Sealing is to prevent leakage, so the principle of valve sealing is also studied from leak prevention. 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 principle of valve sealing is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing principle and valve sealing pair.

1, liquid sealing
The liquid sealing is carried out by the viscosity and surface tension of the liquid. When the leaking capillary of the valve is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. This creates a tangency angle. When the tangency angle is less than 90°, the liquid is injected into the capillary, which causes leakage. The reason for the leakage is the different nature of the medium. Experimenting with different media will give different results under the same conditions. It can be used with water, with air or with kerosene. When the tangency angle is greater than 90°, leakage will also occur. Because it is related to grease or waxy film on the metal surface. Once the film on these surfaces is dissolved, the properties of the metal surface change, and the originally repelled liquid will invade the surface and leak. In view of the above, according to the Poisson formula, it is possible to achieve the purpose of preventing leakage or reducing the amount of leakage while reducing the capillary diameter and the viscosity of the medium.

2. Gas sealing
According to the Poisson formula, the gas sealing is related to the viscosity of gas molecules and gases. The leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and is proportional to the diameter and driving force of the capillary. When the diameter of the capillary is the same as the average degree of freedom of the gas molecules, the gas molecules flow into the capillary with free thermal motion. Therefore, when we are doing the valve sealing test, the medium must be used for water to seal, and the air, that is, the gas, cannot function as a seal. Even if we reduce the capillary diameter below the gas molecules by plastic deformation, it still cannot stop the flow of gas. The reason is that the gas can still diffuse through the metal wall. Therefore, when we do gas test, we must be more strict than liquid test.

3, the sealing principle of the leakage channel
The valve seal consists of two parts: the unevenness spread on the wave surface and the waviness between the peaks. In the case that most of the metal materials in our country have low elastic strain, if the sealing state is to be achieved, the compressive force of the metal material needs to be higher, that is, the compressive force of the material exceeds its elasticity. Therefore, when designing the valve, the sealing pair is combined with a certain hardness difference to match, and under the action of pressure, a certain degree of plastic deformation sealing effect is produced. If the sealing surfaces are all metallic materials, the uneven spots on the surface will appear at the earliest, and the uneven protruding points can be plastically deformed at the beginning with a small load. When the contact surface is increased, the unevenness of the surface becomes plastic-elastic deformation. At this time, the roughness on both sides of the recess will exist. When a load is required to cause severe plastic deformation of the underlying material, and the two surfaces are in close contact, the remaining paths can be brought into close contact along the continuous line and the circumferential direction.

4, valve sealing pair

The valve seal pair is the portion of the valve seat and closure that closes when in contact with each other. Metal sealing surfaces are susceptible to damage from entrainment of media, media corrosion, wear particles, cavitation and erosion during use. Such as wear particles. If the wear particles are smaller than the unevenness of the surface, the surface accuracy is improved when the sealing surface is worn-in, without deterioration. On the contrary, the surface accuracy is deteriorated. Therefore, when selecting wear particles, factors such as material, working condition, lubricity and corrosion to the sealing surface should be considered. Just like wear particles, when choosing a seal, we must consider various factors that affect its performance to prevent leaks. Therefore, materials that are resistant to corrosion, scratching and erosion must be selected. Otherwise, the lack of any requirement will greatly reduce the sealing performance.

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