Specialized in manufacturing compensators, expansion joints, baffle doors

Is the flue expansion joint installed without distinction between positive and negative? I told him that it was a score. Not all expansion joints have a direction, but once there is a direction requirement, they are installed backwards, ranging from wear and air leakage to the whole section of corrugation tearing. Let's break it up and break it down today.

Why do some flue expansion joints have to be oriented?

The key is in the deflector. This thing fits inside the corrugation, like a bell mouth, and functions to guide the flow of the medium and reduce scour and vortex. Think about it, there is much dust in the flue gas pipeline and the flow rate is fast. If the guide tube is installed in the reverse direction, the airflow will go straight to the root of the corrugation, and the wear speed will more than double. Like the one we stand onUniversal corrugated expansion joint、High temperature axial expansion jointThe direction of the guide tube is a hard requirement-the opening must point to the direction of the medium. Why? Because the medium enters from the large port and exits from the small port, the guide tube can smoothly guide the airflow into the corrugation to avoid direct impact.

A high-temperature axial expansion joint was installed backwards. After less than three months of operation, the corrugated root grinded out grooves visible to the naked eye, and the air leakage rate soared to 15%. You say it was wrong or not?

What about the expansion joint without the deflector?

For example, pure rubberrubber compensator, andNon-metallic expansion joints (fabric fiber expansion joints), there is no metal diversion structure inside, and it doesn't matter whether it is positive or negative in theory. However, note that some non-metallic expansion joints will thicken the wear-resistant layer on the upward surface in order to resist erosion. At this time, the manufacturer will mark an arrow on the product, and the arrow points to the flow direction of the medium. If you ignore the arrows, the wear-resistant layer will be installed on the back, which is basically equivalent to wasting money.

In addition,Rubber PTFE compensatorSuch composite products sometimes make a fuss about the lining. In short, don't just look at the appearance, you have to look at the manufacturer's label.

How to judge if there is a direction? Three methods

Look for arrow marks on the outer wall or flange of the product. We often say that "the direction of the arrow of the expansion joint refers to", that is, the direction of the medium flow and the direction of the small mouth of the guide tube. In our stationMetal rectangular expansion joint、Corrugated expansion joint for power station industryWhen you leave the factory, an arrow will be marked in a prominent position, so you can tell it at a glance.

What if there were no arrows? Take apart the flange to see the internal structure. The guide tube is like a bell mouth, and the small mouth is facing the direction of the medium. Otherwise, look forLarge diameter thick wall expansion jointDrawings-This type of heavy equipment generally has installation instructions, and the manufacturer will mark the flow direction.

Then I bought itSleeve type pipe expansion jointIs there any direction? Sleeve type is also divided, but most of them rely on sealing packing, and the direction has little influence. But the corrugated ones are basically particular.

What if I install it backwards? Don't make do

For flue gas pipelines, especially desulfurization systems or high-temperature flues, the inverted guide tube will expose the ripples directly to the dusty gas stream. Doubling the wear rate is light, and in severe cases, it can trigger corrugation tearing, resulting in the outage of the entire pipeline. We handled aDesulfurization flue gas baffle doorThe matching expansion joint was installed backwards by the user, and after half a year, the air leakage exceeded the standard, so it had to be stopped for a new one.

It is recommended to reinstall in the correct direction after shutdown. If the site can't be dismantled (for example, it has been welded or the space is limited), you can consider installing an anti-wear plate on the outside, but it will treat the symptoms rather than the root cause. The guard plate can block for a while, but can't stop long-term scouring. Eventually it will have to be changed.

At the end of the day, take ten minutes to verify directions before installation

It is much more cost-effective than replacing an expansion joint of tens of thousands of dollars in the later period. Whether it isRectangular non-metallic expansion jointStillDirect buried (fully buried) type expansion jointThe rules are the same: if there is an arrow, look at the arrow, if there is no arrow, look at the guide tube, and if there is no mark, ask the manufacturer for drawings. Don't pretend by your feeling, the industry is just like that.

So back to the question of "Is there a positive and negative for the flue expansion joint"-the answer is: those with guide tubes and those marked by arrows must be distinguished between positive and negative; Pure rubber or fabric with no guide tube and no marks, usually not distinguished, but also pay attention to the direction of the wear layer. The heavier the equipment, the worse the working conditions, the less careless. Remember: When you get the right direction, the pipeline can run for the long term.

1. What does the expansion joint at the end of the flue do?

Many people think that the expansion joint at the end of the flue is a "telescopic tube", which is used to offset thermal expansion and contraction. Actually, what it does is more complicated than you think. Especially in power stations, cement and chemical industries, the end of the flue is connected to a dust collector, desulfurization tower or induced draft fan, and the pipeline is full of high-temperature flue gas, as well as dust and corrosive gases. The expansion joint should not only absorb axial displacement, but also carry radial offset and angular displacement. Think about it, the flue is hundreds of meters long. As soon as the temperature rises, the pipe will not only become longer, but also be distorted due to uneven settlement of the support and wind load. If the expansion joint is not selected correctly, it will either get stuck or leak, and in serious cases, the equipment will be directly pulled out.

Many on-site faults are not the poor quality of the expansion joint at all, but the parameters are not calculated correctly. Therefore, don't turn over the sample to select the model as soon as you come up, and honestly find out the three basic parameters: temperature, pressure and displacement.

2. Three bottoms that must be found out before calculation: temperature, pressure and displacement

These three parameters are like oil, salt, sauce and vinegar for cooking. If one dish is missing, it will be wasted.

• Design temperature: It is not the operating temperature, but the limit temperature. For example, the smoke is normal at 250℃, but it may rush to 350℃ when it starts and stops. You have to calculate it according to 350℃. In addition, consider the temperature gradient-the temperature difference between the inner and outer walls of the flue will lead to uneven expansion, especially the fiber layer of non-metallic expansion joints, which will burn up when the temperature is too high.
• Design pressure: Most flue systems are slightly positive or negative pressure, but don't be careless. The entrance area of the desulfurization tower may have an instantaneous positive pressure impact due to the fan surge. One project once selected an ordinary non-metallic expansion joint, and as a result, the pressure fluctuation directly tore the skin. It is an industry rule that the pressure parameter should be 1.5 times the maximum working pressure as the safety margin.
• displacement amount: Here are the most pits. The displacement includes thermal expansion displacement, installation error displacement and foundation settlement displacement. Thermal expansion can be calculated by formula (detailed in the next section), but the installation error and settlement are estimated by experience, and it is generally recommended to reserve a margin of ±10mm. For example, the horizontal section of the flue is 20 meters long, and the thermal expansion is calculated to be 45mm, so the displacement capacity should be at least 60mm when you select the model.

3. How to calculate thermal expansion? The formula is simple, but the pits lie in the details

Δ L = α × L × Δ T. α is the linear expansion coefficient, about 12×10⁻⁶/°C for carbon steel and about 17×10⁻⁶/°C for stainless steel; L is the length of the pipe in mm; Δ T is the temperature difference in °C. For example: a section of carbon steel flue with a length of 15 meters, rising from the installation temperature of 20°C to the operating temperature of 300°C, Δ L =12×10⁻⁶ ×15000×280=50.4mm. ok, the numbers are out, and then what? Here comes the pit.

The pipe is not a whole one, there are elbows, tees, and baffle doors in the middle. The elbow itself will absorb part of the displacement, but in actual calculation, it is usually conservatively calculated according to the length of the straight pipe section, and safety comes first. The second pit: multiple pipes in parallel flue, such as double flue, each pipe has different expansion amount, so the expansion joint can't be selected independently, so the mutual constraints between pipe bundles have to be considered. Third Pit: If the end of the flue is connected toFlue gas baffle door(For example, the desulfurization flue gas baffle door or electric plug-in insulation door of this station), the frame stiffness of the baffle door is limited, and the thrust generated by the expansion joint will deform the door body. At this time, it is necessary to select the expansion joint with a tie rod to offset the pressure thrust.

So don't worry and be happy after calculating the values, ask yourself: Do I have any of these "invisible killers" in my plumbing system?

Fourth, the key to selection: non-metallic expansion joint or metal rectangular expansion joint, which should you use?

Non-metallic expansion joint (fabric fiber expansion joint)AndMetal rectangular expansion joint。 How to choose? In one sentence: large displacement, low pressure, high temperature flue gas, with non-metal; For small displacement, medium and high pressure, wear resistance and erosion resistance are needed, and metal rectangles are used.

The advantages of non-metallic expansion joints are large compensation (a single layer of skin can absorb 100mm axial displacement), high temperature resistance (silica gel cloth + ceramic fiber layer can reach 1000℃), and no thrust-because the fiber layer has basically no stiffness. However, its weakness is poor compression resistance. If the positive pressure exceeds 0.05MPa, it should be cautious, and there should be no sharp particles to scour, otherwise the skin will wear out quickly. Therefore, the end of the dusty flue, such as the electrostatic precipitator outlet, often needs to be addedguide tubeProtection (the guide tube can prevent the flue gas from directly eroding the internal parts of the expansion joint, which is specifically mentioned in the FAQ of this site).

The metal rectangular expansion joint has high stiffness, high pressure and long life, but the manufacturing cost is high, and the compensation per wave is limited (generally, a single wave can only absorb 10~20mm). If your flue size is very large, such as a rectangle of 2 meters by 3 meters, the metal rectangular expansion joint is usually made of a multi-wave structure, and it should be equipped withtie rodTo withstand internal pressure thrust. The function of the tie rod is to limit the excessive stretching of the expansion joint and prevent the instability of the bellows (refer to the question and answer of this site "The function of the expansion joint tie rod").

In addition, don't forget that some projects will also use the two in combination: metal rectangular expansion joints for the axial direction of the flue, and non-metallic ones for angular displacement.

5. Don't be in a hurry to place an order after calculation: do you want to count the guide tube, tie rod and fatigue life?

The parameters have been calculated and the model has been selected, but there are still a few things to confirm before placing an order, otherwise it may break after three days of installation.

• guide tube: High-frequency wear conditions must be matched. The thickness and material of the guide tube should also be selected according to the flue gas temperature. Generally, the temperature resistance of carbon steel guide tube is ≤500℃, which exceeds that of stainless steel.
• Tie rod and limit structure: When the metal rectangular expansion joint is installed, how to adjust the tie rod nut is a technical job (this site has special questions and answers). Remember: After installation, the tie rod nut should be loosened to half of the designed displacement, otherwise the expansion joint will not expand or contract normally.
• Fatigue life: Flues that start and stop frequently, such as peak shaving units in thermal power plants, expansion joints may experience multiple temperature cycles every day. The fatigue life of metal bellows is inversely proportional to the displacement, and generally requires at least 1000 cycles. Although the non-metallic expansion joint has no fatigue problem, the aging life of the skin is usually 3~5 years, so it needs to be replaced regularly.
• Medium corrosivity: The wet flue gas after desulfurization contains sulfuric acid condensate, which has strong corrosion to metals. That's when you have to goPTFE-lined hoseOrPTFE compensatorOr choose a corrosion-resistant non-metallic skin.

Oh yeah, there's another one that's easy to overlook-pressure thrust. The internal pressure of the pipeline acting on the effective area of the expansion joint will generate a large axial thrust. If the tie rod or fixed bracket is not installed, this force will destroy the adjacent equipment. Especially for large-size flues, the pressure thrust can easily reach tens of tons, so don't take it seriously.

VI. A practical case: the complete calculation process from parameters to selection

Two days ago, I met a customer, the kiln tail flue of a cement plant, with a rectangular cross-section of 1.8m ×1.2m, a length of 25 meters, and a material of Q235. Working conditions: Flue gas temperature 350℃, maximum 400℃ at start-up, pressure-0.03MPa (negative pressure), horizontal installation, two ends connected respectivelyRound Flap Door (Double Seal)AndFlue gas baffle door。 Displacement requirements: axial thermal expansion + installation allowance total 80mm, transverse displacement estimated ±5mm.

Step 1: Calculate thermal expansionΔ L =12 x 10⁻⁶ x 25000 x (400-20) =114 mm. With a reserved margin of 10mm, the total displacement is 124mm. However, note that there are baffle doors at both ends of the flue, which are rigid, and the actual absorption of the expansion joint is 120mm.

Step 2: Judge positive and negative pressuresUnder negative pressure conditions, the internal pressure thrust is very small, and the pull rod is not needed. However, negative pressure may cause the skin to be concave, and a support ring needs to be added inside the non-metallic expansion joint.

Step 3: Type selectionConsidering the temperature of 400℃, large displacement, negative pressure and rectangular section, the first choiceRectangular non-metallic expansion jointThe skin structure adopts ceramic fiber + silica gel cloth + stainless steel wire mesh, which has a temperature resistance of 450℃, and a guide tube (carbon steel, δ =3mm) is installed to protect the inner wall. Since the lateral displacement is only ±5mm, the non-metallic expansion energy saving is easily absorbed.

Step 4: CheckCheck the product information of non-metallic expansion joint in this site, select the model with a length of 500mm, and the axial compensation amount can reach 150mm, which meets the requirements. The length of the guide tube is designed according to 1.2 times the inner diameter of the pipe to avoid flushing. Finally, confirm the size of the flange of the baffle door interface, and the order can be placed if it matches.

After walking down the whole process, you will find:How to calculate the expansion joint at the end of the flueThis matter was not just a formula. There are living working condition details behind each parameter of temperature, pressure and displacement, and if one is missed, the car may overturn. But as long as you follow this logic step by step, even doing it the first time can reduce your selection mistakes by 90%.

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.