Valve sealing performance requirements, to prevent leakage point of departure. According to the different parts of the leak and the degree of leakage caused by different valves, therefore, need to propose different leak-proof measures. First, the principle of valve sealing
Sealing is to prevent leakage, then the principle of valve sealing is to prevent leakage from the study. Leakage caused by two main factors, one is the most important factor affecting the sealing performance, that is, there is a gap between the sealing pair, and the other is the pressure difference between the two sides of the sealing pair. Valve sealing principle is also from the liquid tightness, gas tightness, leakage of the sealing principle of the valve and the valve seal from four aspects to analyze.
1, liquid tightness
Liquid tightness is achieved by the viscosity and surface tension of the liquid. When the leaking capillary is filled with 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 °, liquid will be injected into the capillary, which will leak. The reason for the leak is the different nature of the media. Test with different media, under the same conditions, will come to different results. You can use water, air or with kerosene and so on. Leakage can also occur when the tangent angle is greater than 90 °. Because of the oil on the metal surface or waxy film. Once the thin films on these surfaces are dissolved, the properties of the metal surface change. The originally rejected liquid invades the surface and leaks. In view of the above situation, according to Poisson's formula, the purpose of preventing leakage or reducing the leakage can be achieved under the condition of reducing capillary diameter and medium viscosity.
2, gas tightness
According to the Poisson formula, the tightness of the gas depends on the viscosity of the gas molecules and the gas. The leak 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 the gas molecules, the gas molecules flow into the capillary with free thermal motion. Therefore, when we are doing the valve seal test, the media must use water to play the role of sealing, the air that is, the gas can not play a sealing role. Even if we reduce the capillary diameter below the gas molecules by plastic deformation, we still can not stop the gas flow. The reason is that the gas can still diffuse through the metal wall. So we are doing gas test, must be more stringent than the liquid test.
3, Leakage channel sealing principle
The valve seal is composed of two parts: the roughness distributed on the corrugated surface and the corrugation distance between the corrugated surfaces. In our country most of the metal materials are low elastic strain case, if you want to achieve a sealed state, you need to compress the metal material to mention higher requirements, that is, the material compression force to exceed its flexibility. Therefore, the valve design, the seal with a certain degree of hardness to match the difference in the role of pressure, it will have a certain degree of plastic deformation seal effect. If the sealing surfaces are made of a metal material, the bumps on the surface will appear the earliest, and these bumpy bumps will plastically deform initially with only a small load. As the contact area increases, the surface irregularities become plastic - elastic deformation. At this time in the recess on both sides of the roughness there. It is necessary to apply a load that causes severe plastic deformation of the underlying material and to allow the two surfaces to be in close contact with each other in a continuous line and in the circumferential direction to allow these existent paths to close.
4, valve sealing pair
The valve seal is the part where the valve seat and the closure are closed when they contact each other. Metal sealing surface in the process of using, is easy to be caught in the media, media corrosion, wear particles, cavitation and erosion damage. Such as wear particles. If the wear particles than the surface roughness is small, the seal surface wear, the surface accuracy will be improved, without deterioration. On the contrary, it will deteriorate the surface accuracy. Therefore, the choice of wear particles, it is necessary to consider its material, working conditions, lubricity and corrosion of the sealing surface and other factors. As with the wear particles, we select the seal, we must consider the various factors that affect its performance, can play a leak-proof function. Therefore, you must select those materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any one requirement will greatly reduce its sealing performance. Second, affect the valve seal the main factors
Affect the valve seal a lot of factors, mainly in the following categories:
1, seal vice structure
Under the influence of temperature or sealing force, the structure of sealing pair will change. And this change will affect and change the sealing pair of the interaction between each other, so that the valve seal performance decreases. Therefore, the choice of seal, be sure to choose a seal with elastic deformation. At the same time, also pay attention to the width of the sealing surface. The reason is that the sealing surface of the contact surface can not be completely consistent, when the sealing surface width increases, it is necessary to increase the sealing force.
2, sealing surface pressure
Sealing surface than the size of the size of the valve sealing performance and valve life. Therefore, the sealing surface pressure is also very important factor. Under the same conditions, the pressure is too large will cause damage to the valve, but the pressure is too small wine can cause valve leakage. Therefore, we need to take full account of the appropriate pressure than the design.
3, the physical properties of the media
The physical properties of the medium also affect the valve sealing performance. These physical properties include temperature, viscosity and surface hydrophilicity. Temperature changes not only affect the seal of the slack and the size of the changes, but also with the gas viscosity is inextricably linked. Gas viscosity increases or decreases as the temperature increases or decreases. Therefore, in order to reduce the influence of temperature on the valve sealing performance, we design the seal pair as a valve with thermal compensation such as flexible valve seat. Viscosity and fluid permeability related. When under the same conditions, the greater the viscosity, the less the fluid's ability to penetrate. Hydrophilic surface refers to the metal surface has a layer of film, to remove this layer of film. Because this layer of thin oil film, will destroy the hydrophilic surface, leading to block the fluid channel.
4, the quality of the seal
Seal Vice quality mainly refers to the choice of materials, matching, manufacturing precision on the gatekeeper. For example, the flap and seat sealing surface is very consistent, can improve the sealing. Ring to the more features of the waviness, the labyrinth seal performance is good.
Leakage of valves in the life, production is very common, ranging from waste or cause danger to life, such as tap water valve leaks, heavy lead to serious consequences, such as the chemical industry is toxic, harmful, flammable, explosive and Corrosive media leak, etc., serious threat to personal safety, property safety and environmental pollution accidents. A relying on external rotation of the drive open and close the valve is designed with a sealing device is used in the packing culvert installed a certain number of packing rings, so as to achieve the sealing effect, but what about the sealing? Leakage of the valve packing is the most prone to leakage in the valve of the site one, but the reason there are roughly two reasons. Third, the valve seal form
Seals in the valve is also very crucial components. Valve sealing performance refers to the seal of the valve to prevent leakage of the ability of the media, it is the valve of the most important technical performance indicators.
Valve sealing parts are three:
Opening and closing parts and the valve seat between the two sealing surfaces at the contact; packing and the stem and packing box with the Department; body and the valve cover at the junction. One of the leaks is called leakage before, which is commonly referred to as lax, it will affect the ability of the valve to cut off the medium. For cut-off valves, internal leakage is not allowed. After the two leaks called leakage, that is, the medium leaked from the valve to the valve.
Leakage will result in material loss, pollute the environment, serious accidents will result. For flammable, toxic or radioactive media, leakage is even more not allowed, so the valve must have a reliable sealing performance.
How to solve the sealing problem can not be ignored, the valve running, running, dripping, leakage phenomenon, do the night department happen here. Below we will consider valve dynamic seal, static seal problem.
1, moving seal
Valve dynamic seal, the main refers to the valve stem seal. Do not let the valve medium with the stem movement and leakage, valve sealing is the center topic.
① stuffing box form
Valve sealed to stuffing box based. The basic form of stuffing box is:
(1) gland type
This is the most form.
A unified form can distinguish many details. For example, the compression bolts can be divided T-bolts (for pressure ≤ 16 kg / square centimeter low-pressure valve), studs and bolts and so on. From the gland, can be divided into the overall and combined.
(2) compression nut type of such forms, small size, but the pressure is limited, only for small valves.
② packing
Stuffing box, the filler and the stem direct contact and filled stuffing box, to prevent media leakage. The filler has the following requirements:
(1) good sealing;
(2) resistant to erosion
(3) small coefficient of friction;
(4) comply with the media temperature and pressure.
Regular use of fillers are:
(1) Asbestos packing: Asbestos packing, heat resistance Wen Wen erosion resistance are good, but when used alone, the sealing effect is not safe, always impregnated or attached to other materials. Oil impregnated asbestos packing: It has two basic forms, one is twisted, the other is braided. Can be divided into round and square.
(2) Teflon braided packing: Teflon braided packing, has excellent corrosion resistance, but also can be used for cryogenic medium.
(3) Rubber O-ring: Under low pressure, the sealing effect is excellent. The use of temperature is limited, such as natural rubber can only be used at 60 ℃.
(4) plastic molding filler: generally made of three-piece, but also made of other shapes. The plastic used to polytetrafluoroethylene as much.
In addition, for example, steam valve at 250 ℃, with asbestos packing and lead alternately overlap, the leakage will be reduced; a valve, the media often change, such as the use of asbestos packing and Teflon tape together , The sealing effect will be better. To reduce the friction on the stem, there are places, you can add molybdenum disulfide (M0S2) or other lubricants.
For new packing, is being requested. For example, polyacrylonitrile fiber impregnated with PTFE emulsion, and after pre-oxidation, the mold sintering sintering, can be sealed to good shape forming filler; Another example is made of stainless steel sheet and asbestos corrugated filler, high temperature and high pressure And erosion.
③ bellows seal
The rapid growth of the chemical industry and the atomic energy industry, flammable, explosive, highly toxic and radioactive materials with more stringent requirements on the valve seal, there is no place where the use of packing seal, a new seal - bellows seal. This type of seal without filler, also known as no packing seal.
Bellow two, and other parts welded to death. When the valve stem is up and down, bellows telescopic, bellows do not leak, the media will not escape. To be on the safe side, double seals are often used with bellows and packing.
2, static seal
What is static seal called it?
Static sealing usually refers to the seal between two stationary surfaces. The main method of sealing is the use of gaskets.
① gasket material
(1) Non-metallic materials: such as paper, hemp, leather, asbestos products, plastics, rubber and so on. Paper, hemp, leather and the like, with pores, easy to infiltrate, use must be impregnated with oil, wax or other impermeable material. Valve is rarely used.
Asbestos products, there are asbestos belt, rope, board and asbestos rubber sheet. Asbestos rubber sheet which compact structure, pressure performance, temperature performance is also very good, the valve itself and the valve and pipe flange adjoining, the use of very common.
Plastic products, have good erosion resistance, use is also more common. Varieties of polyethylene, polypropylene, soft PVC, PTFE, nylon 66, nylon 1010 and so on.
Rubber products, soft quality, all kinds of rubber divided into a certain acid, alkali, oil, sea water capacity. Varieties are natural rubber, styrene butadiene rubber, nitrile rubber, neoprene, butyl rubber, polyurethane rubber, fluorine rubber.
(2) metal materials: In general, metal materials, high strength, high temperature performance. But lead is actually not so, only take it resistant to dilute sulfuric acid characteristics. Often used varieties brass, copper, aluminum, mild steel, stainless steel, Monel, silver, nickel and so on.
(3) composite materials: such as metal foreskin (internal asbestos) washer, combined wave washer, wrapping around the washer and so on.
② often use gasket performance
When using the valve, often the specific circumstances, the replacement of the original gaskets. Often have gaskets are: rubber flat washers, rubber O-ring, plastic flat washers, PTFE packing gaskets, asbestos rubber gaskets, metal flat washers, metal washers, metal foreskin washers, wave washers, wrapping around washers and so on.
(1) Flat rubber washers: easy to deform, not difficult when pressed, but the pressure resistance, temperature resistance are poor, only for low pressure, the temperature is not high side. Natural rubber has certain acid and alkali resistance, the use of temperature should not exceed 60 ℃; neoprene can also be resistant to some acid, temperature 80 ℃; nitrile rubber oil, available to 80 ℃; fluorine rubber corrosion resistance is good, resistance Temperature performance than the average rubber, 150 ℃ medium can be used.
(2) Rubber O-shaped washer: The cross-sectional shape is a perfect circle with a certain self-tightening effect. The sealing effect is better than the flat washer and the pressing force is smaller.
(3) plastic flat washers: the largest plastic is resistant to erosion, the temperature resistance of most parts of the plastic is not good. Teflon plastic crown, excellent corrosion resistance, temperature range of relatively wide, -180 ℃ ~ +200 ℃ lasting use.
(4) Teflon gasket: To fully demonstrate the advantages of Teflon while making up for the fallacy of its poor flexibility and made of Teflon coated rubber or asbestos rubber gasket. In this way, both with PTFE flat washer erosion-resistant, but also excellent flexibility, and enhance the sealing effect, reducing the pressure.
(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its composition is 60 ~ 80% asbestos and 10 ~ 20% rubber, and fillers, curing agents and so on. It has good heat resistance, cold resistance, chemical stability, abundant sources, the price is cheap. When used, the pressing force is not necessary. It can adhere to metal, preferably coated with a layer of graphite powder, so as not to strenuous demolition.
(6) Metal flat ring: lead, temperature 100 ℃; aluminum 430 ℃; copper 315 ℃; low carbon steel 550 ℃; silver 650 ℃; nickel 810 ℃; Monel (nickel copper) alloy 810 ℃, stainless steel 870 ℃ . Among them, lead has poor pressure resistance, aluminum can withstand 64 kg / cm 2, and other materials can withstand high pressure.
(7) Metal isotropic gasket:
Lens Gasket: Self-tightening effect, used in high-pressure valve.
Oval washers: also belong to high-pressure self-tight washers.
Conical double washer: for high pressure within the prudent seal.
In addition, there are square, diamond, triangle, toothed, dovetail, B-shaped, C-shaped, generally only used in high pressure valves.
(8) metal foreskin washers: metal both excellent temperature and pressure performance, but also excellent flexibility. Leather materials are aluminum, copper, low carbon steel, stainless steel, Monel and so on. Filling material inside asbestos, PTFE, glass fiber and so on.
(9) Wave Washer: With a small pressure, sealing effect is good. Often used metal and non-metallic combination.
(10) around the entanglement washers: is a very thin metal belt and non-metallic belt close together, entangled into a multi-circle around the cross-section was surf-like, with good elasticity and tightness. Metal with 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy construction. Non-metallic belt materials are asbestos, PTFE and so on.
Above, when it comes to gasket performance, a few numbers are listed. It must be explained that these figures are closely related with the form of flanges, medium conditions, installation and repair techniques, etc., and can cross over, reach, withstand voltage and temperature resistance, and also transform into each other. For example, the temperature is high and the withstand voltage capability tends to decrease These subtle problems can only be understood in practice. Fourth, the valve sealing material
Valve seal material is an important part of the valve seal, which acts as a valve seal in direct contact with the surface. What are the valve sealing materials do, we know that the valve seal materials are two categories of metal and non-metallic.
Valve seal material is an important part of the valve seal, which acts as a valve seal in direct contact with the surface. What are the valve sealing materials do, we know that the valve seal materials are two categories of metal and non-metallic.
Here's a brief introduction to the conditions of use of various types of sealing materials, as well as the types of valves commonly used:
1, synthetic rubber
Synthetic rubber oil, temperature, corrosion resistance and other comprehensive performance better than natural rubber. General synthetic rubber temperature t ≤ 150 ℃, natural rubber t ≤ 60 ℃, rubber for nominal pressure PN ≤ 1MPa globe valve, gate valve, diaphragm valve, butterfly valve, check valve, pinch valves and other valves sealed.
2, nylon
Nylon has a small friction coefficient, good corrosion resistance and so on. Nylon used for temperature t ≤ 90 ℃, nominal pressure PN ≤ 32MPa ball valve, globe valve and so on.
3, Teflon
Polytetrafluoroethylene and more for temperature t ≤ 232 ℃, PN ≤ 6.4MPa nominal pressure of the globe valve, valve, ball valve and so on.
4, cast iron
Cast iron for temperature t ≤ 100 ℃, the nominal pressure PN ≤ 1.6MPa, gas and oil gate valve, globe valve, plug valves.
5, Babbitt
Babbitt alloy for temperature t-70 ~ 150 ℃, Nominal pressure PN ≤ 2.5MPa ammonia with the stop valve.
6, copper alloy
Copper alloy materials commonly used 6-6-3 tin bronze and 58-2-2 manganese brass. Copper alloy wear resistance, suitable for temperature t ≤ 200 ℃, nominal pressure PN ≤ 1.6MPa of water and steam, commonly used in gate valves, globe valves, check valves, plug valves.
7, chrome stainless steel
Chrome stainless steel commonly used grades 2Cr13, 3Cr13 quenched and tempered, corrosion resistance is good. Commonly used in the temperature t ≤ 450 ℃, nominal pressure PN ≤ 32MPa of water, steam and oil and other media on the valve.
8, chrome nickel titanium stainless steel
Common chromium-nickel-titanium stainless steel grades 1Cr18Ni9ti, its corrosion resistance, erosion resistance and heat resistance better. For temperature t ≤ 600 ℃, nominal pressure PN ≤ 6.4MPa steam, nitric acid and other media, used in globe valves, ball valves and so on.
9, nitrided steel
The common grade of nitrided steel is 38CrMoAlA, which is carburized and has good corrosion resistance and scratch resistance. Commonly used in temperature t ≤ 540 ℃, nominal pressure PN ≤ 10MPa of the power station gate valve.
10, boronizing
Boronizing body or valve body material directly processed from the sealing surface, and then boronizing surface treatment, sealing surface wear resistance well. For power plant sewage valve.
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