Valve tightness principle

 

Sealing is to avoid leakage, so the principle of valve sealing is also studied from avoiding leakage. There are two main factors that cause leakage, one is the most important factor affecting the sealing function, 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, leaking 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 leaky capillary of the valve is filled with gas, the surface tension can repel the liquid or introduce the 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. Leakage occurs because of the different properties of the medium. Experiments with different media will yield different results under the same conditions. Can use water, air or kerosene etc. When the tangent angle is greater than 90°, leakage will also occur. Because of the relationship with the grease or wax film on the metal surface. Once the film on these surfaces is dissolved, the properties of the metal surface are changed, 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 avoiding leakage or reducing the amount of leakage can be accomplished under the condition of reducing the diameter of the capillary and the viscosity of the medium.

 

2. Gas tightness According to Poisson's formula, gas tightness is related to the viscosity of gas molecules and gas. Leakage is inversely proportional to capillary length and gas viscosity, and proportional to capillary diameter and driving force. 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 use water to achieve the sealing effect, and air, that is, gas, cannot achieve the sealing effect. Even if we reduce the diameter of the capillary below the gas molecules through the plastic deformation method, the movement of the gas still cannot be prevented. The reason is that the gas is still able to disperse through the metal walls. Therefore, when we do gas tests, we must be more strict than liquid tests.

 

3. The sealing principle of the leakage channel The valve seal consists of two parts, the unevenness scattered on the wave surface and the roughness of the distance between the wave crests. Under the condition that most of the metal materials in our country have low elastic strain force, if the sealing state 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 a certain hardness difference, and under the effect of pressure, a certain degree of plastic deformation sealing effect will occur. If the sealing surface is made of metal materials, the uneven protruding points on the surface will appear at the earliest, and only a small load can be used to plastically deform these uneven protruding points at the beginning. When the touch surface is enlarged, 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 serious plastic deformation of the underlying material, and make the two surfaces touch closely, these remaining paths can be made close along the continuous line and circumferential direction.

 

4. Valve sealing pair The valve sealing pair is the part that closes when the valve seat and the closing member touch 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 surface unevenness, the surface accuracy will be improved without deterioration when the sealing surface is run in. On the contrary, the appearance accuracy will be deteriorated. Therefore, when selecting wear particles, factors such as their materials, working conditions, lubricity and corrosion conditions on the sealing surface should be comprehensively considered. Just like wear particles, when we select seals, we must comprehensively consider various factors that affect their performance, so as to prevent leakage. Therefore, materials that are resistant to corrosion, scratches and erosion must be selected. Otherwise, the lack of any one requirement will greatly reduce its sealing performance.