Valve sealing principle:
To ensure that the valve can be a good cut-off medium flow, no leakage, we must ensure that the valve seal is intact. The causes of valve leakage for many reasons, including unreasonable structural design, sealing contact surface defects, fastening parts loose, the valve body and bonnet are not tightly coordinated, etc., all of which may lead to the valve sealing is not good, thus generating leakage problems. Therefore, the valve sealing technology is related to the valve performance and quality of an important technology, the need for systematic and in-depth research.
Valve from the generation to the present, its sealing technology has experienced great development. So far, the valve sealing technology is mainly reflected in two major aspects, namely, static sealing and dynamic sealing.
The so-called static Sealing, usually refers to the seal between the two stationary surfaces, static sealing sealing method is mainly the use of gaskets.
The so-called dynamic sealing, mainly refers to the stem sealing, that is, do not let the valve medium with the stem movement and leakage, dynamic sealing sealing method is mainly the use of packing box.
Static seal refers to the formation of a seal between two stationary sections, the sealing method is mainly the use of gaskets. There are many types of gaskets, often used gaskets, including flat washers, O-rings, packet gaskets, shaped gaskets, wave gaskets and winding gaskets and other major categories, each type of the following can be further divided according to the different materials used.
Flat washers are flat on the two stationary section between the washers, generally based on the use of materials can be divided into plastic flat washers, rubber flat washers, metal flat washers and composite flat washers, each material flat washers have their own scope of application.
O-ring refers to the cross-section shape of the O-shaped gasket, due to its cross-section shape is O-shaped, there is a certain self-tightening effect, so the sealing effect is better than the flat gasket.
③ Packing washer.
Package gasket refers to a certain material wrapped in another material on the gasket, such a gasket generally has good elasticity, can enhance the sealing effect.
④ Shaped washers.
Shaped washers are those irregularly shaped washers, including oval washers, diamond-shaped washers, gear-type washers, dovetailed washers, etc. These washers generally have a self-tightening effect, mostly used in high and medium pressure valves.
⑤ Wave washers.
Wave washer is only a wave shape of the gasket, this type of gasket is usually a combination of metallic and non-metallic materials to form, generally has a small compression force, sealing effect is good.
⑥ Wrap washers.
Wrap washer is a very thin metal and non-metallic tape tightly together, winding the formation of the gasket, this type of gasket has good elasticity and sealing.
The materials used to make the gaskets include three main categories, namely metallic materials, non-metallic materials and composite materials.
Generally speaking, metallic materials have high strength and temperature resistance, and commonly used metallic materials include copper, aluminium and steel. There are many types of non-metallic materials, including plastic products, rubber products, asbestos products, hemp products, etc. These non-metallic materials are widely used and are selected according to specific needs. There are also many types of composite materials, including laminates, composite panels, etc., also selected according to specific needs, generally used in wave washers and winding washers, etc. above more.
Dynamic sealing refers to the sealing that does not allow the medium flow inside the valve to leak with the movement of the valve stem, which is a sealing problem in the process of relative movement, and its sealing method mainly adopts the stuffing box. There are two basic forms of stuffing box, namely, gland type and compression nut type. Gland type is currently the most used form, generally from the form of the gland, can be divided into a combination of two types and the whole type, each form of difference, but basically include the bolt with the compression. The compression nut type is generally used for smaller valves, due to the small size of this form, so the compression force is limited.
In the packing box, because the packing is in direct contact with the valve stem, all the requirements of the packing sealing, friction coefficient is small, can adapt to the medium pressure and temperature, and corrosion resistance. Currently more commonly used fillers include rubber o-ring, PTFE braided packing, asbestos packing and plastic moulding packing, etc., each filler has its own conditions and scope of adaptation, according to the specific needs to be selected.
Sealing is to prevent leakage, then the principle of valve sealing is also studied from the prevention of leakage. There are two main factors that cause leakage, one is the most important factor affecting the sealing performance, that is, there is a gap between the sealing vice, the other is the sealing vice of the pressure difference between the two sides. Valve sealing principle is also from the liquid sealing, gas sealing, leakage channel sealing principle and valve sealing vice and so on four aspects to analyse.
Sealability of liquids:
Sealing of liquids is carried out by the viscosity and surface tension of the liquid. When a capillary tube leaking from a valve is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary tube. This creates an angle of tangency. When the angle of tangency is less than 90°, the liquid is injected into the capillary tube and a leak occurs. Leakage occurs due to the different properties of the media. Tests with different media give different results under the same conditions.
This can be done with water, with air or with paraffin, etc. And leakage also occurs when the tangent angle is greater than 90°. This is because there is a relationship with the greasy or waxy films on the metal surfaces. Once the film on these surfaces is dissolved away, the properties of the metal surface change, and the liquid, which was originally repelled, will invade the surface and leakage occurs. For the above situation, according to Poisson’s formula, it is possible to achieve the purpose of preventing leakage or reducing the amount of leakage by reducing the diameter of the capillary tube and the viscosity of the medium is large.
Sealing properties of gases:
According to Poisson’s formula, the hermeticity of a gas is related to the gas molecules and the viscosity of the gas. Leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and directly proportional to the diameter of the capillary and the driving force. When the diameter of the capillary tube and the average degree of freedom of the gas molecules are the same, the gas molecules will flow into the capillary tube with free thermal movement. Therefore, when we do the valve sealing test, the medium must be water in order to play the role of sealing, with air that is, gas can not play the role of sealing.
Even if we reduce the diameter of the capillary tube to below the gas molecule by plastic deformation, we still cannot stop the flow of gas. The reason is that the gas can still diffuse through the metal wall. That is why we must be stricter in gas testing than in liquid testing.
Leakage channel sealing principle:
Valve sealing consists of two parts: the unevenness scattered on the waveform surface and the roughness constituted by the corrugation of the distance between the crests. In China, most of the metal material elastic strain is low, if you want to achieve the sealing state, it is necessary to metal material compression force to raise higher requirements, that is, the material compression force to exceed its elasticity. Therefore, in the valve design, the sealing vice combined with a certain hardness difference to match, under the action of pressure, it will produce a certain degree of plastic deformation sealing effect.
If the sealing surfaces are all metal materials, then the surface uneven projections will be the earliest appearance, in the initial only need to use a small load can make these uneven projections produce plastic deformation. When the contact surface increases, the unevenness of the surface becomes plastic-elastic deformation. At this point the roughness of the two surfaces that are in the recesses will exist. It is necessary to apply loads that cause severe plastic deformation of the underlying material and to bring the two surfaces into close contact, both along the continuum and in the circumferential direction, in order to make these remaining passes close.
The valve seal is the part of the valve that closes when the valve seat and the closing member are in contact with each other. The metal sealing surface is susceptible to damage by entrained media, media corrosion, wear particles, cavitation and scouring during use. For example, wear particles. If the wear particles are smaller than the unevenness of the surface, the surface accuracy of the sealing surface will be improved without deterioration when the sealing surface is worn. On the contrary, the surface accuracy will deteriorate. Therefore, when selecting wear particles, we should consider its material, working conditions, lubricity and corrosion of the sealing surface and other factors.
As with wear particles, when selecting seals, we need to take into account the various factors affecting their performance in order to provide a leak-proof function. Therefore, it is important to select those materials that are resistant to corrosion, scuffing and scouring. Otherwise, the lack of any one of these requirements will greatly reduce its sealing performance.
WESDOM Group specializes in pipeline fluid systems: R&D, production and sales of valves, pipe fittings, water meters, flow meters, etc. The products cover cast iron, cast steel, stainless steel, copper, plastics and other materials, which are widely used in hydropower stations, heat, buildings, Water supply and drainage, petroleum, chemical industry, electric power, medical and other fields.
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