Valve sealing principle

There are many types of valves, but their basic function is the same, which is to connect or cut off the flow of media. Therefore, the sealing problem of valves becomes very prominent.

To ensure that the valve can cut off the medium flow well and prevent leakage, it is necessary to ensure that the seal of the valve is intact. There are many reasons for valve leakage, including unreasonable structural design, defective sealing contact surfaces, loose fastening parts, loose fit between the valve body and the valve cover, etc. All of these problems may lead to improper valve sealing. Well, thus creating a leakage problem. Therefore, valve sealing technology is an important technology related to valve performance and quality, and requires systematic and in-depth research.

Since the creation of valves, their sealing technology has also experienced great development. So far, valve sealing technology is mainly reflected in two major aspects, namely static sealing and dynamic sealing.

The so-called static seal usually refers to the seal between two static surfaces. The sealing method of static seal mainly uses gaskets.

The so-called dynamic seal mainly refers to the sealing of the valve stem, which prevents the medium in the valve from leaking with the movement of the valve stem. The main sealing method of dynamic seal is to use a stuffing box.

1. Static seal

Static sealing refers to the formation of a seal between two stationary sections, and the sealing method mainly uses gaskets. There are many types of washers. The commonly used washers include flat washers, O-shaped washers, wrapped washers, special-shaped washers, wave washers and wound washers. Each type can be further divided according to the different materials used.
①Flat washer. Flat washers are flat washers that are placed flat between two stationary sections. Generally, according to the materials used, they can be divided into plastic flat washers, rubber flat washers, metal flat washers and composite flat washers. Each material has its own application. range.
②O-ring. O-ring refers to a gasket with an O-shaped cross-section. Because its cross-section is O-shaped, it has a certain self-tightening effect, so the sealing effect is better than that of a flat gasket.
③Include washers. A wrapped gasket refers to a gasket that wraps a certain material on another material. Such a gasket generally has good elasticity and can enhance the sealing effect. ④Special-shaped washers. Special-shaped washers refer to those gaskets with irregular shapes, including oval washers, diamond washers, gear-type washers, dovetail-type washers, etc. These washers generally have a self-tightening effect and are mostly used in high and medium pressure valves.
⑤Wave washer. Wave gaskets are gaskets that only have a wave shape. These gaskets are usually composed of a combination of metal materials and non-metal materials. They generally have the characteristics of small pressing force and good sealing effect.
⑥ Wrap the washer. Wound gaskets refer to gaskets formed by wrapping thin metal strips and non-metal strips tightly together. This type of gasket has good elasticity and sealing properties. The materials for making gaskets mainly include three categories, namely metallic 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 can be selected according to specific needs. There are also many types of composite materials, including laminates, composite panels, etc., which are also selected according to specific needs. Generally, corrugated washers and spiral wound washers are mostly used.

2. Dynamic seal

Dynamic seal refers to a seal 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. The main sealing method is the stuffing box. There are two basic types of stuffing boxes: gland type and compression nut type. The gland type is the most commonly used form at present. Generally speaking, in terms of the form of the gland, it can be divided into two types: combined type and integral type. Although each form is different, they basically include bolts for compression. The compression nut type is generally used for smaller valves. Due to 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, small friction coefficient, be able to adapt to the pressure and temperature of the medium, and be corrosion-resistant. Currently, commonly used fillers include rubber O-rings, polytetrafluoroethylene braided packing, asbestos packing and plastic molding fillers. Each filler has its own applicable conditions and range, and should be selected according to specific needs. Sealing is to prevent leakage, so the principle of valve sealing is also studied from the perspective of preventing leakage. There are two main factors causing leakage. One is the most important factor affecting sealing performance, that is, the gap between the sealing pairs, and the other is the pressure difference between both sides of the sealing pair. The valve sealing principle is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing principle and valve sealing pair.

Liquid tightness

The sealing properties of liquids are determined by the viscosity and surface tension of the liquid. When the capillary of a leaking valve is filled with gas, surface tension may repel the liquid or introduce liquid into the capillary. This creates a tangent angle. When the tangent angle is less than 90°, liquid will be injected into the capillary, and leakage will occur. Leakage occurs due to the different properties of the media. Experiments using different media will yield different results under the same conditions. You can use water, air or kerosene, etc. When the tangent angle is greater than 90°, leakage will also occur. 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 originally repelled liquid will wet the surface and leak. In view of the above situation, according to Poisson’s formula, the purpose of preventing leakage or reducing the amount of leakage can be achieved by reducing the capillary diameter and increasing the viscosity of the medium.

Gas tightness

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

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

The sealing principle of leakage channel

The valve seal consists of two parts: the unevenness spread on the wave surface and the roughness of the waviness in the distance between wave peaks. In the case where most of the metal materials in our country have low elastic strain, if we want to achieve a sealed state, we need to raise 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. Under the action of pressure, a certain degree of plastic deformation sealing effect will be produced.

If the sealing surface is made of metal materials, then the uneven protruding points on the surface will appear earliest. At the beginning, only a small load can be used to cause plastic deformation of these uneven protruding points. When the contact surface increases, the surface unevenness becomes plastic-elastic deformation. At this time, the roughness on both sides in the recess will exist. When it is necessary to apply a load that can cause serious plastic deformation of the underlying material, and make the two surfaces in close contact, these remaining paths can be made close along the continuous line and circumferential direction.

Valve seal pair

The valve sealing pair is the part of the valve seat and closing member that closes when they come into contact with each other. During use, the metal sealing surface is easily damaged by entrained media, media corrosion, wear particles, cavitation and erosion. Such as wear particles. If the wear particles are smaller than the surface roughness, the surface accuracy will be improved rather than deteriorated when the sealing surface is worn in. On the contrary, the surface accuracy will be deteriorated. Therefore, when selecting wear particles, factors such as their materials, working conditions, lubricity, and corrosion on the sealing surface must be comprehensively considered.

Just like wear particles, when we select seals, we must comprehensively consider various factors that affect their performance in order to prevent leakage. Therefore, it is necessary to choose materials that are resistant to corrosion, scratches and erosion. Otherwise, the lack of any requirement will greatly reduce its sealing performance.