Understanding Valve Stem Sealing in Soft-Seal Gate Valves

Soft-seal gate valves are widely used in fluid control systems due to their simple structure, ease of operation, and excellent sealing performance. The sealing of the valve stem is a crucial aspect that ensures the normal functioning of the gate valve. The valve stem is not only the core component that drives the gate valve to open and close but also the key to achieving effective sealing.

Structural Characteristics of Soft-Seal Gate Valves

The basic structure of a soft-seal gate valve typically includes the valve body, valve plate, valve stem, packing chamber, and sealing components. The valve stem connects to the valve body through the packing chamber, responsible for opening and closing the valve. The quality of the valve stem seal directly affects the valve's lifespan and the sealing of the fluid. To ensure effective sealing, soft-seal gate valves commonly use compressed packing as the sealing material.

Principle of Valve Stem Sealing

The principle of valve stem sealing mainly relies on the sealing layer formed by the packing around the valve stem. Compressed packing is placed in the packing chamber and applies pressure around the valve stem to ensure tight contact between the stem and the valve body. The working mechanism of compressed packing is as follows.

1. Function of Packing

When the valve stem is in the open or closed position, the packing expands laterally due to the pressure applied by the packing gland, creating a seal. This pressure must be equal to or greater than the medium's pressure to effectively prevent fluid leakage.

2. Generation of Sealing Pressure

The sealing pressure generated by the tightening of the packing gland causes the packing to expand laterally, forming a good seal. When the packing pressure equals or exceeds the medium pressure, the leakage path is effectively closed.

Packing Materials Used in Soft-Seal Gate Valves

Choosing the right packing material for soft-seal gate valves is crucial, as different materials have various physical and chemical properties suitable for different working environments.

1. Asbestos Packing

Asbestos packing was widely used in soft-seal gate valves due to its high-temperature and corrosion-resistant properties. It conducts heat well, making it suitable for high-temperature media like steam and hot water. However, its use is now restricted due to health concerns, and alternative materials are often chosen.

2. Polytetrafluoroethylene (PTFE)

PTFE is a commonly used sealing material in soft-seal gate valves because of its excellent corrosion resistance and high-temperature performance, with a working temperature range of -150°C to 260°C. Its low friction coefficient effectively reduces wear, extending the valve's lifespan. PTFE's chemical inertness and high-temperature stability make it a key choice for modern valve sealing materials.

3. Flexible Graphite

Flexible graphite is widely used in soft-seal gate valves operating in high-temperature environments, capable of withstanding temperatures up to 600°C and providing strong resistance to most corrosive media. Its excellent compressibility helps maintain good sealing during the movement of the valve stem, making it popular in high-temperature valves in the power and petrochemical industries.

Types of Packing Materials and Their Applications

The types of sealing packing used in soft-seal gate valves mainly include the following.

1. Lip Packing

Lip packing has good elasticity and flexibility, allowing it to expand outward under medium pressure and fit snugly between the valve stem and body. While its sealing action is primarily one-directional, it is widely used in hydraulic systems. Lip packing is typically made from PTFE or filled PTFE, easy to install, and effectively resistant to various media.

2. Extruded Packing

Extruded packing, suitable for O-rings and similar types, maintains a preload force due to the elastic properties of the material when compressed. When the medium enters the packing chamber from the bottom, the packing moves into the gap between the stem and support, preventing leakage. Extruded packing can quickly recover its shape, ensuring long-term sealing effectiveness, especially in high-temperature and high-pressure conditions.

3. Thrust Packing

Thrust packing consists of packing rings and a washer on the valve stem, allowing axial movement between the packing rings. Initial sealing can be provided by a spring or other auxiliary sealing device to ensure tight contact between the stem and packing. The design of thrust packing can significantly reduce friction, extend the valve's lifespan, and is suitable for various fluid control applications.

Conclusion

The sealing technology of soft-seal gate valves plays a vital role in fluid control. By carefully selecting packing materials and sealing structures, it is possible to enhance the valve's sealing capability and lifespan, ensuring safety and efficiency in various industrial applications. With advancements in materials science and increasing environmental requirements, the development and use of new sealing materials will be an essential direction for the future of soft-seal gate valve technology.

In practical applications, designers should consider the characteristics of the medium, working temperature, and pressure when selecting packing materials and designing sealing structures to achieve the best sealing effect and valve performance. This not only helps prevent medium leakage and reduces environmental pollution but also enhances the valve's operational efficiency, ensuring reliable performance in various working conditions.