In the industrial field, valves are indispensable and important components in fluid control systems. Whether in the petroleum, chemical, power, or water treatment industries, valves play a key role in controlling the flow, pressure, and direction of fluids. The sealing performance of valves is directly related to the safety and reliability of the entire system. Once a sealing problem occurs, it will not only lead to fluid leakage, causing waste of resources and environmental pollution, but may also trigger serious safety accidents. Therefore, it is crucial to understand and master valve sealing technology. This article will provide a detailed introduction to several common sealing forms of valves, including stem packing sealing, body cover flange sealing, seat sealing, and pipe flange sealing, as well as their characteristics and advantages in practical applications.
Valve stem packing sealing is a key technology to ensure the formation of a seal between the valve stem and the packing box. Its main function is to prevent the medium from leaking to the outside through the gap between the valve stem and the packing box. The performance of stem packing sealing directly affects the sealing effect and service life of the valve.
Valve stem packing is usually made of pure graphite or modified graphite. Graphite materials have good high-temperature resistance and chemical stability, making them ideal sealing materials. There are various cross-sectional shapes of packing, including square, rectangular, V-shaped, and parallelogram. In practical applications, square and rectangular packing are more common because they have less influence on the lateral stress under the action of axial load and can better maintain the sealing performance.
There are two important performance indicators for valve stem packing: reducing the friction coefficient and increasing the pressure coefficient. Pure graphite packing needs to be specially modified to achieve these goals. The modification methods include changing the structure of graphite and adopting different processing techniques. Through these modifications, the packing can reduce the wear of the valve stem and extend the service life of the valve while ensuring the sealing performance.
In practical applications, graphite compression packing rings are usually not used alone, but in combination with braided packing. This combination can effectively prevent the graphite ring from being "extruded" under the action of medium pressure. The braided packing mainly plays the role of preventing the graphite compression packing ring from being extruded, thereby improving the reliability of the seal. For example, in high-temperature valves, pure graphite compression rings are usually used as the main seal body, with metal wire-reinforced graphite braided packing added above and below. This structural form performs well in high-temperature environments.
Body cover flange sealing is an important part of valve sealing, mainly responsible for the seal between the valve body and the valve cover. This sealing form is very similar to the conventional forced flange sealing, which ensures the contact pressure between the sealing surfaces through the pre-tightening force of bolts to achieve the sealing effect. Body cover flange sealing is particularly important in high-pressure valves because it can increase the sealing working stress with the increase of medium pressure, thereby maintaining good sealing performance.
The flat sealing of the body cover flange usually adopts the MF flange form. In this design, the valve body uses a concave flange (FM), while the valve cover uses a convex flange (M). The combination of concave and convex surfaces provides good sealing performance, especially in high-pressure environments. The design of this structure ensures that the sealing performance remains stable even when the medium pressure changes.
When the flat sealing of the body cover flange adopts the MF flange form, common gaskets include metal wound gaskets, corrugated gaskets, pure graphite gaskets, and graphite reinforced gaskets. The selection of these gaskets depends on the working conditions, medium characteristics, pressure, and temperature ratings. For example, in high-temperature and high-pressure conditions, metal wound gaskets and corrugated gaskets are more ideal choices because they have good high-temperature resistance and anti-squeeze performance.
There is also a form of self-tightening sealing for body cover flanges, similar to the sealing ring of the Wood type sealing. This sealing ring, under the action of medium pressure and the initial pre-tightening force of the initial bolt load, increases its sealing working stress with the increase of medium pressure. This self-tightening sealing form is particularly suitable for high-pressure valves because it can automatically adjust the sealing stress when the medium pressure changes, ensuring the stability of the sealing performance.
Valve seat sealing is another important part of valve sealing, whether it is a gate valve, globe valve, butterfly valve, ball valve, etc., there is seat sealing. The performance of seat sealing directly affects the sealing effect and service life of the valve.
For gate valves and globe valves, the sealing between the valve disc and the seat is usually achieved by welding relevant sealing materials. This sealing method can provide good sealing performance and is suitable for various media and working conditions.
The seat sealing of ball valves is relatively special, and it mainly consists of two parts: one is the sealing between the seat and the valve core, and the other is the sealing between the seat and the valve body. The sealing material between the seat and the valve core can be realized by various polymer materials, or by welding some hard alloys on the seat. This design enables ball valves to maintain good sealing performance even in high-pressure and high-temperature environments.
The sealing working stress of valves can be provided in various ways, including the use of springs, Belleville springs, and flange tightening bolts. For the sealing between the seat and the valve body, metal C-rings or spring-energized rings can be used. Spring-energized rings can also release the pressure in the cavity in the opposite direction when the machine is shut down or needs to be depressurized, thereby improving the safety of the system. In addition, rubber O-rings can also be used for the sealing between the seat and the valve body, and the material selection needs to be determined according to the specific working conditions.
Pipe flange sealing is the last link in valve sealing, and it mainly adopts three ways of surface sealing, line sealing, and quick-sealing, playing a vital role in the sealing of the valve pipe flange.
Surface sealing is the sealing of the contact surface between the gasket and the valve flange. It has good sealing performance and can effectively prevent the leakage of the medium. Surface sealing is achieved through the gasket between the flanges, and the gasket materials can be flexible graphite, rubber, polytetrafluoroethylene, and other materials with good sealing performance. Surface sealing is widely used in the connections of valves, pipes, and equipment, and is a common sealing method.
Line sealing, also known as linear sealing, is a sealing method where the contact between the gasket and the valve flange is linear. It has high reliability and stability and can maintain good sealing performance under harsh conditions such as high temperature and high pressure. Line sealing is mainly achieved by using metal sealing rings, spring-energized seals, etc. In the valve industry, the application of line sealing is becoming more and more widespread and has become the preferred sealing method for many valve products.
Quick-sealing is a sealing method for rapid connection and disassembly, suitable for occasions where the sealing element needs to be frequently replaced. It is mainly realized through quick connection mechanisms such as threads and snap rings, and has the characteristics of easy disassembly and good sealing performance. Quick-sealing is widely used in the sealing of valves, pipes, containers, and other equipment, especially suitable for occasions where quick maintenance and replacement of sealing elements are required.
The sealing performance of valves is one of the key factors to ensure their normal operation. By reasonably selecting and applying sealing forms such as stem packing sealing, body cover flange sealing, seat sealing, and pipe flange sealing, the sealing performance and service life of valves can be effectively improved. In practical applications, it is necessary to select appropriate sealing materials and structural forms according to the specific working conditions and medium characteristics to ensure the reliable operation of valves in various complex environments. With the continuous development and progress of sealing technology, the sealing performance of valves will be further improved, providing strong support for the safe and efficient operation of industrial production.