The Sealing Principles of the Valve (Part Two)

The Sealing Principles of the Valve (Part Two)


Airtightness of liquid
The sealing performance of the liquid is performed by the viscosity and surface tension of the liquid. When the capillary of the valve which is leaked is filled with gas, the surface tension may repel the liquid or make the liquid get 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 leakages. The reason for the leakage is the different nature of the medium. Experiments with different media will have different results under the same conditions. You can use water, air or kerosene as the medium. When the tangent angle is greater than 90°, leakage will also occur. The leakage is related to the grease or waxy film on the metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface change and the originally repelled liquid will wet the surface and cause leakages. In view of the above situation, according to Poisson's formula, the purpose of preventing or reducing leakages can be achieved when the diameter of the capillary is reduced and the viscosity of the medium is great.
 
Airtightness of gas
According to the Poisson formula, the airtightness of gas is related to the gas molecules and viscosity. Leakage is inversely proportional to the length of the capillary tube and the viscosity of the gas, and is proportional to the diameter of the capillary tube 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 the valve sealing test is performed, the medium must be water in order to play a sealing role, and air or gas cannot play a sealing role. Even if we use plastic deformation to reduce the diameter of the capillary below the gas molecule, it still cannot stop the flow of gas. The reason is that the gas can still diffuse through the metal wall. Therefore, when the gas test is performed, it must be stricter than the liquid test.
 
The sealing principles of the leakage channel
The valve seal is composed of two parts, the roughness of the unevenness scattered on the wave surface and the waviness of the distance between the wave crests. In the case that most of the metal materials in China have low elastic strain. If the sealing state is to be achieved, higher requirement of compression force of the metal material needs to be put forward, that is, the compression force of the material must exceed its elasticity. Therefore, in the valve design, the sealing pair is matched with a certain hardness difference, and a certain degree of plastic deformation sealing effect will be produced under the action of pressure.
 
If the sealing surface is made from metal materials, the uneven protruding points will appear first, and only a small load can be used to plastically deform these uneven protruding points. When the contact surface increases, the uneven surface will become plastic-elastic deformation. At this time, the roughness on both sides of the concave part 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, the remaining diameters can be tightly closed along the continuous line and circumferential direction.
 
Valve sealing pairs
The valve sealing pair is the part that closes when the valve seat and closing member are in contact with each other. The metal sealing surface is susceptible to damage by the entrapped medium, medium corrosion, wear particles, cavitation and erosion during use, for example wear particles. If the abrasion particles have less unevenness than the surface, the surface accuracy will be improved without deterioration when the sealing surface is running-in. On the contrary, the surface accuracy will deteriorate. Therefore, when selecting wear particles, factors such as their materials, working conditions, lubricity and corrosion to the sealing surface must be considered comprehensively. Like wear particles, when we select seals, we must comprehensively consider various factors that affect their performance to prevent leakages. Therefore, we must choose those materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any requirement will greatly reduce its sealing performance.