Specialized in manufacturing compensators, expansion joints, baffle doors

At the construction site of urban heating pipelines, you will find an interesting phenomenon: steam pipelines that can be laid straight often have to go around a U-shaped bend. Is this "superfluous" by designers, or is there another mystery? In fact, this section of "redundant" elbow has a professional name in engineering-expansion joint. It seems simple, but it is the core component to ensure the safe operation of industrial pipelines and large equipment. This article will take you through a comprehensive understanding of what an expansion joint is, from the fundamentals, the main types to the core functions.

1. What is the expansion joint? A vivid metaphor

What is an expansion joint? From an engineering definition, expansion joints, also commonly called compensators or expansion joints, are flexible structures placed on vessel shells or pipes to compensate for additional stresses caused by temperature differences and mechanical vibrations。

To better understand what expansion joints are, a life-like metaphor can be used: steam pipes will "elongate" at high temperatures and "retract" at low temperatures, just as people's stomachs will bloat when they are full and deflate when they are hungry. The expansion joint is the "elastic waist" of the pipeline。 Whether the pipe is thermally expanded or cold contracted, it can absorb this part of the dimensional change through its own elastic deformation, thus protecting the pipe from being pulled or crushed by stress.

Why would an expansion joint be needed?

Metals have the physical property of "thermal expansion and cold contraction". Take steam pipes as an example. The operating temperature can usually reach 150 ℃ or even higher. A 100-meter-long steel pipe will elongate about 18 mm when it rises from normal temperature to 150 ℃。 In electric power, chemical industry and other industries, the pipeline temperature can even reach 500-600℃, and the expansion amount caused by temperature difference is more considerable。

If both ends of the pipeline are fixed, this expansion has nowhere to be released, which will generate huge thermal stress inside the pipeline, which will lead to pipeline deformation and bracket damage, and in the worst case, weld cracking or even pipe burst accident. The expansion joint was created to solve this contradiction。 It will be stretched or compressed like a spring, absorbing away the linear elongation of the pipe; When the temperature drops, it springs back to its original state to ensure the safety of the pipe system。

2. What are the main types of expansion joints?

After learning what an expansion joint is, let's take a look at its classification. There are many types of expansion joints, which are mainly divided into the following categories according to their structure and materials:

1. Classification by shape and structure

• Elbow type expansion joint (natural compensation): that is, a U-shaped, L-shaped or Z-shaped bend is directly bent on the pipe, and the elasticity of the pipe itself is used to absorb the displacement。 It has the advantages of simple structure, low cost and convenient maintenance, but the disadvantage is that it occupies more space and is suitable for outdoor or open space occasions。
• Bellows Expansion Joint: This is the most widely used type at present, with circles of metal ripples in the middle part like an organ。 This structure is very compact, can not only absorb axial displacement, but also adapt to lateral and angular offsets, and has excellent performance。
• Sleeve type expansion joint: absorbs axial displacement by sliding of inner and outer cylinders, has good sealing performance, and is suitable for high pressure and high temperature steam system。

2. Classification by materials used

• Metal expansion joint: The main body is stainless steel, with strong pressure bearing capacity and high temperature resistance, which is the main force of industrial pipelines. Its design and manufacture should strictly follow the national standard GB/T 12777-2019 General Technical Conditions for Expansion Joints of Metal Corrugated Pipe。
• Non-metallic expansion joint: Also called fabric compensator, the main body is fibrous fabric and rubber。 Its advantages are large amount of compensation (especially lateral displacement, far exceeding metal material), no reverse thrust (friendly to equipment interface), vibration and noise isolation and corrosion resistance, which is very suitable for flue gas pipelines in power plants and chemical plants。

3. Core functions of expansion joints

In order to understand more fully what an expansion joint is, we need to understand several core "skills" of it. It is not only a joint on a pipe, but also a protector of the whole system.

4. Application fields and importance of expansion joints

Nowadays, expansion joints are found in various industrial fields. In the petrochemical industry, it is subjected to high temperatures and pressures; In the power industry, it is accompanied by the roar of boilers and steam turbines, absorbing huge thermal displacements; In the field of building HVAC, it connects water pumps and air conditioning units to guard the warmth and coldness of the city. In the field of environmental protection, it is an indispensable "flexible joint" in desulfurization and denitrification system, which protects the flue from corrosion and cracking。

It can be said that all fields involving fluid transport and high temperature are inseparable from expansion joints. Although it is only a "joint" in the pipeline system, it is a key link to ensure the long-term, safe and stable operation of the whole system.

sum up

Now back to the original question: What is an expansion joint? It is not only the curious "bend" on the steam pipeline, but also a cold string of provisions in the Code for Design of Industrial Metal Pipelines. Expansion joint is the "safety valve" of industrial pipeline and the "wisdom crystallization" of engineering design. Through the ingenious flexible structure, it solves the most basic physical problem of thermal expansion and contraction, and protects the safe operation of complex pipe network system.

There are countless expansion joints playing a role silently behind every safe industrial operation. If your business is facing excessive pipe stress or vibration problems, check whether the expansion joint in the system is properly selected and installed in place-this small part is often the key to solving the problem.

In the design, procurement and operation and maintenance of industrial pipeline system, the difference between expansion joint and compensator is often confused by many engineers and technicians. Some people say they are the same thing, only by different names; Others insist that there are essential differences between the two in structure and application. Which statement is correct? This article will combine GB/T 12777 national standard and industry practice, systematically analyze the difference between expansion joint and compensator, and help you thoroughly clarify these two concepts.

1. Basic definition: customary "same object with different names"

By definition, the difference between an expansion joint and a compensator is actually not obvious-in most industry contexts, "expansion joint" and "compensator" refer to the same product.

Expansion joints, also known as compensators, or expansion joints, are composed of bellows (an elastic element) and attachments such as end tubes, brackets, flanges and conduits that constitute their working body。 It belongs to a compensation element, which utilizes the effective expansion and contraction deformation of the bellows to absorb the dimensional change of pipelines, conduits, containers, etc. caused by thermal expansion and cold contraction, or to compensate the axial, transverse and angular displacement of pipelines, conduits, containers, etc., and can also be used for noise and vibration reduction。

An expansion joint is a flexible structure arranged on a vessel shell or pipe to compensate for additional stress caused by temperature difference and mechanical vibration。 Expansion joints (compensators) play an irreplaceable role in chemical industry, construction, water supply and drainage, petroleum, electric power or fire protection。

Second, type subdivision: metal and non-metal

Although there is little difference between expansion joints and compensators in terms of name, expansion joints (compensators) can be divided into two broad categories according to material and structure subdivision: metal expansion joints and non-metal expansion joints。 The performance differences between the two are reflected in multiple dimensions.

1. Compensation function

The compensation function of non-metallic expansion joints is much larger than that of metallic expansion joints under the same external dimensions. Especially for the compensation function of lateral displacement, non-metallic expansion joints are almost one order of magnitude larger than metal expansion joints。

2. Compensatory resistance

An expansion joint acts like an elastomer that develops resistance after being compressed or stretched. The resistance of metal expansion joints is much greater than that of non-metal expansion joints-too much resistance is bad for the system and may damage the surrounding equipment。

3. High temperature resistance and corrosion resistance

The yield limit of ordinary carbon structural steel is equal to zero at 600℃, and it will be oxidized and peeled when the temperature is higher. Carbon steel expansion joints are easy to crack in the working process, while stainless steel expansion joints will have low leakage corrosion in low temperature environment。

The material of non-metallic expansion joint not only has good high temperature resistance, but also has acid and alkali resistance, which is more advantageous in corrosive environment。

4. Other performance comparisons

3. The broader connotation of the concept of "compensator"

If we must delve into the difference between expansion joints and compensators, we can think that the extension of the concept of "compensator" is wider than that of "expansion joints".

Compensators refer more to those devices that can compensate for various axial, lateral, and angular displacement changes of pipes due to thermal expansion and contraction or operational vibrations。 Expansion joint specifically refers to the compensation device with bellows as the core elastic element.

Therefore, from the perspective of the inclusion relationship: the bellows expansion joint is a type of compensator, but the compensator may also include other structural forms of compensating devices, such as:

• Casing expansion joint: It adopts inner and outer casing structure, with small friction resistance to fluid flow and compact structure
• Hinged compensator: used to absorb transverse deformation in a single plane of a pipe
• Universal hinge compensator: can absorb the amount of deformation in three dimensions

Differences in Application Scenarios

Special requirements in the marine field

In the marine system, the difference between the expansion joint and the compensator is also reflected in the selection of media. Water and oil pipes are basically made of rubber expansion joints, and smoke are made of metal expansion joints, because the exhaust temperature is unbearable by rubber。

Recommended choices for different working conditions

V. Clarification of common misunderstandings

Myth 1: "Expansion joint" and "compensator" are two completely different devices

This is one of the most common misconceptions. In fact, the two refer to the same product in the vast majority of cases, only by different names. Expansion joints are also customarily called compensators。

Myth 2: All compensators are called expansion joints

This statement is not accurate enough. Expansion joint specifically refers to a device with bellows as the core elastic element. Other forms of compensating devices, such as casing expansion joints, although also belong to the category of compensators, are not generally called expansion joints。

Myth 3: Non-metallic expansion joints cannot be used at high temperatures

In fact, the operating temperature of non-metallic expansion joints covers-20℃ to 1000℃, and the application range is very wide. The use of thermal insulation materials such as aluminum silicate thermal insulation cotton makes it able to run stably under high temperature conditions.

sum up

After systematic combing, the following conclusions can be drawn from the difference between expansion joint and compensator:

In actual engineering communication, neither the use of the terms "expansion joint" nor "compensator" can be misunderstood because they refer to the same class of products. It is important to understand the classification, performance characteristics and applicable working conditions of the product, rather than dwelling on the name itself. When selecting the model, the focus is on "what type and material of expansion joint to choose under what working conditions", rather than "should it be called expansion joint or compensator".

In the industrial flue gas pipeline system, how to keep the flue expansion joint warm is the key technical problem to ensure the safe operation of equipment, reduce the heat loss and prevent personnel from burning. The expansion joint should not only absorb thermal displacement and isolate vibration, but also keep the external surface temperature within a safe range under high temperature conditions (usually requires ≤60℃). Improper insulation design can lead to increased heat loss and energy waste, or overtemperature aging of expansion joint, burning of outer skin and even safety accidents. This paper will systematically explain the professional technical knowledge of how to insulate flue expansion joint from insulation structure, material selection to construction steps.

1. Why does the flue expansion joint need to be insulated?

How to insulate the flue expansion joint First of all, we should understand the necessity of insulation. The thermal insulation of expansion joints is mainly due to the following four considerations:

1. Prevent over-temperature aging

The outer skin of the expansion joint is usually made of non-metallic composite materials (fluororubber, silicone rubber, ceramic fiber cloth, etc.), which will accelerate aging and brittleness when exposed to high temperature for a long time. According to industry experience, the problem of overtemperature of expansion joint not only affects its life, but also affects the operation safety of boiler。 Effective heat preservation can reduce the working temperature of the skin and prolong the service life.

2. Reduce heat loss

High temperature flue heat loss will reduce the system thermal efficiency and increase energy consumption. Standardized insulation can significantly reduce heat loss.

3. Anti-scald protection

According to safety specifications, the surface temperature of equipment accessible to personnel shall not exceed 60°C. Insulation of expansion joint is an important measure to ensure the safety of maintenance personnel.

4. Prevent embrittlement in winter

Under the low temperature environment in winter, non-metallic skin materials are easy to harden and brittle. Low temperature resistant modified materials such as low temperature fluororubber can be adapted to environment ≤ -30℃Cooperate with heat preservation measures to effectively prevent embrittlement.

Second, heat insulation structure design of flue expansion joint

The core of how to insulate the flue expansion joint lies in the reasonable design of the insulation structure. According to the latest patented technology, the insulation structure of the expansion joint mainly includes two parts: the inner insulation layer and the outer skin。

1. Inner insulation layer structure

The inner insulation layer is the core of expansion joint insulation, and the structural design of multi-layer insulation cotton + steel mesh barrier is usually adopted:

Key technical points:

• The adjacent insulation cotton layers are separated by steel wire mesh to prevent the insulation layer from settling
• The heat insulation cotton shall be wrapped and fixed with stainless steel wire mesh, and the heat insulation nails shall be distributed to be fixed smoothly
• The innermost layer is laid with ceramic fiber cloth and fine mesh steel wire mesh to enhance the overall strength

2. Thermal insulation extension of flange part

The metal frames (flanges) at both ends of the expansion joints also need to be insulated. The patent technology proposes that that inner side of each folding plate-shaped flange is fixed with an inner lining folding plate, the inner lining folding plate is filled with heat preservation cotton, and an expansion gap is left between the two inner lining folding plates。

The advantages of this design are:

• The insulation layer is continuous and uninterrupted to prevent heat from escaping from the flange
• The inclined opening of the inner lining folding plate is consistent with the flow direction of smoke gas, so as to reduce the flow of smoke gas into the gap
• The expansion gap meets the thermal displacement requirements

3. Guide for selection of insulation materials

The key link of how to preserve the flue expansion joint is the material selection. Select the appropriate insulation material according to the working condition temperature:

1. Main insulation material

2. Multilayer composite structure of outer skin

A typical outer skin structure (from the inside out) comprises：

3. Supplementary heat insulation in low temperature working conditions

For expansion joints running in winter, anti-embrittlement measures should be considered:

• Avoid using non-metallic skin materials with low molecular weight and easy freezing
• Use low temperature resistant modified materials such as low temperature fluorine rubber to adapt to the environment ≤ -30℃
• Add anti-brittle coating, such as silicon-based protective agent or thermal insulation film, to reduce temperature stress and moisture erosion

Four, flue expansion joint heat insulation construction steps

The construction of how to insulate the flue expansion joint should follow the standard process:

Step 1: Construction Preparation

1. Check that the outer diameter size, skin width and technical parameters of the expansion joint meet the requirements
2. Check the quantity and quality of insulation materials (insulation cotton, wire mesh, skin, bolt assembly)
3. Clean up the surrounding flammable objects and prepare fire extinguishing tools

Step 2: Installation of the inner insulation layer

How to insulate the flue expansion joint, the inner insulation layer is the most critical link:

1. Laying the second wire mesh: fixing the second wire mesh between the left flange and the right flange as the base frame of the insulation layer
2. Laying bottom layer protection: the steel wire mesh is concave to form the installation area. First, a layer of ceramic fiber cloth is laid, and then a layer of fine mesh steel wire mesh is laid
3. Laying insulation cotton layer by layer: Laying insulation cotton layer and first steel wire mesh layer layer by layer, adjacent insulation cotton layers are separated by steel wire mesh
4. Insulation cotton fixation: Fix flat with insulation nails, and insulation cotton should be wrapped with stainless steel wire mesh
5. Outer wrap: Ceramic fiber cloth is laid on the outer part of the outermost insulation cotton, and ceramic fiber cloth is sewn on the edge

Thickness requirements: The total thickness of insulation cotton is determined according to the design temperature. Generally, low-temperature flue (400 °C) at 200-250 mm.

Step 3: Outer skin laying

1. Laying sequence: one layer of steel wire mesh, two layers of ceramic fiber cloth, one layer of PTFE cloth, one layer of ceramic fiber cloth and one layer of PTFE membrane are laid sequentially from the inside to the outside
2. PTFE membrane welding: Weld the PTFE membrane to the previous layer of ceramic fiber cloth to ensure sealing
3. Hemming and stitching: Finally, the outer side is hemmed and stitched with ceramic fiber cloth

Step 4: Skin fixation

1. Flat skin: Flat skin on the flange surface of metal frame to prevent wrinkles
2. Install from bottom to top: Install from the bottom, make sure that the inner insulation cotton has been installed sufficiently, and spread out from the bottom point to the outside in sequence
3. Tightening and compressing: After the skin is tightened, it is compressed with pressure plates and bolts
4. Bolt tightening: All compression bolts are tightened with an electric wrench, and no missing screwing is strictly prohibited

V. Installation precautions

How to insulate the flue expansion joint? The following matters should be paid attention to in the construction:

1. Prevent dust accumulation and blockage

For the non-metallic expansion joint of horizontal flue, the design of anti-plugging structure should be considered to prevent ash from entering the expansion joint and affecting the heat insulation effect。

2. Interface waterproof

For the expansion joint skin of horizontal flue, the upper interface should be pressed down at the interface of the outermost skin to prevent water seepage at the interface。

3. Welding protection

If welding is required during thermal insulation construction, protective measures must be taken to prevent welding slag splashing from damaging the skin.

4. Drainage design

According to the technical requirements, the expansion joint in contact with wet flue gas and located in the horizontal flue section shall be provided with a drainage hole, the drainage hole shall be at least DN150, and the drainage fitting shall be made of FRP or nickel-based alloy steel material。 Drainage design should be carried out in coordination with heat insulation construction, and drainage channels should be reserved.

5. Insulation thickness verification

After the installation of the insulation layer, the surface temperature shall be measured to confirm that the external surface temperature is ≤60℃ (or the design requirement value).

VI. Maintenance and Inspection

After the heat insulation construction is completed, how to insulate the flue expansion joint needs to be included in daily maintenance:

1. Regular inspection: check the outer skin for damage, aging and cracking
2. Inspection of insulation layer: Local overheating indicates collapse or missing insulation cotton
3. Winter inspection: Check the skin for embrittlement cracks during winter operation, and repair it in time
4. Drainage inspection: Check whether the drainage hole is unobstructed to prevent the accumulation of acidic condensate [ci^tation:2]

sum up

How to keep the flue expansion joint warm is the key technology to ensure the long-term safe operation of the expansion joint. The core points can be summarized as follows:

A flue expansion joint with reasonable heat insulation design and in-place construction specification can effectively reduce external surface temperature, prevent over-temperature aging, reduce heat loss, and realize long-term safe and stable operation under harsh working conditions such as high temperature and corrosion.

In the engineering construction and maintenance of boiler flue gas system, the acceptance of boiler flue expansion joint is the last barrier to ensure the safe operation of equipment and eliminate hidden dangers in operation. After the expansion joint is installed, if the acceptance is not strict and the standard is unclear, it will lead to air and smoke leakage and energy consumption after operation, and in the worst case, it will lead to expansion joint failure, unplanned furnace shutdown or even safety accidents. So, what exactly should be checked for boiler flue expansion joint acceptance? What is the standard basis? This paper will systematically explain the professional and technical points of acceptance of boiler flue expansion joint from acceptance basis, inspection items to test methods.

1. Standard basis for acceptance of boiler flue expansion joint

The acceptance of boiler flue expansion joint shall strictly follow the following national standards and industry norms:

According to the provisions of Article 10.2.1 of GB 50517-2010, the expansion joint shall participate in the system test: the expansion joint of the self-restraining device shall participate in the system test; Externally constrained expansion joints shall participate in system tests; All expansion joints shall be subjected to system test during leakage test。

In the waste heat power generation project of cement factory, GB51005-2014 Code for Construction and Quality Acceptance of Waste Heat Power Generation Project of Cement Factory clearly lists "Installation of Expansion Joints of Smoke and Air Ducts" as an independent acceptance chapter, covering the installation quality inspection of expansion joints of smoke and air ducts。

2. Installation quality acceptance: full inspection from appearance to size

1. Appearance inspection

Appearance inspection for acceptance of boiler flue expansion joint shall include the following:

2. Dimensional inspection

The accuracy of the mounting dimensions directly affects the compensation ability of the expansion joint:

• Installation length: Check whether the distance from the flange surface to the flange surface of the expansion joint meets the design value, and the deviation shall not exceed ±5mm
• Coaxiality: The expansion joint shall be concentric with the pipe with a deviation ≤3mm
• Levelness: horizontally mounted expansion joint, level deviation ≤3‰
• Perpendicularity: Expansion joint installed vertically, perpendicularity deviation ≤3‰

3. Direction confirmation

This is the "one-vote veto item" in the acceptance of boiler flue expansion joint:

• Flow direction identification: The flow direction arrow on the expansion joint must coincide with the flue gas flow direction
• Direction of guide tube: The small end of guide tube must be facing the direction of flue gas flow
• Inner sleeve position: the end of the inner sleeve with weld, which should be located at the inflow end of the medium on the horizontal pipeline; It should be placed on the upper part of the vertical pipe

Special warning: A 410t/h boiler ignition air duct expansion joint fault case shows that the installation misalignment amount of 80mm is not reserved according to the design requirements during installation, and the expansion joint bellows is seriously deformed during operation, and the internal pouring material is damaged and fallen off, which finally leads to the expansion joint damage。

3. Inspection of transportation and limiting device

In the acceptance of boiler flue expansion joint, the treatment of transportation protection device is the most easily overlooked link.

1. Removal of transport tie rod

According to the provisions of Article 7.0.3 of GB 50753-2012, after the installation of the compensator, all the protective screws of the original belt shall be loosened, and the loosening distance shall be greater than the designed maximum compensation amount of the compensator.

Acceptance inspection points:

• Whether the transport protection tie rod of the metal expansion joint has been completely removed
• Whether the positioning limit pull rod is retained (not removed)
• Whether the release nut has been adjusted to the pre-bias position required by the design

2. Limit device adjustment

• Whether the limit tie rod nut has been adjusted in place according to the design drawing
• Whether the limit clearance meets the design requirements (generally about 1.2 times the axial compensation amount)
• Is the spring hanger reset (if any)

3. Temporary support removal

Transportation temporary supports added to large-diameter expansion joints when they leave the factory shall be completely removed after installation.

4. Sealability inspection and pressure test

1. Sealability Check

The tightness inspection of boiler flue expansion joint acceptance is the key link to verify the installation quality:

• Inspection method: Soap water application method or leak detector can be used
• Inspection site: Flange connection surface, weld, skin joint, pressure plate bolt
• Acceptance criteria: No bubble generation or no alarm from the detector is qualified

According to GB 50517-2010, all expansion joints shall participate in system test during leak test。

2. Pressure test precautions

When the expansion joint participates in the system pressure test, attention should be paid to:

• Confirm that the limit device of the expansion joint is set correctly before pressure test
• The expansion joint of the self-restraining device can directly participate in the system test
• The water used for hydraulic test shall be clean and non-corrosive, and the chloride ion content shall not exceed 25ppm

3. Airtightness test

For flue gas systems, air tightness tests should also be performed:

• The test pressure is 1.1 times the design pressure
• Hold time ≥30 minutes
• Pressure not reduced to qualified

V. Special acceptance items of non-metallic expansion joints

Acceptance of boiler flue expansion joints for non-metallic expansion joints requires additional attention to the following items:

1. Bolt tightening inspection

The skin of the non-metallic expansion joint is fixed by a platen bolt. Due to the large length of the platen (up to 4-6 meters), the distal bolt will loosen due to the compression deformation of the skin after one-time tightening.

Acceptance points:

• Check that all bolts have been tightened as per "diagonal tightening, fractional tightening"
• Use a hand hammer to gently tap the pressure plate, the sound should be crisp and consistent, without loosening
• Whether the bolt torque meets the design requirements (generally 50-80N·m)

2. Drain hole inspection

For horizontally installed boiler flue expansion joints (especially wet flue sections), drainage holes must be provided:

• Check Location: Whether the drain hole is located at the lowest point of the frame
• Pore size: ≥ DN50 (recommended DN50-DN100)
• Smoothness: Whether the drainage pipe is unobstructed and the dust net is intact
• Material: Whether drainage fittings are made of corrosion-resistant materials (FRP or stainless steel)

3. Skin lamination examination

The skin of the non-metallic expansion joint is a multi-layer composite structure, and the following shall be confirmed during acceptance:

• Whether the number of skin layers meets the design requirements
• Whether the layers are tightly attached and without delamination
• No wrinkles or scratches on the skin surface

VI. Acceptance of quality certification documents

The acceptance of boiler flue expansion joint should not only check the physical objects, but also check the documents. Complete quality certification documentation.

Common Acceptance Problems and Treatment

8. Work after acceptance

After the boiler flue expansion joint passes the acceptance, the following work shall be completed:

1. Establish equipment ledger: record the model, specification, installation date and acceptance person of the expansion joint
2. Photo archive: Take photos of the expansion joint after installation (including nameplate and flow direction identification)
3. Formulate maintenance plan: specify the first tightening time (1 month, 3 months after commissioning) and inspection cycle
4. Handover to Operation Department: Provide acceptance report and maintenance instructions

sum up

The acceptance of boiler flue expansion joint is the key link to ensure the safe operation of flue gas system. The standard acceptance shall follow the five-step method of "checking the appearance, nuclear size, checking the direction, removing the tie rod and testing the seal":

Same Ensure that the quality certification documents are complete, establish the equipment ledger, and formulate the maintenance plan. A standardized acceptance can prevent the early failure of the expansion joint caused by installation quality problems from the source, and ensure the long-term safe and stable operation of the boiler flue gas system.

In the design of industrial flue gas pipeline system, the reasonable arrangement of flue expansion joint directly determines whether the pipeline can operate safely and whether the expansion joint can play the normal compensation function. Improper arrangement can lead to premature failure of expansion joints, pipe deformation and even equipment damage. The expansion joint is not randomly installed in any position of the pipe. It needs to work together with the fixing bracket, guide bracket and limit bracket to form a complete compensation system. This article will systematically explain the technical key points of flue expansion joint arrangement from the arrangement principle, bracket configuration to typical examples.

1. Basic principles of flue expansion joint arrangement

The core objective of flue expansion joint arrangement is to absorb the axial and radial displacement of all connected equipment and flue with the most reasonable number and position of expansion joints, and at the same time control the thrust and torque of pipe to equipment within the safe range。

1. Principle of segmented compensation

The long straight pipeline is divided into several independent compensation pipe sections, and fixed brackets are arranged at both ends of each pipe section, and an expansion joint is arranged in the middle. Each expansion joint is only responsible for absorbing the amount of thermal elongation of this pipe section. The principle of dividing the length of the pipe section is: the upper limit of the compensation amount of each expansion joint determines the maximum length of the pipe section. Taking carbon steel pipe as an example, when the design temperature is 400℃, the thermal elongation of the pipe per 10 meters is about 45-50mm, and the length of the pipe section can be determined accordingly.

2. Principle of proximity to fixed bracket

The expansion joint should be arranged as close to the fixed bracket as possible. According to the engineering experience, when arranging the flue expansion joint, ensure that the distance from the expansion joint to the fixed bracket should be controlled within 4 times the pipe diameter (4D)。 The purpose of this requirement is to prevent transverse instability of the pipeline and to ensure that the expansion joint only bears axial displacement and does not bear additional bending moments.

3. Guide bracket matching principles

Both ends of the expansion joint must be provided with guide brackets, and the distance between the guide brackets and the expansion joint is strictly regulated. The first guide bracket shall be installed at a distance of 4D from the expansion joint, and the distance between the second guide bracket and the first guide bracket shall be 14D。 The function of the guide bracket is to ensure that the pipe expands and contracts along the axis direction and prevents lateral swing.

2. Arrangement requirements of fixed brackets

The fixing bracket is the most important component in the flue expansion joint arrangement. Its function is to limit pipe displacement and divide pipe sections into independent compensation units. The fixed bracket must be able to withstand the vector sum of the following loads:

The research published in Pressure Vessel, Issue 2, 2024 clearly points out that the selection and rational arrangement of pipeline supports are the key to ensuring the safe and effective service of expansion joints。 In high temperature flue gas pipeline, if the strength of the fixed bracket is insufficient, the blind plate force will lead to the failure of the bracket and the instability of the expansion joint.

Third, the spacing of the guide bracket and the golden combination

The arrangement spacing of the guide brackets directly affects the working state of the expansion joint. The correct flue expansion joint arrangement should follow the following "golden combination":

Guide bracket spacing formula

According to the industrial pipeline construction specifications, the bracket configuration of flue expansion joint arrangement shall follow the combination mode of "fixed bracket-expansion joint-first guide-second guide-sliding bracket":

1. Fixed bracket: end point of pipe section, bearing full load
2. Expansion joint: mounted against fixed bracket
3. First guide bracket: ≤4D from expansion joint
4. Second guide bracket: distance from first guide bracket =14D
5. Sliding brackets: The spacing is determined according to specification, allowing the pipe to slide freely

This "4D-14D" rule is derived from the engineering practice of diesel generator smoke exhaust pipe, and also applies to general industrial flue。

4. Difference between horizontal flue and vertical flue

1. Horizontal flue expansion joint arrangement

Horizontal flues are the most common scenario for flue expansion joint arrangements. In addition to following the above stent rules, note that:

• Drainage slope: When installing horizontal pipelines, the drainage slope should be ≥3‰ to ensure that the condensate can flow to the drainage point
• Drainage hole setting: Expansion joint contacting wet flue gas and located in horizontal flue section, drainage hole is at least DN150, located on the center line of horizontal flue section
• Material of drainage fittings: Drainage pipes should be made of corrosion-resistant materials such as FRP or nickel-based alloy steel

2. Vertical flue expansion joint arrangement

The expansion joint arrangement of a vertical flue is more complicated than that of a horizontal pipe because gravity factors can significantly affect the pipe stress:

• Guide brackets are required to prevent pipeline instability
• Load-bearing device: Consider the self-weight of the pipeline bearing by the expansion joint, and set a spring hanger if necessary
• Direction of inner sleeve: One end of the bellows expansion joint with welding seam, which should be placed on the upper part of the vertical pipe

3. Special arrangement of high-temperature flue

For high temperature flue such as SCR flue gas system, there are special requirements for flue expansion joint arrangement. According to DL/T 5121-2020 "Technical Regulations for Design of Flue, Wind and Pulverized Coal Pipeline in Thermal Power Plant", due to the high flue gas temperature, the lateral displacement and horizontal displacement of the flue are large, and the flue is not long enough, the metal expansion joint generally cannot meet the requirements of safe use, so the non-metal expansion joint is preferred。

Pre-offset setting: In order to ensure the uniformity of flue gas flow field, the expansion joint should be 100% pre-offset during installation, that is, the flue on both sides of the expansion joint is misaligned during installation, and the axis of the flue on both sides is on a straight line during operation。

V. Typical Arrangement Examples and Bracket Spacing Reference

Example 1: Smoke Exhaust Pipe for Diesel Generator

A 30-meter-long metal smoke pipe has a temperature rise of 520℃ and an expansion amount of 176mm. The correct arrangement scheme of flue expansion joint is as follows: a fixed bracket is arranged near the equipment side, the expansion joint is installed close to the fixed bracket, the first guide bracket is located at 4D, the second guide bracket is located at 14D, and the other positions are arranged with sliding brackets according to specifications。

Example 2: SCR Denitrification System Flue

The flue at the inlet and outlet of SCR reactor is subjected to three-way displacement, and non-metallic expansion joint is adopted. Special attention should be paid to the arrangement: fixing brackets with sufficient strength are set on both sides of the expansion joint; The spacing of guide brackets is according to the 4D principle; 100% pre-bias at installation。

Example 3: FRP Flue Gas Pipe

According to SH/T 3522-2017, the expansion joint arrangement of FRP flue gas pipe also follows the principle of distance from fixed bracket ≤4D. Set drainage slope ≥3‰ when installing horizontal pipeline。

6. Key control points in construction and installation

1. Pre-stretching/Pre-compression

Before installation, the expansion joint shall be pre-stretched or compressed according to the design documents, and the force shall be uniform。 The purpose of pre-stretching is to give the bellows an initial deformation in the cold state, so that the bellows are in optimal working condition in the hot state.

2. Temporary restraint device

When installing the bellows expansion joint, a temporary restraining device shall be installed, which shall be removed after the pipeline is installed and fixed。 After installation, the transport protection tie rod must be removed immediately (the positioning tie rod must not be removed) so that the expansion joint can be expanded and contracted freely.

3. Coaxiality Control

During installation, the bellows expansion joint shall be concentric with the pipe and shall not be skewed, and circumferential torsion of the expansion joint caused by installation shall be avoided。 The installation error of the pipeline shall not be adjusted by means of making the pipeline deform or compensation for the expansion joint.

4. Welding protection

During installation, welding slag is not allowed to splash on the surface of the bellows, temporary supports are not allowed to be welded on the wave nodes, and hoisting fixtures such as wire ropes are not allowed to be directly tied on the wave nodes, so as to avoid mechanical damage to the wave nodes。

VII. Common mistakes and correction of layout design

sum up

The arrangement of flue expansion joint is a systematic project. The correct arrangement should be closely integrated with the bracket system, and follow the cooperative arrangement principle of "fixed bracket-expansion joint-guide bracket":

• Section determination of fixed bracket: the pipeline is divided into several independent compensation pipe sections, and the fixed bracket shall bear blind plate force
• The expansion joint is close to the fixing frame: the distance is ≤4D, and the high-temperature flue should be 100% pre-biased
• The guide brackets are sequentially arranged: a first guide distance expansion joint 4D, a second guide distance first 14D
• Drainage design should not be neglected: DN150 drainage hole for horizontal wet flue expansion joint, material FRP or nickel-based alloy steel
• Welding specifications must be followed: Splash of welding slag is strictly prohibited, and the transport tie rod is removed after installation

The selection and reasonable arrangement of the support is the key to ensure the safety and effective service of the expansion joint。 A reasonably designed expansion joint arrangement scheme can avoid the failure problem from the source and ensure the long-term safe operation of the flue system.