The Sealing Principles of the Valve (Part One)

The Sealing Principles of the Valve (Part One)

There are many types of valves, but the basic function is the same, that is, connecting or cutting off the medium flow. Therefore, the sealing problem of the valve is very prominent. To ensure that the valve can cut off the flow of the medium well without leakage, it is necessary to make sure that the valve is well sealed. There are many reasons for valve leakages, including unreasonable structural design, defective sealing contact surfaces, loosening of fastening parts and inadequate fit between the valve body and bonnet. All these problems may lead to improper sealing of the valve, resulting in leakage problems. Therefore, the valve sealing technology is an important technology related to valve performance and quality, which requires in-depth and systematic research.
From the beginning of the valve to the present, its sealing technology has also experienced great development. So far, the valve sealing can be classified into two types, namely static sealing and dynamic sealing. The so-called static sealing usually refers to sealing between two static surfaces, and sealing methods of static sealing mainly adopt gaskets. The so-called dynamic sealing mainly refers to sealing of the valve stem, that is, preventing the medium in the valve from leaking with the movement of the valve stem. The sealing method of the dynamic seal mainly uses a stuffing box.
1. Static sealing
Static sealing refers to the formation of a seal between two static sections. The sealing method is mainly to use gaskets. There are many types of gaskets. Frequently used gaskets include flat gaskets, O-rings, wrap gaskets, special-shaped gaskets, wave gaskets and spiral wound gaskets. Each type can be further divided according to the different materials used.
Flat gaskets
Flat gaskets are attached between two static sections. Generally, they can be divided into a plastic flat gasket, a rubber flat gasket, a metal flat gasket and a composite flat gasket according to the used materials. Each flat gasket with a different material has its own scopes of applications.
O-rings refer to a gasket whose cross-section has the O shape. Because their cross-sections have the O shape, they have a certain self-tightening effect. Therefore, the sealing effect of the O ring is better than that of a flat gasket.
Wrap gaskets
Wrapped gaskets refer to a gasket that wraps a certain material on another material. Such gaskets generally have good elasticity and can enhance the sealing effect.
Special-shaped gaskets
Special-shaped gaskets are those with irregular shapes, including oval gaskets, diamond gaskets, gear gaskets and dovetail gaskets. These gaskets generally have a self-tightening effect and are mostly used in high and medium pressure valves.
Wave gaskets
Wave gaskets are gaskets that only have a wave shape. Such gaskets are usually composed of a combination of metal and non-metal materials, and generally have the characteristics of small pressing force and good sealing effect.
Spiral wound gaskets
Spiral wound gaskets refer to a gasket formed by tightly adhering a thin metal belt and a non-metallic belt together and winding. This type of gasket has good elasticity and sealing.
The materials for gaskets mainly include three major categories, namely metallic materials, non-metallic materials and composite materials. Generally speaking, metal materials have high strength and high temperature resistance. Commonly used metal materials are copper, aluminum and steel. There are many types of non-metallic materials, including plastic, rubber, asbestos and hemp products. These non-metallic materials are widely used and selected according to specific needs. There are also many types of composite materials, including laminates and composite panels, which are also selected according to specific needs. Generally, wave gaskets and spiral wound gaskets are used more.
2. Dynamic sealing
Dynamic sealing refers to the sealing that prevents the medium flow in the valve from leaking with the movement of the valve stem. This is a sealing problem during relative movement. There are two basic forms of stuffing boxes, namely the gland type and compression nut type. The gland type is currently the most used form. Generally speaking, the gland type can be divided into two types, namely combined types and integral types. Although each type is different, it basically contains bolts for pressing. The compression nut type is generally used for small valves. Because of the small size of this type, the compression force is limited.
In the stuffing box, since the packing is in direct contact with the valve stem, the packing is required to have good sealing performance, low friction coefficient, ability to adapt to pressure and temperature of the medium and corrosion resistance. At present, the more commonly used fillers include rubber O-rings, PTFE braided packing, asbestos packing and plastic molding packing. Each kind of packing has its own conditions and scopes, and is selected according to specific needs.
Sealing is to prevent leakages, so the principle of valve sealing is also studied from leakage prevention. There are two main factors that cause leakages. One is the most important factor affecting the sealing performance, that is, there is a gap between the seal pairs. The other is the pressure difference between the two sides of the seal pair. The principles of valve sealing are also analyzed from four aspects: liquid sealing, gas sealing, sealing principles of leakage channels and valve sealing pairs.