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

01. Sealing of liquids

The sealing performance of a liquid is determined by its viscosity and surface tension. When the capillary tube leaked from the valve is filled with gas, surface tension may repel the liquid or introduce the liquid into the capillary tube. This forms a tangent angle. When the tangent angle is less than 90 °, the liquid will be injected into the capillary tube, causing leakage.

The reason for the leakage is due to the different properties of the medium. Experiments conducted with different media will yield different results under the same conditions. You can use water, air, kerosene, etc. 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 thin films on these surfaces are dissolved, the characteristics of the metal surface change, and the previously repelled liquid will wet the surface and leak. In response to the above situation, according to Poisson's formula, the goal of preventing leakage or reducing leakage can be achieved by reducing the diameter of the capillary and the viscosity of the medium.

02. Gas sealing performance

According to Poisson's formula, the sealing performance of a gas is related to its molecules and viscosity. Leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and directly proportional to the diameter and driving force of the capillary. When the diameter of the capillary tube is the same as the average degree of gas molecules, gas molecules will flow into the capillary tube through thermal motion.

Therefore, when conducting valve sealing tests, the medium needs to be sealed with water, and air or gas cannot provide sealing. Even if we reduce the diameter of the capillary to below the gas molecules through plastic deformation, it still cannot prevent the flow of gas. The reason is that gases can still diffuse through metal walls. So when conducting gas tests, we need to be more rigorous than liquid tests.

03. Sealing principle of leakage channel

The valve seal consists of two parts: roughness caused by unevenness scattered on the waveform surface and roughness caused by the distance between wave peaks. In the case where the elastic strain force of most metal materials in our country is low, in order to achieve a sealed state, higher requirements need to be placed on the compressive force of the metal material, that is, the compressive force of the material must exceed its elasticity.

Therefore, when designing valves, 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 surface is made of metal material, uneven protrusions will appear on the surface, and plastic deformation can be caused by these uneven protrusions with only a small load. When the contact surface is large, the unevenness of the surface will become plastic elastic deformation. At this point, roughness will exist on both sides of the concave area. When a load that can cause severe plastic deformation of the underlying material needs to be applied, and the two surfaces must be in close contact, the remaining diameters must be tightly fitted along the continuous line and circumferential direction.

04. Valve sealing pair

The valve sealing pair is the part where the valve seat and the closing member close when in contact with each other. Metal sealing surfaces are susceptible to damage from trapped media, media corrosion, abrasive particles, cavitation, and erosion during use. For example, wear particles. If the wear particles are smaller than the surface roughness, the surface accuracy will be improved during the running in of the sealing surface without deterioration. On the contrary, it will deteriorate the surface accuracy.

Therefore, when selecting wear particles, factors such as material, working conditions, lubricity, and corrosion of the sealing surface should be comprehensively considered. Like abrasive particles, when choosing seals, we need to consider various factors that affect their performance comprehensively in order to achieve leak prevention. Therefore, choose materials that are corrosion-resistant, scratch resistant, and erosion resistant. Otherwise, the lack of any requirement will greatly reduce its sealing performance.