Two Types of Sealing Methods for Plug Valves
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Plug valves are widely used in fluid control systems, operating by rotating the plug to control the flow of the medium. Based on different sealing methods, plug valves are categorized into two types: packing type and oil-sealed type. Each of these sealing methods has unique characteristics, making them suitable for specific operating conditions. This article explores the structural features, working principles, applications, and advantages and disadvantages of these two common plug valve sealing methods.
Packing Type Plug Valve
The packing type plug valve is a classic sealing design widely used in low- to medium-pressure conditions due to its simple structure and ease of maintenance. The sealing effect is achieved through the mechanical compression of the packing, making it ideal for systems requiring high sealing performance at relatively low pressures.
1. Structural Features
In packing type plug valves, the seal between the plug and the valve body is achieved by compressing the packing material. The packing is housed in the valve’s packing chamber, and external force applied to the packing gland tightens the packing against both the plug and the valve body, forming an effective sealing layer. Since the plug does not extend outside the valve body, the design minimizes the risk of media leakage.
2. Working Principle
The sealing performance of this valve relies on the mechanical compression force between the packing, valve body, and plug. When the valve is closed, the pressure of the medium helps further compress the packing, enhancing the sealing effect. In high-pressure or dynamic conditions, when the packing chamber wall thickness exceeds 8 mm, holes can be drilled in the chamber to inject sealant for repairs. The injected sealant serves the same function as the packing, increasing sealing reliability under high-pressure conditions.
3. Applications
Packing type plug valves are commonly used in low- to medium-pressure systems, especially those with pressure ratings less than or equal to 1 MPa. Due to their simple design and easy maintenance, they are prevalent in industries like oil, natural gas, and chemicals. These valves are ideal for systems requiring high sealing but operating at lower pressures.
4. Advantages and Disadvantages
The advantages of packing type plug valves include their simple structure, reliable sealing performance, and low cost. They perform stably in low- to medium-pressure environments and are easy to maintain, particularly under dynamic conditions where sealant injection allows for on-site repairs.
However, in high-temperature, high-pressure, or highly corrosive environments, their sealing performance may be insufficient. This is because the physical properties of the packing can change with temperature and pressure, leading to potential sealing failures. Additionally, wear and aging of the packing are common issues during prolonged use.
Oil-Sealed Plug Valve
Oil-sealed plug valves rely on forced lubrication to form an oil film between the plug and valve body, achieving efficient sealing. By injecting sealant, the valve remains lubricated during rotation, reducing opening and closing resistance and preventing wear. Compared to packing type, oil-sealed valves are suitable for high-pressure, high-temperature, and corrosive media environments, ensuring reliable long-term operation.
1. Structural Features
The oil-sealed plug valve achieves sealing through a forced lubrication system. An oil film forms between the plug and valve body from the injected sealant. These valves have oil grooves that store sealant, which is injected through bolts or an oil gun into the check valve inside the plug stem. The sealant then reaches the contact surface between the plug and the valve body, creating a lasting seal. As the valve rotates, the sealant in the grooves is automatically replenished, keeping the sealing surfaces lubricated.
2. Working Principle
Oil-sealed plug valves use the oil film created by the sealant between the plug and valve body for sealing. The sealant not only lubricates but also continually replenishes during operation, ensuring a tight fit between the plug and valve body. Even under high pressure, the oil film effectively reduces wear, maintaining sealing performance over extended periods.
When the pre-tightening force is insufficient, and the sealing effect is not optimal, injecting additional sealant or tightening the packing gland bolts can enhance the pre-tightening force to ensure proper sealing.
3. Applications
Oil-sealed plug valves are particularly suited for conditions requiring high sealing and lubrication, excelling in environments like oil fields and gas fields that contain abrasive media. These valves perform well in media containing sand, water, and other impurities, making them widely used in main pipelines of the oil and gas industries. Due to their lubricating and sealing properties, they are ideal for abrasive media transportation but are not suitable for throttling operations, as the sealant can be quickly lost, affecting the sealing performance.
4. Advantages and Disadvantages
The key advantage of oil-sealed plug valves is their excellent sealing performance. The lubrication system not only reduces frictional wear but also protects the sealing surfaces from media corrosion. The sealing surfaces are isolated from the medium when fully open, reducing wear. Additionally, the lubrication system allows for easy operation under high-pressure conditions.
However, the complex structure of oil-sealed plug valves increases maintenance difficulty, and the sealant requires regular replenishment. If the sealant is depleted or not properly injected, the sealing performance can degrade significantly. Furthermore, these valves are unsuitable for throttling operations, limiting their range of applications.
Comparison and Conclusion
A detailed analysis of the packing type and oil-sealed plug valves reveals clear differences in their sealing principles, applications, and performance. The following comparison highlights their strengths and weaknesses based on sealing methods, working conditions, and maintenance requirements.
1. Sealing Principle Comparison
The packing type plug valve relies on mechanical compression of the packing to achieve sealing, making it suitable for low- to medium-pressure conditions with a simpler structure. On the other hand, the oil-sealed plug valve uses an oil film formed by injected sealant for sealing, providing better lubrication and sealing performance, especially in high-pressure and abrasive media environments.
2. Application Comparison
Packing type plug valves are more suited for low- to medium-pressure conditions, such as standard oil and gas pipelines. Oil-sealed plug valves excel in high-pressure or abrasive media environments, such as oil fields and gas fields, and perform particularly well in pipelines containing sand and water.
3. Maintenance and Cost Comparison
The simple structure of packing type plug valves makes them easy to maintain and suitable for systems requiring frequent repairs. However, the packing must be periodically replaced due to wear and aging. Oil-sealed plug valves, while offering superior lubrication and sealing, have higher maintenance costs due to their complex structure and the need for regular sealant replenishment.
Both types of plug valves offer distinct sealing advantages. Packing type valves are ideal for low- to medium-pressure applications, while oil-sealed valves are best for high-pressure and abrasive media environments. Selecting the appropriate valve type based on operating conditions can enhance system sealing performance, extend valve life, and ensure stable and safe operation.
