There are many types of valves, but their basic functions are the same, that is, to connect or cut off the medium flow. Therefore, the sealing problem of valves is very prominent.

To ensure that the valve can well cut off the medium flow and no leakage occurs, it is necessary to ensure that the valve seals well. There are many reasons for valve leakage, including unreasonable structural design, defects in the sealing contact surface, loose fasteners, loose fit between the valve body and the valve cover, etc. All these problems may lead to poor valve sealing and leakage. Therefore, valve sealing technology is an important technology related to valve performance and quality, and requires systematic and in-depth research.

From the birth of valves to the present, their sealing technology has also experienced great development. So far, valve sealing technology is mainly reflected in two aspects, namely static sealing and dynamic sealing.

The so-called static sealing usually refers to the sealing between two stationary surfaces. The main sealing method of static sealing is to use gaskets.

The so-called dynamic sealing mainly refers to the sealing of the valve stem, that is, to prevent the medium in the valve from leaking with the movement of the valve stem. The main sealing method of dynamic sealing is to use stuffing boxes.

1. Static seal

Static seal refers to the formation of a seal between two static sections. The main sealing method is to use gaskets. There are many types of gaskets. Commonly used gaskets include flat gaskets, O-rings, wrapped gaskets, special-shaped gaskets, corrugated gaskets and spiral-wound gaskets. Each type can be further divided according to the different materials used.

① Flat gasket. A flat gasket is a gasket that is flatly attached between two static sections. Generally, it can be divided into plastic flat gaskets, rubber flat gaskets, metal flat gaskets and composite flat gaskets according to the materials used. Each material has its applicable range.

② O-ring. An O-ring refers to a gasket with an O-shaped cross-section. Because its cross-section shape is O-shaped, it has a certain self-tightening effect, so the sealing effect is better than that of a flat gasket.

③ Wrapped gasket. A wrapped gasket refers to a gasket that wraps a certain material around another material. Such a gasket generally has good elasticity and can enhance the sealing effect.

④ Special-shaped gasket. Special-shaped gaskets refer to those with irregular shapes, including oval gaskets, diamond gaskets, gear-shaped gaskets, dovetail gaskets, etc. These gaskets generally have self-tightening effects and are mostly used in high and medium pressure valves.

⑤ Wave-shaped gaskets. Wave-shaped gaskets are gaskets with only wavy shapes. This type of gasket is usually composed of a combination of metal materials and non-metallic materials. It generally has the characteristics of small clamping force and good sealing effect.

⑥ Wound gaskets. Wound gaskets refer to gaskets formed by tightly wrapping and winding very thin metal and non-metallic strips. This type of gasket has good elasticity and sealing.

The materials used to make gaskets mainly include three categories, namely metal materials, non-metallic materials and composite materials. Generally speaking, metal materials have high strength and strong temperature resistance. Commonly used metal materials include copper, aluminum, steel, etc. There are many types of non-metallic materials, including plastic products, rubber products, asbestos products, hemp products, etc. These non-metallic materials are widely used and are selected according to specific needs. There are many types of composite materials, including laminates, composite plates, etc., which are also selected according to specific needs. Generally, they are used more on corrugated gaskets and spiral wound gaskets.

2. Dynamic seal

Dynamic seal refers to the seal that prevents the medium flow in the valve from leaking with the movement of the valve stem. This is a sealing problem in the relative movement process. Its sealing method mainly uses stuffing box. There are two basic forms of stuffing box, namely gland type and clamping nut type. Gland type is the most commonly used form at present. Generally speaking, from the form of gland, it can be divided into combined type and integral type. Although each form is different, it basically contains bolts for clamping. The clamping nut type is generally used for smaller valves. Due to the small size of this form, the clamping force is limited.

In the stuffing box, since the packing is in direct contact with the valve stem, all packings are required to have good sealing performance, small friction coefficient, ability to adapt to the pressure and temperature of the medium, and corrosion resistance. At present, the more commonly used packings include rubber O-rings, polytetrafluoroethylene braided packing, asbestos packing and plastic molded packing, etc. Each packing has its own adaptability conditions and range, and is selected according to specific needs.

Sealing is to prevent leakage, so the valve sealing principle is also studied from the perspective of preventing leakage. There are two main factors that cause leakage. One is the most important factor affecting the sealing performance, that is, there is a gap between the sealing pairs, and the other is the pressure difference between the two sides of the sealing pair. The valve sealing principle is also analyzed from four aspects: the sealing of the liquid, the sealing of the gas, the sealing principle of the leakage channel and the valve sealing pair.

Liquid sealing

The sealing of the liquid is carried out through the viscosity and surface tension of the liquid. When the capillary of the valve leakage is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. This forms a tangent angle. When the tangent angle is less than 90°, the liquid will be injected into the capillary, and leakage will occur. The reason for the leakage is the different properties of the medium. When testing with different media, different results will be obtained under the same conditions.

You can use water, air or kerosene. When the tangent angle is greater than 90°, leakage will also occur. This is because it is related to the grease or wax film on the metal surface. Once these surface films are dissolved, the properties of the metal surface change, and the liquid that was originally repelled will wet the surface and leak. In view of the above situation, according to Poisson's formula, the purpose of preventing leakage or reducing leakage can be achieved by reducing the capillary diameter and the viscosity of the medium.

Gas sealing

According to Poisson's formula, the sealing of the gas is related to the gas molecules and the viscosity of the gas. Leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and is proportional to the diameter of the capillary and the driving force. When the diameter of the capillary is the same as the average degree of freedom of the gas molecules, the gas molecules will flow into the capillary with free thermal motion. Therefore, when we are doing a valve sealing test, the medium must be water to play a sealing role, and air, that is, gas, cannot play a sealing role.

Even if we reduce the capillary diameter to below the gas molecule through plastic deformation, we still cannot stop the flow of gas. The reason is that the gas can still diffuse through the metal wall. So when we do gas tests, we must be more strict than liquid tests.

Sealing principle of leakage channel

The valve seal consists of two parts: the unevenness scattered on the wave surface and the waviness of the distance between the wave peaks. In the case that the elastic strain of most metal materials in my country is low, if you want to achieve a sealed state, you need to put forward higher requirements on the compression force of the metal material, that is, the compression force of the material must exceed its elasticity. Therefore, when designing the valve, the sealing pair is matched with a certain hardness difference, and under the action of pressure, a certain degree of plastic deformation sealing effect will be produced.

If the sealing surface is all metal material, then the uneven protrusions on the surface will appear first, and at the beginning, only a small load is needed to cause these uneven protrusions to produce plastic deformation. When the contact surface increases, the surface unevenness will become plastic-elastic deformation. At this time, the roughness of the two sides in the concave will exist. When a load that can cause severe plastic deformation of the underlying material is applied, and the two surfaces are in close contact, these remaining paths can be sealed along the continuous line and the circumferential direction.

Valve seal pair

The valve seal pair is the part of the valve seat and the closing member that closes when they contact each other. During use, the metal sealing surface is easily damaged by the inclusion of media, media corrosion, wear particles, cavitation and erosion. For example, wear particles. If the wear particles are smaller than the surface roughness, the surface accuracy will be improved when the sealing surface is run-in, and will not deteriorate. On the contrary, it will deteriorate the surface accuracy. Therefore, when selecting wear particles, factors such as its material, working conditions, lubricity and corrosion to the sealing surface should be comprehensively considered.

Just like wear particles, when we select seals, we must comprehensively consider various factors that affect their performance in order to play a leak-proof function. Therefore, materials that are resistant to corrosion, abrasion and erosion must be selected. Otherwise, the lack of any one of the requirements will greatly reduce its sealing performance.