# Valve tightness principle

### Sealing is preventing leakage, so the principle of valve sealing is also studied from preventing leakage.There are two main factors that cause leakage, one is the main 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 tightness is also analyzed from four aspects: liquid tightness, gas tightness, leakage channel sealing principle and valve sealing pair.

1. Liquid tightness

The tightness of the liquid is carried out by the viscosity and surface tension of the liquid. When the valve leaking capillary is filled with gas, surface tension may repel the liquid or introduce 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, which will cause leakage.

Leaks occur because of the different properties of the media. Experiments with different media will yield different results under the same conditions. Water, air or kerosene can be used. 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 the film on these surfaces is dissolved, the characteristics 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 the Poisson formula, the purpose of preventing leakage or reducing leakage can be achieved under the condition of reducing the diameter of the capillary and the viscosity of the medium.

2. 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 and the viscosity of the gas, and 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 the gas molecules, the gas molecules flow into the capillary with thermal motion.

Therefore, when we are doing the valve sealing test, the medium needs water to play the role of sealing, and air, that is, gas, cannot play the role of sealing. Even if we reduce the diameter of the capillary below the gas molecules by plastic deformation, the flow of the gas still cannot be stopped. The reason is that the gas can still diffuse through the metal wall. So when we do the gas test, we are more strict than the liquid test.

3. The sealing principle of the leakage channel

The valve seal consists of two parts, the roughness that is spread on the wave surface and the waviness of the distance between the wave crests. Under the circumstance that most of the metal materials in our country have low elastic strain force, if the state of sealing is to be achieved, it is necessary to raise higher requirements for the compressive force of the metal material, that is, the compressive force of the material must exceed its elasticity.

Therefore, when designing the valve, the sealing pair is matched with the difference in hardness, and under the action of pressure, it will produce a degree of plastic deformation sealing effect. If the sealing surfaces are all metal materials, uneven protrusions on the surface will appear, and these uneven protrusions can be plastically deformed with a small load. When the contact surface is large, the unevenness of the surface becomes plastic-elastic deformation. At this time, the roughness on both sides of the recess will exist. When it is necessary to apply a load that can cause severe 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 hoop direction.

4. Valve sealing pair

The valve sealing pair is the part that closes when the valve seat and the closing member are in contact with each other. During use, the metal sealing surface is easily damaged by intercalated media, media corrosion, wear particles, cavitation and erosion. such as wear particles. If the wear particles are smaller than the unevenness of the surface, the surface accuracy will be improved without deterioration when the sealing surface is run-in. On the contrary, the surface accuracy will be deteriorated.

Therefore, when selecting wear particles, factors such as their material, working conditions, lubricity and corrosion to the sealing surface should be comprehensively considered. Just like wear particles, when we choose seals, we must comprehensively consider various factors that affect their performance in order to play the function of preventing leakage. Therefore, choose materials that are resistant to corrosion, scratches and erosion. Otherwise, the lack of any one of the requirements will greatly reduce its sealing performance.

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