Bellows Seals in Valves: Design, Materials & Maintenance

With the rapid development of modern industries such as chemical and nuclear sectors, the number of pipeline systems involving flammable, explosive, toxic, and radioactive substances has been steadily increasing. These pipeline systems are typically located in high-risk environments, where any leakage could lead to severe accidents. As a result, the sealing requirements for industrial valves used in these pipelines have become more stringent, and conventional packing seals can no longer meet these high demands. To ensure the safety of these pipelines, valves with bellows seals on the industrial valve stem have become an important and effective solution.

Bellows Seal Structure Design

The bellows seal structure ensures that fluid media do not leak by using the bellows to create a sealed barrier inside the valve. The bellows is usually connected to the valve body or other components by roll welding or argon arc welding, forming a reliable sealing system. The design of the bellows must ensure long-term functionality even after compression, providing excellent elasticity and durability. This structure not only effectively isolates the external environment but also adapts to the valve's working conditions in high temperatures, high pressures, and corrosive environments.

The connection between the bellows and the valve must be absolutely leak-proof. Over time, the bellows may experience damage due to stress or corrosion. Therefore, the design should include provisions for easy replacement of the bellows, ensuring no significant modifications are needed to the valve structure during replacement. This simplifies the maintenance process. Additionally, the bellows often comes with a tail sleeve for easy installation, although a single-layer bellows may not require one during installation.

Materials and Connection Methods for Bellows

As a key component of the valve sealing system, the material and connection method of the bellows are crucial for its sealing performance and service life. The selection of materials and connection methods is determined by the working environment and application requirements. Below are some common bellows materials and their connection methods.

1. Copper-Zinc Alloy Bellows (with approximately 20% zinc content) 

Copper-zinc alloy bellows are suitable for low-temperature, low-pressure environments. They offer good corrosion resistance and are relatively inexpensive. The connection is typically made by brazing with soft lead-tin solder. For critical connections, hard solder (such as silver) is often used to ensure a more secure bond.

Connection Methods

Brazing: The bellows is brazed to the inner cylindrical surface of the connected components. This method is suitable for applications with high sealing requirements but where the connection's lifespan does not need to be extended.
Roller/Expansion Connection: Another method involves expanding or rolling the bellows metal to tightly fit the connecting parts, followed by brazing. This method enhances the connection's sturdiness, making it suitable for high-demand valve applications.

2. 06Cr19Ni10 Stainless Steel Bellows

06Cr19Ni10 stainless steel bellows have excellent high-temperature resistance, capable of withstanding up to 450°C, and are suitable for valves used in corrosive media. This material's corrosion resistance and high-temperature capabilities make it an ideal choice for the chemical and nuclear industries.

Connection Methods

Arc Welding: The bellows typically comes with a welded flange, which is attached to valve components (such as the valve cover or valve stem) by arc welding. This connection method is stable and suitable for high-temperature and high-pressure environments, providing long-term sealing.
Welded Flange: The flange connection can further enhance sealing performance, ensuring safety in extreme environments.

3. Single-Layer Stainless Steel Bellows

Single-layer stainless steel bellows are widely used in general valve sealing, especially in low-pressure or less corrosive environments. Their main advantage lies in the material's corrosion resistance and high strength.

Connection Methods

Short Pulse Welding: The cylindrical sleeve of the bellows is positioned at the proper location on the connecting parts and welded using short-pulse welding technology. This method is suitable for components with thin walls (0.05–0.5mm thick). During welding, the bellows must completely cover the part being welded to ensure good welding quality and prevent leakage.
Sleeve Connection: In some applications, the bellows is connected to the components by a sleeve connection, ensuring stable sealing.

4. Multi-Layer Stainless Steel Bellows

Multi-layer stainless steel bellows offer superior pressure resistance and longer service life, making them suitable for high-pressure systems or extreme fluid flow conditions.

Connection Methods

Threaded Sleeve Connection: Multi-layer bellows are usually equipped with a threaded sleeve that connects to other parts. Threaded connections provide a firm seal and allow for position adjustment during installation.
Electro-pulse Welding: Electro-pulse welding or argon arc welding is used to weld the pipe joint to the connecting parts or seams. While this method can make bellows replacement more difficult, it generally ensures strong and leak-proof connections.
Mechanical Fixing: Another method involves mechanically securing the bellows to the components, making it easier to maintain and replace, as it requires minimal welding during maintenance.

Maintenance and Replacement of Bellows

As an essential part of the valve sealing system, the maintenance and replacement of bellows require special attention. Over time, bellows may become damaged due to factors such as medium corrosion, mechanical wear, or temperature fluctuations. To ensure the system's safety and reliability, regular inspections of the bellows should be conducted. If damage or leakage is found, replacement should be done promptly.
During the replacement of bellows, it is advisable to minimize changes to the valve structure to reduce maintenance complexity. It is also recommended to use original manufacturer parts to ensure compatibility with the valve system's connection method, maintaining the original sealing performance.

Conclusion

As industrial sealing technology continues to evolve, bellows seal structures have become a key solution for meeting the stringent sealing requirements of high-demand valves. The use of various materials and connection methods allows bellows to meet the sealing needs of different working environments, enhancing the safety and longevity of valve systems. Through careful design, material selection, and precise installation processes, bellows can effectively prevent harmful media leakage and ensure the reliable operation of valve systems.