For valve seal performance, prevent leakage Angle. According to the different parts and degree of leakage, the leakage situation of the valve is different. Therefore, different leakage prevention measures are required. 1. Valve sealing principle
 
Sealing is to prevent leakage, so the valve sealing principle is also from the prevention of leakage research. Cause leakage there are two main factors, one is the main factors influencing the sealing performance, namely gap exists between the sealing pair, the other is seal pressure difference exists between the both sides of the deputy. Valve sealing principle is also analyzed from four aspects including the sealing of liquid, sealing of gas, sealing principle of leakage channel and valve seal.
 
1. Sealing of liquid
 
The sealability of the liquid is carried out by the viscosity and surface tension of the liquid. When the valve is filled with gas, the surface tension may be repulsive to the liquid or the liquid is introduced into the capillary. So you have a tangent Angle. When the tangent Angle is less than 90 °, the liquid will be injected into capillary, it will leak. The cause of leakage is the different properties of the medium. Experiment with different media, under the same conditions, different results can be obtained. You can use water, air or kerosene. The tangent Angle is greater than 90 °, and also will leak. Because it has to do with grease or waxy films on metal surfaces. Once the membranes of these surfaces are dissolved, the properties of the metal surface change, and the previously rejected liquid will then invade the surface and leak. For the above situation, according to the poisson formula, the purpose of preventing leakage or reducing leakage can be achieved by reducing capillary diameter and medium viscosity.
 
Gas sealing
 
According to poisson's formula, the gas seals are related to the viscosity of the gas molecules and gases. The leakage is inversely proportional to the length of the capillary and the viscosity of the gas, proportional to the diameter and drive 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 heat. Therefore, when we are doing the valve seal test, the medium must be used for the sealing effect of water, which can not be sealed by air. Even if we move the capillaries down to the gas molecules in the form of plastic deformation, we still can't stop the flow of gas. The reason is that gases can still spread through the metal walls. So when we do a gas test, it's definitely more rigorous than the liquid test.
 
3. Sealing principle of leakage channel
 
The valve seal consists of two parts, the roughness of the waviness of the distance between the unflatness and the peak of the wave. In our country, most of the elastic resilience of metallic materials are lower case, if you want to reach the state of the seal you need regarding the compression strength of the metal material is higher requirements, namely material than its elastic compression force. Therefore, when valve design is carried out, the sealing side combines a certain hardness to match, and under the action of pressure, a certain degree of plastic deformation sealing effect is produced. If the sealing surface is metal material, surface smooth convex point will be the emergence of the earliest in the original with the smaller load can make these uneven protruding point to produce plastic deformation. When the contact surface increases, the uneven surface becomes plastic-elastic deformation. And then the two surfaces of the concave are there. It is necessary to apply a load that can produce severe plastic deformation to the underlying material, and to make the two surfaces close together, along the continuous line and the direction of the loop, to make the remaining diameter close together.
 
4. Valve seal pair
 
Valve seals are the part of the seat and the closing part that is closed when contact with each other. The metal sealing surface is easy to be clamped into medium, medium corrosion, wear and tear, cavitation and erosion damage. Like wear particles. If the abrasive particles are less than the surface of the surface, the surface accuracy of the surface will improve without worsening. On the contrary, the surface accuracy can be deteriorated. Therefore, when selecting the abrasive particles, the factors such as the material, operating condition, lubrication and corrosion of the sealing surface should be taken into consideration. As with the abrasive particles, we must consider the various factors that affect its performance when selecting the sealing part, which can be used to prevent leakage. Therefore, it is necessary to select materials that are resistant to corrosion, abrasion and erosion resistance. Otherwise, the lack of any requirement will make its sealing performance greatly reduced. Ii. Main factors affecting valve seal
 
There are many factors affecting valve seal, mainly including the following:
 
1. Seal sub-structure
 
Under the change of temperature or seal force, the structure of the seal is changed. Moreover, this change can affect and change the force between the seals, thus reducing the performance of the valve seal. Therefore, when choosing seals, be sure to choose the seals with elastic deformation. Also pay attention to the width of the cover. The reason is that the contact surface of the seal is not completely consistent, and when the width of the sealing cover is increased, the force needed for sealing will be increased.
 
The cover is more than the pressure
 
The size of the sealing surface affects the sealing performance of the valve and the service life of the valve. Therefore, the sealing surface is also a very important factor. Under the same conditions, the valve may be damaged by the pressure of the assembly, but the valve is leaking when the pressure is too small. Therefore, we need to be able to design with adequate consideration of the ratio of pressure.
 
The physical properties of the medium
 
The physical properties of the media also affect valve sealing performance. These physical properties include temperature, viscosity and surface affinity. Temperature change not only affects the looseness of the seal and the change of parts size, but also has an inseparable relationship with the viscosity of gas. The gas viscosity increases or decreases as the temperature increases or decreases. Therefore, in order to reduce the influence of temperature on valve seal performance, we shall design the valve with thermal compensation in the design of elastic seat. Viscosity is related to the permeability of fluid. When under the same conditions, the greater the viscosity, the lower the permeability of the fluid. Surface hydrophilicity refers to the removal of the film when there is a thin film on the metal surface. Because of this thin film of oil, it can damage the hydrophilicity of the surface, leading to the blockage of the fluid.
 
4. The quality of the seal
 
Seal quality mainly refers to the selection of materials, matching and manufacturing precision. For example, the disc fits well with the seat airtight cover and can improve the seal. The characteristic of the ring to waviness is its labyrinth sealing property is good.
 
Valve leakage is common in the life and production, light can cause waste, or bring danger to life, such as tap water valve leak, or causing serious consequences, such as chemical industry of poisonous and harmful, inflammable, explosive and corrosive medium leakage in the nature of, serious threat to personal safety and property safety, and environmental pollution accidents. A rely on outside force rotating transmission opening and closing of the valve design have a sealing device is used in the stuffing box containing a certain amount of packing seal, to achieve sealing effect, but seal situation is how? The leakage of the valve is one of the most easily leaky parts of the valve, but there are two main reasons for this. Iii. Valve sealing form
 
Seals are also critical components in valves. The sealing performance of the valve is the ability of the valve seal to prevent the leakage of the medium. It is the most important technical performance indicator of the valve.
 
There are three sealing parts of the valve:
 
The contact between the opening and the two sealing surfaces of the seat; The coordination of packing and stem and stuffing box; The connection of the valve body to the bonnet. The first leak is called an internal leak, which is often said to be lax, and it will affect the valve's ability to truncate the medium. For truncated valve classes, internal leakage is not allowed. The latter two leaks are called external leakage, that is, the medium leaks out of the valve to the outside of the valve.
 
Leakage can cause material loss, environmental pollution and serious accidents. For flammable, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.
 
How to solve the sealing problem is not neglect, the valve runs, runs, drops, leaks, and the department happens all the time. Below we will discuss valve operation seal, static seal problem.
 
1. Dynamic seal
 
Valve movement seal, main finger stem seal. The valve inner medium is not leaking with stem movement. It is the center of valve action seal.
 
Form of stuffing box
 
Valve dynamic sealing, mainly packing letter. The basic form of the stuffing box is:
 
(1) gland type
 
This is the most common form.
 
A uniform form can be distinguished by many details. For example, from the compression bolt, the separable T bolt (for pressure is less than 16 kg/cm low pressure valve), double head bolts, and live bolt, etc. From the gland, it can be integrated and combined.
 
(2) the compact nut type is small, but the compression force is restricted and only used in small valves.
 
(2) packing
 
In the stuffing box, contact with the stem directly and fill the stuffing box to prevent leakage. There are the following requirements for packing:
 
(1) the sealing is good;
 
(2) corrosion resistance;
 
(3) small friction coefficient;
 
(4) adapt to medium temperature and pressure.
 
Frequently used fillers are:
 
(1) asbestos packing: asbestos, resistant to erosion and resistance to corrosion, but the sealing effect is not good when it is used, always impregnated or attached to other materials. Oil immersion asbestos packing: it has two basic structural forms, one is twist and the other is braid. It can be rounded and square.
 
(2) polytetrafluoroethylene braid: the polytetrafluoroethylene strip is woven into a disk root, which has excellent corrosion resistance and can be used in cryogenic media.
 
(3) rubber o-ring: under low pressure, the sealing effect is excellent. Use temperature is restricted, such as natural rubber can only be used for 60 ℃.
 
(4) plastic molding filler: generally made into three parts, it can also be made into other shapes. The plastic used is polytetrafluoroethylene.
 
In addition, for example, in 250 ℃ steam valves, lamination, alternately with asbestos packing and lead lap steam leakage situation will ease; There are valves, the medium often transforms, such as asbestos packing and polytetrafluoroethylene raw material belt to use, the sealing effect is better. In order to reduce the friction of the stem, there is a place where you can add molybdenum disulfide (M0S2) or other lubricants.
 
The novel packing is being carried out. For example, the polyacrylonitrile fiber is impregnated with polyfluoroethylene emulsion, and after preoxidation, the mould is sintered and pressed, and the sealing performance is excellent. It is also made of stainless steel sheet and asbestos into corrugated packing, which can be resistant to high temperature, high pressure and erosion.
 
The bellows seal
 
Rapid growth and atomic energy industry, chemical industry flammable, explosive, toxic and radioactive materials increased, the valve seal more harsh requirements, have already can't use packing seal, a new seal form - bellows seal. This kind of seal does not need filler, also called packless seal.
 
The two sides of the bellows are welded to the dead. When the stem is raised, the bellows scale, the bellows are not leaking, and the medium can't be escaped. For the sake of insurance, often use bellows and packing double seal.
 
Static seal
 
What is a static seal?
 
A static seal is usually a seal between two stationary surfaces. The sealing method is mainly use gasket.
 
Gasket material
 
(1) non-metallic materials: such as paper, flax, cowhide, asbestos products, plastics, rubber, etc. Paper, flax, cowhide, etc., have capillaries, permeable, must be impregnated with oil, wax or other permeable material. General valves are rarely used.
 
Asbestos products, asbestos belt, rope, plate and asbestos rubber sheet, etc. The structure of the asbestos rubber sheet is compact, the pressure resistance is good, the temperature resistance is good, the valve and the valve are connected to the pipe flanges, which is very common.
 
Plastic products, have very good erosion resistance performance, use is also more common. The varieties are polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010, etc.
 
Rubber products, soft and soft, various kinds of rubber have certain acid, alkali resistance, oil resistance, seawater resistance. The varieties have natural rubber, butadiene rubber, butadiene rubber, chloroprene rubber, isobutyl rubber, polyurethane rubber, fluorine rubber and so on.
 
(2) metal material: generally speaking, metal materials have high strength and high temperature resistance. But lead doesn't. It just takes its dilute sulfuric acid. Frequently used varieties are brass, copper, aluminum, low carbon steel, stainless steel, monel alloy, silver, nickel, etc.
 
(3) composite materials: such as metal foreskin (inner asbestos) gasket, combination of waveform gaskets, surrounding entanglement gaskets, etc.
 
Use gasket property frequently
 
When using the valve, often specific situation, replace the original belt gasket. Common gasket are: flat washer, rubber o-ring, rubber plastic flat gasket, ptfe gasket, asbestos rubber gasket, metal flat washer, metal profile gasket, metal wrapping washers, wave washers, surrounding the entanglement washer, etc.
 
(1) rubber flat gasket: it is easy to deform, not hard when pressed, but it has poor resistance and temperature resistance, only for low pressure and low temperature. Natural rubber is, to some extent, acid-proof alkaline can use temperature is unfavorable across 60 ℃; Neoprene also ability some acid and alkali, temperature 80 ℃; Nitrile rubber oil resistant, available to 80 ℃; Fluorine rubber corrosion resistance performance is good, heat resistance performance is better than general rubber, can be used in 150 ℃ medium.
 
(2) rubber o-type gasket: the profile of the section is positive and has a certain self-tightening effect. The sealing effect is better than the flat washers, and the tension force is smaller.
 
(3) plastic flat gasket: the annual feature of plastic is good corrosion resistance, the annual night department plastic temperature is not good. Ptfe plastic crown, good erosion resistance, heat resistance than force a wide range, can be - lasting use within 180 ℃ ~ + 200 ℃.
 
(4) polytetrafluoroethylene bag washer: enrich the advantages of polytetrafluoroethylene, and make up for its less elastic error, and make the polytetrafluoroethylene rubber or asbestos rubber gasket. In this way, it has the same corrosion resistance as ptfe, and excellent elasticity, enhancing the sealing effect and reducing the compression force.
 
(5) asbestos rubber gasket: made of asbestos rubber sheet. Its component is 60 ~ 80% asbestos and 10 ~ 20% rubber, and filling agent, vulcanization agent etc. It has good heat resistance, cold resistance, chemical stability, plentiful supply and cheap price. When used, the pressure is not necessary. It can stick to metal, preferably with a sheet of graphite powder, so as not to be disassembled.
 
(6) metal flat hot laps: lead, 100 ℃ heat resistance; Aluminum 430 ℃; Copper 315 ℃; Low carbon steel 550 ℃; Silver 650 ℃; Nickel 810 ℃; Monel nickel copper alloy is 810 ℃, stainless steel 870 ℃. The lead tolerance is poor, the aluminum is 64 kg/cm2, and other materials can withstand high pressure.
 
(7) metal heterosexual gaskets:
 
Lens gasket: it has a self-tightening action, which is used in high pressure valve.
 
Elliptical gasket: also high pressure self-tightening gasket.
 
Tapered double gasket: used for high pressure internal self-sealing.
 
In addition, there are square, diamond, triangle, tooth shape, dovetail shape, B shape, C shape, etc., generally used in high school pressure valve.
 
(8) metal cover gasket: the metal has excellent durable-temperature resistance and excellent elasticity. The material is aluminum, copper, low carbon steel, stainless steel, monel alloy etc. inside