In industrial flue gas treatment system, the design specification of high temperature flue gas expansion joint is the key technical basis to ensure the safe operation of equipment and prevent the damage of thermal stress. High temperature flue gas pipelines (such as boiler outlet, catalytic cracking regeneration flue gas, MTO regeneration flue gas, etc.) have the characteristics of high temperature (up to 600-1000℃), large pipe diameter, significant thermal displacement and complicated working conditions, which put forward much higher requirements for the design of expansion joints than ordinary working conditions. Once the design specification of high-temperature flue gas expansion joint is not implemented in place, it will lead to deformation and failure of the expansion joint, and in the worst case, it will lead to pipeline tearing, equipment damage and even safety accidents. This paper will systematically explain the core content of the design specification of high temperature flue gas expansion joint from standard basis, design key points, material selection to installation acceptance.

1. "Four beams and eight columns" of design specifications: the basis of core standards

The design specification of high-temperature flue gas expansion joint is mainly formulated according to the following national standards and industry regulations:

According to the provisions of DL/T 5121-2020, the expansion joint of SCR flue gas system should be preferred for non-metallic expansion joint with inner baffle and high temperature resistance because it is necessary to bear axial, radial and angular displacement and effectively absorb vibration. Article 7.0.3 of GB 50753-2012 clearly stipulates: "A compensator shall be provided in the high-temperature flue gas pipeline, and brackets shall be provided at both ends of the compensator. After the compensator is installed, all the protective screws of the original belt shall be relaxed, and the relaxation distance shall be greater than the designed maximum compensation amount of the compensator."

INDUSTRY BODY STANDARDS

The Group Standard T/HEBQIA 271 — 2024 "Expansion Joints for High Temperature Gas Pipelines" issued in June 2024 further standardizes the basic parameters, technical requirements, test methods and inspection rules for expansion joints for high temperature gas pipelines. This standard is applicable to the expansion joints of high temperature gas pipelines and has important reference value for the design of expansion joints under high temperature working conditions.

2. "Accurate handling" of design parameters: temperature, displacement and blind plate force

1. Design temperature and pressure

The first task of the high temperature flue gas expansion joint design specification is to determine the correct design parameters:

According to practical engineering experience, the expansion joint can absorb the axial and lateral displacement of all connecting equipment and flue under all operation and accident conditions, and all expansion joint belts can withstand high temperature of flue gas and strong acid corrosion of gas and liquid.

2. Calculation of thermal displacement

Thermal displacement is the most basic parameter in the design specification of high temperature flue gas expansion joint. The amount of thermal elongation is calculated as follows:

Δ L = α × L × Δ T

Among them:

• Δ L: thermal elongation (mm)
• α: Line expansion coefficient of pipeline (12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel)
• L: length of pipe section between two fixed brackets (mm)
• Δ T: Difference between operating temperature and installation temperature (℃)

Example: A 10-meter-long carbon steel high-temperature flue gas pipeline with an installation temperature of 20℃ and an operating temperature of 600℃, then:
Δ L =12×10⁻⁶ ×10000×580=69.6 mm

3. Blind plate force calculation

Blind plate force is a load that must be considered in the design specification of high temperature flue gas expansion joint. The formula for calculation is:

F = P × A

Among them:

• F: blind plate force (N)
• P: working pressure (Pa)
• A: Effective area of bellows (m²)

According to the research of "Stability Failure Analysis and Design Optimization of Expansion Joints in High Temperature Flue Gas Pipelines" published in Pressure Vessel, Issue 2, 2024, the selection and reasonable arrangement of pipeline supports are the key to ensure the safe and effective service of expansion joints. In high-temperature flue gas pipelines, the blind plate force can reach several tons or even tens of tons, which must be borne by fixed brackets.

3. "Heat resistance test" of materials: selection of bellows and insulation layer

1. Bellows Material Selection

Selecting corrugated pipe material according to working temperature is the core content of high-temperature flue gas expansion joint design specification:

Special warning: The case of stable failure of the expansion joint of the regenerated flue gas pipeline of a methanol-to-olefin (MTO) plant shows that the material degradation of austenitic stainless steel used for a long time at high temperature will occur. When the operating temperature exceeds 480℃, the high-temperature durable strength of the material will decrease significantly. Therefore, high-temperature resistant alloys must be selected for high-temperature working conditions, and the materials should not be degraded.

2. Thermal insulation structure design

The high-temperature flue gas expansion joint must be provided with an effective heat insulation layer to prevent the high temperature from directly radiating the bellows. The requirements of the design specification for high temperature flue gas expansion joints for heat insulation structures include:

• Inner lining insulation layer thickness ≥100mm (determined according to temperature)
• High temperature resistant materials such as aluminum silicate fiber cotton are adopted
• Installation of thermal insulation layer anti-collapse structure
• Leave expansion gap between that guide tube and the heat insulation layer

4. The "golden rule" of bracket arrangement: fixation, guidance and limitation

1. Fixed bracket

According to Article 7.0.3 of GB 50753-2012, brackets shall be provided at both ends of the compensator. The fixed bracket must be designed to be strong enough to withstand:

• Elastic reaction force generated by expansion joint
• Blind plate force
• Pipe frictional resistance

2. Guide bracket

The setting requirements of the guide bracket in the design specification of high-temperature flue gas expansion joint:

• The distance between the first guide bracket and the expansion joint is ≤4 times the pipe diameter
• The second guide bracket is less than 14 times the tube diameter from the first guide bracket
• Guide clearance 2-5mm

3. Limit device

The temperature of high-temperature flue gas pipeline changes drastically in the process of starting and stopping the machine, so a limit device should be set to prevent the expansion joint from exceeding the limit deformation.

V. The "last step" of installation acceptance: pre-offset and protective screw

1. Prebias Setting

For the high-temperature flue of SCR flue gas system, due to the high flue gas temperature, the lateral displacement and horizontal displacement are both large, and the expansion joint should be 100% pre-biased when installing-that is, the flues on both sides of the expansion joint are misaligned during installation, and the axes of the flues on both sides are on a straight line during operation.

2. Protective screw treatment

According to 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. This is an easily overlooked link in the design specification of high-temperature flue gas expansion joint-if the transportation rod is not relaxed, the expansion joint will lose its compensation ability.

3. Welding requirements

• Temporary supports shall not be welded to bellows
• The skip welding method is adopted during welding to control the interlayer temperature
• Remove the weld slag after welding and check the appearance of the weld

4. Airtightness test

• The test pressure is 1.5 times the design pressure
• Hold time ≥30 minutes
• No leakage is qualified

6. The "Advanced Road" of Design Optimization: Failure Analysis and New Structure

1. Failure analysis and prevention

The research in the journal "Pressure Vessel" points out that the selection and reasonable arrangement of pipeline supports are the key to ensure the safe and effective service of expansion joints. For high-temperature flue gas pipelines, we should focus on:

• Avoid excessive lateral displacement of pipeline
• Reasonable spacing of guide brackets
• Fully consider the influence of variation of operating conditions on expansion joint

2. New thermal insulation structure

In the papers of the 17th National Conference on Expansion Joints, some scholars put forward a design scheme of high-temperature expansion joint insulation structure. By optimizing the material and structure of insulation layer, the working temperature of bellows can be effectively reduced and the service life can be prolonged.

3. Flexible connection section design

For the high temperature flue gas exhaust system, the flexible connection section design scheme can be adopted. By reasonably arranging multiple expansion joints and brackets, the multi-directional thermal displacement can be absorbed and the system stress level can be reduced.

sum up

The design specification of high temperature flue gas expansion joint is the technical cornerstone to ensure the safe operation of high temperature flue gas pipeline system. The specification design should follow the following core principles:

• Standards first: Strictly implement the current standards such as GB/T 12777-2019 and DL/T 5121-2020, and refer to T/HEBQIA 271-2024 Group Standard for high temperature gas pipelines
• Accurate parameters: Correct calculation of thermal displacement and blind plate force is the basis of bracket design
• Material suitable temperature: select the corrugated pipe material according to the working temperature, above 450℃ must be upgraded to 321 or nickel-based alloy
• Reliable thermal insulation: Set sufficient thickness of thermal insulation layer to prevent high temperature radiation bellows
• Reasonable brackets: Brackets must be set at both ends of the compensator, and the spacing between the guide brackets shall be strictly according to the specifications
• Pre-bias specification: High temperature flue expansion joint shall be pre-biased by 100% during installation
• Protection removal: Relax the protective screw after installation, and the relaxation distance is greater than the maximum compensation amount

A high temperature flue gas expansion joint with reasonable design and standard installation can run stably for a long time under high temperature alternating working conditions. It is suggested that the parameter calculation and structure design should be carried out strictly according to the requirements of the specification in the design stage, so as to avoid the failure of the expansion joint from the source.