Valve sealing knowledge

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Knowledge of valve sealing: The requirements for valve sealing performance should be considered from the perspective of preventing leaks. Depending on the location and degree of the leakage, the valve will exhibit different leakage patterns; therefore, different leak prevention measures need to be implemented. The principle of valve sealing: Sealing is about preventing leaks, and therefore the principle of valve sealing is also based on the prevention of leaks. There are mainly two factors that cause leakage: one is the most significant factor affecting the sealing performance, namely the gap existing between the sealing surfaces; the other is the pressure difference between the two sides of the sealing surfaces. The principle of valve sealing is also analyzed from four aspects: the sealing of liquids, the sealing of gases, the sealing principles of leakage paths, and the valve sealing pairs. 1. Sealing property of liquids: The sealing property of liquids is determined by their viscosity and surface tension. When the capillaries of a leaking valve are filled with gas, surface tension may repel the liquid or draw it into the capillaries. This creates the tangent angle. When the tangency angle is less than 90°, liquid is injected into the capillary, resulting in leakage. The reason for the leakage lies in the different properties of the media. Conducting experiments with different media will yield different results under the same conditions. Water, air, or kerosene can be used. And when the tangency angle is greater than 90°, leakage also occurs. Because it is related to a greasy or waxy film on the metal surface. Once this surface film is dissolved, the properties of the metal surface change; liquids that were previously repelled by it now wet the surface, leading to leaks. In light of the above situation, according to Poisson’s formula, it is possible to prevent leakage or reduce its amount by decreasing the capillary diameter and the viscosity of the medium. 2. Gas sealing performance: According to Poisson’s formula, the gas sealing performance is related to the gas molecules and the viscosity of the gas. Leakage is inversely proportional to the length of the capillary and the viscosity of the gas, and directly proportional to the diameter of the capillary and the driving force. When the diameter of the capillary is equal to the average degree of freedom of the gas molecules, these molecules flow into the capillary as a result of free thermal motion. Therefore, when conducting valve sealing tests, water must be used as the medium in order to achieve sealing; air or any gas cannot serve that purpose. Even if we use plastic deformation to reduce the capillary diameter below the size of gas molecules, we still cannot prevent the flow of gas. The reason is that gas can still diffuse through the metal wall. Therefore, when conducting gas tests, we must be even more stringent than when doing liquid tests. 3. Sealing principle of leakage paths: Valve sealing consists of two components: irregularities scattered across the wavy surface and the roughness caused by the wavelength between peaks. In most of our country’s metal materials, the elastic strain stress is relatively low; therefore, to achieve a sealed state, it is necessary to impose higher compression forces on these metals, meaning that the compression force must exceed their elastic limit. Therefore, when designing valves, the sealing pair is matched based on a certain difference in hardness; under pressure, this results in a certain degree of plastic deformation, thereby achieving a sealing effect. If all the sealing surfaces are made of metal, then the uneven protrusions on the surface will appear first; initially, only a small load is required to cause plastic deformation of these uneven protrusions. When the contact area increases, surface irregularities undergo plastic-elastic deformation. At this point, roughness on both sides of the recess will be present. When a load is required to cause severe plastic deformation of the underlying material, and to ensure tight contact between the two surfaces, these remaining passages can be sealed only along the longitudinal and circumferential directions. 4. Valve sealing pair: The valve sealing pair is the part that enables the valve to close when the valve seat and the closing element come into contact with each other. During use, metal sealing surfaces are prone to damage caused by entrapped media, media corrosion, wear particles, cavitation, and erosion. Such as wear particles. If the wear particles are smaller than the surface irregularities, the surface precision will improve rather than deteriorate during the running-in of the sealing surfaces. Conversely, it will degrade the surface precision. Therefore, when selecting wear particles, factors such as their material, operating conditions, lubricity, and corrosiveness to the sealing surface must be considered comprehensively. Just like wear particles, when selecting seals, we must consider all factors that affect their performance in order for them to fulfill their leak-proof function. Therefore, materials that are resistant to corrosion, scratches, and erosion must be selected. Otherwise, the absence of any one of these requirements will **reduce** its sealing performance. II. Main factors affecting valve sealing There are many factors that affect valve sealing; the main ones are as follows: 1. Structure of the sealing pair Under changes in temperature or sealing force, the structure of the sealing pair can change. Moreover, this change affects and alters the force between the sealing surfaces, thereby reducing the sealing performance of the valve. Therefore, when selecting a seal, it is essential to choose one that can undergo elastic deformation. At the same time, pay attention to the width of the sealing surface. The reason is that the contact surfaces of the sealing pair cannot fit together perfectly; as the width of the sealing surfaces increases, more force is required to achieve sealing. 2. Specific pressure of the sealing surface: The specific pressure of the sealing surface affects both the sealing performance of the valve and its service life. Therefore, the specific pressure on the sealing surface is also a very important factor. Under the same conditions, an excessively high specific pressure can cause damage to the valve, while an excessively low specific pressure can lead to valve leakage. Therefore, we must fully consider the appropriateness of the specific pressure during the design process. 3. Physical properties of the medium: The physical properties of the medium also affect the sealing performance of the valve. These physical properties include temperature, viscosity, and surface hydrophilicity, etc. Temperature changes not only affect the relaxation of the sealing pair and the variation in part dimensions, but are also closely related to the viscosity of the gas. Gas viscosity increases or decreases as the temperature rises or falls. Therefore, in order to reduce the impact of temperature on the sealing performance of valves, we design the sealing pairs as valves with thermal compensation features, such as elastic valve seats. Viscosity is related to the fluid’s ability to penetrate. Under the same conditions, the greater the viscosity, the lower the fluid’s permeability. Surface hydrophilicity refers to the need to remove a thin film that forms on the metal surface. Because this very thin oil film disrupts the surface’s hydrophilicity, it causes blockages in the fluid channels. 4. Quality of the sealing pair: The quality of the sealing pair refers primarily to the need to pay attention to material selection, matching, and manufacturing precision. For example, when the valve disc fits well with the valve seat sealing surface, it improves the sealing performance. The feature of having many circumferential ripples is its excellent labyrinth sealing performance. Valve leaks are quite common in daily life and industrial processes. In minor cases, they can lead to waste or pose dangers to people’s safety; for example, a leak in a tap water valve. In severe cases, they can result in serious consequences, such as leaks of toxic, harmful, flammable, explosive, or corrosive substances in the chemical industry. Such incidents pose significant threats to personal safety, property, and the environment. A valve that relies on external force to rotate and thus open or close is equipped with a sealing mechanism; a certain number of packing rings are installed in the packing box to achieve a sealing effect. But what is the actual level of sealing? Leakage at the valve packing is one of the most common areas where leaks occur in valves, and there are roughly two main reasons for this. III. Valve sealing types: Seals are also very crucial components in valves. The sealing performance of a valve refers to the ability of its various sealing elements to prevent the leakage of the medium, and it is the most important technical parameter for valves. There are three sealing areas in a valve: the contact point between the moving part and the two sealing surfaces of the valve seat; the interface where the packing fits around the valve stem and the packing box; and the joint between the valve body and the valve cover. The leakage in the former case is called internal leakage, which is what is commonly referred to as poor sealing; it affects the valve’s ability to block the flow of the medium. For cut-off valves, internal leakage is not allowed. The leaks in the latter two locations are called external leaks, that is, the medium leaks from inside the valve to outside it. Leaks can result in material loss, environmental pollution, and in severe cases, accidents. For flammable, explosive, toxic, or radioactive media, leaks are absolutely unacceptable; therefore, valves must have reliable sealing properties. Solving sealing problems cannot be taken lightly; most cases of leakage from valves occur here. Next, we will discuss the issues of dynamic and static sealing for valves. 1. Dynamic sealing: The dynamic sealing of a valve refers primarily to the sealing of the valve stem. Preventing the medium inside the valve from leaking as the valve stem moves is the core challenge of valve dynamic sealing. 1) Packing box type: The valve uses dynamic sealing, with the packing box being the main component. The basic forms of stuffing boxes are: (1) gland type, which is the most commonly used form. A unified format allows for many details to be distinguished. For example, regarding packing bolts, there are T-shaped bolts (used for low-pressure valves with a pressure ≤ 16 kg/cm²), double-ended bolts, swivel bolts, etc. In terms of gland types, they can be divided into integral and modular types. (2) Compression nut type: This type has small external dimensions, but the clamping force is limited; it is only used for small valves. 2) Packing: Inside the packing box, the packing is in direct contact with the valve stem and fills the entire packing box, preventing the medium from leaking out. The filler must meet the following requirements: (1) Good sealing performance ； (2) Erosion resistance ； (3) Low friction coefficient ； (4) Adapt to medium temperature and pressure. Commonly used packing materials include: (1) Asbestos packing: Asbestos packing offers excellent heat resistance and corrosion resistance, but when used alone its sealing performance is poor; it always needs to be impregnated or combined with other materials. Oil-impregnated asbestos packing: There are basically two structural forms, one being twisted and the other being braided. It can also be divided into circular and square shapes. (2) Polytetrafluoroethylene braided packing: Polytetrafluoroethylene strips are woven into packing, offering excellent corrosion resistance and being suitable for use with cryogenic media. (3) Rubber O-ring: Excellent sealing performance at low pressures. The operating temperature is limited; for example, natural rubber can only be used at 60°C. (4) Plastic molding fillers: Generally made in a three-piece design, but can also be made in other shapes. Polytetrafluoroethylene is the most commonly used plastic. Furthermore, for example, in steam valves at 250°C, alternating layers of asbestos packing and lead rings can reduce steam leakage ； With valves and frequent changes in the medium, using a combination of asbestos packing and PTFE tape yields a better sealing effect. To reduce friction on the valve stem, molybdenum disulfide (MoS2) or other lubricants can be added in some cases. There is a demand for novel fillers. For example, by impregnating polyacrylonitrile fibers with polytetrafluoroethylene emulsion and then subjecting them to pre-oxidation, followed by sintering and pressing in a mold, molding fillers with excellent sealing properties can be obtained ； For example, wave-shaped packing made of stainless steel sheets and asbestos can withstand high temperatures, high pressures, and corrosion. With the rapid growth of the chemical and nuclear industries, there has been an increase in flammable, explosive, highly toxic, and radioactive materials, which has imposed stricter requirements on valve sealing. In some applications, packing seals can no longer be used, leading to the development of a new type of seal – the bellows seal. This type of seal does not require a filler and is also known as a fillerless seal. The two ends of the bellows are welded firmly to other components. As the valve stem moves up and down, the bellows expand and contract; since the bellows itself does not leak, the medium cannot escape. As a precaution, a dual seal using both a bellows and a packing is often employed. 2. Static seal: What is a static seal? A static seal generally refers to a seal between two stationary surfaces. The main method of sealing is to use washers. 1) Gasket material (1) Non-metallic materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc. Materials such as paper, linen, and cowhide have capillaries and are permeable; therefore, they need to be impregnated with oil, wax, or other impermeabilizing materials when used. They are rarely used in ordinary valves. Asbestos products include asbestos tapes, ropes, sheets, and asbestos rubber sheets. Among them, asbestos rubber sheets have a dense structure, excellent pressure resistance, and good temperature resistance. They are widely used in valves themselves as well as in the connections between valves and pipe flanges. Plastic products have excellent resistance to erosion and are widely used. The varieties include polyethylene, polypropylene, flexible PVC, polytetrafluoroethylene, nylon 66, nylon 1010, etc. Rubber products are soft in texture. Different types of rubber exhibit certain resistance to acids, alkalis, oils, and seawater. The varieties include natural rubber, styrene-butadiene rubber, nitrile rubber, neoprene, isobutylene rubber, polyurethane rubber, fluororubber, etc. (2) Metal materials: Generally speaking, metal materials have high strength and excellent temperature resistance. But lead actually isn’t like that; only its resistance to dilute sulfuric acid is utilized. Commonly used materials include brass, copper, aluminum, low-carbon steel, stainless steel, Monel alloy, silver, nickel, etc. (3) Composite materials: such as metal-clad (asbestos-filled inside) gaskets, composite wave-shaped gaskets, helically wound gaskets, etc. 2) When valves whose gasket performance is frequently utilized are used, it is often necessary, depending on the specific circumstances, to replace the original gasket. Common types of gaskets include: rubber flat gaskets, rubber O-rings, plastic flat gaskets, PTFE-coated gaskets, asbestos rubber gaskets, metal flat gaskets, metal specialty-shaped gaskets, metal-clad gaskets, wave-shaped gaskets, spiral-wound gaskets, etc. (1) Rubber flat washers: They deform easily and are simple to compress; however, their pressure and temperature resistance are relatively poor. They are only suitable for applications with low pressure and moderate temperatures. Natural rubber has certain resistance to acids and alkalis; the operating temperature should not exceed 60°C℃ ； Neoprene can also resist certain acids and alkalis; its operating temperature is 80℃ ； Nitrile rubber is oil-resistant and can be used up to 80℃ ； Fluororubber has excellent resistance to corrosion and better heat resistance than ordinary rubbers; it can be used in media at 150°C. (2) Rubber O-ring: Its cross-section is perfectly circular; it has a certain self-tightening effect. Its sealing performance is better than that of flat gaskets, while requiring less clamping force. (3) Plastic flat washers: The greatest characteristic of plastics is their good corrosion resistance; however, most plastics have poor heat resistance. Polytetrafluoroethylene is regarded as the king of plastics; it has excellent corrosion resistance and a relatively wide temperature tolerance range, allowing it to be used continuously within the range of -180°C to +200°C. (4) PTFE-coated gaskets: These fully utilize the advantages of PTFE while compensating for its poor elasticity; they are essentially rubber or asbestos-rubber gaskets encased in PTFE. In this way, it is as resistant to erosion as PTFE flat gaskets, while also possessing excellent elasticity, thereby enhancing the sealing effect and reducing the clamping force. (5) Asbestos rubber gasket: Cut from asbestos rubber sheet. Its components are 60–80% asbestos and 10–20% rubber, along with fillers, vulcanizing agents, etc. It has excellent heat resistance, cold resistance, and chemical stability. It is readily available in abundant quantities and at a low price. When in use, the clamping force doesn’t need to be very large. It can adhere to metal; it’s best to apply a layer of graphite powder on its surface to facilitate disassembly later on. (6) Metal flat heat rings: Lead, temperature resistant up to 100℃ ； Aluminum 430℃ ； Copper 315℃ ； Low-carbon steel 550℃ ； Silver 650℃ ； Nickel 810℃ ； Monel (nickel-copper) alloy: 810°C; stainless steel: 870°C. Among them, lead has a poor voltage resistance; aluminum can withstand 64 kilograms per square centimeter, while other materials can handle high pressures. (7) Metal non-conformal washers: Lens washers, which have a self-tightening effect and are used in high-pressure valves. Elliptical washer: Also belongs to high-pressure self-tightening washers. Tapered double washer: used for high-pressure internal sealing. In addition, there are also square, rhombus, triangular, toothed, dovetail, B-shaped, C-shaped, etc., which are generally used only in medium and high-pressure valves. (8) Metal cladding washers: Metals possess excellent temperature and pressure resistance as well as good elasticity. The materials for the prepuce include aluminum, copper, low-carbon steel, stainless steel, Monel alloy, etc. The filling materials inside include asbestos, polytetrafluoroethylene, glass fiber, etc. (9) Wave washer: It features low compressive force and good sealing performance. A combination of metals and non-metals is commonly used. (10) Twisted gasket: It consists of very thin metal strips and non-metallic strips that are pressed together and twisted into multiple layers in a circular shape; its cross-section has a wavy appearance, and it offers excellent elasticity and sealing properties. The metal strip can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non-metallic strip materials include asbestos, polytetrafluoroethylene, etc. Above, when discussing the performance of sealing gaskets, some figures were listed. It must be noted that these figures are closely related to factors such as the type of flange, the nature of the medium, and the techniques used for installation and maintenance. The pressure and temperature resistance capabilities can vary; for example, as temperature rises, the pressure resistance often decreases. Such subtle issues can only be understood through practical experience. IV. Valve sealing materials: Valve sealing materials are an important component of valve sealing, serving as the surface that is in direct contact with the seal. So what are the sealing materials for valves? We know that valve seal materials can be divided into two categories: metallic and non-metallic. The valve sealing material is an important component of valve sealing, serving as the surface that is in direct contact with the seal. So what are the sealing materials for valves? We know that valve seal materials can be divided into two categories: metallic and non-metallic. Next, a brief introduction will be given to the operating conditions of various sealing materials, as well as the common types of valves. 1. Synthetic rubber: Synthetic rubber has superior comprehensive properties such as oil resistance, temperature resistance, and corrosion resistance compared to natural rubber. Generally, the operating temperature for synthetic rubber is t≤150°C, while for natural rubber it is t≤60°C. Rubber is used for sealing valves such as globe valves, gate valves, diaphragm valves, butterfly valves, check valves, and clamp valves with a nominal pressure of PN≤1MPa. 2. Nylon: Nylon features a low coefficient of friction and good corrosion resistance. Nylon is mostly used in ball valves, globe valves, etc. for temperatures t≤90°C and nominal pressures PN≤32MPa. 3. Polytetrafluoroethylene: Polytetrafluoroethylene is commonly used in stop valves, gate valves, ball valves, etc., in applications where the temperature is t≤232°C and the nominal pressure is PN≤6.4MPa. 4. Cast iron: Cast iron is used for gate valves, globe valves, plug valves, etc., intended for use with gas and oils at temperatures of t≤100°C and nominal pressures of PN≤1.6MPa. 5. Babbitt alloy: Babbitt alloy is used in ammonia stop valves operating at temperatures of t-70~150°C and with a nominal pressure of PN≤2.5MPa. 6. Copper alloys: Common copper alloy materials include 6-6-3 tin bronze and 58-2-2 manganese brass, among others. Copper alloys have good wear resistance and are suitable for use in water and steam at temperatures of t≤200°C and nominal pressures of PN≤1.6MPa. They are commonly used in gate valves, globe valves, check valves, plug valves, etc. 7. Chromium stainless steel: Common grades of chromium stainless steel include 2Cr13 and 3Cr13; after heat treatment, they exhibit good corrosion resistance. It is commonly used on valves for media such as water, steam, and oil at temperatures of t≤450℃ and nominal pressures of PN≤32MPa. 8. Chromium-nickel-titanium stainless steel: The common grade for this type of stainless steel is 1Cr18Ni9Ti, and it exhibits good corrosion resistance, erosion resistance, and heat resistance. Suitable for use in media such as steam and nitric acid at temperatures of t≤600℃ and nominal pressures of PN≤6.4MPa, for use in stop valves, ball valves, etc. 9. Nitrided steel: The common grade for nitrided steel is 38CrMoAlA; after carburizing treatment, it exhibits good corrosion resistance and scratch resistance. It is commonly used for power station gate valves in applications where the temperature t≤540℃ and the nominal pressure PN≤10MPa. 10. Boron infiltration: In this process, the sealing surface is created directly from the material of the valve body or valve disc, and then a boron infiltration treatment is applied; as a result, the sealing surface has excellent wear resistance. Used for power plant drain valves.