Specialized in manufacturing compensators, expansion joints, baffle doors

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

1. Why should we formulate a professional replacement plan?

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

2. Working condition diagnosis and data acquisition before replacement

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

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

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

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

Standardized replacement operation process

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

Step 1: Kiln shutdown, temperature reduction and isolation

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

Step 2: Remove old expansion joints and accessories

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

Step 3: Cylindrical correction and support repair

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

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

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

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

Step 5: Liner Refractory Repair

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

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

Step 6: Anti-corrosion and Acceptance

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

Quality Acceptance Criteria

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

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

6. Typical cases and experience

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Common Selection Error Alert

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

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

The integral vulcanization expansion joint is a flexible connection device used in pipeline systems, mainly used to compensate the displacement of the pipeline due to temperature changes, vibrations or installation errors. Its core feature is that the rubber is tightly combined with the metal components through the vulcanization process to form a monolithic structure with excellent sealing and durability.

Key Features:

1. Integral vulcanization process：

• Rubber and metal parts are tightly bonded through high-temperature vulcanization to ensure seamless joints and improve sealing performance.

• The vulcanized rubber has better elasticity and anti-aging ability.

1. Compensation capacity：

• It can absorb the displacement of the pipe due to thermal expansion and contraction, vibration or installation error, and protect the pipe system.

• Suitable for axial, lateral and angular displacement compensation.

1. Corrosion resistance：

• The rubber material can be selected according to the medium and adapted to different corrosive environments, such as acid, alkali or oil resistance.

1. Vibration and noise reduction：

• The elasticity of rubber effectively reduces pipe vibration and noise, and improves system stability.

1. Easy to install：

• Compact construction, easy installation and maintenance, reducing downtime.

Application area:

• Chemical industry: Piping systems for corrosive media.

• Power industry: Suitable for high temperature and high pressure steam pipeline.

• Water supply and drainage system: It is used in urban water supply and drainage pipelines to compensate for thermal expansion and contraction.

 Precautions for selection:

1. Properties of media: Select the appropriate rubber material according to the corrosiveness, temperature and pressure of the medium.

2. Displacement requirements: Select the appropriate compensation capability according to the displacement amount of the pipe.

3. Installation Environment: Consider the installation space and pipe layout to ensure the expansion joint is working properly.

Maintenance and maintenance:

• Check the rubber parts regularly for aging or damage.

• Avoid over-pressure or over-temperature use to prevent rubber failure.

Integral vulcanized expansion joints have become an indispensable component in pipeline systems due to their excellent performance and wide application.

Vulcanized expansion joint is an expansion joint made of non-metallic materials treated by vulcanization process. It is mainly used to compensate the displacement caused by thermal expansion, mechanical vibration or installation error of pipeline system, while providing good sealing and durability. The following are several materials commonly used for vulcanized expansion joints:

        1. Silicone cloth

Silicone cloth is a high-performance synthetic material with excellent high-temperature, low-temperature, corrosion and aging resistance. It is often used to make the skin of vulcanized expansion joints to improve the durability and reliability of expansion joints.

        2. Fluorine tape

Fluoroadhesive tape is a cloth made of fluororubber with excellent chemical resistance and high temperature stability. It is also widely used in the skin material of vulcanized expansion joints, especially in environments that need to be resistant to strong acids, strong bases or organic solvents.

        3. Polytetrafluoroethylene (PTFE)

PTFE is a high-performance plastic material known for its excellent chemical resistance and low coefficient of friction. In vulcanized expansion joints, PTFE materials are commonly used in liners or coatings to improve their chemical resistance and wear resistance.

        4. Alkali-free glass fiber cloth

Alkali-free glass fiber cloth has good high temperature resistance, corrosion resistance and mechanical strength, and is often used to make skeleton materials or reinforcement materials of vulcanized expansion joints to improve the overall strength and stability of expansion joints.

        5. Ultrafine glass wool and aluminum silicate wool

These materials are mainly used for the insulation layer of vulcanized expansion joints, and they have excellent insulation properties, which can effectively reduce heat loss and improve the energy efficiency of the system.

        6. Fluororubber fiberglass cloth

Fluororubber glass fiber cloth is a composite material that combines the advantages of fluororubber and glass fiber, with excellent high temperature resistance, corrosion resistance and mechanical strength. It is widely used in the skin material of vulcanized expansion joints, especially in environments where resistance to high temperatures and corrosive media is required.

        7. Glass fiber needle felt

Glass fiber needle felt is a porous material made of glass fiber with good filtration performance and high temperature resistance. It is commonly used to vulcanize the internal filling material of the expansion joint to improve its filtration effect and high temperature resistance.

After the above materials are treated by the vulcanization process, their physical properties, such as hardness, elasticity and heat resistance, can be significantly improved, thus enhancing the sealing performance, service life and mechanical stability of the vulcanized expansion joint. Vulcanized expansion joints are widely used in chemical, electric, steel, oil and gas industries because of their excellent properties.

A vulcanized expansion joint is a special type of expansion joint whose skin (i.e., non-metallic fabric layer) is vulcanized. Vulcanization is a chemical process commonly used in rubber products by adding sulfur or similar compounds under heating conditions to cross-link rubber molecules, thereby enhancing their physical properties such as hardness, elasticity, and heat resistance. The following are the main roles of vulcanized expansion joints:

Compensatory effect

Due to thermal expansion and contraction, during the installation process, the pipeline will inevitably be deformed, resulting in leakage at the pipeline connection. The vulcanization expansion joint can play the role of telescopic compensation, compensating the displacement caused by thermal expansion, mechanical vibration or installation error of the pipeline.

Sealing action

The vulcanized material has better sealing properties and can prevent gas or liquid leakage more effectively. This is especially important in long-distance pipelines in petroleum, chemical industry, electric power and other industries, which can replace flange joints to eliminate the interface leakage problem caused by temperature difference and mechanical vibration.

Absorption displacement

The vulcanized expansion joint can effectively prevent axial expansion and contraction caused by temperature difference and axial thrust generated by rotation or oscillation at any angle. It is especially suitable for the pipeline system with shock absorption requirements, which can greatly reduce the resonance frequency and noise value of the pipeline system.

Noise reduction effect

The elastic element of the vulcanization expansion joint has good sound absorption effect, can effectively reduce the amount of noise pollution generated by the operation of power equipment such as water pump and fan, and avoid the occurrence of water hammer phenomenon.

Silencing and noise prevention

&n bsp; After installing and using vulcanization expansion joints in water supply pipe network system, it can effectively reduce the amount of noise pollution, improve the living environment of residents, and improve the living standard and working efficiency. In addition, it can make the valve open more smoothly and smoothly without jamming feeling.

Thermal insulation

When the medium is hot water, choosing the insulation layer made of rubber and plastic material with good insulation performance and not easy to corrode can not only keep the water temperature within a certain range without drastic temperature change, but also keep the quality of the conveying medium unaffected.

Extended service life

Vulcanization increases the durability of the material and helps the expansion joint to work stably for a long time under harsh working conditions, thereby extending its service life.

Improved mechanical stability

The vulcanization process improves the stability and consistency of the material, allowing the expansion joint to perform more reliably when subjected to pressure and displacement.

Better weather resistance

Vulcanized non-metallic expansion joints are better resistant to UV rays, ozone, and weather aging, making them suitable for outdoor and extreme weather conditions applications.

In summary, vulcanized expansion joints play an important role in the industrial field, especially in situations where thermal expansion compensation, sealing, noise reduction and service life extension are required.