Causes and Measures for Bellows Sealed Globe Valve Failure

The bellows sealed globe valve, a modern variant of the traditional globe valve, utilizes a unique bellows sealing design. This design, with its dual sealing structure comprising both the bellows and packing, offers superior sealing performance compared to conventional globe valves. Bellows sealed globe valves are widely used in applications where high sealing reliability is critical, particularly in high-pressure, high-temperature environments like petrochemical, chemical, and steam systems. However, despite their superior design, bellows failure remains a major cause of globe valve malfunction, threatening both the valve's sealing ability and the overall safety of the system. Therefore, understanding the causes of bellows failure and implementing appropriate protective measures is crucial for improving the reliability and safety of these globe valves.

Main Causes of Bellows Failure

While bellows sealed globe valves are known for their superior sealing performance, bellows failure is still a common issue. The main causes of bellows rupture include column instability, plane instability, welding defects, and corrosion and wear. These issues directly affect the valve's sealing performance and the stability of the system. Understanding these causes is vital for improving the reliability of bellows sealed globe valves.

1. Column Instability

The sealing performance of the bellows sealed globe valve heavily depends on the deformation properties of the metal bellows. When subjected to pressure or temperature, the bellows can undergo lateral displacement, causing instability in the bellows' convolute connections, which may lead to rupture. Column instability is primarily caused by insufficient bellows thickness or the failure to account for the bellows' stability under pressure in the design phase. If the bellows' convolute connections shift under internal pressure, permanent deformation or rupture becomes more likely.

2. Plane Instability

Plane instability is another common cause of bellows failure, particularly when the bellows is in a compressed state. Under internal pressure, the bellows may undergo plastic deformation. The combined force of meridional bending stress and circumferential membrane stress caused by the internal pressure reduces the bellows' ability to withstand pressure, ultimately leading to failure. Plane instability not only shortens the service life of the bellows but may also result in valve sealing failure, causing leakage.

3. Welding Defects and Material Issues

The quality of welding during the manufacturing process of the bellows sealed globe valve directly affects its overall strength. Improper welding techniques or defects like heat cracks and pores during the welding process can lead to stress concentration, which may cause rupture in the bellows under operation. Additionally, if inappropriate materials are chosen—those with poor pressure resistance or fatigue strength—failure may occur under high-temperature, high-pressure conditions. Corrosion and wear on the bellows material also increase the risk of failure.

4. Corrosion and Wear

Corrosion and wear become particularly problematic under high-temperature, high-pressure conditions and when the medium being controlled is corrosive. The metal surface of the bellows may become susceptible to corrosion or erosion, leading to a reduction in local strength. Over time, this may result in rupture. To mitigate this, the bellows material should have excellent corrosion resistance, and measures should be taken to reduce the impact of high-speed erosion on the bellows surface.

Protective and Improvement Measures

To effectively prevent bellows failure, a series of protective and improvement measures should be implemented. These measures include optimizing the design, selecting pressure-resistant materials, enhancing welding processes, and improving valve operating conditions. By taking these actions, the lifespan and reliability of the bellows can be significantly improved, reducing the risk of rupture and ensuring the stable operation of the valve in high-pressure and high-temperature environments.

1. Optimizing Bellows Design

The design of the bellows should fully consider both internal and external pressures. Key factors such as the thickness, shape, and material of the bellows should be optimized to improve stability and pressure resistance. Increasing the thickness of the bellows, using high-strength alloys, and designing an appropriate convolute shape can help prevent column and plane instability. Additionally, the thickness should be carefully designed based on the specific operating conditions to ensure that the bellows does not undergo excessive deformation or rupture under pressure.

2. Strict Control of Welding Process

The quality of welding is critical to the sealing performance of bellows sealed globe valves. During manufacturing, precision automated welding techniques should be employed to minimize human error. The heat input during welding should be controlled to prevent the material from overheating and altering its properties. Rigorous quality control checks, such as X-ray or ultrasonic non-destructive testing, should be used to ensure that weld joints are free from defects and maintain the integrity of the bellows.

3. Enhancing Corrosion and Wear Resistance

To prevent failure due to corrosion or wear, bellows should be made from high-corrosion-resistant and wear-resistant alloy materials. Additionally, surface treatments such as coating or surface hardening can be applied to increase the corrosion resistance of the bellows. In environments where erosion is a concern, protective covers or materials designed to resist wear can be added to the surface of the bellows, extending its service life.

4. Regular Maintenance and Inspection

Regular maintenance is crucial, particularly in high-temperature, high-pressure, and corrosive environments. Periodic inspections of the bellows should be carried out to detect potential cracks, corrosion, or wear early. Non-destructive testing techniques should be used for regular checks to prevent rupture. Additionally, valve seat and stem seals should be inspected to ensure sealing integrity and avoid damage to the bellows.

5. Proper Control of Pressure and Temperature

Controlling the operating pressure and temperature of the bellows sealed globe valve is essential to prevent rupture. Especially in high-pressure and high-temperature environments, the materials and design of the bellows should be carefully selected to match the specific requirements of the system. In steam systems, for example, high-speed flow should be avoided to reduce the impact of erosion on the sealing surface of the bellows.

Conclusion

Bellows sealed globe valves play a crucial role in many industries due to their excellent sealing performance. Despite the advantages of their unique bellows sealing design, bellows failure remains a significant issue that can impact valve performance. By optimizing the bellows design, strictly controlling the welding process, improving corrosion and wear resistance, and ensuring regular maintenance, the lifespan and reliability of bellows sealed globe valves can be significantly extended. This will ensure stable operation in complex environments such as high-temperature, high-pressure, and corrosive conditions.