Valve sealing principle

Sealing is to prevent leakage, so the valve sealing principle is also from the study of leakage prevention. There are two main factors causing the 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 sealing is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing and valve sealing pair.

1. Sealing property of liquid

The sealing property of liquid is carried out by viscosity and surface tension of liquid. When the capillary tube leaking from the valve is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. This creates a tangent angle. When the tangent angle is less than 90 degrees, the liquid will be injected into the capillary, which will cause leakage.

The reason for the leakage lies in the different properties of the medium. When different media are used to test, different results will be obtained under the same conditions. You can use water, air or kerosene. When the tangent angle is greater than 90 °, the leakage will also occur. Because it has something to do with the grease or wax film on the metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface will change, and the 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 leakage can be realized under the condition of reducing capillary diameter and medium viscosity.

valve

2. Gas tightness

According to Poisson's formula, the sealing property of gas is related to the viscosity of gas molecules and gas. 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 will flow into the capillary with free thermal movement.

Therefore, when we do the valve sealing test, the medium must use water to play the role of sealing, air or gas can not play the role of sealing. Even if we reduce the capillary diameter to below the gas molecules by plastic deformation, we still can't stop the gas flow. The reason is that the gas can still diffuse through the metal wall. So when we do gas test, we must be more strict than liquid test.

3. Sealing principle of leakage channel

The valve seal is composed of two parts: the unevenness scattered on the wave surface and the waviness of the distance between wave peaks. Under the condition that the elastic strain force of most metal materials in our country is low, if we want to achieve the sealing state, we need to put forward higher requirements for the compression force of metal materials, that is, the compression force of materials should exceed its elasticity.

Therefore, when the valve is designed, the sealing pair is matched with a certain hardness difference, which will produce a certain degree of plastic deformation sealing effect under the effect of pressure.

4. Valve sealing pair

The valve sealing pair is the part of the valve seat that closes when the valve seat and the closing part contact each other. The metal sealing surface is easy to be damaged by entrapped medium, medium corrosion, wear particles, cavitation and erosion. For example, wear particles. If the surface roughness of wear particles is smaller than that of the surface, the surface accuracy will be improved when the sealing surface is running in. On the contrary, it will deteriorate the surface accuracy.

Therefore, when selecting wear particles, the material, working condition, lubricity and corrosion of sealing surface should be considered comprehensively. Like wear particles, we should consider all kinds of factors that affect the performance of seals when we choose seals, so as to play the function of anti leakage. Therefore, choose materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any requirement will greatly reduce its sealing performance.