In petroleum, chemical, energy and other process industries, thermal stress compensation of pipeline system is the key link to ensure the long-term safe operation of equipment. Expansion joints in chemical industry, as the core flexible components that absorb the thermal displacement of pipelines and reduce the stress on the nozzles of equipment, are widely used in complicated working conditions such as high temperature, high pressure and corrosive media. This paper will systematically explain the classification and selection, design and calculation, standards and specifications and engineering application points of expansion joints in chemical industry, and provide professional reference for engineering design and equipment operation and maintenance personnel.

1. Why is the chemical industry inseparable from expansion joints?

Chemical pipeline system has the characteristics of large pipe diameter, high temperature and complicated pressure. Taking the propane dehydrogenation unit as an example, the design of thermal compensation scheme of high temperature and large diameter pipeline system in the reaction zone is directly related to the long-term safe and stable operation of the unit。 When the pipe arrangement is compact and cannot absorb the thermal expansion by natural compensation (e.g. L-shaped, Z-shaped elbow), it is necessary to rely on the expansion joint of the chemical industry to solve the thermal stress problem。

In addition, the nozzles of equipment in chemical plants (such as reactors, heat exchangers, towers) have strict restrictions on the force. If the thrust generated by the thermal expansion of the pipeline is directly transmitted to the nozzle of the equipment, it may lead to serious accidents such as nozzle cracking and equipment deformation. Reasonable setting of expansion joints can effectively isolate pipeline stress and protect core equipment.

2. Classification and structural characteristics of expansion joints in chemical industry

According to the structural type and displacement compensation mode, expansion joints in chemical industry are mainly divided into the following categories:

2.1 Axial Expansion Joint

• Internal pressure type (Z): Most commonly used type, absorbing axial displacement by bellows expansion and contraction
• External pressure type (ZW): suitable for working conditions that need to withstand external pressure or avoid medium retention
• Pressure balanced type (ZPZ/ZPW): can eliminate the blind plate force, suitable for the situation where the fixed point is limited

2.2 Transverse Expansion Joint

• Unidirectional equation (H): absorbing transverse displacement in a single direction
• Universal ring/tie rod (HW/HL): absorbs lateral displacement in any direction of space

2.3 Angular Expansion Joint

• Unidirectional hinged type (J): absorbs angular displacement in a single plane
• Universal hinge type (JW): absorbs angular displacement in any direction of space

2.4 Comparison of typical application cases

In the design of the raw acid treatment unit, the engineers compared and selected two expansion joint schemes: the double tie rod type and the single universal hinge type are used in pairs. The results show that the latter is superior to the former in reducing the stress on the nozzle of the equipment, and it is especially suitable for the large pipe diameter, high temperature and severe allowable load of the equipment nozzle。

3. Technical points of selection and design

3.1 Follow current standard specifications

When selecting expansion joints in chemical industry, the following standards should be strictly followed:

Among them, the T/CPCIF 0344-2024 standard, jointly drafted by more than 20 units including China Global, Sinopec Guangzhou Engineering and CSSC Shuangrui, covers the basic requirements for flexible compensation design of expansion joints, pipe frame setting, design, manufacturing, inspection and installation and maintenance requirements, and is an important technical specification in the field of energy and chemical industry。

3.2 Bellows Structure and Material Selection

Multi-layer structure design

• Multilayer metal bellows are recommended for high temperature and high pressure conditions (standard JB/T 6171-1992 covers multilayer structures)
• Multi-layer structure can reduce single-layer stress, improve fatigue life, and have better flexibility

Key Points of Material Selection

• 304/316L stainless steel: conventional chemical medium, temperature 20~400℃
• Alloys such as Inconel: Extreme high temperature or strong corrosion conditions
• The welding quality of bellows, end pipe and flange shall comply with the requirements of GB 3323 Class II

It should be noted that the safety pressure and cycle life of the expansion joint are two core verification indicators. According to the regulations, the product should be able to withstand 1.5 times the nominal pressure without damage; At nominal pressure, operating temperature and total compensation, the number of allowable cycles shall meet the requirements of the user, usually not less than 1000 times。

3.3 Blind plate force calculation and pipe rack setting

A key problem in the selection of expansion joint is the calculation of blind plate force. When the expansion joint is set in the pipeline, the internal pressure acting on the effective area of the bellows will produce a huge blind plate force, which must be borne by the main fixing bracket. If the pipe frame is not properly set, it may lead to instability or even damage of the pipeline system.

From the failure case of a regenerative flue gas pipeline in a methanol-to-olefin (MTO) plant, the key to ensure the safe and effective service of the expansion joint is the selection and reasonable arrangement of the pipeline support. In the design, engineering and technical personnel should combine the calculation of thermal displacement and blind plate force of pipeline, and reasonably allocate fixed bracket, guide bracket and intermediate bracket。

4. Key points of installation and maintenance

• Inspection before installation: Check that the model, specification and pressure level of the expansion joint are consistent with the design documents
• Handling of transportation protective parts: The positioning screw or support used to protect the bellows during transportation must be removed after installation in place and pipe fixation, otherwise the expansion joint will not be able to expand and contract freely, and even lead to damage to the bellows
• Avoid mechanical damage: The surface of the bellows shall not have scratches, pits, welding slag splashing and other defects
• Forced centering is strictly prohibited: pipeline deviation must not be forcibly adjusted through flanges
• Regular inspection: Focus on checking the bellows for cracks, corrosion and leakage; Check the stent for loosening or displacement

V. Summary

The expansion joint in chemical industry is the core component to solve the thermal stress of pipeline and protect the safety of equipment. Its selection and design need to comprehensively consider multiple factors such as displacement type, temperature and pressure, media corrosiveness and standards and specifications.

The core points are summarized as follows:

1. Classification and selection: Select the appropriate structural type according to the displacement direction (axial/transverse/angular) and whether the blind plate force needs to be eliminated. The engineering practice of the raw acid treatment unit shows that the single-type universal hinge type has advantages in reducing the force on the nozzle of the equipment。
2. Standards first: Give priority to the current industry specifications such as T/CPCIF 0344-2024 and SH/T 3421-2025 to ensure design compliance 。
3. Structural verification: Multi-layer bellows structure can improve flexibility and life, and two core indicators of safety pressure (≥1.5 times nominal pressure) and cycle life (≥1000 times) should be paid attention to when selecting the type。
4. Bracket design: Reasonable arrangement of fixed bracket and guide bracket, accurate calculation of blind plate force, which is the key to prevent the failure of expansion joint. The failure case of MTO device fully proves the importance of pipe rack design。
5. Standardized installation: Correct dismantling of transportation protective parts, forced centering is strictly prohibited, and regular inspection and maintenance are necessary measures to ensure the long-term stable operation of expansion joints.

It is suggested that engineering designers carry out complete thermal stress analysis at the initial stage of the project, carry out special design of expansion joint in combination with process conditions and standard specifications, and strictly implement technical requirements in the installation stage. Only by closed-loop control of selection, calculation, installation and maintenance can the compensation efficiency of expansion joints in chemical industry be truly brought into play, and the safe, stable and long-term operation of chemical plants can be ensured.