Technology Sharing

Valve sealing principle

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Update time : 2020-06-11 10:39:40

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There are many types of valves, but the basic function is the same, that is to connect or cut off the flow of the medium. Therefore, the sealing problem of the valve is very prominent.
 
To ensure that the valve can intercept the medium flow well, and no leakage occurs, it is necessary to ensure that the valve is intact. There are many reasons for valve leakage, including unreasonable structural design, defective sealing contact surface, loosening of fastening parts, inadequate cooperation between valve body and bonnet, etc. All these problems may lead to valve sealing failure Well, there is a leakage problem. Therefore, valve sealing technology is an important technology related to valve performance and quality, which requires systematic and in-depth research.
 
Since its creation until now, its sealing technology has also experienced great development. So far, valve sealing technology has mainly been embodied in two major aspects, namely static sealing and dynamic sealing.
The so-called static seal usually refers to the seal between two static surfaces. The sealing method of static seal mainly uses gaskets.
The so-called dynamic seal mainly refers to the sealing of the valve stem, that is, the medium in the valve is not allowed to leak with the movement of the valve stem. The sealing method of the dynamic seal is mainly to use a stuffing box.
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1. Static seal
Static seal refers to the formation of a seal between two static sections. The sealing method mainly uses gaskets. There are many types of washers. The commonly used washers include flat washers, O-rings, wrap washers, special-shaped washers, wave washers and wound washers. Each type can be further divided according to the materials used.
① Flat washer. Flat washers are flat washers attached between two static sections. Generally, they can be divided into plastic flat washers, rubber flat washers, metal flat washers and composite flat washers according to the materials used. The flat washers of each material have their own Scope of application.
②O-ring. O-ring refers to a gasket with an O-shaped cross-section. Since its O-shaped cross-section has a certain self-tightening effect, the sealing effect is better than that of a flat gasket.
③ Packed washer. Wrapped gasket refers to a gasket that wraps a certain material on another material. Such a gasket generally has good elasticity and can enhance the sealing effect.
④ Shaped washer. Special-shaped washers refer to those with irregular shapes, including oval washers, diamond washers, gear washers, dovetail washers, etc. These washers generally have a self-tightening effect and are mostly used in high and medium pressure valves.
⑤ Wave washer. Wave washers are washers that only have a wavy shape. These washers usually consist of a combination of metallic and non-metallic materials. They generally have the characteristics of low compression force and good sealing effect.
⑥ Wrap the washer. Wound gasket refers to a gasket formed by winding a very thin metal belt and a non-metal belt together. This type of gasket has good elasticity and tightness.
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2. Dynamic seal
Dynamic sealing refers to a seal that does not allow the medium flow in the valve to leak with the movement of the valve stem. This is a sealing problem during relative movement. The sealing method mainly uses a stuffing box. There are two basic forms of stuffing box, namely gland type and compression nut type. The gland type is currently the most used form. Generally speaking, from the gland form, it can be divided into two types: combined type and integral type. Although each type is different, it basically contains bolts for pressing. The compression nut type is generally used for smaller valves. Due to the small size of this form, the compression force is limited.
In the stuffing box, because the packing is in direct contact with the valve stem, all require a good sealing of the packing, a small friction coefficient, be able to adapt to the pressure and temperature of the medium, and resistant to corrosion. The most commonly used fillers at present include rubber O-rings, PTFE braided packing, asbestos packing and plastic molding packing, etc. Each packing has its own suitable conditions and scope, and is selected according to specific needs.
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Sealing is to prevent leakage, then the principle of valve tightness is also studied from preventing leakage. There are two main factors causing leakage, one is the most important factor affecting the sealing performance, that is, there is a gap between the sealing pair, and the other is the pressure difference between the two sides of the sealing pair. The valve sealing principle is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing principle and valve sealing pair.
 
Liquid tightness
 
The liquid tightness is carried out by the viscosity and surface tension of the liquid. When the leaking capillary of the valve is filled with gas, 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°, the liquid will be injected into the capillary, and leakage will occur. The reason for the leakage is the different nature of the medium. Experiments with different media will give different results under the same conditions.
You can use water, air or kerosene. And when the tangent angle is greater than 90°, leakage will also occur. Because it is related to the grease or waxy film on the metal surface. Once the films on these surfaces are dissolved, the characteristics of the metal surface change, and the originally 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 the amount of leakage can be achieved when the diameter of the capillary and the viscosity of the medium are large.
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Gas tightness
 
   According to Poisson's formula, the tightness of the 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 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 motion. Therefore, when we do the valve sealing test, the medium must use water to play the role of sealing, and air or gas cannot play the role of sealing.
   Even if we reduce the capillary diameter below the gas molecules through plastic deformation, it still cannot stop the flow of gas. The reason is that the gas can still diffuse through the metal wall. So when we do gas test, it must be more strict than liquid test.

Leakage channel sealing principle
 
The valve seal is composed of unevenness scattered on the wave surface and waviness of the distance between the peaks to form roughness. In the case where most of the metal materials in my country have low elastic strain, if they want to achieve a sealed state, they need to put higher demands on the compressive force of the metal material, that is, the compressive force of the material should exceed its elasticity. Therefore, in the design of the valve, the sealing pair is matched with a certain hardness difference, and under the action of pressure, a certain degree of plastic deformation sealing effect will be produced.
If the sealing surfaces are all made of metal materials, the uneven projections on the surface will appear at the earliest. At the beginning, only a small load is needed to make these uneven projections plastically deformed. When the contact surface increases, 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 severe plastic deformation of the underlying material, and make the two surfaces contact closely, along the continuous line and the hoop direction, these remaining diameters can be tightly closed.
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