The Application of Bellows in Control Valves

 
In industrial automation and control systems, bellows serve as a critical component of control valves. They effectively prevent medium leakage, ensure operational safety, and protect internal valve components from external contamination. Consequently, the design and application of bellows are increasingly emphasized. This article explores the multi-functionality of bellows, analyzes their installation positions and material selection in control valves, and aims to deepen the reader's understanding of this vital element in modern industry.

The Role of Bellows in Control Valves

 
Bellows are cylindrical thin-walled structures with multiple transverse corrugations, widely used in control valves to isolate media, protect the valve, and enhance safety. Their functions can be detailed as follows:

1. Preventing Medium Leakage

 
The primary function of bellows is to prevent fluid leakage within the valve, especially for chemically reactive, toxic gases (such as chlorine) and volatile liquids (like silicone oil). By sealing the interior of the valve, bellows stop these substances from escaping into the environment, thereby protecting human health and the ecological system, while also preventing resource loss and ensuring safe and compliant operations.
Application Case: In the petrochemical and pharmaceutical industries, leaks can lead to significant accidents. The effective sealing capability of bellows mitigates these risks. For instance, using bellows in chlorine transport pipelines ensures safe containment, protecting worker safety and the environment.

2. Preventing External Contamination

 
Bellows also prevent external impurities and air from entering the valve. In environments with dust, particulate matter, or corrosive gases, bellows effectively block these contaminants, preventing them from entering the valve with the stem's movement. This helps maintain the purity and stable performance of the system.
Influencing Factors: In certain industrial settings, particularly in the food and pharmaceutical industries, controlling the purity of the medium is crucial. By blocking external pollutants, bellows ensure product quality and safety.

3. Protecting Internal Valve Components

 
Bellows play a significant role in protecting the valve stem. The medium's flow may erode or corrode the stem, but the presence of bellows reduces this impact, extending the valve's lifespan. By protecting the stem, bellows also decrease maintenance costs associated with stem damage.
Long-Term Benefits: By minimizing stem wear, bellows not only prolong the valve's operational life but also enhance system reliability, reducing downtime and maintenance expenses.

Installation Position and Material of Bellows

 
To maximize the effectiveness of bellows in control valves, choosing the appropriate installation position and material is essential. These factors directly impact the bellows' performance and lifespan, as well as the overall safety and efficiency of the valve system.

1. Bellows Installation Position

 
Bellows can be installed in two ways.
Internal Installation: Typically made from polytetrafluoroethylene (PTFE), the upper end integrates with the fluorine lining of the valve cover, while the lower end connects to the valve core, securing it in place. This design effectively protects the stem while maintaining structural stability.
External Installation: Often made from metal, one end attaches to the upper part of the stem, with the other end free and surrounded by a protective layer. This layer not only safeguards the bellows but also ensures that if the bellows fail, the internal medium does not contaminate the environment, enhancing system safety.

2. Classification of Bellows Materials

 
Bellows can be classified based on materials into two categories.

Plastic Bellows

 
Typically made from PTFE, they are easy to produce and cost-effective, suitable for specific applications. While they offer good flexibility, their use is limited by environmental temperature, internal pressure, and the chemical nature of the medium, usually not exceeding 180°C. They perform well with highly corrosive substances (e.g., formic acid, hydrochloric acid) but are unsuitable for media with similar molecular structures (e.g., silicon tetrachloride).

Metal Bellows

 
Not constrained by temperature and pressure, metal bellows have a broad range of material options and are suitable for harsh conditions. Thus, they are often used in high-temperature and high-pressure environments, offering greater durability and reliability.

Bellows Selection and Processing Types

 
In the selection process for control valves, choosing the right bellows is crucial. Although there is no dedicated selection software, many parameters (such as volume change rate, axial stiffness, and pressure resistance) can be calculated using established formulas. These parameters are essential for the valve's performance and its suitability for specific applications.
Metal bellows can be categorized based on processing methods:
Formed Bellows: Also known as mechanically formed or single-use bellows, these are created through mechanical stretching and pressing, offering good structural stability suitable for high-pressure and high-temperature applications.
Welded Bellows: Composed of two welded bellows components, these are designed for high-pressure and special environment applications. The welding process ensures the integrity and durability of the connection.

Conclusion

 
Bellows play a vital role in control valves, and their design and application must be rationally configured according to specific working conditions. By selecting the appropriate bellows, the safety, stability, and durability of the valve can be significantly improved, optimizing overall system performance and efficiency. In modern industry, as demands for safety and reliability increase, the importance of bellows continues to grow, with ongoing advancements in technology and materials promising further innovation and improvement in the field.