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Sealing principle of valve

Sealing principle of valve

There are many kinds of valves, but the basic function is the same, that is to connect or cut off the medium flow. Therefore, the valve sealing problem is very prominent.
To ensure that the valve can well cut off the medium flow, no leakage, it is necessary to ensure that the valve seal is intact. There are many reasons for valve leakage, including unreasonable structure design, defective sealing contact surface, loose fastening parts, loose fitting between valve body and valve cover, etc. all these problems may lead to poor sealing of valve, resulting in leakage. Therefore, valve sealing technology is an important technology related to valve performance and quality, which needs to be studied systematically.
The sealing technology of valve has also experienced great development from its birth to now. Up to now, valve sealing technology is mainly reflected in two aspects: 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 is mainly using gasket.
The sealing technology of valve has also experienced great development from its birth to now. Up to now, valve sealing technology is mainly reflected in two aspects: static sealing and dynamic sealing.

Static seal

Static seal refers to the formation of a seal between two static sections, and its sealing method is mainly the use of gaskets. There are many kinds of gaskets. The commonly used gaskets include flat washer, O-ring, wrapped washer, special-shaped washer, corrugated washer and spiral wound washer. Each type can be further divided according to the different materials used.

  • Flat washer. Flat washer is a kind of flat washer attached between two static sections. Generally, it can be divided into plastic flat washer, rubber flat washer, metal flat washer and composite flat washer according to the materials used. Flat washer of each material has its applicable scope.
  • ② O-ring. O-ring refers to the gasket with O-shaped cross-section. Due to its O-shaped cross-section shape and certain self tightening effect, the sealing effect of O-ring is better than that of flat washer.
  • ③ Wrap the gasket. Packing gasket refers to a kind of gasket wrapped on another material. Such gasket generally has good elasticity and can enhance sealing effect.
  • ④ Special shaped washer. Special shaped gasket refers to those gaskets with irregular shape, including oval washer, diamond washer, gear washer, dovetail washer, etc. These gaskets generally have self tightening effect and are mostly used in high and medium pressure valves.
  • ⑤ Corrugated washer. Corrugated washer is a kind of gasket with wave shape only. This kind of washer is usually composed of metal and non-metallic materials, which has the characteristics of small compression force and good sealing effect.
  • Spiral wound gasket. Spiral wound gasket is a kind of gasket formed by winding thin metal belt and non-metallic belt. This kind of gasket has good elasticity and sealing property.

There are three main types of gasket materials, namely metal materials, non-metallic materials and composite materials. Generally speaking, metal materials have high strength and high temperature resistance. The commonly used metal materials are copper, aluminum, steel, etc. There are many kinds of non-metallic materials, including plastic products, rubber products, asbestos products, hemp products and so on. These non-metallic materials are widely used and can be selected according to the specific needs. There are many kinds of composite materials, including laminated plate and composite plate, which are also selected according to the specific needs. Generally, the corrugated washer and spiral wound washer are used more.

Dynamic seal

Dynamic seal is a kind of seal that prevents the leakage of the medium flow in the valve with the movement of the valve stem. It is a sealing problem in the process of relative movement, and its sealing method is mainly using stuffing box. There are two basic forms of stuffing box, namely gland type and compression nut type. The gland type is the most commonly used form at present. Generally speaking, the gland type can be divided into combined type and integral type. Although each form is different, it basically contains bolts for pressing. Compression nut type is generally used for smaller valves, because of the small size of this form, so the clamping force is limited.
In the stuffing box, as the packing is directly in contact with the valve stem, all requirements of packing have good sealing performance, small friction coefficient, can adapt to the pressure and temperature of medium, and corrosion resistance. At present, the commonly used fillers include rubber O-ring, PTFE braided packing, asbestos packing and plastic molding packing. Each kind of packing has its own suitable conditions and scope, which can be selected according to the specific needs.
Sealing is to prevent leakage, so the valve sealing principle is also from the study of leakage prevention. There are two main factors causing the leakage. One is the most important 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 pair. The principle of valve sealing is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing and valve sealing pair.

Sealing property of liquid

The sealing property of liquid is carried out by viscosity and surface tension of liquid. The capillary tube, or the gas introduced into the capillary tube, may repel the liquid when the surface is filled with liquid. This creates a tangent angle. When the tangent angle is less than 90 degrees, the liquid will be injected into the capillary, which will cause leakage. The reason for the leakage lies in the different properties of the medium. When different media are used to test, different results will be obtained under the same conditions.
You can use water, air or kerosene. When the tangent angle is greater than 90 °, the leakage will also occur. Because it has something to do with the grease or wax film on the metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface will change, and the 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 leakage can be realized under the condition of reducing capillary diameter and medium viscosity.

Gas tightness

According to Poisson’s formula, the sealing property of gas is related to the viscosity of gas molecules and gas. The leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and is 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 movement. Therefore, when we do the valve sealing test, the medium must use water to play the role of sealing, air or gas can not play the role of sealing.
Even if we reduce the capillary diameter to below the gas molecules by plastic deformation, we still can’t stop the gas flow. The reason is that the gas can still diffuse through the metal wall. So when we do gas test, we must be more strict than liquid test.

Sealing principle of leakage channel

The valve seal is composed of two parts: the unevenness scattered on the wave surface and the waviness of the distance between wave peaks. Under the condition that the elastic strain force of most metal materials in our country is low, if we want to achieve the sealing state, we need to put forward higher requirements for the compression force of metal materials, that is, the compression force of materials should exceed its elasticity. Therefore, when the valve is designed, the sealing pair is matched with a certain hardness difference, which will produce a certain degree of plastic deformation sealing effect under the effect of pressure.
If the sealing surface is made of metal materials, the bumps on the surface will appear at the earliest time. At the beginning, only a small load is needed to make these uneven bulges produce plastic deformation. When the contact surface increases, the surface roughness will become plastic elastic deformation. At this time, the roughness of both sides in the concave will exist. When it is necessary to apply a load which can cause serious plastic deformation of the underlying material, and make the two surfaces contact closely, the remaining path can be closed along the continuous line and circumferential direction.

Sealing pair of valve

The sealing pair of the valve is the part that the valve seat and the closing part close when they contact each other. The metal sealing surface is easy to be damaged by entrapped medium, medium corrosion, wear particles, cavitation and erosion. For example, wear particles. If the surface roughness of wear particles is smaller than that of the surface, the surface accuracy will be improved when the sealing surface is running in. On the contrary, it will deteriorate the surface accuracy. Therefore, when selecting wear particles, the material, working condition, lubricity and corrosion of sealing surface should be considered comprehensively.
Like wear particles, we should consider all kinds of factors that affect the performance of seals when we choose seals, so as to play the function of anti leakage. Therefore, it is necessary to select materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any requirement will greatly reduce its sealing performance.

Source: China Valve Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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