1, the principle of valve sealing
Sealing is to prevent leakage, so the principle of valve sealing is also studied from leak prevention. 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.
The tightness of the liquid is carried out by the viscosity and surface tension of the liquid. When the leaking capillary of the valve is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. This creates a tangency angle.
When the tangency angle is less than 90°, the liquid is injected into the capillary, which causes leakage. The reason for the leakage is the different nature of the medium. Experimenting with different media will give different results under the same conditions. It can be used with water, with air or with kerosene. When the tangency angle is greater than 90°, leakage will also occur.
Because it is related to grease or waxy film on the metal surface. Once the film on these surfaces is dissolved, the properties of the metal surface change, and the originally repelled liquid will invade the surface and leak. In view of the above, according to the Poisson formula, it is possible to achieve the purpose of preventing leakage or reducing the amount of leakage while reducing the capillary diameter and the viscosity of the medium.
According to the Poisson formula, the gas tightness is related to the viscosity of gas molecules and gases. The leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and is 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 sealing test, the medium must be used for water to seal, and the air, that is, the gas, cannot function as a seal. Even if we reduce the capillary diameter below the gas molecules by plastic deformation, it still cannot stop the flow of gas. 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 spread on the wave surface and the waviness between the peaks. In the case that most of the metal materials in our country have low elastic strain, if the sealing state is to be achieved, the compressive force of the metal material needs to be higher, that is, the compressive force of the material exceeds its elasticity.
Therefore, when designing the valve, the sealing pair is combined with a certain hardness difference to match, and under the action of pressure, a certain degree of plastic deformation sealing effect is produced. If the sealing surfaces are all metallic materials, the uneven spots on the surface will appear at the earliest, and the uneven protruding points can be plastically deformed at the beginning with a small load.
When the contact surface is increased, the unevenness of the surface becomes plastic-elastic deformation. At this time, the roughness on both sides of the recess will exist. When a load is required to cause severe plastic deformation of the underlying material, and the two surfaces are in close contact, the remaining paths can be brought into close contact along the continuous line and the circumferential direction.
The valve seal pair is the portion of the valve seat and closure that closes when in contact with each other. Metal sealing surfaces are susceptible to damage from entrainment of media, media corrosion, wear particles, cavitation and erosion during use. Such as wear particles.
If the wear particles are smaller than the unevenness of the surface, the surface accuracy is improved when the sealing surface is worn-in, without deterioration. On the contrary, the surface accuracy is deteriorated. Therefore, when selecting wear particles, factors such as material, working condition, lubricity and corrosion to the sealing surface should be considered.
Just like wear particles, when choosing a seal, we must consider various factors that affect its performance to prevent leaks. Therefore, materials that are resistant to corrosion, scratching and erosion must be selected. Otherwise, the lack of any requirement will greatly reduce the sealing performance.
2, the main factors affecting the valve seal
There are many factors affecting the valve seal, mainly the following:
Sealed secondary structure
The structure of the seal pair changes under the influence of temperature or sealing force. Moreover, this change affects and changes the force between the seal pairs, thereby reducing the performance of the valve seal. Therefore, when selecting a seal, it is necessary to select a seal with elastic deformation.
Also, pay attention to the width of the sealing surface. The reason is that the contact surface of the sealing pair cannot be completely matched. When the width of the sealing surface is increased, the force required for sealing is increased.
Sealing surface pressure
The specific pressure of the sealing surface affects the sealing performance of the valve and the service life of the valve. Therefore, the sealing surface specific pressure is also a very important factor. Under the same conditions, the specific pressure will cause damage to the valve, but the pressure will be too small to cause the valve to leak. Therefore, we need to fully consider the appropriateness of the specific pressure in the design.
Physical properties of the medium
The physical properties of the media also affect valve sealing performance. These physical properties include temperature, viscosity, and hydrophilicity of the surface. Temperature changes not only affect the slack of the seal pair and the dimensional change of the part, but also have an inseparable relationship with the viscosity of the gas.
The viscosity of the gas increases or decreases as the temperature increases or decreases. Therefore, in order to reduce the degree of influence of temperature on the sealing performance of the valve, we design it as a thermally compensated valve such as an elastic valve seat when designing the sealing pair.
Viscosity is related to the ability of the fluid to penetrate. When under the same conditions, the greater the viscosity, the smaller the ability of the fluid to penetrate. The hydrophilicity of the surface means that the film is removed when there is a film on the metal surface. Because of this very thin oil film, it will damage the hydrophilicity of the surface, leading to the passage of fluids.
Quality of the seal pair
The quality of the seal pair mainly means that we have to check the selection, matching and manufacturing precision of the materials. For example, the valve flap fits well with the seat sealing surface to improve sealing. The characteristic of the circumferential waviness is that the labyrinth sealing performance is good.
Valve leakage is very common in life and production. It can cause waste or bring danger to life. For example, the leakage of tap water valves can lead to serious consequences, such as toxic, harmful, flammable and explosive in the chemical industry. Corrosive dielectric leakage, etc., seriously threaten personal safety, property safety and environmental pollution accidents.
A valve that relies on external force rotary drive to open and close is designed with a sealing device that uses a certain number of packing seals in the packing culvert to achieve the sealing effect, but what about the sealing situation? Leakage at the packing of the valve is one of the most prone to leakage faults in the valve, but there are two reasons for this.
3, valve sealing form
Seals are also a critical component in valves. The sealing performance of the valve refers to the ability of the sealing parts of the valve to prevent the leakage of the medium. It is the most important technical performance index of the valve.
There are three sealing parts of the valve:
The contact between the sealing surface of the opening and closing member and the valve seat; the joint of the packing with the valve stem and the stuffing box; the connection between the valve body and the valve cover. The leak in the former place is called endoleak, which is commonly referred to as the lack of tightness, which will affect the ability of the valve to cut off the medium. For shut-off valves, internal leakage is not allowed. The latter two leaks are called external leaks, ie the medium leaks from the valve to the outside of the valve.
External leakage can cause material loss, pollute the environment, and cause accidents in severe cases. For flammable, explosive, toxic or radioactive media, external leakage is not allowed, so the valve must have a reliable sealing performance.
How to solve the sealing problem is not negligible, the valve runs, runs, drops, leaks, the department happens here. Below we will consider the problem of valve dynamic sealing and static sealing.
The valve is dynamically sealed and the main finger is sealed. It does not allow the medium in the valve to leak with the movement of the valve stem, which is the center of the valve dynamic seal.
1) Stuffing box form
The valve is dynamically sealed and is mainly filled with stuffing. The basic form of the stuffing box is:
(1) gland type
This is the most used form.
The unified form can be distinguished by many details. For example, from the compression bolts, T-bolts (for low pressure valves with pressure ≤ 16 kg / cm 2 ), studs and joint bolts can be divided. From the gland, it can be divided into integral and combined.
(2) compression nut type
This type of form has a small size, but the pressing force is limited and is only used for small valves.
In the stuffing box, the filler is in direct contact with the valve stem and filled with a stuffing box to prevent leakage of the medium. The following requirements apply to the packing:
(1) good sealing;
(2) corrosion resistance;
(3) The friction coefficient is small;
(4) Comply with the medium temperature and pressure.
Frequently used fillers are:
(1) Asbestos packing: Asbestos packing, resistance to temperature and corrosion resistance are very good, but when used, the sealing effect is not good, always dipping or adding other materials. Oil-impregnated asbestos packing: There are two basic structural forms, one is twisting and the other is braiding. It can be divided into circles and squares.
(2) Polytetrafluoroethylene braided packing: The PTFE strip is woven into a packing, which has excellent corrosion resistance and can be used for deep cooling medium.
(3) Rubber O-ring: Excellent sealing effect under low pressure. The use temperature is limited, such as natural rubber can only be used at 60 ° C.
(4) Plastic molding filler: generally made of three-piece, can also be made into other shapes. The plastic used is mostly polytetrafluoroethylene.
In addition, for example, in a 250°C steam valve, the asbestos packing and the lead ring are alternately stacked, and the steam leakage situation is alleviated; there are valves, and the medium is often changed, such as using asbestos packing and polytetrafluoroethylene raw material belt. The sealing effect will be better. In order to reduce the friction on the valve stem, there is a place where molybdenum disulfide (M0S2) or other lubricant can be added.
For new fillers, we are asking for it. For example, the polyacrylonitrile fiber is impregnated with a polytetrafluoroethylene emulsion, and after pre-oxidation, the mold is sintered and pressed to obtain a sealing material with excellent sealing performance; and the stainless steel sheet and asbestos are used to form a wave filler, which can withstand high temperature and high pressure. With erosion.
3) Bellows seal
The chemical industry and the atomic energy industry have grown rapidly, flammable, explosive, highly toxic, and with radioactive materials. There are more stringent requirements for valve sealing. Some places have no way to use packing seals, and new sealing forms have emerged – bellows seals. This type of seal does not require a packing, which is also called a packing-free seal.
Both ends of the bellows are welded to other parts. When the valve stem is lifted and lowered, the bellows expands and contracts, the bellows does not leak by itself, and the medium cannot escape. For the sake of safety, a double seal of bellows and packing is often used.
What is a static seal called?
A static seal is usually a seal between two stationary faces. The sealing method is mainly to use a gasket.
1) Gasket material
(1) Non-metallic materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc.
Paper, hemp, cowhide and the like, with pores, easy to penetrate, must be immersed in oil, wax or other anti-penetration materials. Typical valves are rarely used.
Asbestos products, asbestos tape, rope, board and asbestos rubber sheet. Among them, the asbestos rubber sheet has a compact structure, good pressure resistance and good temperature resistance. The valve itself and the valve are adjacent to the pipe flange, and the use is extremely common.
Plastic products have good corrosion resistance and are commonly used. The varieties are polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010 and the like.
Rubber products, soft, all kinds of rubber are divided into acid, alkali, oil and sea water resistance. The varieties include natural rubber, styrene butadiene rubber, nitrile rubber, neoprene rubber, butyl rubber, urethane rubber, and fluoro rubber.
(2) Metal materials: Generally speaking, metal materials have high strength and high temperature resistance. But lead is not the case, only take its resistance to dilute sulfuric acid. Commonly used varieties are brass, copper, aluminum, low carbon steel, stainless steel, Monel, silver, nickel and so on.
(3) Composite materials: for example, metal sheath (internal asbestos) gaskets, combined wave washers, wraparound entangled gaskets, and the like.
2) Frequent use of gasket performance
When using a valve, it is often the case that the original gasket is replaced. Commonly available gaskets include: rubber flat washers, rubber O-rings, plastic flat washers, Teflon gaskets, asbestos rubber washers, metal flat washers, metal profile washers, metal sheathed washers, wave washers, wraparound entangled washers, etc.
(1) Rubber flat washer: It is easy to deform, not difficult to press when pressed, but the pressure resistance and temperature resistance are poor, and it is only used for low pressure and low temperature. Natural rubber has certain acid and alkali resistance, the use temperature should not exceed 60 °C; neoprene can also resist certain acids and alkalis, use temperature 80 ° C; nitrile rubber resistant to oil, can be used up to 80 ° C; fluororubber corrosion resistance is very good, resistant The temperature performance is also stronger than normal rubber, and can be used in 150 ° C medium.
(2) Rubber O-shaped washer: The cross-sectional shape is a perfect circle, which has a certain self-tightening effect, and the sealing effect is better than that of the flat washer, and the pressing force is smaller.
(3) Plastic flat washers: The most characteristic of plastics is good corrosion resistance, and the temperature resistance of plastics in the department is not good. Teflon is the crown of plastic, excellent in corrosion resistance, wide temperature range, and durable for use within -180 ° C ~ +200 ° C.
(4) Teflon-coated gasket: Enrich and demonstrate the advantages of PTFE, and make up for its poor flexibility and error. It is made of Teflon-wrapped rubber or asbestos rubber gasket. In this way, it is as resistant to erosion as Teflon flat washers, and has excellent elasticity, which enhances the sealing effect and reduces the pressing force.
(5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its composition is 60-80% asbestos and 10-20% rubber, and fillers, vulcanizing agents and the like. It has good heat resistance, cold resistance, chemical stability, abundant supply and low price. When used, the pressing force does not need to be very large. It can adhere to the metal, and it is best to apply a layer of graphite powder in order to avoid the trouble of disassembly.
(6) Metal flat hot ring: lead, temperature resistance 100 ° C; aluminum 430 ° C; copper 315 ° C; low carbon steel 550 ° C; silver 650 ° C; nickel 810 ° C; Monel (nickel copper) alloy 810 ° C, stainless steel 870 ° C . Among them, lead has poor pressure resistance, aluminum can withstand 64 kg/cm 2 , and other materials can withstand high pressure.
(7) Metallic gaskets:
Lens washer: self-tightening, used in high pressure valves.
Oval washer: Also a high pressure self-tightening washer.
Cone double washer: for self-care sealing inside high pressure.
In addition, there are square, diamond, triangle, tooth, dovetail, B, C, etc., generally only used in high and medium pressure valves.
(8) Metal sheathed gasket: Metal has excellent temperature and pressure resistance and excellent elasticity. The sheath material is aluminum, copper, low carbon steel, stainless steel, Monel and the like. The filling materials are asbestos, PTFE, fiberglass and the like.
(9) Wave washer: It has the characteristics of small pressing force and good sealing effect. Metal and non-metal combinations are often used.
(10) Surrounding entanglement washer: It is a thin metal strip and a non-metallic strip that are closely attached together, and are entangled into a multi-layered circular shape with a wave-shaped cross section, which has good elasticity and sealing. The metal strip can be constructed of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, monel or the like. Non-metallic strip materials are asbestos, polytetrafluoroethylene, and the like.
Above, when describing the performance of the gasket.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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