Specialized in manufacturing compensators, expansion joints, baffle doors

In the daily operation and maintenance and regular maintenance of industrial flue system, expansion joint, as a wear part, will have problems such as aging, leakage or decreased compensation ability after long-term operation, so it needs to be replaced in time. So, how to remove the expansion joint in the flue? This is a practical problem that many field engineers and overhaul workers often face. It seems simple to remove the expansion joint, but if it is not operated properly, it may not only damage the flue interface, but also pose a great safety hazard. This paper will systematically explain the disassembly method, preparation work, operation steps and matters needing attention of the expansion joint in the flue, and provide practical guidance for the field operation.

First, why do you need to know the disassembly method of the expansion joint in the flue?

Before starting the discussion "How to remove the expansion joint in the flue", it is necessary to clarify the circumstances under which the expansion joint needs to be removed:

• Damage to the expansion joint body: cracks, perforations, bulging or severe aging, resulting in flue gas leakage
• Achieve the designed service life: Non-metallic expansion joints are usually 5~8 years, and metal expansion joints need to be replaced 8~12 years
• System retrofit and upgrade: flue routing change or replacement of different types of expansion joints
• Internal maintenance requires: cleaning up dust accumulation, checking the guide tube or replacing the insulation layer

Whatever the reason, mastering the correct disassembly method can effectively shorten the downtime, avoid secondary damage and ensure the safety of workers.

Preparation before disassembly

The first step in how to remove the expansion joint in the flue is to be fully prepared. Do not work blindly, otherwise it may cause safety accidents or equipment damage.

1. Safety measures

• Shutdown Cooling: Ensure that the flue system has been completely shut down and the internal temperature has dropped below 50℃
• Ventilation replacement: Forced ventilation of flue to eliminate residual toxic and harmful gases (such as CO, SO₂)
• Energy isolation: Cut off the fan power supply and lock the tag (LOTO), close the upstream and downstream baffle doors
• Gas detection: Use four-in-one gas detector to confirm oxygen content (19.5% ~23.5%) and no toxic gas

2. Preparation of tools and materials

3. Site Investigation and Marking

• Take photos to record the original condition of the expansion joint before installation
• Mark the relative position of the expansion joint to the flue (may be aligned with sample punch or paint at the flange)
• Check whether there is dust accumulation on both sides of the expansion joint, and clean it in advance if so

Third, step-by-step explanation: How to remove the expansion joint in the flue?

The following is a standard disassembly process for the most common flanged expansion joints.

Step 1: Clean the area around the expansion joint

Use a blade or compressed air to remove dust, corrosion and debris from both ends of the expansion joint and flange bolts. Pay special attention to cleaning the blockage in the bolt hole to avoid the bolt jamming during disassembly.

Key points: Wear a dust mask during cleaning to prevent inhalation of flue dust containing heavy metals.

Step 2: Remove the insulation layer and outer guard

If the outside of the expansion joint is covered with insulation layer and outer guard plate, it needs to be removed first:

• Use a screwdriver or driver to remove the outer guard fixing screws
• Carefully remove the outer guard plate to avoid deformation
• Clean the heat preservation cotton to expose the connection part between the expansion joint body and the flue

Step 3: Remove the connecting bolts

The core step of how to remove the expansion joint in the flue is to remove the flange connecting bolt:

• First, try to loosen the first bolt with a wrench. If the rust is serious, spray loosening agent (WD-40, etc.) and wait for 10~15 minutes
• Loosen all bolts gradually in diagonal order to avoid flange deformation caused by eccentric load
• For bolts that are rusted and cannot be loosened, the following methods can be used:
  • Angle grinder cutting nut
  • Acetylene flame heating (metal flanges only, and pay attention to fire protection)
  • Hydraulic nut splitter

Note: The removed bolts and gaskets should be placed in order, and anti-rust treatment should be done if reuse is planned.

Step 4: Separate the expansion joint from the flue flange

After all the bolts are removed, the expansion joint and the flue flange may not be separated due to rust or sealant sticking:

• Gently insert a crowbar into the flange gap and pry it evenly around
• If it still cannot be separated, gently tap the edge of the flange with a hand hammer and a copper rod (it is prohibited to directly tap the expansion joint body)
• For firmly bonded non-metallic expansion joints, use a utility knife to cut the sealant along the flange surface

Step 5: Lift or remove the expansion joint

Select the appropriate removal method according to the weight and installation position of the expansion joint:

• Small expansion joint (
• Medium expansion joint (50~200kg): lifted by hand hoist or forklift
• Large expansion joint (> 200kg): crane is required and center of gravity position is calculated in advance

Safety reminder: It is strictly forbidden to stand under heavy objects during lifting. Although the non-metallic expansion joint is light, it is large in volume and easy to deform. When hoisting, a wide sling should be used to avoid the rope strangling the flexible body.

Step 6: Check the flue flange interface

After the expansion joint is removed, do not rush to install new parts. You should first check:

• Whether the flange surface is smooth and there are corrosion potholes
• Whether the bolt hole is deformed or the thread is damaged
• Is the flow guide tube inside the flue intact

If there are defects, they need to be repaired first (sanding, repair welding, tapping, etc.).

4. Disassembly differences of different types of expansion joints

How to remove the expansion joint in the flue will vary slightly depending on the type of expansion joint:

For the welded expansion joint, the answer to "How to remove the expansion joint in the flue" involves cutting operation: use gas cutting or plasma cutting to cut along the edge of the weld, and pay attention to controlling the cutting depth to avoid damaging the flue base metal.

5. Common problems and coping strategies

Problem 1: The bolts are all rusty and cannot be removed

Response: Cut nuts in batches and replace new bolts. It is not recommended to forcibly use a large wrench afterburner, which can easily lead to bolt breakage or flange damage.

Problem 2: The expansion joint is stuck in the flange and cannot be removed

Response: Check for hidden weld spots or retaining pins. If not, use a three-jaw pull horse or mechanical jack to eject from the inside.

Problem 3: Nonmetallic expansion joint flexible cloth is aged and broken

Response: The flexible cloth may tear itself when removed, which is normal. However, care should be taken to clean up all debris to avoid leftovers entering the flue.

Question 4: Difficulty working at height

Response: It is necessary to set up a reliable working platform or use a lift truck, and 100% of the operators should wear seat belts. Operations should be stopped in bad weather (strong wind, rain and snow).

Safety Reminders and Environmental Protection Requirements

1. Anti-scald: Even if the machine is stopped for cooling, the temperature of the flue wall may still be higher than 80℃, and high-temperature-resistant gloves should be worn when contacting
2. Anti-asphyxiation: There may be oxygen deficiency or nitrogen replacement residue in the flue, and continuous ventilation and gas detection must be made
3. Fire and explosion prevention: Clean up combustible materials before cutting operations and equip fire extinguishers. If gas has been conveyed into the flue, it must be thoroughly purged
4. Environmental protection disposal: Scrapped expansion joints (especially non-metallic composite cloth) shall be disposed of according to industrial solid waste classification and shall not be discarded at will

VII. SUMMARY

Regarding the practical problem of "how to remove the expansion joint in the flue", the core points can be summarized as follows:

1. Safety priority: shutdown, cooling, ventilation, gas detection and energy isolation are indispensable, which is the prerequisite of all disassembly operations
2. Sufficient preparation: cleaning, marking, tools in place, protective wear, on-site investigation to understand the type of expansion joint and connection method
3. Disassembly sequence specification: cleaning → removing heat insulation → loosening bolts (diagonal) → separating flanges → lifting and removing, each step has specific skills
4. Classification: Flange connection type is the most common, just follow the above steps; Welded type needs to be cut; Plug-in type needs to remove the pressure plate first. Different types of expansion joints have different answers to "How to remove expansion joints in flue"
5. Flange inspection is indispensable: after removing the old parts, the condition of the flange surface must be checked and repaired if necessary, otherwise the new expansion joint may still leak after installation
6. Team cooperation: Disassembly operation requires at least 2~3 people to cooperate, one person to operate and one person to monitor, and command signal is required in lifting process

Mastering the correct disassembly method can not only safely and efficiently complete the replacement work of the expansion joint in the flue, but also protect the flue interface from being intact, thus laying a good foundation for the subsequent installation of new expansion joint. It is recommended that the above-mentioned procedures be prepared as an operation instruction, and special training should be carried out for maintenance personnel to ensure that every disassembly operation is standardized, safe and reliable.

In the technical exchange of industrial pipelines and flue gas treatment systems, the terms "shock absorber", "expansion joint" and "compensator" are often heard overlapping. Someone asked: Is the expansion joint installed at the inlet and outlet of the fan? What exactly should that fabric soft connection in the flue be called? Therefore, the question "Is the flue shock absorber the expansion joint?" has become the common doubt of many technicians. Their functions do overlap, but strictly speaking, they are two concepts in different dimensions, intersecting but not equivalent. This article will help you clarify these two concepts thoroughly, from functional positioning, structural principles to practical applications.

1. Functional analysis: the core task of expansion joint and shock absorber

To answer "Is the flue shock absorber an expansion joint?", we should first dismantle it from the most essential function:

• Expansion Joint (Compensator): The core task is thermal displacement compensation. When the pipeline transports high-temperature steam or flue gas, the pipe wall will elongate (thermal expansion and contraction) when heated. If there is no flexible link, huge thermal stress will crack the pipeline or equipment. The expansion joint is the use of its corrugated structure or fabric material to absorb the thermal expansion and contraction of the pipe。
• Flue Shock Absorber: The core task is to absorb shock and noise. The operation of equipment (such as fans) will generate high-frequency mechanical vibrations. If not blocked, these vibrations will be transmitted along the rigid flue steel plate to the whole building, which will not only disturb the people with noise, but also cause fatigue damage to the pipe connections. The original intention of the shock absorber is to cut off the propagation path of vibration。

From this dimension, the expansion joint is "mainly inside"-to resolve thermal stress; Shock Absorber "Main Outer"-Isolates mechanical vibrations. The focus of the two is not exactly the same.

2. Shock absorption and compensation: overlapping functions on rubber expansion joints

Since the expansion joint and the shock absorber have different functions, why do people still ask "Is the flue shock absorber the expansion joint"? Because in engineering practice, there is one product that perfectly blends the functions of the two, and this is the rubber compensator.

Rubber compensator (aka rubber expansion joint, flexible rubber joint, shock absorber, pipe shock absorber, shock throat), the name itself reflects its dual identity.

• Structural realization: It is composed of high elasticity rubber sphere and metal flange. The rubber itself has good vibration absorption and damping characteristics, which can greatly reduce noise。
• Displacement compensation: At the same time, it makes use of the flexibility of rubber to compensate the trace axial expansion and contraction of the pipeline and the discentricity (i.e., lateral displacement) during installation.

In this particular category, the role of the expansion joint (to compensate for displacement) is combined with the role of the shock absorber (to absorb shock and noise). Therefore, when the technician points to the rubber compensator and says "this is the flue shock absorber", this answer holds true in the specific context.

3. Non-metallic expansion joint: the "flexible all-around champion" in the flue

Apart from rubber, another structure that is more common in smoke and air systems is a non-metallic expansion joint (also known as a fabric compensator). Returning to the question "Is the flue shock absorber an expansion joint?", we can see more adequate evidence on non-metallic expansion joints.

Non-metallic expansion joints are mainly composed of flexible materials such as fiber fabric, silicone cloth, fluorine adhesive cloth and thermal insulation cotton。 Its construction is perfectly suited to the needs of "shock absorption":

• Seismic isolation function: Fiber fabric and insulation cotton itself have sound absorption and vibration isolation functions, which can effectively reduce and eliminate the transmission of noise and vibration in boiler, fan and other systems。
• Compensation displacement: It can provide large multi-dimensional direction compensation in a smaller size range and absorb the thermal displacement of the pipe。

The thrust-free characteristic is the key feature to distinguish: because the main material is fiber fabric, the non-metallic expansion joint has weak transmission (no reverse thrust) when it runs. This means that it can easily cut off the conduction of equipment vibration to the pipe system, which is not possible with metal hard connections。 Therefore, installing non-metallic expansion joints at the inlet and outlet of the fan completely plays the physical effect of "shock absorber" in actual engineering.

4. Concept analysis: how to accurately select according to the scene

Although the expansion joint often covers the demand of shock absorption in function, the two still have different emphases in engineering selection. To help you sort out "Is the flue shock absorber an expansion joint?", the following table shows the typical configuration in different scenarios:

Note: Although pure metal corrugated expansion joint is also called expansion joint, it mainly absorbs heat displacement by metal corrugated deformation, and its vibration isolation effect is far inferior to that of rubber or fabric. If pure metal expansion joints are selected only for "shock absorption", the effect may not be satisfactory.

V. Summary: Is the flue shock absorber the expansion joint? The key depends on the occasion

At this point, a clear conclusion can be given for the question "Is the flue shock absorber an expansion joint?"

"Flue shock absorber" is a function that expansion joints implement in a specific scenario, and it can't be said that all expansion joints are shock absorbers.

• When the expansion joint uses rubber or fiber fabric as the main material (such as rubber compensator and non-metallic fabric compensator), it naturally has excellent seismic isolation and noise absorption function. In this context, it is the ideal flexible shock absorber for flue systems。
• When the expansion joint is a pure metal bellows structure, it is mainly used to absorb heat displacement, and the vibration isolation effect is weak. At this time, it is not appropriate to call it a shock absorber。

Selection suggestion: For the vibration and noise problems of fans and flues, non-metallic fabric compensator or rubber air duct compensator should be preferred; Aiming at the problem of thermal expansion of high temperature steam pipeline, the metal corrugated compensator is selected with the guide bracket.

In the criss-crossing heating pipe network underground in cities, on the steam pipes of large factories, a metal device like an accordion bellows can often be seen, or a "U"-shaped bend specially wound on the pipes. They look like "joints" of pipes and actually have a professional name – compensators. So, what is a compensator? Why is it essential? This article will systematically explain the definition, type, working principle and core function of compensator.

I. What is a compensator? A vivid metaphor

What is a compensator? In terms of engineering definition, compensator is also customarily called expansion joint or expansion joint. It is a flexible structure arranged on the container shell or pipe in order to compensate for additional stress caused by temperature difference and mechanical vibration. It belongs to a compensation element, which can absorb the dimensional change caused by thermal expansion and cold contraction of pipelines, conduits, containers and the like by using its own elastic deformation.

In order to better understand what a compensator is, 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 compensator is the "elastic waist" of the pipeline. Whether the pipe is thermally expanded or coldly 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.

The compensator can compensate various axial, transverse and angular displacement changes caused by thermal expansion and contraction or running vibration of the pipeline, at the same time absorb equipment vibration, reduce noise, and facilitate the installation and disassembly of the valve.

Second, why is the pipeline system inseparable from the compensator?

The key to understanding what a compensator is lies in understanding the problem of thermal stress in pipes. Metals have the physical property of "thermal expansion and cold contraction". In the case of steam pipes, the operating temperature can usually reach 150°C 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 caused by temperature difference is even more considerable.

If both ends of the pipe are fixed, this expansion has nowhere to be released, which will create huge thermal stress inside the pipe. This force is amazing, which can lead to pipeline twisting and deformation, bracket damage, weld cracking, and serious accidents such as pipe burst and equipment damage. Compensators (expansion joints) were created to solve this contradiction. It will be compressed or stretched 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.

3. The core function of the compensator

The complete answer to what a compensator is contains its multiple functions:

4. Common Types of Compensators

1. Bellows compensator (metal expansion joint)

Bellows compensator is currently the most widely used type of compensator, consisting of stainless steel bellows, end pipe, flange and guide tube. The bellows resemble the bellows of an accordion, with displacement compensation by compressing and stretching the corrugations.

Features: Compact structure, large compensation, good sealing performance, can absorb multi-directional displacement.

2. Sleeve compensator (packing box type)

The sleeve compensator consists of inner and outer sleeves that absorb axial displacement by relative sliding, and the sleeves are sealed with packing between them. The structure is simple and the friction resistance is small, but the sealing packing needs to be replaced regularly.

3. Spherical Compensator

The spherical compensator absorbs the angular displacement through the rotation of the sphere, and usually needs two or three combinations to be used, which is suitable for the working conditions of large rotation angle and high pressure.

4. Square compensator (natural compensation)

Rather than stand-alone devices, square compensators bend the pipe itself into a U, L, or Z shape, using the tube's own elasticity to absorb displacement. It has the advantages of simple structure and no maintenance, and the disadvantage is that it takes up a large space.

5. Non-metallic compensator (fabric compensator)

The elastic element of the non-metallic compensator is made of non-metallic materials such as fiber fabric and rubber, which has the advantages of large compensation amount, no reverse thrust, corrosion resistance and good vibration isolation effect. It is widely used in flue gas pipelines in power plants, metallurgy, cement and other industries.

V. Classification by material: metal and non-metal

In the selection of what is a compensator, the material is an important dimension.

6. Working principle and selection of compensator

What does a compensator work: When the pipe is elongated by heat, the bellows is compressed; When the pipe cools and shrinks, the bellows is stretched. Through this elastic deformation, the bellows converts the thermal stress into its own elastic potential energy, thus protecting the interface between the pipe and the equipment from damage.

Key points of selection:

• Select compensator type and material according to pipe operating temperature, pressure and medium characteristics
• Calculate the thermal displacement of the pipe: Δ L = α × L × Δ T (α is the linear expansion coefficient, L is the pipe length, Δ T is the temperature difference)
• Ensure that the rated compensation of the compensator is ≥1.2× calculated thermal displacement
• Consider installation space constraints and bracket configuration conditions

Implementation standard: The design, manufacture and inspection of metal bellows compensator shall follow the national standard GB/T 12777-2019 General Technical Specifications for Expansion Joints of Metal Bellows.

VII. Installation and maintenance points of compensator

1. Check before installation

• Check that the model, specification and design are consistent
• Check bellows/skin for mechanical damage
• Verify that the direction of the guide tube is consistent with the direction of the medium flow

2. Installation Points

• It is strictly prohibited to adjust the deviation of pipeline installation by deforming the compensator
• Install concentrically with the pipe without deflection
• The amount of pre-tension or pre-compression shall be performed according to the design requirements
• Cover with fireproof cloth during welding during installation to prevent welding slag from splashing and damaging the bellows

3. Transportation Tie Rod Handling

The compensator has temporary fixed tie rods during transportation and installation. After the pipe system is installed, the transport tie rod must be removed so that the compensator can expand and contract freely. This is the most overlooked part of installation-if the transport tie rod is not removed, the compensator will lose its ability to compensate.

4. Operation and maintenance

• Regularly check the surface of the compensator for cracks, corrosion and deformation
• Monitor operating temperatures and pressures to ensure they are within safe ranges
• Discover leakage in time, serious damage should be replaced as a whole

VIII. Summary

The answer to what a compensator is can be summarized as the following core points:

Whether you are drawing at a design institute, installing at a construction site, or maintaining equipment in a factory, compensators are the key components to ensure the safe operation of the system whenever you encounter high-temperature pipelines or long-distance pipelines. Understanding what a compensator is is to understand the basis of thermal stress management in pipeline systems. A compensator with reasonable design and standard installation can run stably for a long time under harsh working conditions such as high temperature and high pressure, and provide reliable "telescopic joint" for the whole pipe network system.

In the installation of thermal piping systems, it is common to hear that "the compensator needs to be pre-stretched" or "the corrugated compensator needs to be cold tightened". So, why does the compensator stretch? What are the consequences of not stretching? How to determine the amount of stretch? This paper will systematically explain the principle, function and operation method of compensator pre-stretching, and help engineers and technicians to correctly understand and execute this key process.

I. Basic concept of compensator pre-stretching

The question of why the compensator stretches, first of all, we need to understand the working state of the compensator in the pipeline system. The compensator is installed between two fixed brackets, and its core function is to absorb the thermal expansion and contraction of the pipe due to temperature changes.

When the pipe is elongated by heat, the compensator is compressed; When the pipe cools and contracts, the compensator is stretched. In the installed state, the compensator is in an initial position, and the selection of this position directly affects its working performance.

Pre-stretching (also called cold tightening) refers to the application of a deformation amount opposite to the direction of thermal expansion to the compensator in advance during pipeline installation, so that the compensator is in a pre-compressed or pre-stretched state when cold。 In this way, when the pipe is put into operation and subjected to thermal expansion, the amount of deformation of the compensator will be reduced, thus reducing its working stress.

Second, why does the compensator stretch? Three core roles

1. Reduce the stress on pipes and equipment supports

The primary reason why the compensator is stretched is to reduce the load-bearing capacity of the supports on the pipes and equipment。

When the corrugated compensator is deformed, an elastic reaction force is generated that is proportional to the amount of deformation-the greater the deformation, the greater the reaction force. This reaction force will act on the pipe holding bracket and the equipment interface. If pre-stretching is not carried out, the deformation of the compensator in the hot state is the largest, the reaction force is also the largest, and the load requirement on the support and equipment is the highest.

Core principle: The pre-stretching amount is generally 1/2 of the design compensation amount (the maximum thermal elongation of the pipe). In this way, when the pipe is heated to the rated temperature, the compensator is compressed from the pre-tension state to the intermediate position, the deformation amount is only half of the maximum deformation amount, and the generated reaction force is only half of the maximum value。

In short: pre-tensioning allows the compensator to be in a better stress range during operation, the fixed bracket can be designed to be more economical, and the equipment interface is less stressed.

2. Extending the fatigue life of the compensator

Another important reason why the compensator stretches is to extend the fatigue life of the bellows.

The fatigue life of bellows is closely related to the stress amplitude when it works. The greater the stress amplitude, the greater the damage to the material per cycle, and the fewer cycles allowed. The fatigue life can be increased exponentially by reducing the working stress amplitude by half by pre-stretching。

According to industry experience, pre-stretching of compensator is a necessary process to increase the number of fatigue uses. Generally, the pre-stretching measure is one half of the compensation amount。 This is the core value of the compensator that needs to be pre-stretched.

3. Compensate for deviation of installation temperature from design zero

The design of the compensator is based on the "zero temperature" (i.e., the midpoint of the maximum and minimum temperature of the design). When the installation ambient temperature is not equal to the zero temperature, pre-stretching or pre-compression is required to compensate for this deviation。

Rules:

• When the installation ambient temperature is equal to zero temperature: No pre-tension or pre-pressing
• When installation ambient temperature is above zero temperature: shall be pre-compressed
• When the installation ambient temperature is below zero temperature: shall be pre-stretched

For pipes with medium temperatures above 400°C, the amount of stretch may be greater than Δ L/2; When the medium temperature is above 500°C, the amount of stretch may be Δ L。 The higher the temperature, the greater the thermal expansion of the pipe, and the more prominent the significance of pre-stretching.

3. Operation methods and key points of pre-stretching

1. Determination of pre-stretch amount

The understanding of why the compensator should be stretched should ultimately be implemented to the operational level. The amount of pre-stretching shall be determined in accordance with the design requirements and the ambient temperature at the time of installation, and the allowable deviation shall be ±10mm。

The amount of pre-stretching is not the larger the better, nor the smaller the better, but it is accurately calculated according to the actual working conditions of the pipeline. Within the allowable compensation range, pre-stretching will not affect the compensation amount of the compensator and its own life。

2. Construction method of pre-stretching

Typical construction steps for on-site pre-stretching are as follows：

Step 1: Leave the required clearance in the pipe for pre-stretching. Usually on the straight pipe sections at both ends of the compensator, 2-2.5m away from the compensator, a gap of Δ L/4 is left.

Step 2: Fixly connect one end of the compensator to the pipe (welded or flanged).

Step 3: Using machines such as pipe drawers, jacks or chain cranes, stretch the pipe in the opposite direction of thermal expansion until the reserved gap is filled.

Step 4: Maintaining the tensile state, welding and fixing the live port and the other end of the compensator.

Step 5: Loosen the stretching implement and check that the compensator is in the correct pre-stretched state.

3. Construction precautions

• Uniform force: during pre-stretching, the force should be gradually increased to ensure that the circumferential surface of each node of the bellows is uniformly stressed, and the deviation should be less than 5mm
• Concentricity: The waveform compensator must be concentric with the pipe and must not be skewed
• Temporary fixation: It should be fixed immediately after stretching in place to prevent rebound
• Safety protection: For large and medium-sized compensators, the nodes and joints should be protected from damage when using hoisting machinery

4. What are the consequences of not pre-stretching?

After understanding why the compensator is stretched, it is natural to ask: What happens if it is not pre-stretched?

1. Excessive force on the bracket: the fixed bracket and the equipment interface bear the maximum thrust, which may cause deformation of the bracket and damage to the equipment interface
2. Shortened fatigue life of compensator: Bellows are prone to early fatigue cracking when working at maximum stress amplitude
3. Compensator pull-off risk: In extreme cases, if the fixing bracket is not properly set or the tie rod device is not adjusted in place, the compensator may be pulled into a straight cylinder, completely losing the compensation function

According to the failure analysis, the common causes of compensator pulling failure include: improper setting of fixed bracket, non-adjustment of tie rod device in place, restricted pipe shrinkage, improper design and use, etc. 。 And correct pre-stretching is an important measure to avoid these problems.

V. Pre-stretching requirements of different types of compensators

VI. Clarification of common misunderstandings

Myth 1: Pre-stretching can increase the amount of compensation

Positive solution: Pre-stretching can not increase the maximum compensation amount of the compensator, it only translates the available working area of the compensator, so that the working deformation amount in the hot state is reduced。

Myth#2: All compensators require pre-stretching

Positive solution: The requirements of pre-stretching depend on the type of compensator and installation conditions. When the installation ambient temperature is equal to the zero temperature, the waveform compensator may not be pre-stretched。

Myth 3: The larger the amount of pre-stretch, the better

Correct solution: The amount of pre-stretching must be accurately controlled according to the design requirements. Too large or too small may affect the compensation effect and even cause damage to the bellows.

Myth 4: The transport tie rod is not removed after installation

Correct solution: The compensator has a temporary fixed tie rod during transportation and installation, which must be removed after installation, otherwise the compensator cannot expand and contract freely, and will be pulled out in hot state。

sum up

The core answer to why the compensator stretches can be summarized as three points:

Operation Points:

• The amount of pre-stretching is generally 1/2 of the design compensation amount, and the allowable deviation is ±10mm
• Bear the force evenly and maintain concentricity during construction, and fix it immediately after stretching into place
• Installation ambient temperature should be pre-compressed when above zero and pre-stretched when below zero
• The transport tie rod and temporary fixtures must be removed after installation

A compensator that performs pre-stretching correctly can be in the optimum stress state during hot operation, which both reduces the load on the pipeline system and extends its own life. This is why pre-stretching is regarded as an indispensable and critical process in the thermal pipeline construction code.

In wet flue gas desulfurization system, the condensate problem of flue outlet expansion joint is one of the most headaches for operation and maintenance personnel. The net flue gas temperature at the outlet of the desulfurization tower drops to 45-55℃, which is in a completely saturated state, carrying a large amount of water mist and acidic droplets. When these wet flue gases pass through the expansion joint at the flue outlet, they condense into strong acidic liquid when exposed to condensation, which accumulates in the groove of the expansion joint, resulting in skin corrosion, loose bolts, acid leakage, equipment corrosion and environmental pollution in the slightest case, and icing in winter in the worst case, threatening personnel safety. This paper will systematically explain the professional technical knowledge of condensate water in flue outlet expansion joint from the cause mechanism, hazard analysis to treatment scheme.

1. How is condensate produced?

The formation of condensate in the flue outlet expansion joint has its specific physicochemical mechanism. In the wet desulfurization process, after the original flue gas is sprayed and washed by limestone slurry in the absorption tower, acidic gases such as SO₂ and SO₃ are effectively removed, and the flue gas temperature drops sharply, usually to 45-55℃. At this time, the clean flue gas becomes saturated wet flue gas, carrying a large amount of water mist and tiny droplets.

When the saturated wet flue gas passes through the clean flue and flue outlet expansion joints, the water vapor in the flue gas condenses into liquid water when condensed because the temperature of the metal frame and skin of the expansion joint is usually lower than the temperature of the flue gas. At the same time, the residual acidic gases such as SO₂, SO₃, Cl⁻¹ and F⁻¹ in the flue gas are dissolved in the condensed water to form a strongly acidic condensate with a pH value as low as 2-3.

Key Features: When the non-metallic expansion joint is installed, an annular groove will naturally form between the skin and the platen. It is this structural defect that has become a "hotbed" for condensate accumulation. Once the condensate enters the groove, it cannot be discharged naturally by gravity, and it soaks the skin and bolts for a long time, gradually causing osmotic corrosion.

2. The harm of condensate should not be underestimated

The hazards of condensate in flue outlet expansion joint are many aspects, which seriously threaten the safety of equipment and the health of operators.

1. Skin penetration and leakage

Soaking the skin of the expansion joint with acid water for a long time will slowly penetrate through the multi-layer fabric layer and reach the position of the fixed screw, causing the screw to loosen and corrode and break. Finally, the acid liquid flows out from the broken screw hole and the damaged part of the skin, resulting in a serious "waterfall" phenomenon. Judging from the operation situation, the expansion joint without preventive measures will generally be corroded and damaged in one to two years.

2. Metal frame and bolt corrosion

Acidic condensate with high concentration of Cl⁻¹ is extremely corrosive to stainless steel bolts and metal frames. The results show that the average life of 316L expansion joint is not more than two years in the practical use of desulfurization wet flue, and the pitting corrosion and stress corrosion cracking caused by chloride ions are the main failure modes.

3. Potential safety hazards of icing in winter

Condensate leaks from flue outlet expansion joints can pose serious safety concerns in cold northern regions. The acidic liquid leaked from the expansion joint will freeze rapidly in winter and accumulate on the equipment platform and channel, which seriously affects the passage safety of operators and maintenance personnel, and may even cause slip and fall accidents.

4. Environmental impact and equipment corrosion

The leaked acidic condensate will seriously corrode the steel structure, platform and insulation layer of the equipment, and at the same time produce pungent acid mist, which will affect the on-site working environment.

3. Governance plan: from emergency treatment to radical repair

Aiming at the condensate problem of flue outlet expansion joint, the industry has developed a variety of mature and effective treatment schemes, which can be selected according to the actual situation.

Option 1: Groove filling technology-radical solution

To completely solve the problem of condensate leakage, the core lies in filling the water-accumulated groove of the expansion joint, so that acid water can't contact the skin and bolts.

Construction steps:

1. Shutdown cleaning: remove dust around expansion joint, remove damaged skin
2. Skeleton repair: Clean up the floating ash of the metal skeleton and weld and repair the corroded part
3. Bottom layer anti-corrosion: High elasticity tung oil gel is applied to both sides of the groove of the expansion joint
4. Filler filling: Fill with high-temperature and corrosion-resistant sponge-shaped closed-cell foamed rubber filler (compression ratio 7:1), and compact in layers
5. Surface sealing: the top layer is smoothed with high elasticity tung oil gel, and the thickness is controlled at about 5mm
6. Perimeter anti-corrosion: Use high elastic flexible glass flakes to prevent corrosion at the joints around the expansion joints, and the thickness should be more than 3mm

Technical advantages: This scheme makes the acid water in the flue not contact the skin part of the expansion joint, which does not affect the absorption of the expansion amount, and completely solves the problem of skeleton corrosion and leakage of the expansion joint.

Scheme 2: Set up drainage device

For horizontally mounted flue outlet expansion joints, a drainage system must be provided at the lowest point of the frame.

Manual drainage scheme:

• Set drainage holes at the lowest point of the expansion joint frame (at least DN150)
• Install drain stub and valve
• Drain pipe leads to gutter or acid collection system
• The valve is normally closed, and the drainage is opened regularly

Operation requirements: Start drainage for 1-2 minutes per shift, observe the properties of discharged liquid, and close the valve after drainage to prevent negative pressure operation from inhaling outside air.

Scheme 3: Structural design of water baffle

An effective structural optimization scheme is that a water retaining device is arranged inside the expansion joint, and a water retaining plate and an inner lining plate are arranged on the metal frame to respectively double-block the condensed water refluxing in the pipeline and the condensed water dripping above, thereby effectively reducing the probability of the condensed water flowing to the inner ring belt and accumulating. This design reduces the erosion of the skin by condensate from the structural source.

IV. Preventive measures and maintenance suggestions

The long-term prevention and treatment of condensate problem in flue outlet expansion joint needs to establish a perfect prevention and maintenance system.

1. Prevention during the installation phase

• When installing the horizontal flue expansion joint, ensure that the drain hole is at the lowest point
• Install strictly according to the flow direction mark, with the small end of the guide tube facing the incoming flow
• The bolts are tightened diagonally and tightened in fractional intervals, and are tightened once in 1 month and once in 3 months after operation

2. Monitoring during the operation phase

• Check the appearance of the expansion joint every week for water seepage and bulge
• Open the drain valve regularly and observe the properties of the discharged liquid
• Clear and transparent is normal condensed water; Yellow or reddish brown indicates that the frame is beginning to corrode

3. Maintenance cycle

• Full tightening of platen bolts quarterly
• Clean drain holes monthly to prevent clogging
• Check groove fill layer and anticorrosive coating integrity periodically

4. Inspection during shutdown

Each time the furnace is shut down for maintenance, the expansion joint at the flue outlet should be checked:

• Check the skin for damage and aging
• Check bolts for looseness and corrosion
• Check the groove filling layer for cracking and detachment

V. Summary

The problem of condensation in flue outlet expansion joint is a common technical problem in wet desulfurization system, which is rooted in the condensation of saturated wet flue gas when cooled and the groove structure of expansion joint. The correct prevention and control strategy should follow the principle of "combining prevention and discharge and comprehensive management":

A flue outlet expansion joint with reasonable design and proper maintenance can solve the stubborn problem of condensate leakage from the root cause, ensure the safe and stable operation of desulfurization system, and eliminate the hidden safety hazard caused by icing in winter. It is suggested that enterprises should include the condensate water problem of expansion joint into the key inspection items every time the furnace is shut down for maintenance, so as to achieve early detection and treatment.