Sealing Principle of Valves

Valves play a crucial role in controlling the flow of media, and ensuring a proper seal is essential to prevent leakage. The sealing technology of valves has been developed significantly over time, and valve sealing technology is critical for both valve performance and quality.

Types of Valve Seals

There are two main types of valve sealing: static sealing and dynamic sealing.

1. Static Seal

Static sealing refers to the seal formed between two static surfaces. This seal is typically achieved using gaskets, which come in various types:

• Flat Gasket: Flat gaskets are placed between two static sections. They can be made from plastic, rubber, metal, or composite materials, with each material offering different application ranges.
• O-Ring: O-rings have a circular cross-section and provide a self-tightening effect, making them more effective than flat gaskets.
• Packing Washer: This washer wraps a material around another, enhancing elasticity and sealing effectiveness.
• Shaped Washer: Specially shaped washers, such as oval, diamond, gear, and dovetail washers, are used in high-pressure applications and provide a self-tightening effect.
• Wave Washer: These washers, typically made from a combination of metal and non-metal materials, provide a good sealing effect with low compression force.
• Wrap Washer: Made from thin metal and non-metallic materials wound together, wrap washers offer excellent elasticity and sealing properties.

Materials for Gaskets

Gaskets are generally made from three types of materials: metal, non-metallic, and composite. Metal materials, such as copper, aluminum, and steel, are strong and offer high temperature resistance. Non-metallic materials, including plastics, rubber, and asbestos, are widely used for various applications. Composite materials are typically used in wave and wrap washers, offering unique benefits depending on the specific needs.

2. Dynamic Seal

Dynamic sealing prevents the leakage of media through the valve stem during movement. This type of sealing is crucial during the relative movement of the valve stem, and is achieved using a stuffing box. There are two main forms of stuffing boxes:

• Gland Type: Commonly used and consists of a cap, which may be either combined or integrated. This type includes compression bolts.
• Compression Nut Type: Typically used in smaller valves.

Stuffing Box Fillers

The packing material in the stuffing box is in direct contact with the valve stem. To ensure sealing, the packing must have low friction, resistance to pressure and temperature, and corrosion resistance. Common fillers include rubber O-rings, braided coils, asbestos coils, and plastic molding fillers. The choice of filler depends on the specific operating conditions.

Factors Affecting Valve Sealing

The main factors influencing valve sealing are:

1. Gap Between Sealing Surfaces: The gap between the sealing pair is a critical factor affecting sealing performance.
2. Pressure Difference: A pressure difference across the sealing pair also plays a significant role in sealing effectiveness.

Sealing is designed to prevent leakage, and the principle of valve sealing can be analyzed from four aspects: sealing of liquids, sealing of gases, leakage channel principles, and sealing pair performance.