Valve Stem Sealing: Principles, Structures & Applications

 
The stem sealing system plays a critical role in industrial valve performance, particularly during the opening and closing processes. Wear and tear on the stuffing box can lead to valve leakage. The design of the valve stem and its sealing structure directly affect the performance and service life of industrial valves, making it essential to impose strict requirements on the material and surface treatment. This article will delve into the principles of stem sealing, explore different types of packing seal structures, and examine their applications.

Importance of Stem Sealing

 
In daily valve operation, the stem controls fluid flow through opening and closing actions. Due to frequent operation and the erosive nature of working fluids, the stem is prone to wear and corrosion. This is particularly true at the stuffing box, where the stem's motion can cause wear on the packing, creating gaps that lead to fluid leakage. As a result, the stem must possess suitable hardness and elasticity, along with excellent resistance to corrosion and abrasion, to withstand fluid erosion. To further enhance wear resistance, surface treatments such as chrome plating or nitriding are typically applied to the stem, improving its durability and corrosion resistance. This helps prevent seal failure caused by bending, corrosion, or surface tension changes on the stem.

Working Principle of Stem Sealing

 
Stem sealing is a multi-component system that relies on radial contact pressure between the packing and the stem to achieve a tight seal. The system applies axial pressure to the packing through bolts or a gland, which then deforms radially to snugly fit against the stem's surface, forming a seal. Due to the stem's micro-rough surface, contact between the packing and the stem produces uneven lubrication. At the points of contact, the packing reaches a boundary lubrication state, while in non-contact areas, small oil grooves form an oil film that reduces friction and enhances sealing.
This lubrication state is crucial for prolonging the service life of both the stem and the packing. When axial pressure compresses the packing, its high plasticity causes radial residual stress, ensuring tight contact with the stem surface. Lubricating oil enters the interface between the packing and the stem during compression, creating a lubricating film that reduces friction and wear. Additionally, this labyrinth-like seal structure, characterized by irregular contact points and gaps, forms a sealing channel that effectively prevents fluid leakage.

Pressure Distribution in Packing Seals

 
The pressure distribution within the packing seal directly impacts its sealing effectiveness. Radial pressure increases from the inside of the stuffing box outward, while average pressure gradually decreases. Typically, maximum compressive force occurs at two or three packing rings near the gland’s radial location. This distribution helps ensure effective radial sealing while reducing packing wear.

Types of Packing Seal Structures

 
Valve stem packing seal structures generally fall into three categories: external compression, internal compression, and bi-directional compression. Each type offers varying sealing performance and is suited to different applications.

1. External Compression Soft Packing Seal Structure

 
In this structure, sealing is achieved by compressing the packing externally through a gland. When the gland applies axial force, radial pressure is generated at the contact point between the packing and the stem, creating a seal. This structure is commonly used in medium- and low-pressure soft-sealed valves, such as wafer soft-seal butterfly valves. The advantage of external compression is its simple manufacturing and maintenance, making it suitable for low-precision sealing scenarios. However, due to lower radial pressure at the packing layer near the fluid medium, the sealing performance is not as strong. In some cases, increasing the packing compression force improves sealing, but this leads to higher operating torque and increased wear, ultimately shortening the valve's lifespan.

2. Internal Compression Soft Packing Seal Structure

 
In this structure, greater radial contact pressure is applied to the packing near the medium side, achieving a better seal. Since the radial contact pressure aligns with the direction of fluid leakage, good sealing is maintained as long as the contact pressure exceeds the medium pressure at each cross-section. This structure is ideal for high-pressure valves and offers advantages such as lower required compression force, fewer packing rings, smaller operating torque, and reduced wear between the stem and packing. It excels in high-pressure environments where reliability and durability are paramount.

3. Bi-Directional Compression Soft Packing Seal Structure

 
This structure uses glands on both the upper and lower sides of the stem to compress the packing from both internal and external directions. Compared to single-sided compression structures, bi-directional compression creates more uniform radial contact pressure along the axial length of the packing, allowing each ring of packing to contribute to sealing. This forms an expanded-constricted leakage channel, providing a stable seal. The large pressure drop across the channel ends makes it easier to form a reliable seal, while the thicker oil film in the middle lubricates the stem, reducing wear and extending valve life.

Selecting the Right Packing Seal Structure

 
Selecting the appropriate packing seal structure depends on various factors, including working pressure, fluid type, and operating frequency. External compression structures are suitable for medium- and low-pressure soft-sealed valves and are easier to manufacture and maintain, though their sealing performance is relatively weaker. Internal compression structures are ideal for high-pressure valves, providing excellent sealing with minimal wear. Bi-directional compression structures are preferred for applications requiring high sealing reliability and extended service life, offering superior sealing and lubrication performance.

Conclusion

 
The design of the valve stem packing seal has a direct impact on the overall performance and service life of the valve. By selecting the appropriate packing structure, optimizing surface treatments, and ensuring proper lubrication, valve life can be significantly extended, and leakage risks minimized. In practical applications, choosing the right sealing structure based on operational conditions will greatly enhance the reliability and safety of the valve.