In the pipeline system, there is a seemingly ordinary but indispensable component-it can both bear pressure and temperature like a steel pipe, and telescopic and flexible like a spring. It is the bellows. From high-temperature steam pipelines in chemical plants to ultra-low-temperature pipelines in liquefied natural gas (LNG) receiving stations, from sensors in precision instruments to propulsion systems in spacecraft, bellows have become one of the most widely used elastic elements in industrial systems due to their unique "rigid and soft" characteristics. This paper will systematically analyze the classification, material selection, design criteria and typical application scenarios of bellows.

I. Definition and working principle of bellows

Corrugated pipe is a cylindrical thin-walled shell with transverse corrugation. Its core feature is that the corrugated structure gives the pipe wall flexibility, so that it can produce considerable elastic deformation while withstanding certain pressure。 According to the different corrugated shapes, bellows are mainly divided into two categories: annular bellows (good elasticity and low stiffness) and spiral bellows (the corrugated shape is spiral, with balanced stiffness and strength)。

The working principle of corrugated pipe is based on its unique geometry: when subjected to axial tension or pressure, the corrugated part undergoes elastic deformation, and the wave pitch of the corrugation increases or decreases, thus absorbing the thermal elongation or mechanical displacement of the pipe. This feature makes it an ideal element for thermal compensation of pipes, shock absorption of equipment and displacement adjustment.

2. Core classification of bellows

1. Metal bellows

Metal bellows is the main product of industrial pipeline system, and the main material is usually austenitic stainless steel (such as 304, 304L, 321, 316L, etc.)。 It has that characteristics of good flexibility, strong corrosion resistance, long service life and the like, and the connection mode includes flange connection, welding, threaded connection and quick connection。

2. Non-metallic bellows

Non-metallic bellows (e.g. plastic/rubber bellows) are mainly used for electrical cable protection and low pressure fluid delivery. PA6, PA12, PP and PE are common plastic bellows materials。 Its advantages lie in light weight, corrosion resistance, easy installation, and suitable for medium and low pressure working conditions.

3. Key points of material selection of corrugated pipe

According to the working environment and media characteristics, the following factors should be comprehensively considered in the material selection of corrugated pipes:

1. Corrosion Resistance

The service environment of corrugated pipe is complex, which requires stable material structure and performance to meet the corrosion requirements of different regions。 It is recommended to use 316L stainless steel under the condition of large deformation and bad corrosion environment: it has good corrosion resistance in heavily polluted atmospheric environment above C4, and no plastic induced phase transformation (TRIP effect) occurs, and the hardness increase is small。

In addition, for the special application scenarios of electrical equipment, various stainless steel materials will produce different degrees of magnetism after deformation. Therefore, special attention should be paid to material selection when non-magnetism is required。

2. Temperature Adaptability

The material of the bellows should be selected according to the working temperature. For high temperature working conditions, 321 stainless steel contains titanium stabilizing element, excellent intergranular corrosion resistance, suitable for 450-600℃ range. For ultra-low temperature operating conditions (such as LNG transport), austenitic stainless steel has good low temperature toughness and is the preferred material.

For non-metallic bellows, PE/PA material should be selected for temperature resistance 120-150℃, and PP flame retardant material should be preferred for high fire protection requirements。

4. Design standards of corrugated pipes

The design, manufacture and inspection of corrugated pipes are subject to strict national standards. The core standard of metal bellows expansion joints is GB/T 12777-2019 General Technical Conditions for Metal Bellows Expansion Joints, which was officially implemented on December 1, 2019, replacing the old GB/T 12777-2008。

The standard is under the control of the National Marine Machinery Standardization Technical Committee, and the main drafting units include the 725th Research Institute of China Shipbuilding Industry Corporation, Luoyang Shuangrui Special Equipment Co., Ltd., Nanjing Chenguang Dongluo Corrugated Pipe Co., Ltd. and other authoritative organizations。 The standard covers:

• Classification and labeling
• Material requirements (bellows, pressure-bearing parts, force-bearing parts)
• Dimensions and deviations
• Design conditions and calculation methods
• Manufacturing process and welding requirements
• Inspection and test methods (pressure test, airtightness test)
• Marking, packaging, transportation and storage
• Selection Guide
• Installation and Use Requirements and Safety Recommendations

In addition, GB/T 35979-2018 Technical Specification for Selection, Installation, Use and Maintenance of Expansion Joints of Metal Corrugated Pipe provides systematic technical guidance for the engineering application of corrugated pipe。

V. Application fields of corrugated pipes

1. Pipeline system displacement compensation

In industrial pipelines, the core application of corrugated pipes is to absorb the thermal displacement of the pipe due to temperature changes. Take a section of carbon steel steam pipeline with a length of 20 meters as an example. When the temperature rises from 20℃ to 400℃, the thermal elongation can reach more than 90mm, which must be compensated by bellows.

2. Equipment shock and noise reduction

The bellows can absorb vibration energy and play the role of vibration reduction and noise silencing。 Installing bellows in vibration-sensitive parts such as pump inlet and outlet, compressor pipeline, etc. can effectively reduce vibration transmission and protect equipment interface.

3. Protection of electrical equipment

In the electrical field, bellows are widely used for cable protection. The stainless steel threading hose can be bent freely at various angles, and has good flexibility, corrosion resistance and electromagnetic shielding properties。 Slit bellows (such as PA6 double-slit bellows) facilitate later cable installation and maintenance。

4. Building HVAC system

In HVAC water piping systems, corrugated metal hoses are used to connect pumps, chillers and other equipment to compensate for pipe displacement and absorb vibration. 13K204 "Selection and Installation of Soft Connection of HVAC Water Pipe" Atlas provides standardized selection and installation guidance。

5. Power Equipment GIS/GIL

Corrugated pipe is an important component of GIS (Gas Insulated Switchgear) and GIL (Gas Insulated Transmission Line) in power equipment. The service environment is complicated, which requires stable material structure and small temperature rise。

VI. Suggestions on the installation and maintenance of bellows

1. Check before installation

• Check that the bellows model, specification and design are consistent
• Check the bellows surface for mechanical damage and corrosion
• Confirm the flow direction identification, the direction of the guide tube must be consistent with the flow direction of the medium

2. Installation precautions

• It is strictly prohibited to adjust the deviation of pipeline installation by deforming the bellows
• Welding slag is not allowed to splash onto the bellows surface during installation
• Transport protection tie rod must be removed after installation

3. Operation and maintenance

• Check the bellows surface regularly for cracks and corrosion
• Monitor operating temperatures and pressures to ensure they are within safe ranges
• Regular non-destructive testing for multi-layer bellows

sum up

Corrugated pipe is an engineering element that combines rigidity and flexibility, and its value lies in the integration of two seemingly contradictory characteristics of "elastic deformation" and "pressure-bearing seal". The core points can be summarized as follows:

• Select the structure according to the work requirements: annular bellows is preferred for large displacement and low pressure scenes; Spiral bellows is preferred for high pressure and high rigidity requirements; Multi-directional displacement recommended multilayer structure
• Select materials according to working conditions and environment: 316L stainless steel is recommended for corrosive environment; 321 stainless steel is recommended for high temperature working conditions; 304 stainless steel is recommended for general working conditions; Pay attention to material selection for non-magnetic applications
• Manufacturing and acceptance according to design standards: The manufacture of bellows shall strictly follow GB/T 12777-2019 standard, and the installation and use shall refer to GB/T 35979-2018 specification
• Standardized installation and regular maintenance: It is strictly prohibited to use bellows deformation adjustment deviation during installation, and aged products should be replaced in time by regular inspection

A bellows with reasonable design, proper material selection and standard installation can run stably for more than ten years under harsh working conditions such as high temperature, high pressure and corrosion. It is suggested that a professional organization should be entrusted to conduct stress analysis and special selection of bellows in the design stage, so as to ensure the safety and reliability of the system from the source.