A Comprehensive Guide to Bellows Sealed Globe Valves

In the industrial sector, valves are essential components in fluid control systems, and their performance and quality directly impact the stability and safety of the entire production process. Bellows sealed globe valves, as a novel type of globe valve, are gradually becoming the preferred choice in numerous industrial scenarios due to their unique design and superior performance. This article will delve into the structural characteristics, working principles, common issues and solutions, as well as how to properly select and utilize these efficient and reliable valves.

Structure and Advantages of Bellows Sealed Globe Valves

The core of the bellows sealed globe valve lies in its stem sealing, which employs a dual-sealing design combining bellows and packing. This design not only enhances the sealing performance but also significantly boosts the valve's safety, energy efficiency, and environmental friendliness. The metallic barrier formed by the bellows effectively prevents leakage, ensuring the valve's sealing performance even under extreme operating conditions. The packing provides additional sealing assurance in case of bellows failure, making the bellows sealed globe valve capable of achieving virtually zero leakage in practical applications.

The key component of the bellows sealed globe valve is the metallic bellows. The bellows are connected to the stem through an automatic rolling welding process, ensuring the leak-tightness of the bellows assembly. This connection method not only improves the reliability of the bellows but also extends their service life. However, despite the numerous advantages of bellows sealed globe valves, bellows rupture remains one of the most common issues encountered in actual use.

Causes and Prevention of Bellows Rupture

Bellows sealed globe valves are highly favored in the industrial field for their excellent sealing performance and reliability. However, to fully leverage their advantages, it is crucial to understand the potential issues that may arise during use. Among these, bellows rupture is a significant problem that cannot be overlooked. Next, we will explore the causes of bellows rupture and propose corresponding preventive measures to ensure the long-term stable operation of bellows sealed globe valves.

1. Bellows Sealed Globe Valve Column Instability

Column instability refers to the lateral displacement of the bellows' corrugations, causing the actual axis of the displaced bellows to become arc-shaped or S-shaped. This phenomenon is usually caused by factors such as insufficient total thickness of the bellows, excessive number of bellows corrugations, and significant deviation in the coaxiality of the bellows. In steam systems, water hammer can also cause deformation of the stem or displacement of the valve head, leading to column instability.

To prevent column instability, it is essential to ensure that the material selection and design parameters of the bellows meet the actual operating conditions. During the design phase, factors such as the thickness, number of corrugations, and coaxiality of the bellows should be fully considered to ensure that they can withstand the expected test and operating pressures. Additionally, for steam systems, effective water hammer protection measures should be implemented, such as installing water hammer eliminators or optimizing pipeline design, to reduce the impact of water hammer on the bellows.

2. Bellows Sealed Globe Valve Planar Instability

Planar instability is characterized by one or several bellows planes tilting relative to the bellows axis due to rotation, although the center of the bellows remains essentially on the axis. This type of instability usually occurs when the bellows are under compression and is caused by the combined action of the meridional bending stress and circumferential membrane stress generated by internal pressure, resulting in plastic deformation of the bellows. Planar instability often occurs when the bellows are in a compressed state. If column or planar instability is not controlled, the shape of the bellows will change significantly, deviating from the original calculation model, causing displacement to concentrate on one or several bellows, and ultimately leading to fatigue rupture of the bellows.

To avoid planar instability, the structural design of the bellows should be optimized to ensure their stability under compression. During the manufacturing process, the machining accuracy and quality of the bellows should be strictly controlled to ensure they meet the design requirements. Additionally, during use, the bellows should not be kept in a compressed state for extended periods. Regular inspections of the bellows' deformation should be conducted to promptly identify and address potential issues.

3. Bellows Sealed Globe Valve Welding Defects

The material of the bellows and the stem may differ, leading to differences in weldability. Improper welding processes can easily result in welding defects, causing holes in the bellows. To address this issue, appropriate welding materials and processes should be selected to ensure welding quality. Before welding, the welding area should be thoroughly cleaned and treated to remove surface oils, impurities, and other contaminants, thereby enhancing the reliability of the weld. During welding, welding parameters should be strictly controlled to avoid the formation of welding defects. After welding, the welded area should be inspected to ensure it is free of defects.

Causes and Solutions for Valve Trim and Seat Sealing Failure

Valve trim and seat sealing failure is another common issue encountered during the use of bellows-sealed globe valves. Users often report that when using bellows-sealed globe valves for media with significant pressure differences, such as steam and high-pressure water, it is difficult to close the valve. Even when force is applied to close it, leakage often occurs. This problem is primarily due to the valve's structural design and the insufficient maximum torque that can be applied by a person, as well as unreasonable design of the valve trim and seat sealing pair's structure, material, heat treatment, and machining accuracy.

1. Conical Valve Disc Design

To address the issue of valve trim and seat sealing failure, the latest conical valve disc design has been introduced. When the valve is closed, the conical sealing surface with a "scraping effect" can automatically remove surface impurities, ensuring reliable sealing and a longer service life for the valve disc. The conical valve disc can be designed in various forms, such as throttling type, globe check type, and flat soft sealing, to meet different user requirements based on different operating conditions.

2. Erosion Issues in Steam Systems

During the startup of a steam system, it is necessary to preheat the steam pipeline, which typically requires a very small flow of steam to slowly and evenly heat the pipeline. However, during this process, the valve opening is often very small, leading to an erosion rate that is much higher than normal, severely reducing the service life of the valve sealing surface. Watt Energy-Saving Steam Engineers have found that damage to the valve trim and seat often occurs at the throttling port. When the valve is about to close, the steam velocity increases sharply, and the scouring force on the sealing surface multiplies, resulting in linear grooves on the surfaces of the valve trim and seat, significantly reducing the valve's lifespan.

Additionally, according to the principle of energy conservation, as velocity increases, pressure will drop sharply, causing flashing at the outlet. The instantaneous generation of powerful pressure shock waves can impact the surfaces of the valve trim and seat, creating honeycomb-like small holes and causing vibration and noise, which is known as cavitation. Therefore, the hardness and erosion resistance of the valve disc and seat materials must be excellent to ensure the valve's service life.

To address this issue, appropriate materials for the valve disc and seat should be selected, such as stainless steel and hard alloys, which have high hardness and good erosion resistance. During the design phase, the structure of the valve trim and seat should be optimized to reduce the impact of erosion on the sealing surface. During use, the valve opening should be reasonably controlled to avoid prolonged operation at a small opening, thereby reducing erosion.

Selection and Installation of Bellows sealed globe Valves

Next, we will focus on how to properly select and install bellows sealed globe valves to ensure their optimal performance in practical applications. Proper selection and installation are key to the long-term stable operation of the valve.

1. Selection Key Points

When selecting a bellows sealed globe valve, the actual operating conditions should be considered comprehensively, including the valve's size, pressure, temperature, and media. For products with a nominal diameter (DN) above 150, a triangular positioning device must be installed to effectively prevent the valve from trembling, making noise, and causing damage to internal components during use, thereby increasing the service life by more than 150%. Additionally, the valve body is preferably cast using film sand casting or silica sol casting technology to improve the surface finish and mechanical properties of the valve body, reduce the occurrence of porosity, sand holes, and cracks, and enhance the tensile strength of the casting.

2. Handwheel Design

The design of the valve handwheel is also very important. The handwheel should be easy to grip and free of burrs or other protrusions. The diameter of the handwheel should not be too large or too small. An oversized handwheel can easily cause artificial damage to the valve trim and seat when closing the valve, while an undersized handwheel makes it difficult to close the valve. The use of a lever can lead to deformation of the stem and damage to the sealing pair. Therefore, when designing the handwheel, the user's operating habits and convenience should be fully considered.

3. Stem Treatment

The stem is preferably treated with processes such as nitriding to increase its wear resistance. The stem threads should be fine-pitch rolled threads to avoid dead-zone leakage after the valve is closed. These treatment processes can effectively improve the service life and reliability of the stem, ensuring the long-term stable operation of the valve.

4. Pressure Testing

Before leaving the factory, the valve must undergo 100% pressure testing at 1.5 times the design pressure. When used for steam, a 100% tightness test is required, and the sealing grade must be higher than the Grade 4 leakage standard. These stringent testing procedures ensure the reliability and safety of the valve in actual use, providing users with confidence.

Application Scope and Limitations of Bellows sealed globe Valves

Bellows sealed globe valves are widely used in industries such as oil, chemical, power, pharmaceutical, and food, especially in applications where high sealing performance is required. However, there are limitations to the application scope of bellows sealed globe valves. Generally, the maximum nominal diameter of bellows sealed globe valves does not exceed 350mm. When the diameter is greater than 350mm, the size, weight, complexity, and operational difficulty of the bellows sealed globe valve will increase significantly, making it no longer a suitable choice. In such cases, selecting other types of valves, such as bellows gate valves or eccentric butterfly valves, would be more economical and reliable.

Conclusion

As a novel type of globe valve, the bellows sealed globe valve has gained widespread application in the industrial field due to its excellent sealing performance, reliability, and safety. However, during actual use, attention must still be paid to issues such as bellows rupture and valve trim and seat sealing failure, and corresponding preventive and solution measures should be taken. Through proper selection, correct installation, and maintenance, bellows sealed globe valves can provide users with efficient and reliable fluid control solutions, contributing to the stable operation of industrial production.