Control Valve Sealing Technology: Principles & Materials

In industrial production, control valves are key devices for regulating fluid flow and pressure. The quality of the sealing performance directly determines the reliability and service life of the control valve. This article will delve into the common forms of control valve sealing, material selection, and considerations in practical applications, helping readers better understand and choose suitable sealing solutions.

The plunger valve core is one of the common sealing forms in control valves. Its sealing surface is a 60° small cone, and the valve seat is also a 60° small cone, with the cone width typically ranging from 0.5mm to 2mm. This face-to-face sealing method can theoretically achieve good sealing effects, but in practice, there are many issues.

Firstly, machining errors are the main factors affecting the sealing effect of the plunger valve core. Errors such as concentricity, roundness, and tilt can prevent the two cones from fully mating, resulting in leakage. Even with precision grinding, the leakage rate can only reach the 10^-6 level, making it difficult to achieve zero leakage. Additionally, the plunger valve core is prone to erosion and wear from the medium during use, further reducing sealing performance. Therefore, although the plunger valve core sealing is structurally simple, it has significant shortcomings in terms of sealing effect and reliability.

To overcome the limitations of plunger valve core sealing, spherical sealing has emerged. The core of spherical sealing lies in the tangential contact between the spherical surface of the valve core and the small cone of the fixed valve seat, forming a line contact. This line contact method significantly reduces the contact area of the sealing surface, thereby improving sealing performance. Control valves using spherical sealing, such as fully functional ultra-light valves and spherical sealing butterfly valves, can achieve leakage rates in the range of 10^-6 to 10^-8. High-performance triple-eccentric butterfly valves can even achieve zero leakage.

The advantage of spherical sealing lies in its unique structural design. During rotation, the spherical valve core can automatically adjust its contact position with the valve seat, ensuring that the sealing surface maintains good contact. Even with fluctuations in medium pressure or temperature changes, spherical sealing can maintain stable sealing performance. Additionally, the machining precision requirements for spherical sealing are relatively low, making it easier to achieve high-precision machining and assembly. Therefore, spherical sealing has been widely used in modern control valves, especially in applications with high sealing performance requirements.

In addition to plunger valve core sealing and spherical sealing, soft sealing and hard sealing are also common sealing forms in control valves. They each have different characteristics and applicable ranges.

Soft sealing refers to the sealing form where at least one of the valve core or valve seat is made of non-metallic materials (such as polytetrafluoroethylene (PTFE), rubber, etc.). Soft sealing materials have good elasticity and adaptability, allowing them to tightly mate with metal sealing surfaces, thereby achieving the best sealing effect. Under ideal conditions, soft sealing can achieve zero leakage, making it widely used in applications with extremely high sealing performance requirements.

However, soft sealing also has obvious disadvantages. Firstly, the wear resistance and corrosion resistance of soft sealing materials are relatively weak. In media containing particles, soft sealing is easily scratched, leading to a sharp increase in leakage. Additionally, in high-temperature, high-pressure, or highly corrosive media, the performance of soft sealing materials will significantly deteriorate. Therefore, when choosing soft sealing, the cleanliness of the medium and the system's flushing conditions must be considered. During pipeline installation and system cleaning, it is essential to avoid the ingress of foreign matter into the control valve to ensure the reliability of soft sealing.

Compared to soft sealing, hard sealing has stronger durability and reliability. Hard sealing is usually made of metallic materials (such as copper alloys, hard alloys, etc.) and improves the wear resistance and corrosion resistance of the sealing surface through processes such as hardfacing with wear-resistant alloys. Although the leakage rate of hard sealing cannot reach the zero-leakage level of soft sealing, it can generally meet the requirements for strict shutoff, with factory specifications typically in the range of 10^-6 to 10^-8.

The advantage of hard sealing lies in its good wear resistance and corrosion resistance. Even in media containing particles or harsh working environments, hard sealing can maintain stable sealing performance. Additionally, hard sealing has a longer service life, which is more cost-effective from an economic perspective. For example, the leakage rate of hard sealing in fully functional ultra-light valves can reach 10^-7 to 10^-8, and triple-eccentric butterfly valves can even achieve zero leakage. Therefore, hard sealing should be preferred when conditions permit.

The selection of sealing materials is crucial for the sealing performance of control valves. Different sealing materials have different physical and chemical properties and are suitable for different operating conditions. Here are some common sealing materials and their characteristics.

Copper Alloys: Copper alloys have good corrosion resistance and wear resistance in water or steam but have low strength and are not resistant to ammonia and ammoniacal corrosion. They are suitable for applications with medium temperatures ≤250℃.

Babbitt Metal: Babbitt metal is resistant to ammonia and ammoniacal corrosion, has a low melting point and low strength, and is suitable for ammonia valves with temperatures ≤70℃ and pressures ≤160 kg.

Carbon Steel: Carbon steel has good corrosion resistance and is commonly used in non-corrosive media such as water, steam, and oil products, with working temperatures ≤425℃. However, it has poor resistance to scoring, especially under high specific pressure.

Hard Alloys: Hard alloys (such as cobalt-based hard alloys) have advantages in corrosion resistance, wear resistance, and resistance to scoring, and can maintain sufficient hardness even at high temperatures. Their operating specific pressure is 800-1000 kg, applicable temperature range is 196-650℃, and can reach up to 816℃ in special cases. However, hard alloys are not corrosion-resistant in sulfuric acid, high-temperature hydrochloric acid, and some chlorides.

Polytetrafluoroethylene (PTFE): PTFE has excellent corrosion resistance and lubricity and is suitable for chemical media such as acids and alkalis.

Polyethylene (PE), Polypropylene (PP): Suitable for simple media and low-temperature environments.

Rubber: Such as nitrile rubber (NBR), silicone rubber, etc., suitable for low-temperature applications requiring good sealing.

Composite sealing materials combine the advantages of metallic and non-metallic materials, providing higher sealing performance under different conditions. For example, combinations of metals with PTFE or other elastic materials can ensure sealing effects in high-temperature, high-pressure, or complex operating conditions.

In practical applications, to further improve the sealing performance of control valves, it is often necessary to optimize the sealing structure. Here are some common optimization methods:

For shut-off control valves handling high-temperature media, temperature changes can cause thermal expansion of the sealing surface, altering the sealing specific pressure established during assembly. If it is too tight, the sealing surface will be stuck, and severe scoring will occur during movement; if it is too loose, the sealing pressure will be insufficient. Both situations will lead to a sharp increase in leakage. The best solution to this problem is to use elastic valve seats to absorb the differences caused by temperature changes. Common elastic solutions include:

Spring Type: Compensate for thermal expansion through the elasticity of springs to maintain the contact pressure of the sealing surface.

Thin Flexible Metal Ring: Utilize the elastic deformation of the flexible metal ring to adapt to thermal expansion.

Combination of Rigid and Flexible Elastic Valve Seat: Combine the advantages of flexible and rigid materials, with flexibility to overcome thermal expansion and rigidity with hardfacing of wear-resistant alloys to improve the reliability of the wear-resistant surface.

For closing parts with low surface hardness, such as the ball in a ball valve, surface treatment methods can be used to increase surface hardness and wear resistance. Common surface treatment methods include:

Hard Chrome Plating: Plate a layer of hard chrome on the part surface to increase surface hardness and wear resistance.

Electroless Nickel Plating: Plate a layer of nickel-phosphorus alloy on the part surface through a chemical reaction to increase surface hardness and corrosion resistance.

Nickel-Phosphorus Alloy Plating: Similar to electroless nickel plating but with different plating compositions, offering better wear resistance and corrosion resistance.

Nitriding: Form a nitride layer on the part surface through nitriding treatment to increase surface hardness and wear resistance.

Multi-Element Composite Nitriding: Combine multiple nitriding processes to further increase surface hardness and wear resistance.

Control valve sealing technology is an indispensable part of industrial production. By deeply understanding the principles and characteristics of plunger valve core sealing, spherical sealing, soft sealing, and hard sealing, and reasonably selecting sealing materials and structures, while paying attention to details such as medium characteristics, actuator matching, and system flushing in practical applications, the sealing performance and service life of control valves can be effectively improved. It is hoped that this article will help readers better understand and apply control valve sealing technology, providing a strong guarantee for the efficient operation of industrial production.