Sealing principle of valve
The requirements of valve sealing performance should be based on the prevention of leakage. According to the different parts and degrees of leakage, the leakage of the valve is different. Therefore, it is necessary to put forward different leakage prevention measures.
Seal 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 on both sides of the sealing pair. The principle of valve sealing is analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing and valve sealing pair.
Sealing of liquid
The sealing of liquid is through the viscosity and surface tension of liquid. When the capillary leaking from the valve is full of gas, the 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 when the pressure is high, which will cause leakage. The reason for the leakage is the different properties of the medium. Different results can be obtained by using different media under the same conditions.
You can use water, air, kerosene, etc. When the tangent angle is greater than 90 ° Leakage can 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 originally excluded liquid will wet the surface and leak. In view of the above situation, according to Poisson formula, the purpose of preventing leakage or reducing leakage can be achieved under the condition of reducing capillary diameter and medium viscosity.
According to Poisson’s formula, the tightness of gas is related to gas molecules and gas viscosity. The 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 equal to 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, air that is, gas can not play the role of sealing.
Even if we reduce the capillary diameter below the gas molecule 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 passage
The valve seal is composed of two parts: roughness, which is composed of unevenness scattered on the wavy surface and waviness of distance between wave crests. In the case of low elastic strain of most metal materials in our country, 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 the material should exceed its elasticity.
Therefore, in the design of the valve, the sealing pair is matched with a certain hardness difference, which will produce a certain degree of plastic deformation sealing effect under the action of pressure.
If the sealing surface is made of metal material, the protruding points with uneven surface will appear first. At the beginning, only a small load can make these protruding points produce plastic deformation. When the contact surface increases, the surface roughness will become plastic elastic deformation. At this time, the roughness on both sides of the concave will exist.
It is necessary to apply a load that can cause serious plastic deformation of the underlying material, and make the two surfaces contact closely, so as to make the remaining path close along the continuous line and circumferential direction.
Sealing pair of valve
The valve sealing pair is the part of the valve seat and the closing part that closes when they contact each other. The metal sealing surface is easy to be damaged by medium, medium corrosion, wear particles, cavitation and erosion in the process of use. Such as wear particles.
If the roughness of the 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 make the surface accuracy worse. Therefore, in the selection of wear particles, the material, working condition, lubricity and corrosion of sealing surface should be considered comprehensively.
Just like the wear particles, when we choose the seal, we should consider all kinds of factors that affect its performance in order to play the function of anti leakage. Therefore, it is necessary to choose those materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any requirement will greatly reduce its sealing performance.
Main factors affecting valve sealing
There are many factors affecting valve sealing, mainly as follows:
Structure of sealing pair
Under the change of temperature or sealing force, the structure of sealing pair will change. And this change will affect and change the force between the sealing pair, thus reducing the performance of the valve seal.
Therefore, in the selection of seals, we must choose seals with elastic deformation. At the same time, also pay attention to the width of the sealing surface. The reason is that the contact surface of the sealing pair can not be completely matched, when the width of the sealing surface increases, it is necessary to increase the force required for sealing.
Specific pressure of sealing surface
The specific pressure of the sealing surface affects the sealing performance and service life of the valve. Therefore, the sealing surface pressure is also a very important factor. Under the same conditions, too high specific pressure will cause valve damage, but too low specific pressure will cause valve leakage. Therefore, we need to fully consider the suitability of specific pressure in the design.
Physical properties of media
The physical properties of the medium also affect the sealing performance of the valve. These physical properties include temperature, viscosity and surface hydrophilicity. The change of temperature not only affects the relaxation of sealing pair and the change of part size, but also has an inseparable relationship with the viscosity of gas. The gas viscosity increases or decreases with the increase or decrease of temperature.
Therefore, in order to reduce the influence of temperature on the sealing performance of the valve, when we design the sealing pair, we should design it as a valve with thermal compensation such as elastic seat. Viscosity is related to the permeability of the fluid.
Under the same conditions, the greater the viscosity, the smaller the permeability of the fluid. Hydrophilicity of the surface means that when there is a film on the metal surface, the film should be removed. Because this thin oil film will destroy the hydrophilicity of the surface, resulting in blocking the flow channel.
Quality of sealing pair
The quality of sealing pair mainly means that we should check the material selection, matching and manufacturing accuracy. For example, the disc and seat sealing surface is very consistent, can improve the sealing. Its labyrinth sealing performance is good because of its more circumferential waviness.
Valve leakage is very common in life and production, which may cause waste or bring danger to life. For example, the leakage of tap water valve may lead to serious consequences, such as the leakage of toxic, harmful, flammable, explosive and corrosive medium in chemical industry, which may seriously threaten personal safety, property safety and environmental pollution.
A valve which is opened and closed by external rotary drive is designed with a sealing device, which is used to install a certain number of packing rings in the packing culvert to achieve sealing effect. But what is the sealing situation?
Valve packing leakage is one of the most prone parts of the valve leakage failure, but there are two reasons.
Sealing form of valve
Seal in the valve is also a very key component. The sealing performance of the valve refers to the ability of each sealing part of the valve to prevent medium leakage, which is the most important technical performance index of the valve.
There are three sealing parts of the valve:
- The contact between the opening and closing parts and the two sealing surfaces of the valve seat;
- The matching place of packing, valve stem and stuffing box;
- The joint between valve body and valve cover.
One of the previous leakage is called internal leakage, which is usually said to be not closed tightly, it will affect the ability of the valve to cut off the medium. For block valves, internal leakage is not allowed. The leakage of the latter two places is called external leakage, that is, the medium leaks from the inside of the valve to the outside of the valve.
Leakage will cause material loss, pollute the environment, and even cause accidents. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.
Next we will talk about the valve dynamic seal, static seal.
Valve dynamic seal, mainly refers to the stem seal. It is the central task of valve dynamic sealing to prevent the medium in the valve from leaking with the movement of valve stem.
Stuffing box form: valve dynamic seal, mainly stuffing box. The basic form of stuffing box is as follows:
1. Gland type: This is the most used form.
The unified form can distinguish many details. For example, from the perspective of hold down bolt, it can be divided into T-shaped bolt (used for low-pressure valve with pressure ≤ 16 kg / cm2), stud bolt and looper bolt, etc. From the gland, it can be divided into integral type and combined type.
2. Compression nut type: this type of form, small size, but the compression force is limited, only used in small valves.
Packing: in the stuffing box, the packing is in direct contact with the valve stem and is covered with the stuffing box to prevent medium leakage. The requirements for fillers are as follows:
- Good sealing performance;
- Corrosion resistance;
- The friction coefficient is small;
- Comply with medium temperature and pressure.
With the rapid growth of chemical industry and atomic energy industry, the number of inflammable, explosive, highly toxic and radioactive materials has increased, and the valve seal has more stringent requirements. In some places, it is impossible to use packing seal, and a new sealing form, bellows seal, has taken place. This kind of seal does not need packing, also known as no packing seal.
Both ends of bellows are welded with other parts. When the valve rod rises and falls, the bellows will expand and shrink, and the bellows will not leak, so the medium can not be released. For the sake of safety, bellows and packing are often used for double sealing.
What is static seal? Static seal usually refers to the seal between two stationary surfaces. The main sealing method is to use gasket.
Properties of frequently used gaskets:
When using the valve, the original gasket is often replaced according to the specific situation. Common gaskets are: rubber flat washer, rubber O-ring, plastic flat washer, PTFE coated washer, asbestos rubber washer, metal flat washer, metal special-shaped washer, metal clad washer, corrugated washer, wrapping washer, etc.
1. Rubber flat washer: easy to deform, not hard to press, but poor pressure and temperature resistance, only used for low pressure and low temperature. Natural rubber has a certain acid-base resistance, and its service temperature should not exceed 60 ℃; Chloroprene rubber is also resistant to some acids and bases, and its service temperature is 80 ℃; NBR is oil resistant and can be used up to 80 ℃; Fluorine rubber has good corrosion resistance and temperature resistance, and can be used in 150 ℃ medium.
2. Rubber O-shaped washer: the section shape is round, has a certain self tightening effect, the sealing effect is better than the flat washer, and the compression force is smaller.
3. Plastic flat washer: the biggest characteristic of plastic is good corrosion resistance, most of the plastic temperature resistance is not good. Polytetrafluoroethylene (PTFE) is the top of plastics. It has excellent corrosion resistance and wide temperature range. It can be used for a long time from – 180 ℃ to + 200 ℃.
4. Polytetrafluoroethylene wrapped gasket: fully play the advantages of polytetrafluoroethylene, at the same time make up for its poor elasticity error, made of polytetrafluoroethylene wrapped rubber or asbestos rubber gasket. In this way, it not only has the same corrosion resistance as the PTFE flat washer, but also has excellent elasticity, enhances the sealing effect and reduces the compression force.
5. Asbestos rubber gasket: cut from asbestos rubber sheet. It consists of 60-80% asbestos, 10-20% rubber, fillers, vulcanizers, etc. It has good heat resistance, cold resistance, chemical stability, abundant supply and low price. When in use, the pressing force need not be large. It can adhere to metal, it is better to apply a layer of graphite powder, so as to avoid the difficulty of disassembly.
6. Metal flat hot ring: lead, temperature resistance 100 ℃; Aluminum 430 ℃; Copper 315 ℃; Low carbon steel 550 ℃; Silver 650 ℃; Nickel 810 ℃; Monel (Ni Cu) alloy 810 ℃, stainless steel 870 ℃. Among them, the pressure resistance of lead is poor, aluminum can withstand 64 kg / cm2, and other materials can withstand high pressure.
7. Metal dissimilar gasket:
- Lens washer: self tightening, used in high pressure valve.
- Oval washer: also belongs to high pressure self tightening washer.
- Conical double washer: used for high pressure internal self tightening seal.
- In addition, there are square, diamond, triangle, tooth shape, dovetail shape, B shape, C shape, etc., generally only used in high and medium pressure valves.
8. Metal sheathed gasket: metal has excellent temperature and pressure resistance, and excellent elasticity. Prepuce materials include aluminum, copper, low carbon steel, stainless steel, monel alloy, etc. It is filled with asbestos, polytetrafluoroethylene, glass fiber, etc.
9. Wave washer: it has the characteristics of small pressing force and good sealing effect. The combination of metal and nonmetal is often used.
10. Encircling entangled gasket: it is a thin metal belt and non-metal belt close together, encircling entangled into a multi-layer circle, the section is wave shaped, with good elasticity and sealing. The metal belt can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non metallic belt materials include asbestos, polytetrafluoroethylene, etc.
Above, when talking about the performance of sealing washer, some figures are listed. It must be noted that these figures are closely related to the flange form, medium condition and installation and repair technology. They can be crossed and can not be reached. The pressure resistance and temperature resistance are also mutually transformed. For example, when the temperature is high, the pressure resistance often decreases. These subtle problems can only be realized in practice.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(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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