Valve Stem Sealing: Principles, Materials & Applications

In industrial production, valves, as a key fluid control device, have their sealing performance directly related to the safe operation of process pipelines. Valve stem sealing, as an important part of valve sealing, has a crucial impact on the overall performance of the valve. This article will delve into the three main structural forms of valve stem sealing, the selection methods for packing materials, and the precautions in practical applications, helping readers gain a comprehensive understanding of valve stem sealing technology.

In the field of industrial valves, valve stem sealing is a critical link in ensuring the sealing performance of valves. There are various structural forms of valve stem sealing, but the three most common ones are: external compression soft packing sealing structure, internal compression soft packing sealing structure, and bidirectional compression soft packing sealing structure. Each of these structures has its own characteristics and is suitable for different working conditions and types of valves. Below, we will introduce these three main valve stem sealing structures one by one.

The external compression soft packing sealing structure is one of the most common forms of valve stem sealing. Its working principle is to compress the packing from the outside using a gland, generating radial contact pressure between the soft packing and the valve stem, thereby achieving sealing. The advantages of this structure include relatively simple manufacturing, processing, and maintenance, as well as lower costs. It is suitable for medium and low-pressure valves, such as wafer-type soft-seated butterfly valves.

However, this structure also has some drawbacks. Since the radial pressure of the packing is not evenly distributed along the axial direction, the bottom layer of packing in contact with the medium experiences the least radial pressure, which may lead to poor sealing performance. Moreover, if the compression force on the packing is excessively increased, although the sealing effect can be improved, it will significantly increase the opening and closing torque of the valve stem, exacerbate the wear of the valve stem, and thus affect the service life of the valve.

The main difference between the internal compression soft packing sealing structure and the external compression structure lies in the direction of packing compression. In this structure, the gland compresses the packing from the inside, allowing the packing near the medium side to obtain a greater radial contact force. This design ensures that the radial contact pressure and the leakage pressure of the medium attenuate in the same direction. As long as the radial contact pressure on each cross-section of the packing is slightly higher than the leakage medium pressure at that location, a good sealing effect can be achieved.

The internal compression structure is usually used for high-pressure valves. Its advantages include a smaller required compression force, fewer packing rings, which in turn result in a smaller opening and closing torque of the valve stem and less wear between the valve stem and the packing. These features give the internal compression structure a significant advantage in high-pressure working conditions.

The bidirectional compression soft packing sealing structure is a more advanced design. It achieves sealing by setting upper and lower packing glands to compress the packing from both the inside and the outside. This structure ensures a more uniform distribution of radial contact pressure between the packing and the valve stem along the axial direction. Each ring of packing can effectively participate in sealing, forming an expansion-convergence leakage channel.

In the bidirectional compression structure, the pressure drop of the leakage medium at both ends of the leakage channel is relatively large, which makes the sealing effect more significant. At the same time, in the middle part of the leakage channel, the contact between the packing and the valve stem is relatively loose, resulting in higher leakage, which serves as good lubrication. This can reduce the wear of the packing on the valve stem and extend the service life of the valve.

The selection of packing for valve stem sealing is one of the key factors in ensuring sealing performance. The properties of packing materials directly affect the reliability, durability, and applicability of the seal. Common packing materials can be divided into three major categories: non-metallic, semi-metallic, and metallic. Each type of material has its unique properties and application conditions.

Flexible graphite is an excellent sealing material with advantages such as high-temperature resistance, corrosion resistance, and good self-lubrication. It can withstand a temperature range from -200°C to 600°C and is suitable for working conditions with a nominal pressure not exceeding 32 MPa. Flexible graphite can almost withstand all media, except for strong oxidizing media such as fuming sulfuric acid and concentrated nitric acid. Therefore, flexible graphite is widely used in the sealing of high-pressure steam, water, and oil media.

Plastic packing mainly includes polytetrafluoroethylene (PTFE) and impregnated PTFE. These materials have excellent strong corrosion resistance and are suitable for sealing gases and highly fluid liquids. Their working temperature range is from -200°C to 200°C, with a pressure not exceeding 32 MPa. PTFE packing, due to its good chemical stability and low friction coefficient, is often used in situations where high sealing performance is required.

Asbestos fiber is a traditional sealing material with good heat resistance, capable of withstanding high temperatures up to 480°C. It has better tolerance to weak acids and strong alkalis, high strength, and good adsorption properties. However, asbestos fiber has poor resistance to strong acids, a higher friction coefficient, and poor thermal conductivity. To improve these properties, asbestos fiber is often impregnated with PTFE emulsion or lubricating grease, and metal wires are added to enhance wear resistance and heat resistance.

Carbon fiber is a high-performance sealing material with advantages such as high strength, high-temperature resistance, wear resistance, and low relative density. It also has good elasticity, softness, and self-lubrication. It can work stably in a temperature range from -120°C to 350°C and is suitable for working conditions with a pressure not exceeding 32 MPa. Carbon fiber packing, due to its excellent comprehensive properties, is gradually becoming one of the commonly used sealing materials in modern industry.

Metal packing can be divided into five types based on its structure: metal foil tape wound packing, metal foil folded and pressed packing, metal corrugated packing, lead wire twisted packing, and aluminum ring packing. Metal foil tape wound packing and metal foil folded and pressed packing have advantages such as high-temperature resistance, erosion resistance, wear resistance, high strength, and good thermal conductivity. They are suitable for high-temperature and high-pressure working conditions. Metal corrugated packing is a better-performing high-temperature and high-pressure packing, suitable for extreme working conditions. Lead wire twisted packing is mainly used for strong oxidizing media such as concentrated sulfuric acid at temperatures not exceeding 90°C. Aluminum ring packing is often used alternately with asbestos packing and is suitable for oil media at temperatures not exceeding 550°C.

Bellows sealing is a special sealing structure. By fixing one end of the bellows to the valve stem and the other end to the valve bonnet, it achieves a complete seal between the valve stem and the valve bonnet. This sealing form has extremely high sealing performance and is suitable for the sealing of highly toxic, flammable, and other hazardous media. For example, in the hydrogenation unit's carbon disulfide solution and the liquefied petroleum gas used in gas separation units, the bellows globe valve plays an important role in bellows sealing.

The practical application of valve stem sealing requires the consideration of various factors, including media conditions, working requirements, and valve types. When selecting the valve stem sealing form and packing materials, it is essential to ensure reliable sealing and economic rationality to guarantee the long-term safe operation of petrochemical installations.

The nature of the media (such as temperature, pressure, corrosiveness, etc.) is a key factor in selecting the valve stem sealing form and packing materials. For example, in high-temperature and high-pressure working conditions, metal packing or bellows sealing should be prioritized; for media with strong corrosiveness, packing materials with good corrosion resistance, such as plastic or graphite, should be chosen.

As a pressure pipeline component, the valve stem not only has to withstand the media pressure during operation but also has to deal with pressure fluctuations caused by various uncertain factors. Therefore, the valve stem should have appropriate hardness and elasticity, be able to withstand media erosion, be resistant to scratching, and have excellent machinability. In addition, to increase wear resistance and corrosion resistance, the valve stem usually needs surface treatment, such as hard chrome plating or nitriding, to avoid media leakage caused by valve stem bending, corrosion, surface tension, and other issues.

The quality of packing installation has a direct impact on sealing performance. When installing packing, attention should be paid to the number of packing layers, the uniform distribution of compression force, and the lubrication treatment of the packing. The number of packing layers should be reasonably selected based on the working conditions. Too many or too few packing layers can affect the sealing effect. The uniform distribution of compression force can be achieved through the rational design of packing glands and packing rings. At the same time, adding an appropriate amount of lubricating oil to the packing can improve the lubrication conditions between the packing and the valve stem, reducing wear.

During the use of valves, it is necessary to regularly check for leakage of the valve stem seal. If leakage is detected, repair measures should be taken promptly. Repair methods include re-tightening the packing, replacing the packing, or repairing the valve stem. For serious leakage problems, it may be necessary to replace the entire valve stem sealing component.

Valve stem sealing technology plays a vital role in industrial production. By reasonably selecting the valve stem sealing structure and packing materials, and paying attention to installation and maintenance, the sealing performance and service life of valves can be effectively improved. In practical applications, it is essential to consider the reliability, economy, and safety of sealing based on specific media conditions, working requirements, and valve types, to ensure the long-term safe operation of petrochemical installations. With the continuous development of material science and sealing technology, valve stem sealing technology will also keep advancing, providing more reliable guarantees for the safety and efficiency of industrial production.