures in the installation of non-metallic expansion joints

The non-metallic expansion joint itself has no welding parts, and its connection to the flue relies on flanges and bolts. However, during installation, it is necessary to weld flanges or angle steel platen frames at the end of the flue. This part of the welding also requires caution.

3.1 Flange welding

How to weld the expansion joint of the flue For non-metallic expansion joints, it refers to welding the flange ring or frame used to compress the skin.

Step:

1. First, spot weld the flange ring and fix it at the end of the flue to ensure that the flange surface is perpendicular to the axis of the flue, and the deviation is ≤2mm/m.
2. Adopt segmented jump welding method: 100mm per weld, skip 100mm, and repair welding after cooling. Prevent flange deformation and warpage caused by continuous welding.
3. The weld should be continuous, pore-free, and the weld slag should be ground to smooth, without sharp bumps-otherwise it will puncture the skin.

3.2 Plate Bolt Seat Welding

Some non-metallic expansion joints need to be welded with nut plates or bolt seats. Care should be taken when welding:

• Remove splashes after welding and re-pass the thread with a tap to prevent weld slag from clogging the thread.
• The position of the bolt seat must correspond to the hole position of the skin pressure plate, and the deviation shall be ≤1.5mm.

3.3 Deflector welding

The guide tube is welded on the inside of the flue, which belongs to a concealed process, and is especially important:

• The fixed end of the guide tube shall be continuously welded with the inner wall of the flue, and no spot welding shall be allowed.
• The free end is strictly prohibited from welding and must be kept in a free sliding state.
• Before welding, confirm the installation direction of the guide tube: the bell mouth or overlap end faces the incoming flue gas.

Welding quality inspection

After completing the construction of how to weld the expansion joint of the flue, the following inspections must be carried out:

4.1 Appearance inspection

• There are no cracks, pores, slag inclusions and biting edges on the weld surface (depth ≤0.5mm).
• The weld residual height is ≤3mm, and the transition is smooth.

4.2 Dimensional inspection

• The parallelism of the flanges or end tubes at both ends of the expansion joint after welding is ≤3mm.
• The overall length variation of flue shall be within the design allowable range (the non-metallic expansion joint shall be kept in the pre-compressed state).

4.3 Sealability Test

After completion of welding and before heat insulation, conduct air tightness test:

• Methods: Apply soapy water to the weld, and fill the flue with compressed air to 1.1 times the working pressure (but not exceed the pressure resistance limit of the expansion joint).
• Criteria: No bubbles are continuously generated as qualified.
• For non-metallic expansion joints, it is also necessary to check the flange pressure plate bolts for leakage.

4.4 Non-destructive testing (according to design requirements)

• The butt welds of important flues (such as denitrification inlet and absorption tower inlet) shall be subjected to 20% ~100% radiographic inspection (RT) or ultrasonic inspection (UT) according to NB/T 47013 standard, and are qualified for Class II.

V. Common welding problems and prevention

6. Welding matching of different flue materials

Special Note: When welding dissimilar steel, the ground wire must be clamped on the carbon steel side to avoid the tendency of intergranular corrosion in stainless steel.

Welding Safety and Protection

1. Fire prevention: combustible gas (gas, VOCs) may remain in the flue. Gas detection and hot fire ticket must be carried out before welding.
2. Anti-scalding: Bellows and skin will heat up under welding heat radiation. The surface of non-metallic skin should not be exposed to high temperature (> 80℃) for a long time, and wet asbestos cloth can be used to cool down.
3. Ventilation: When welding in the flue, forced ventilation and dust mask must be worn.

VIII. Summary

The core principle of how to weld the expansion joint of the flue can be summarized as three sentences: "protecting flexible parts, controlling deformation by sections, and strictly checking and sealing":

• Protect flexible parts: Cover with asbestos cloth (non-metal) or protective plate (metal) before welding, the grounding wire is strictly prohibited from crossing the expansion joint, and the arc shall not touch the bellows or skin.
• Sectional deformation control: Symmetric sectional jump welding method is adopted to control the interlayer temperature and prevent flange warping or bellows instability.
• Strict sealing inspection: Air tightness test must be carried out after welding, and non-destructive testing must be carried out for important welds.

For non-metallic expansion joints, the so-called "welding" is actually the welding of the flange frame and the guide barrel, and the skin itself is not involved in the welding. Workers must distinguish the process differences of different types of expansion joints, and avoid directly applying the welding method of metal expansion joints to non-metallic products. Correct welding process is the first guarantee for the long-term reliable operation of expansion joints, and any negligence may lead to premature failure of equipment or even safety accidents.

In flue system design, the expansion joint not only needs to absorb thermal displacement, but also produces a significant "pressure thrust" due to internal medium pressure. If this thrust is neglected in the design stage, it may lead to the failure of the fixed bracket, flue deformation and even the instability of the expansion joint itself. Therefore, mastering the calculation formula of flue expansion joint thrust is the key link to ensure the safety of flue structure. This paper will systematically explain the thrust source, calculation formula and engineering application examples of metal expansion joint and non-metal expansion joint.

1. Why do you need to calculate the thrust of the expansion joint

The expansion joint is installed in the flue, and when there is pressure (positive or negative) acting inside, the pressure creates an axial force on the effective area of the bellows or skin. This force will be transmitted to the fixed brackets at both ends and will also act on the expansion joint body.

The core value of the thrust calculation formula of flue expansion joint lies in:

• Determining the structural dimensions and anchoring mode of the fixed bracket
• Check the pressure stability of the expansion joint itself
• Prevent flue interface cracking or expansion joint inversion due to thrust exceeding limit

The consequences of ignoring the thrust calculation are often disastrous: a power plant did not calculate the pressure thrust of the metal expansion joint, which led to the flue fixing bracket at the outlet of the induced draft fan being pushed 30mm away from the foundation, and the flue welds cracked in many places.

2. Thrust calculation of expansion joint of metal bellows

2.1 Sources of Pressure Thrust

Under the action of internal pressure, the effective area of the metal bellows expansion joint will produce an axial expansion force. The magnitude of this force is proportional to the pressure value and the effective area of the bellows.

The basic form of the calculation formula of the thrust of the flue expansion joint (metal bellows) is:

F_p = P × A_eff

Among them:

• F_p — — Pressure thrust, unit: N
• P — — Working pressure in flue (gauge pressure), unit: Pa (Note: the thrust direction is opposite under negative pressure)
• A_eff-Effective area of bellows in m²

2.2 Determination of effective area A_eff

The effective area of the bellows is not equal to the cross-sectional area of the flue because the corrugated structure of the bellows makes its pressure-bearing area between the inner diameter area and the outer diameter area. The following methods are commonly used in engineering to obtain:

Method 1: Check the product sample
The A_eff value is given directly in the technical parameter sheet provided by the manufacturer.

Method 2: Empirical Formula
For standard U-bellows:

A_eff ≈ (π/4) × (D_m) ²

Where D_m is the mean diameter of the bellows = (D_in + D_out) /2, D_in is the inner diameter, and D_out is the peak outer diameter.

Method 3: Inverse calculation by stiffness method
For installed expansion joints, it can be calculated back from the length change under pressure:

A_eff = K × Δ L/P

Where K is the axial stiffness of the bellows (N/mm) and Δ L is the elongation under pressure (mm).

2.3 Corrections in actual operating conditions

Metal bellows expansion joints are usually equipped with tie rods or hinges. The role of the tie rod is to withstand the pressure thrust, thus protecting the bellows. Therefore, the thrust calculation needs to distinguish between two cases:

Key conclusion: For metal expansion joints with tie rods, the pressure thrust is balanced by the tie rods themselves and is not transmitted to the external bracket. However, the design strength of the tie rod must be able to withstand F_p (usually taking 1.5 times the safety factor).

2.4 Calculation Examples

Known:

• Circular flue diameter DN1200mm, metal bellows expansion joint
• Inner diameter D_in =1200mm, crest outer diameter D_out =1320mm
• Operating pressure P = +5000Pa (5kPa positive pressure)
• Try to calculate the pressure and thrust and judge whether the tie rod needs to be installed

Calculation:

1. Average diameter D_m = (1200+1320) /2=1260mm =1.26m
2. Effective area A_eff = π/4× (1.26) ² =1.247 m²
3. Pressure thrust F_p =5000×1.247=6235 N ≈ 636 kgf

Conclusion: If the free expansion joint is used, the fixed bracket at both ends needs to bear a thrust of about 636kgf, which must be included in the design of the bracket. If the type with tie rods is adopted, it can be easily withheld by 4 M16 tie rods (each with a bearing capacity of about 3000kgf).

Thrust calculation of non-metallic fabric expansion joint

A non-metallic expansion joint has a different source of thrust than a metal. Because the fabric skin is so soft that it can barely withstand pressure thrust, the thrust is all borne by the external metal frame and platen.

The calculation formula of flue expansion joint thrust (non-metal) is:

F_p = P × A_duct

That is, the effective area is directly taken as the internal cross-sectional area of the flue (instead of the average area of the bellows).

For rectangular flue:

A_duct = W × H

For circular flues:

A_duct = π/4× D²

3.1 Thrust transmission path of non-metallic expansion joint

The thrust of the non-metallic expansion joint is not borne by the skin, but is transmitted through the following path:

1. The flue gas pressure acts on the end face of the flue
2. The end plate transmits force to the flange connected to the expansion joint
3. The flange is transmitted to the outer metal frame by a platen bolt
4. The frame is then transmit to the flue fixing bracket by a pull rod or bracket

Therefore, for non-metallic expansion joints, installation must ensure that the platen bolts have sufficient strength and pre-tightening force to prevent internal pressure from blowing off the skin.

3.2 Calculation Examples

Known:

• Rectangular flue, width 1500mm, height 1200mm
• Operating pressure P = -8000Pa (8kPa negative pressure, i.e. suction)
• Trial calculation of the thrust to be withstood by the fixed bracket

Calculation:

1. Flue cross-sectional area A_duct =1.5×1.2=1.8 m²
2. Thrust F_p = P × A_duct = (-8000) ×1.8= -14400 N (negative sign indicates directional inward contraction)
3. About 1469 kgf in absolute

Conclusion: The fixed bracket must withstand a tensile force of about 1470kgf (because the negative pressure is inward suction). This value is required for anchor checking during bracket design.

4. Elastic reaction force generated by temperature load

In addition to the pressure thrust, the expansion joint also produces an elastic reaction force when absorbing thermal displacement. This force also needs to be factored into the total load.

Calculation formula of elastic reaction force:

F_e = K × Δ L

Among them:

• K-axial stiffness of the expansion joint (N/mm), supplied by the manufacturer
• Δ L — — Thermal displacement absorbed after actual installation (mm)

For metal bellows expansion joints, the K value is usually 100~500 N/mm; For non-metallic expansion joints, the K value is small (typically

The total thrust (acting on the fixed bracket) is:

F_total = F_p + F_e (When the metal expansion joint has no tie rod)
F_total = F_e (when the metal expansion joint is equipped with a tie rod, the pressure thrust is balanced by the tie rod)
F_total = F_p (non-metallic expansion joint, elastic reaction force can be ignored)

V. Precautions in engineering application

5.1 Thrust direction under negative pressure

When the flue is under negative pressure (such as behind the induced draft fan), the direction of thrust is opposite to the positive pressure, which is the "suction" of inward contraction. At this time, the fixed bracket needs to be subjected to tension instead of pressure. Many engineers only focus on positive pressure thrust and ignore negative pressure suction, resulting in insufficient pull-out ability of the bracket and being pulled out of the foundation.

5.2 Effect of temperature change on thrust

For metal expansion joints, if cold pre-compression/pre-stretching is not performed at the design temperature during installation, the actual Δ L will deviate from the design value, resulting in more than expected elastic reaction force F_e. For example, if the thermal elongation is designed to be 40mm, if it is not pre-compressed during installation, the actual Δ L may reach 2 times the design value and F_e may double, which may lead to overload of the fixed bracket.

5.3 Introduction of safety factors

Regardless of pressure thrust or elastic reaction force, when finalizing the bracket load, the safety factor shall be multiplied by:

• Normal operating load: safety factor 1.5
• Extreme working conditions (e.g. start-stop, failure): Safety factor 1.2 (check according to material yield strength)

That is:

F_design ≥ F_total ×1.5

5.4 Coupling Effect of Multiple Expansion Joints

When multiple expansion joints are arranged in series on the same section of flue, the forces on the fixed bracket are not simply superimposed. A pipe flexibility analysis is required because the stiffness of the expansion joints interacts with each other and the displacement distribution may not be consistent with the initial design. At this point it is recommended to use CAESAR II or AutoPIPE software for simulation.

6. Quick table look-up for thrust calculation

To facilitate engineering site estimation, the following table gives the pressure thrust F_p (non-metallic expansion joint) at ±5kPa pressure for common flue sizes:

Instructions for use: The values in the table are approximate values (converted to 9.8 N/kgf). In practical application, for the expansion joint of metal bellows, the effective area A_eff should be calculated instead of the flue cross-sectional area A_duct, and the value will be slightly lower.

Common Mistakes and Avoidance

VIII. Summary

The calculation formula for flue expansion joint thrust varies depending on the type of expansion joint:

• Metal bellows expansion joint: thrust F_p = P × A_eff (A_eff is the effective area of bellows). When there is no pull rod, F_p is carried by the supports at both ends; When there is a tie rod, it is balanced inside the tie rod, and the bracket only bears the elastic reaction force F_e = K × Δ L.
• Expansion joint of non-metallic fabric: Thrust force F_p = P × A_duct (A_duct is the internal cross-sectional area of flue), and elastic reaction force can be ignored. The thrust force is all transmitted from the external metal frame to the fixed bracket.

Correct calculation of expansion joint thrust is an indispensable step in flue structure design. In engineering practice, the process of "first distinguishing the types of expansion joints, then selecting the correct effective area, and finally counting the elastic reaction force and safety factor" should be strictly followed. For complex pipeline systems, it is recommended to use professional stress analysis software for overall calibration. Through scientific calculation and reasonable type selection, serious accidents such as fixed bracket damage, expansion joint inversion and flue cracking can be effectively avoided.

In flue system design, the core function of the expansion joint is to absorb the thermal expansion displacement caused by the temperature change of the pipe. However, many engineers have little understanding of the calculation of "compensation amount" when designing or selecting models, resulting in insufficient compensation capacity or excessive redundancy of the selected expansion joints. How to calculate the compensation amount of flue expansion joint is directly related to the system safety and investment economy. Based on the principle of thermal expansion, this paper will systematically explain the calculation methods of axial, transverse and angular compensation quantities, and demonstrate the complete calculation process with examples.

I. Basic concept of compensation amount

Compensation amount, also known as displacement compensation ability, refers to the amount of thermal displacement of the pipeline that the expansion joint can absorb, which usually includes three directions:

• Axial compensation amount (Δ L): The amount of elongation or compression along the centerline of the flue.
• Lateral compensation amount (Δ y): horizontal displacement in the direction perpendicular to the flue centerline.
• Angular compensation amount (Δθ): the angle at which the flue axis is deflected.

For most straight flues, the core of how to calculate the compensation amount of flue expansion joint is the calculation of axial thermal expansion. The lateral and angular displacements are not negligible in L, Z or π bend sections.

2. Calculation formula and steps of axial compensation amount

2.1 Basic Formula

The axial thermal expansion Δ L (unit: mm) is calculated as follows:

Δ L = α × L × Δ T

Among them:

• α — — Linear expansion coefficient of flue material, mm/ (m·℃)
  Common values: carbon steel α =0.012 mm/ (m·℃); Stainless steel (304/316L) α =0.017 mm/ (m·℃)
• L-the length of the flue between the two fixed brackets (i.e. the distance to be compensated by the expansion joint), unit: m
• Δ T — — Difference between flue operating temperature and installation temperature, unit: ℃
  Δ T = t_work-t_install

2.2 Key points of parameter values

Temperature difference Δ T:

• The operating temperature T_work shall be the highest possible continuous operating temperature of the flue (excluding instantaneous overtemperature).
• The installation temperature T_install is generally taken as the local annual average temperature or the ambient temperature at the time of actual installation on site, usually calculated at 20℃. If it is installed in winter and no preheating measures are taken, the value should be taken according to the actual lower temperature.

Length L:
Not the full length of the entire flue, but the spacing between the two fixed brackets. The expansion joint should be arranged at one end or in the middle.

Safety Factor:
The calculated Δ L is the theoretical thermal elongation. In actual type selection, the rated compensation amount of the expansion joint should meet the following requirements:

Rated compensation amount ≥ Δ L × K
K is the safety factor, which is generally taken as 1.2~1.5. Take the upper limit for the system with large temperature fluctuation and frequent start-and-stop.

3. Example calculation: axial compensation of straight section flue

Known Conditions:

• Carbon steel round flue, spacing between two fixed brackets L =25 m
• Maximum operating smoke temperature T_work =320℃
• Installation temperature T_install =20℃
• Try to calculate the minimum axial compensation amount of the expansion joint.

Calculation steps:

1. Calculate temperature difference : Δ T =320-20=300 °C
2. Check the linear expansion coefficient of carbon steel: α =0.012 mm/ (m·℃)
3. Substitute into the formula: Δ L =0.012×25×300=90 mm
4. Take the safety factor 1.3: Δ L_required =90×1.3=117 mm

Conclusion: The expansion joint with rated axial compensation not less than 117mm should be selected. If a single expansion joint is adopted, the model with compensation amount of 120mm ~130mm can be selected; If the standard product is only up to 100mm, two expansion joints (each compensating approximately 60mm) need to be arranged within a 25m spacing.

4. Calculation of lateral and angular compensation amount

The expansion joint also needs to absorb the lateral displacement (Δ Y) and the angular displacement (Δ θ) when there is an elbow in the flue, the diameter reduction or the equipment interface is not on the same axis.

4.1 Lateral compensation amount

For L-flues (a 90° elbow), thermal expansion causes lateral displacement on the outside of the elbow. The simplified calculation formula is:

Δ Y = Δ L_vertical × (H/L_horizontal) (approximation)

More accurate calculation requires the force analysis of the fixed bracket and the guide bracket. The following are common experience values:

In practical engineering, the transverse part of how to calculate the compensation amount of flue expansion joint is usually aided by stress analysis software (such as CAESAR II), but the following simplified method can be used for small systems:

Δ y = β × L_vertical × α × Δ t

Where β is the conversion factor, usually taken from 0.5 to 0.8 (depending on elbow stiffness and bracket arrangement).

4.2 Angular compensation amount

When the two expansion joints are arranged in Z-type or π-type, the intermediate pipe segment will generate angular displacement. Calculation formula of angular compensation amount Δθ (unit: degrees or radians):

Δθ = arctan (Δ Y/L_arm)

Where L_arm is the length of the arm that produces the lateral displacement. In actual selection, the angular compensation ability of non-metallic expansion joints is usually ±3° ~ ±6°; Product samples should be checked for metal bellows expansion joints.

V. Comparison of compensation amount between non-metal and metal expansion joints

The compensation ability of different types of expansion joints is significantly different:

After understanding how to calculate the compensation amount of flue expansion joint, it is necessary to choose the appropriate type according to the displacement direction. For example, if the calculation result is 120mm in the axial direction and ±15mm in the transverse direction, priority should be given to non-metallic expansion joints or double metal bellows, which cannot be satisfied by single metal bellows.

VI. Common Calculation Errors and Pit Avoidance Guide

6.1 Error 1: Ignore cold pre-compression

Many installers install the expansion joint directly at the free length without cold pre-compression. Correct practice: For high temperature flues, pre-compressed (or pre-stretched) should be installed ) Δ L_PRE = -0.5× Δ L_CALC. For example, if the thermal elongation is calculated to be 120 mm, the cold compression is installed to be 60 mm, and the other half of the elongation space is reserved.

6.2 Error 2: Confusion of adjacent expansion joint spacing

Some designers mistakenly substitute the length of the whole flue into the formula. In practice, each expansion joint is only responsible for compensating for the pipe segment between the fixed brackets at its two ends. If the total length of the flue is 100m, and four fixed brackets are set to be divided into three sections (30+30+40m), it will be calculated as 30m and 40m respectively instead of 100m.

6.3 Error 3: Ignoring the influence of installation temperature difference

A difference of 45°C in Δ T between winter installation (-10°C) and summer installation (35°C) can result in a difference of approximately 54mm in Δ L for 100m flue. If not corrected to the actual installation temperature, the expansion joint may get stuck in the summer or tear in the winter.

6.4 Error 4: Metal expansion joint does not count pressure thrust

The metal bellows expansion joint generates a pressure thrust (F = P × A_eff) under internal pressure, which needs to be withstood by the bracket. If the influence of pressure thrust on the stability of the bellows is neglected when calculating the compensation amount, the bellows may be unstable and inverted. This problem is particularly critical in the negative pressure condition of desulfurization net flue.

VII. Compensation amount calculation list and verification

Once the calculation is complete, it is recommended to use the following checklist to verify itemized:

1. Is there a clear distinction between axial, lateral and angular displacements?
2. Is the value of material linear expansion coefficient α correct (carbon steel/stainless steel)?
3. Is the length L the spacing between the two fixed brackets and not the total flue length?
4. Does the temperature difference Δ T take into account the minimum installation temperature?
5. Has the safety factor been multiplied (1.2~1.5)?
6. Rated compensation of the selected expansion joint ≥ calculated value?
7. Is the amount of cold pre-compression marked on the installation drawing?
8. Has the pressure thrust been checked for metal bellows?

VIII. Summary

The core of how to calculate the compensation amount of flue expansion joint is the axial thermal expansion formula Δ L = α × L × Δ T, and at the same time, whether the lateral or angular displacement needs to be taken into account according to the flue layout form. It is important to note when calculating that the length L is the spacing between fixed brackets rather than the total length; The temperature difference Δ T shall adopt the highest continuous operation temperature minus the lowest installation temperature; The safety factor of 1.2~1.5 should not be omitted. For non-metallic expansion joints, it is recommended that the axial compensation amount of a single piece be controlled within 80mm, and the number of expansion joints should be increased when it exceeds. The influence of pressure and thrust on the stability of metal bellows also needs to be checked additionally.

After mastering the above calculation method, engineers and technicians can quickly complete the preliminary selection of flue expansion joint. For complex arrangements (multiple elbows, variable sections, limited displacement of equipment interface), it is recommended to use professional stress analysis software for accounting. Correct calculation of compensation quantity can not only avoid flue cracking caused by insufficient compensation, but also prevent cost waste caused by excessive selection, which is the cornerstone of reliability design of flue system.

1. Discussion on the Necessity of High Temperature Flue Expansion Joint

In the design of high-temperature flue systems such as industrial furnaces, boilers and roasters, a frequently asked question is: Does the high-temperature flue need to be equipped with expansion joints? The answer: must be set in the vast majority of cases. During the operation of high-temperature flue, as the temperature rises from normal temperature to hundreds or even thousands of degrees Celsius, the flue material will undergo significant thermal expansion. If the flue is a rigid continuous structure and no expansion joint is set to absorb thermal elongation, the huge thermal stress will lead to flue weld cracking, bracket failure, flange leakage and even overall flue instability deformation. However, whether each high-temperature flue must be provided with expansion joints, how many to set them, and what type to adopt need to be comprehensively judged according to the pipe length, working temperature, direction layout and bracket form. This paper will systematically answer this question from the calculation of thermal expansion, the feasibility evaluation of no expansion joint to the selection of setting scheme.

Calculation of thermal expansion: the basis for judging whether an expansion joint is needed

2.1 Basic Formula of Thermal Expansion

Answer high-temperature flue need to set expansion joint, first of all, calculate the thermal expansion of the flue. The calculation formula is as follows:

Δ L = α × L × (T_WORK-T_INST)

Among them:

• Δ L: thermal elongation (mm)
• α: Coefficient of linear expansion of material (/℃)
• L: Calculate the length of the pipe section (m)
• T_work: Operating temperature (℃)
• T_inst: Installation or initial temperature (℃)

2.2 Thermal expansion coefficient of common materials in high temperature flue

2.3 Calculation Example

A section of carbon steel flue with a length of 20m is set, the working temperature is 450 DEG C and the installation temperature is 20 DEG C, then:

Δ L =12×10⁻⁶ ×20000× (450-20) =103.2mm

This means that a 20m-long flue, after warming up to 450°C, elongates by about 103mm – equivalent to a displacement of 10cm. If both ends of the flue are rigid and fixed, this displacement has nowhere to be released, which will inevitably produce huge internal stress. Therefore, in this case, the answer to whether the expansion joint needs to be set in the high-temperature flue is clear: it must be set.

2.4 The critical length of the expansion joint needs to be set

3. Feasibility conditions of not setting expansion joints

Although the vast majority of high-temperature flues need to be provided with expansion joints, they may not be provided under certain conditions.

3.1 Natural Compensation (Flexible Pipe Design)

The flexibility of the pipe itself can absorb part of the thermal displacement when there are sufficiently long straight sections and elbows in the flue stroke. This is the most common alternative to high-temperature flues. Conditions for natural compensation include:

• The flue is arranged in an L-, Z-or U-shape, with elbows providing flexibility
• The length of the pipe between the two fixed points does not exceed the critical value (see table above)
• The pipe wall thickness is thin (≤6mm) and the stiffness is low

3.2 Short-distance straight pipe sections

For straight pipe sections of very short length (e.g. length ≤3-5m from the outlet of the equipment to the first turn), the amount of thermal elongation is very small (usually ≤10mm), which can be absorbed by the clearance of the connecting flange, the flexibility of the equipment interface or the elastic deformation of the pipe. At this time, no expansion joint can be provided.

Typical example: The connecting section from the roaster outlet to the settling chamber is usually only 2-4m long, and the working temperature is 800-900℃. It can adopt thick-walled tube + large flange structure, and the elasticity of flange bolts is used to absorb a small amount of heat displacement, without separate expansion joints.

3.3 Sliding bracket and elastic connection

For longer flues, if expansion joints are not provided, sliding brackets can be arranged throughout the length, allowing the flue to extend freely, with elastic connections at the ends (e.g. packing box seals). This scheme is commonly used in horizontal directly buried thermal pipes, but it is less used in high temperature flues because its sealing reliability is not as good as that of expansion joints.

4. Typical working conditions where expansion joints need to be set

The answer to whether the expansion joint needs to be set in the high-temperature flue under the following working conditions is yes:

4.1 Long-distance straight pipe sections

When the long straight section of the flue exceeds the critical length (see Section 2.4) and goes straight without turning, axial expansion joints must be provided. Common in:

• Connecting flue from boiler outlet to dust collector
• Straight section of annular flue of roaster
• Original flue at inlet of desulfurization tower

4.2 Connections between High Temperature Equipment

When both devices are fixed independently (such as gas turbine exhaust port and waste heat boiler inlet), there is no common basis between the two, and the relative thermal displacement difference is significant, expansion joints must be set. Typical operating conditions:

• Gas turbine boiler inlet flue: the gas turbine exhaust temperature is 500-650℃, the boiler inlet is about 120℃, and the displacement difference is 30-60mm
• Connecting flue between roaster and cooler

4.3 Where the pipeline changes direction

When the flue changes direction at the elbow, thermal expansion causes lateral displacement of the elbow, creating lateral thrust on adjacent equipment. At this time, hinge type or universal expansion joints should be set on both sides of the elbow.

4.4 Where the flue passes through the wall or floor

When a flue crosses a building structure, wall or floor constraints will limit the axial displacement of the flue, and expansion joints must be provided on both sides of the crossing.

5. Risks and consequences of not setting expansion joints

If the operating conditions that should be set are not set, the following problems will arise:

6. Suggestions on the selection of high-temperature flue expansion joint

6.1 Select Material by Temperature

After confirming whether the expansion joint needs to be set in the high-temperature flue and deciding to set it, the material of the bellows should be selected according to the flue temperature:

6.2 Selecting Structure by Displacement Direction

6.3 Special configuration of high temperature expansion joint

For high temperature flues (≥600℃), the expansion joint requires the following special configuration:

• Guide tube: Prevent high-temperature smoke from directly washing the inner wall of the bellows
• Insulation layer: filled with ceramic fibers to reduce the temperature of the outer wall
• Multi-layer bellows: Reduce single-layer stress and disperse heat load
• Air-cooled or water-cooled structure: External cooling is required under extreme high temperature conditions

VII. Example in which the expansion joint is not required

To give a more comprehensive answer to whether an expansion joint needs to be set in a high-temperature flue, the following example does not need to be set:

1. Short-distance connection section: the distance from the outlet of the equipment to the first fixed point is ≤3m, the working temperature is 500℃, the thermal elongation is ≤12mm, and can be absorbed by the elasticity of the pipeline
2. Fully suspended flexible flue: The flue is suspended by a hanger, and the full length can swing and telescope freely
3. Masonry flue with expansion joints: lined with refractory bricks, with expansion gaps reserved in the brick joints, thermal expansion absorbed by the brick joints, and the metal shell separated in sections
4. Small-diameter pipes with bellows compensator as connectors: such as instrument pipes, sampling pipes and other pipes with diameter ≤100mm

VIII. Summary

The core judgment basis of whether the expansion joint needs to be set in the high-temperature flue is whether the thermal expansion amount of the flue exceeds the bearing capacity of itself and the support. When the amount of thermal elongation exceeds 10 mm or the length of the pipe section exceeds a critical value (20-30 m, depending on the temperature), setting the expansion joint is a necessary and economical solution; For the flue with short distance (≤5m), natural compensation elbow or flexible suspension structure, it may not be set.

After deciding to set the expansion joint, the bellows material should be selected according to the working temperature (321/316L is recommended above 450℃, 310S or Inconel is recommended above 600℃), and the expansion joint structure (axial type, large tie rod type or hinge type) should be selected according to the displacement direction. For the high-temperature flue above 600℃, it is necessary to configure a guide tube, heat insulation layer and multi-layer bellows.

It needs to be emphasized that the pipeline stress damage caused by blindly omitting the expansion joint often appears after a period of operation-it may take months or even years for the crack to expand from microscopic to macroscopic leakage, but once it happens, the repair cost far exceeds the investment of the original configuration of the expansion joint. Therefore, in the design, the calculation results of thermal expansion should be used as the basis, and the scientific decision should be made whether the expansion joint should be set in the high-temperature flue, so as to avoid the long-term potential safety hazard due to saving initial investment.