Upper Sealing of Globe Valves and Its Optimization

Globe valves are important fluid control devices widely used in various piping systems. Their main function is to regulate or stop the flow of fluid. The sealing performance of these globe valves directly affects their efficiency and lifespan, making the design and application of the upper sealing device particularly crucial. The upper sealing device, commonly referred to as the inverted seal, effectively prevents fluid leakage into the packing chamber while ensuring the normal operation of the globe valve.

Working Principle of the Upper Sealing Device

When the globe valve is fully open, the upper sealing device creates an effective barrier that prevents fluid from leaking into the packing chamber. In the closed position, the flow direction of the fluid is from below the valve disc to above it, resulting in lower pressure in the valve body cavity. In this scenario, the pressure on the packing is less than the working pressure, which helps reduce the burden on the packing, prolong its lifespan, and lower the risk of external leaks.

1. Mechanism to Prevent Leakage

The upper sealing device is specially designed to prevent the working pressure from acting directly on the packing when the valve is open. This design not only extends the lifespan of the sealing material but also reduces the probability of external leakage. Particularly, after long-term use, issues arising from the aging or wear of the sealing materials can be effectively managed through the upper sealing design.

2. Advantages for Maintenance

In practical applications, if the packing leaks, the valve can be fully opened, allowing the upper sealing device to function and preventing fluid from leaking outside. In this case, operators can loosen the packing gland or packing nut, add more packing, and re-tighten the gland to ensure the sealing performance of the valve. This design makes maintenance more convenient and lowers repair costs.

Challenges in Extending Sealing Lifespan

Despite the many advantages of the upper sealing design in globe valves, there are still challenges in actual operation. When the globe valve is closed, a certain amount of pressure must be applied to ensure that the sealing surfaces do not leak. The resistance that needs to be overcome during operation includes the friction between the valve stem and packing as well as the thrust created by the fluid pressure. Since the force required to close the valve is typically greater than that to open it, the valve stem's diameter must be designed sufficiently large to prevent bending failures.

Wear of Sealing Surfaces

The unique structure of globe valves can cause relative sliding between sealing surfaces during opening and closing, leading to wear and scratching of the sealing components. In practice, the upper seal of the valve may leak after only a few cycles of opening and closing. Minor scratches may require disassembly for repair, while severe damage could lead to valve failure, increasing manufacturing costs and affecting valve performance.

Solutions and Improvements

To enhance the lifespan of the upper seal in globe valves, various experiments and technological improvements have been conducted within the industry. Here are some common solutions.

1. Adjusting the Upper Sealing Area

One approach is to move the upper sealing area of the valve stem to above the valve disc to ensure that the upper sealing surface on the valve disc remains stationary and free from sliding during the application of force. While this improvement increases the lifespan of the upper seal, it complicates the structure due to the additional sealing between the valve cover and disc, resulting in a relatively low cost-performance ratio.

2. Changing Sealing Materials

Another method is to weld wear-resistant hard alloy materials, such as Stellite, onto the sealing taper of the valve stem. This method improves sealing performance to some extent, but the high cost of hard alloys and the complexity of welding processes (which require preheating and insulation) pose challenges. Additionally, ensuring the dimensional accuracy of the valve stem may necessitate adding rough machining processes, further increasing production costs.

3. Using New Sealing Materials and Technologies

In recent years, advancements in material science have led to the development of new sealing materials and technologies. For example, using high-performance sealing materials like polytetrafluoroethylene (PTFE), which boasts excellent wear resistance and corrosion resistance, can significantly enhance the durability of seals. Furthermore, improving the design of the valve stem and disc to stabilize their relative motion can reduce friction and extend sealing lifespan.

Conclusion

The upper sealing device of globe valves is vital for prolonging their lifespan and improving reliability. While improvements in sealing structures and materials can help reduce leakage and extend service life, it is essential to consider cost-effectiveness and manufacturing complexity when selecting materials and designs. Through continuous technological innovation and optimization, the sealing performance of globe valves is expected to improve further, providing more reliable solutions for various fluid control systems.