Industrial Valve Stem Structures and Functions

The valve stem is a critical component of industrial valves, primarily responsible for transmitting operating forces or power to open or close the valve and ensuring precise positioning of the valve disc. Depending on the valve's design, the stem may provide either linear or rotary motion. In gate valves and globe valves, the stem drives the valve disc through linear motion to regulate fluid flow. Conversely, in ball valves, plug valves, and butterfly valves, the stem controls the flow by rotary motion. Valve stems are typically made from high-strength forged materials and require a high surface finish to ensure leak-tight performance in sealing areas. This article provides a detailed analysis of various valve stem structures, their features, advantages, disadvantages, and suitable applications to aid in a deeper understanding of their use.

Outside Screw and Yoke (OS&Y) Rising Stem

This valve stem design is characterized by its external threads, which do not come into contact with the fluid medium. The smooth portion of the stem is isolated from the fluid by a packing assembly, effectively reducing the risk of medium leakage.

Structural Features: The stem threads are external and isolated from the fluid medium through the yoke. The packing assembly seals against the smooth portion of the stem, ensuring leak prevention.

Design Variations: The handwheel and stem are integrated, moving synchronously. The handwheel rotates a threaded sleeve, which, in turn, drives the stem upward.

Advantages: Threads are isolated from the fluid, offering superior corrosion resistance and extended service life. Suitable for high-pressure and high-temperature applications; maintenance and replacement are convenient.

Disadvantages: Relatively complex structure, leading to higher manufacturing costs.

Applications: Commonly used in large-diameter industrial valves (NPS 2 and above), such as high-pressure gate valves and globe valves. Performing exceptionally in demanding conditions like petrochemical, power generation, and water supply industries.

Inside Screw Rising Stem

In this design, the valve stem threads are located within the valve body, directly exposed to the fluid medium, while the stem packing is exposed to the external atmosphere.

Structural Features: The threads are directly exposed to the fluid, making them susceptible to erosion and corrosion. The stem and handwheel are connected as a single unit, with the stem moving linearly during operation.

Advantages: Compact structure suitable for quick operation. Lower manufacturing costs, ideal for medium and low-pressure systems.

Disadvantages: Threads exposed to the fluid are prone to corrosion and wear, requiring specialized materials or protective treatments.

Applications: Widely used in low-pressure or non-corrosive fluid systems, such as municipal water networks or industrial cooling systems. Suitable for applications requiring high operational speed.

Inside Screw Non-Rising Stem

Non-rising stem designs feature a stationary stem, where the valve disc moves along the stem threads during operation.

Structural Features: The stem remains stationary while the disc moves along the threads. The threads are exposed to the fluid, necessitating corrosion-resistant materials.

Advantages: Space-saving design, ideal for installations with restricted space. No external movement of the stem, facilitating integration with other equipment or pipelines.

Disadvantages: Exposed threads are prone to corrosion or wear, increasing maintenance costs. Limited applicability to non-corrosive or low-pressure systems.

Applications: Commonly used in underground or confined installations, such as municipal pipelines or residential heating systems.

Sliding Stem

Sliding stems use linear motion instead of rotary motion to control valve opening and closing, often employed in automated control systems requiring rapid response.

Structural Features: Linear Sliding Design Controls valve movement through sliding instead of rotation, simplifying operation. Operated by hydraulic, pneumatic, or manual lever systems.

Advantages: Quick operation, suitable for high-frequency opening and closing. Ideal for automation systems requiring rapid response.

Disadvantages: Sliding components are prone to wear, necessitating regular lubrication and maintenance. Sealing parts may degrade over time, increasing maintenance frequency.

Applications: Extensively used in pneumatically or hydraulically actuated control valves, such as emergency shutoff valves or high-frequency automated equipment.

Rotary Stem

Rotary stems are commonly used in ball valves, plug valves, and butterfly valves, where a quarter-turn rotation suffices for opening or closing.

Structural Features: A 90° rotation completes the valve's opening or closing. Sealing design ensures reliable sealing between the stem and valve body to prevent leakage.

Advantages: Simple and quick operation, suitable for frequent use. Compact design, saving space, particularly for large-diameter pipelines.

Disadvantages: Sealing materials may be affected by medium or temperature, requiring regular inspection.

Applications: Widely applied in oil, chemical, and natural gas pipelines, especially in scenarios demanding operational convenience and fast response.

Conclusion

The structural design of valve stems directly impacts valve performance, operation, and application suitability. When selecting a valve, considerations should include stem material, corrosion resistance, sealing performance, motion type, and maintenance requirements. For instance, OS&Y stems are ideal for high-pressure, large-diameter systems, while non-rising stems suit space-constrained environments. For corrosive or erosive fluids, corrosion-resistant materials such as stainless steel or alloy steel should be prioritized. A well-chosen valve stem design enhances system safety, reliability, and efficiency while reducing maintenance costs, ensuring long-term support for industrial processes.