Valve sealing principle

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 sealing problem of the valve is very prominent.

To ensure that the valve can well cut off the medium flow without leakage, it is necessary to ensure that the sealing of the valve is intact. There are many reasons for valve leakage, including unreasonable structural design, defective sealing contact surface, loose fastening Ling parts, loose fit between valve body and bonnet, etc. all these problems may lead to poor valve sealing and leakage. Therefore, valve sealing technology is an important technology related to valve performance and quality, which needs systematic and in-depth research.

The sealing technology of valve has also experienced great development since its emergence. So far, valve sealing technology is mainly reflected in two 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 gasket.

The so-called dynamic seal mainly refers to the sealing of the valve stem, that is, the medium in the valve will not leak with the movement of the valve stem. The sealing method of dynamic seal is mainly to use the stuffing box.

1. 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 washers. The commonly used washers include flat washers, O-rings, wrapped washers, special-shaped washers, corrugated washers and wound washers. Each type can be further divided according to the different materials used.

① Flat washer. Flat washer is a flat washer pasted 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. The flat washer of each material has its scope of application.

② O-ring. O-ring refers to the gasket with O-shaped section shape. Because its section shape is O-shaped and has a certain self tightening effect, the sealing effect is better than that of flat gasket.

③ Wrap washer. Wrapped gasket refers to a gasket that wraps one material on another. Such a gasket generally has good elasticity and can enhance the sealing effect.

④ Special washer. Special shaped washer refers to those washers with irregular shape, including oval washer, diamond washer, gear washer, dovetail washer, etc. These washers generally have self tightening effect and are mostly used in high and medium pressure valves.

⑤ Corrugated washer. Corrugated washer is a washer with only wave shape. This kind of washer is usually composed of metal materials and non-metallic materials. It generally has the characteristics of small compression force and good sealing effect.

⑥ Spiral wound washer. Spiral wound gasket refers to the gasket formed by winding a very thin metal belt and non-metal belt together. This kind of gasket has good elasticity and tightness.

Gasket materials mainly include three categories: metal materials, non-metallic materials and composite materials. Generally speaking, metal materials have high strength and strong temperature resistance. Common metal materials include copper, aluminum, steel, etc. There are many kinds of non-metallic materials, including plastic products, rubber products, asbestos products, hemp products, etc. These non-metallic materials are widely used and selected according to specific needs. There are many kinds of composite materials, including laminates and composite plates, which are also selected according to specific needs. Generally, they are used more on corrugated washers and spiral wound washers.

2. Dynamic seal

Dynamic seal refers to the seal that does not allow the medium flow in the valve to leak with the movement of the valve stem, which is a sealing problem in the process of relative movement. Its sealing method is mainly using stuffing box. There are two basic forms of stuffing box, namely gland type and compression nut type. Gland type is the most Zui widely used form at present. Generally speaking, the form of gland can be divided into combined type and integral type. Although each form is different, it basically includes bolts for pressing. 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, since the packing is in direct contact with the valve stem, all the packing is required to have good sealing, low friction coefficient, be able to adapt to the pressure and temperature of the medium, and be corrosion-resistant. At present, the commonly used fillers include rubber O-ring, Teflon braided packing, asbestos packing and plastic molding packing. Each packing has its suitable conditions and scope, which can be selected according to specific needs.

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

Liquid tightness

The sealing of liquid is carried out through the viscosity and surface tension of liquid. When the capillary leaking from the valve is filled with gas, the surface tension may 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. The reason for the leakage lies in the different properties of the medium. Different results will be obtained when different media are used for the test under the same conditions.

You can use water, air or kerosene. When the tangent angle is greater than 90 °, leakage will also occur. Because it is related to grease or wax film on the metal surface. Once the films on these surfaces are dissolved, the characteristics of the metal surface will change, and the excluded liquid will invade the surface and leak. In view of the above situation, according to Poisson formula, the purpose of preventing leakage or reducing leakage can be realized under the condition of reducing capillary diameter and large medium viscosity.

Gas tightness

According to Poisson formula, the tightness 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 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 gas molecules, gas molecules will flow into the capillary with free thermal motion. Therefore, when we are doing the valve sealing test, the medium must use water to play the role of sealing, and air, that is, gas, can not play the role of sealing.

Even if we reduce the capillary diameter below the gas molecules through plastic deformation, we still can't stop the gas flow. The reason is that the gas can still diffuse through the metal wall. Therefore, when we do gas test, we must be more strict than liquid test.

Sealing principle of leakage channel

The valve seal consists of two parts: the unevenness scattered on the wave surface and the waviness of the distance between wave peaks. In the case of low elastic strain of most metal materials in China, if we want to achieve the sealed state, we need to put forward higher requirements for the compressive force of metal materials, that is, the compressive force of materials should exceed its elasticity. Therefore, in the valve design, the sealing pair is matched with a certain hardness difference. Under the action of pressure, it will produce a certain degree of plastic deformation sealing effect.

If the sealing surfaces are all metal materials, the uneven protruding points on the surface will Zui appear early. At the beginning of Zui, only a small load can make these uneven protruding points produce plastic deformation. When the contact surface increases, the unevenness of the surface will become plastic elastic deformation. At this time, the roughness of both sides in the concave will exist. When the load that can cause severe plastic deformation of the underlying material needs to be applied, and the two surfaces are in close contact, these remaining drift diameters can be closed along the continuous line and circumferential direction.

Valve sealing pair

The valve sealing pair is the part where the valve seat and the closing member close when they are in contact with each other. During the use of metal sealing surface, it is easy to be damaged by medium inclusion, medium corrosion, wear particles, cavitation and scouring. Such as wear particles. If the unevenness of wear particles is smaller than that of the surface, the surface accuracy will be improved without deterioration when the sealing surface is run in. On the contrary, it will deteriorate the surface accuracy. Therefore, when selecting wear particles, we should comprehensively consider their materials, working conditions, lubricity and corrosion to the sealing surface.

Just like wear particles, when selecting seals, we should comprehensively consider various factors affecting their performance in order to prevent leakage. Therefore, materials that are resistant to corrosion, abrasion and erosion must be selected. Otherwise, the lack of any requirement will greatly reduce its sealing performance.