Selecting and Ensuring Zero Leakage in Control Valves

 
Control valves play a crucial role in industrial systems, but they often face issues such as poor sealing performance and significant leakage in practice. These problems usually stem from structural defects and improper selection of control valves, rather than just issues like clogging or insufficient pressure differential. With modern industry’s increasing demands for valve performance, especially the strict requirement for zero leakage, achieving this standard has become a necessary function and quality criterion. The following key standards should be considered during valve selection to enhance sealing performance and reliability.

Selection of Valve Structure

 
When choosing the structure of a control valve, it is essential to consider the advantages and disadvantages of different types. The structure impacts not only the sealing performance but also the valve’s reliability and maintenance complexity.

1. Dual Sealing vs. Single Sealing

 
The valve structure directly affects its sealing performance. Dual-sealing valves (e.g., double-seated valves, dual-seal sleeve valves) have historically faced issues with large leakage rates due to their complex design and multiple components. Although sleeve valves were widely used in the 1980s, their complex structure, poor reliability, and challenging maintenance led to less-than-ideal sealing performance.
 
To address these issues, single-seal valves are recommended. Single-seal valves simplify the design, improving sealing, reliability, maintenance, and parts availability. Problems related to unbalanced forces can be managed with powerful piston actuators and reinforced valve stems, which are more straightforward and effective compared to internal modifications.
 
In the 1990s, there was significant interest in simpler and more reliable cut-off valves. Rotary control valve offered superior performance compared to linear control valves (e.g., single-seated valves, sleeve valves, gate valves), leading to the development of new products like triple-eccentric butterfly valves, full-function cut-off valves, and hard-sealed cut-off ball valves.

2. Advantages of Cut-off Valves

 
Cut-off valves are designed with a focus on sealing performance and reliability, particularly in triple-eccentric butterfly valves, full-function cut-off valves, and hard-sealed cut-off ball valves. These valves provide excellent sealing performance and structural stability, meeting higher industrial standards and operating conditions.

Selection of Sealing Surfaces

 
Choosing the appropriate sealing surface is critical for the sealing performance of control valves. Different sealing designs impact leakage rates and sealing effectiveness. Here are the main types of sealing surfaces and their characteristics.

1.  Face-to-Face Sealing

Face-to-face sealing is common in plunger valve cores, where the sealing surface is typically a 60° small cone on both the valve core and seat. This design requires good contact between the two cone surfaces, but machining errors (such as concentricity, roundness, and tilt) can affect sealing effectiveness. Leakage rates for these valves are usually around 10^-4 but can reach 10^-6 with precision grinding, though zero leakage remains challenging.

2. Spherical Sealing

 
Spherical sealing involves a spherical surface on the valve core making line contact with a small cone on the fixed valve seat. This method generally offers better performance than face-to-face sealing, achieving leakage rates from 10^-6 to 10^-8. Full-function ultra-light valves and spherical-sealing butterfly valves use this design to achieve higher sealing performance, potentially reaching zero leakage.

Selection of Sealing Materials

 
The choice of sealing materials is crucial for the performance and reliability of control valves. Appropriate sealing materials can significantly enhance sealing effectiveness and durability. Common sealing materials and their characteristics include:

1. Soft Seals

 
Soft seals typically use non-metallic materials (e.g., PTFE, rubber) for sealing. These valves offer good sealing performance but are unsuitable for media with particles. Surface damage can lead to significant leakage, so hard seals are preferable when conditions allow.

2. Hard Seals

 
Hard-seal valves use wear-resistant alloys and are suitable for high-sealing applications. Hard-seal technology in full-function ultra-light valves and triple-eccentric butterfly valves can achieve leakage rates of 10^-7 to 10^-8 and even zero leakage. Hard seals offer better wear resistance and longer service life, making them the best choice for cut-off valves.

Consideration of Sealing Wear Resistance

 
For control valves requiring high sealing wear resistance, using wear-resistant alloys for welding can enhance the sealing's resistance to wear, erosion, and scratching, thereby significantly extending the valve’s service life.

1. Handling High-Temperature Media

 
For high-temperature media, the seal is assembled at room temperature, but temperature variations during operation can alter the sealing pressure established during assembly. Thermal expansion may cause the sealing surface to bind or the sealing pressure to decrease, increasing leakage. The best solution is to use elastic valve seats to absorb these changes. Common elastic solutions include spring-type, thin metal ring-type, and combined elastic valve seats, with the combined type offering the best overall performance.

2. Handling Particulate Media

 
For media containing particles, valves that prevent direct erosion from high-speed media are preferred. Linear-stroke valves and butterfly valves generally lack this feature, whereas full-function ultra-light valves and eccentric rotary valves offer better erosion protection and are suitable for such media.

Actuator Requirements

 
The actuator for a control valve should have adequate thrust and rigidity to ensure smooth valve operation. Increasing actuator thrust and rigidity can improve overall valve performance and ensure stable and reliable operation under various conditions.
 
By carefully considering these standards, you can significantly enhance the sealing performance of control valves, ensuring zero leakage across diverse operating conditions. This not only meets the modern industrial demand for high standards but also improves the overall reliability and longevity of the equipment. Understanding the characteristics and suitability of different types of control valves during selection and design will help in making the best choice.