External Sealing of Valves: Key Aspects and Applications

External sealing is a crucial component in ensuring a valve maintains its sealing performance during operation. Typically, external sealing consists of seals at the bonnet and stem, with the bonnet seal being particularly important. Seals at the bonnet can be divided into two main types: forced sealing and pressure sealing. These types of external sealing for valves differ significantly in their structural characteristics, working principles, and applicable ranges. The following will explore the characteristics and applications of these two sealing methods in detail.

Forced Sealing

 
Forced sealing achieves sealing through the application of mechanical clamping force, primarily relying on tightening bonnet bolts to apply preload to the seal gasket and fill small gaps on the sealing surface. This sealing method is suitable for medium to low-pressure systems and medium to small-diameter valves, offering advantages of simple structure and good sealing performance. However, its limitation is that the sealing performance may decrease under high pressure.

1. Structural Characteristics of Forced Sealing

 
Forced sealing uses mechanical force to ensure sealing effectiveness. The core of this method is applying preload to the seal gasket by tightening the bonnet bolts. When compressed, the gasket fills the small gaps on the sealing surface, creating an initial seal known as preload. Common structures for forced sealing include:
 
Flat Gasket Seals: Simple and commonly used, suitable for low-pressure applications.
Spiral Wound Gasket Seals: Made from metal strips and filler, offering good sealing and high-temperature resistance.
Grooved Gasket Seals: Provides strong sealing effects with a grooved structure to enhance stability.
These sealing methods are mainly used for medium to low-pressure and medium to small-diameter valves, suitable for industrial applications where sealing requirements are not extreme.

2. The Working Principle of Forced Sealing

 
In forced sealing, the gasket will rebound under medium pressure. As the medium pressure increases, the preload on the sealing surface decreases. However, if the gasket has sufficient rebound force, the working sealing preload can still be higher than the medium pressure, maintaining a good sealing state.
 
A notable feature of forced sealing is that increasing medium pressure reduces the preload on the sealing surface, which may lead to a decrease in sealing performance under high medium pressure. To maintain effective sealing, the gasket’s rebound force must be sufficient to accommodate pressure changes.

3. Application Areas of Forced Sealing

 
Forced sealing is primarily used in medium to low-pressure systems and medium to small-diameter valves, such as:
 
Chemical Industry: Provides stable sealing for handling chemicals and medium to low-pressure media.
Water Treatment Systems: Effectively prevents leakage in medium-pressure water flow processing.
HVAC Systems: Meets conventional sealing needs for medium to small-diameter valves.
In these applications, the structure and working principle of forced sealing ensure the sealing performance of the system and maintain normal valve operation. Understanding the structural characteristics, working principles, and application areas helps achieve the best sealing effect.

Pressure Seal

 
Pressure seal is a high-pressure sealing technology that enhances sealing performance by increasing medium pressure. The core structure is typically a wedge-shaped gasket combination. In this sealing method, the initial seal is provided by the preload created by tightening bolts. As medium pressure increases, the bonnet moves upward, increasing the sealing preload on the sealing surface and thus improving sealing performance.

1. Pressure Seal Structural Features

 
Pressure seal is a high-pressure sealing technology, typically used in high-temperature and high-pressure conditions, such as high-temperature and high-pressure forged steel gate valves. The typical structure of pressure seal is a wedge-shaped gasket combination, where an initial sealing state is formed by tightening bolts. As medium pressure increases, the bonnet moves upward under the pressure, enhancing the sealing preload on the sealing surface.
 
The advantage of pressure seal is that increasing medium pressure enhances sealing performance because the working sealing preload on the sealing surface is a combination of preload and medium pressure, with medium pressure directly improving sealing performance.
 
To maintain good sealing performance at high temperatures, pressure seal often features austenitic stainless steel or hard alloy overlays on the sealing surfaces of the valve body and bonnet, creating a high-temperature and corrosion-resistant coating. This coating is usually 2 mm thick to improve hardness, corrosion resistance, and erosion resistance. Additionally, the choice of wedge-shaped gasket material is crucial, typically using materials with surface hardness lower than the valve body and bonnet sealing layers to avoid damaging the sealing surface.

2. Pressure Seal Working Principle

 
The working principle of pressure seal involves increasing medium pressure, which enhances the sealing preload on the sealing surface. Specifically:
 
Pressurization Stage: Initial sealing state is formed by tightening bolts.
High-Pressure Stage: Medium pressure pushes the bonnet upward, increasing the sealing preload and thus enhancing sealing performance.
In high-pressure environments, pressure seal provides excellent sealing effects because increasing medium pressure automatically enhances sealing performance.

3. Pressure Seal Application

 
Pressure seal is widely used in high-pressure and high-temperature scenarios, such as:
 
Refinery Equipment: For processes such as hydrocracking, hydrotreating, and residue hydrodesulfurization.
Chemical Reactors: Handling high-pressure chemical reactions.
Power Equipment: Sealing in high-temperature and high-pressure systems.
In these scenarios, pressure seal effectively handles high-temperature and high-pressure environments, ensuring stable system operation.
 
Forced sealing and pressure sealing each suit different working environments and pressure ranges. Forced sealing is appropriate for medium to low-pressure systems and relies on mechanical clamping force to maintain sealing, while pressure sealing enhances sealing performance through medium pressure and is suitable for high-pressure and high-temperature environments. Choosing the appropriate sealing method is crucial for ensuring valve sealing performance and effectively enhancing system stability and reliability. Selecting the right sealing method based on specific operating conditions such as pressure, temperature, and medium properties helps achieve optimal sealing performance and long-term equipment stability.