There are many kinds 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.

In order to ensure that the valve can cut off the medium flow well and no leakage occurs, it is necessary to ensure that the valve is sealed in good condition. There are many reasons for valve leakage, including unreasonable structural design, defective sealing contact surfaces, loose fastening parts, poor fit between the valve body and the valve cover, etc. All of these problems can lead to poor valve sealing, which can create leakage problems. Therefore, valve sealing technology is an important technology related to valve performance and quality, which requires systematic and in-depth research.

From its emergence to the present, the sealing technology of valves has also developed greatly. So far, valve sealing technology is mainly reflected in two aspects, namely static sealing and dynamic sealing.

Static sealing: The so-called static seal usually refers to the sealing between two static surfaces. The sealing method of static seal is mainly to use gaskets.

Dynamic sealing: 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 mainly uses the stuffing box.

Static sealing

Static sealing refers to the sealing between two static sections, and the sealing method is mainly the use of gaskets. There are many types of gaskets. The frequently used gaskets include flat gaskets, O-rings, cladding gaskets, special-shaped gaskets, corrugated gaskets and wound gaskets. Each type can be further divided according to the different materials used.

  • Flat gaskets: The flat gasket is a gasket that is flatly attached between two static sections. Generally, according to the materials used, it can be divided into plastic flat gaskets, rubber flat gaskets, metal flat gaskets and composite material flat gaskets, each material flat gasket has its applicable range.
  • O-rings: O-ring refers to a gasket with an O-shaped cross-section. Because its cross-sectional shape is O-shaped and has a certain self-tightening effect, the sealing effect is better than that of flat gaskets.
  • Cladding gaskets: Cladding gasket is a gasket that wraps a certain material on another material. Such gaskets generally have good elasticity and can enhance the sealing effect.
  • Special-shaped gaskets: Special-shaped gaskets refer to those with irregular shapes, including oval gaskets, diamond gaskets, gear gaskets, dovetail gaskets, etc. These gaskets generally have a self-tightening effect and are mostly used in high and medium pressure valves.
  • Corrugated gaskets: Corrugated gasket refers to a gasket with a wave shape. This type of gasket is usually composed of a combination of metal materials and non-metal materials. Generally, it has the characteristics of low pressing force and good sealing effect.
  • Wound gaskets: Wound gasket refers to a gasket formed by winding a thin metal belt and a non-metallic belt together. This type of gasket has good elasticity and sealing performance.

There are three main categories of gasket materials, namely metallic materials, non-metallic materials and composite materials. Generally speaking, metal materials have high strength and strong temperature resistance. Commonly used metal materials include copper, aluminum, steel and so on. There are many types of non-metallic materials, including plastic products, rubber products, asbestos products, linen products, etc. These non-metallic materials are widely used and can be selected according to specific needs. There are also many types of composite materials, including laminates, composite boards, etc., which are also selected according to specific needs, and are generally used in corrugated gaskets and wound gaskets.

Dynamic sealing

Dynamic sealing refers to the sealing that prevents the medium flow in the valve from leaking with the movement of the valve stem. This is a sealing problem in the process of relative movement. The sealing method is mainly to use stuffing boxes. There are two basic forms of stuffing boxes, namely gland type and compression nut type. The gland type is the most used form at present. Generally speaking, from the form of the gland, it can be divided into two types: combined type and integral type. Although each type is different, basically all include bolts for compression. The compression nut type is generally used for smaller valves. Due to the smaller size of this type, the compression force is limited.

In the stuffing box, since the packing is in direct contact with the valve stem, the packing is required to have good sealing performance, small friction coefficient, be able to adapt to the pressure and temperature of the medium, and be resistant to corrosion. At present, the commonly used packings include rubber O-rings, teflon braided packing, asbestos packing and plastic molding packing, etc. Each packing has its own suitable conditions and ranges, which can be selected according to specific needs.

Sealing is preventing leakage, so the principle of valve sealing is also studied from preventing 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 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 tightness of the liquid is carried out by the viscosity and surface tension of the liquid. When the valve leaks and the capillary tube is filled with gas, surface tension may repel the liquid or introduce 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, which will cause leakage. Leaks occur because of the different properties of the media. Experiments with different media will get different results under the same conditions.

Water, air or kerosene can be used. 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 film on these surfaces is dissolved, the properties of the metal surface change, and the previously repelled liquid wets the surface and leaks. In view of the above situation, according to the Poisson formula, the purpose of preventing leakage or reducing leakage can be achieved under the condition of reducing the diameter of the capillary tube and the high viscosity of the medium.

Gas Tightness

According to Poisson’s formula, the gas tightness is related to the viscosity of the gas molecules and the gas. Leakage is inversely proportional to the length of the capillary tube and the viscosity of the gas, and proportional to the diameter of the capillary tube and the driving force. When the diameter of the capillary tube is the same as the average degree of freedom of the gas molecules, the gas molecules will flow into the capillary tube with free thermal motion. Therefore, when we do the sealing test of the valve, the medium must be water to play the role of sealing, and air, that is, gas, cannot play the role of sealing.

Even if we reduce the diameter of the capillary tube below the gas molecules by plastic deformation, the flow of the gas still cannot be stopped. The reason is that the gas can still diffuse through the metal wall. So when we do the gas test, we must be more strict than liquid test.

The Sealing Principle of The Leakage Channel

The valve seal consists of two parts, the degree of unevenness spread over the corrugated surface and the roughness constituted by the waviness of the distance between crests. Under the circumstance that most of the metal materials in our country have low elastic strain force, if the sealing state is to be achieved, it is necessary to raise higher requirements for the compressive force of the metal material, that is, the compressive force of the material must exceed its elasticity. Therefore, when designing the valve, the sealing pair needs to be 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 appear first, and these uneven protrusions can be plastically deformed initially with a small load. 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. It is necessary to apply a load that can cause severe plastic deformation of the underlying material, and to make the two surfaces in close contact, along the continuous line and hoop direction to make these surviving paths close.

Valves Sealing Pair

The valve sealing pair is the part that closes when the valve seat and the closing member are in contact with each other. During use, the metal sealing surface is easily damaged by intercalated media, media corrosion, wear particles, cavitation and scour. Such as wear particles, if the wear particles are smaller than the unevenness of the surface, the surface accuracy will be improved without deterioration when the sealing surface is run-in. On the contrary, the surface accuracy will be deteriorated. Therefore, when selecting wear particles, factors such as their material, working conditions, lubricity and corrosion to the sealing surface should be comprehensively considered.

Just like wear particles, when we choose seals, we must comprehensively consider various factors that affect their performance in order to play the function of preventing leakage. Therefore, materials that are resistant to corrosion, scratches and scour must be selected. Otherwise, the lack of any one of the requirements will greatly reduce its sealing performance.