How to solve valve sealing problems

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How to solve the sealing problems of valves: It is important to address these sealing issues, as most cases of leakage in valves occur in the following areas: 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. Types of stuffing boxes: Currently, stuffing boxes are the primary means of dynamic sealing for valves. The basic forms of stuffing boxes are: (1) gland type: this is the most commonly used form. There are also many subtle differences within the same format. For example, regarding clamping bolts, there are T-bolts (used for low-pressure valves with a pressure ≤ 16 kg/cm²), double-ended bolts, swivel bolts, etc. In terms of gland covers, they can be divided into integral and modular types. (2) Compression nut type: This type has small external dimensions, and its compressive force is limited, so it can only be used for small valves. In terms of the packing, inside the packing box, the packing is in direct contact with the valve stem and fills the packing box to prevent the medium from leaking out. The filler must meet the following requirements: good sealing performance ； Corrosion resistance ； Low friction coefficient ； Adapt to medium temperature and pressure. 2. Common packing materials include: (1) Asbestos packing: Asbestos packing has excellent heat and corrosion resistance, but its sealing performance is poor when used alone; therefore, it is always impregnated with or combined with other materials. Oil-impregnated asbestos packing: It comes in two basic structural forms, one being twisted and the other being braided. It can also be divided into circular and square types. (2) Polytetrafluoroethylene braided gasket: Polytetrafluoroethylene strips are woven into a gasket; it possesses excellent corrosion resistance and can also be used with cryogenic media. (3) Rubber O-ring: Provides good sealing performance at low pressures. However, 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, with nylon 66 and nylon 1010 also being used. In steam valves at 250°C, alternating the use of asbestos gaskets and lead rings can reduce steam leakage; for some valves where the medium changes frequently, using a combination of asbestos gaskets and PTFE tape yields a better sealing effect. To reduce friction on the valve stem, molybdenum disulfide (MoS2) or other lubricants can be used in some cases. Currently, research is being conducted on novel fillers. By impregnating polyacrylonitrile fibers with a polytetrafluoroethylene emulsion and then subjecting them to pre-oxidation, followed by sintering and pressing in a mold, a molded packing with excellent sealing properties can be obtained. Similarly, corrugated packing made from stainless steel sheets and asbestos can withstand high temperatures, high pressures, and corrosion. Bellows sealing: With the rapid development of the chemical and nuclear industries, there has been an increase in flammable, explosive, highly toxic, and radioactive substances, which has led to stricter requirements for valve sealing. In some applications, packing seals can no longer be used; as a result, a new type of seal was developed – bellows sealing. This type of seal requires no packing, so it is also called a packingless seal. The two ends of the bellows are welded to other components. As the valve stem moves up and down, the bellows expand and contract; as long as the bellows themselves do not leak, the medium cannot escape. As a precaution, a dual seal using a bellows and packing is often employed. Static seal A static seal generally refers to a seal between two stationary surfaces. The main sealing method is to use washers. Washer materials include: (1) Non-metallic materials: such as paper, linen, cowhide, asbestos products, plastics, rubber, etc. Materials like paper, linen, and cowhide have capillaries and are prone to permeation; therefore, they must be impregnated with oil, wax, or other impermeabilizing materials before use. They are rarely used in ordinary valves. Asbestos products include asbestos tape, rope, 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 both the valves themselves and in the flange connections between valves and pipes. Plastic products have excellent corrosion resistance and are widely used. The types include polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010, etc. Rubber products are soft in texture, and different types of rubber possess varying degrees of 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 heat resistance. But lead is not 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) washers, composite wave washers, spiral washers, etc. Performance of common washers: When using valves, the original washers are often replaced depending on the specific circumstances. Common washers include: rubber flat washers, rubber O-rings, plastic flat washers, PTFE-lined washers, asbestos-rubber washers, metal flat washers, metal custom-shaped washers, metal-clad washers, wave washers, and spiral wound washers. (1) Rubber flat gaskets: They deform easily and can be compressed without much effort, but they have poor pressure and temperature resistance; they are only suitable for use in environments with low pressure and moderate temperatures. Natural rubber has a certain degree of resistance to acids and alkalis, but its operating temperature should not exceed 60°C; neoprene can also resist certain acids and alkalis, with an operating temperature of up to 80°C; nitrile rubber is resistant to oils and can be used at temperatures of up to 80°C; fluororubber has excellent corrosion resistance as well as higher temperature tolerance than ordinary rubbers, allowing it to be used in environments at 150°C. (2) Rubber O-ring: Its cross-sectional shape is circular, and it has a certain self-tightening effect; its sealing performance is better than that of flat gaskets, with less compressive force required. (3) Plastic flat washers: The greatest advantage of plastics is their good corrosion resistance; however, most plastics have poor temperature resistance. Polytetrafluoroethylene is the king of plastics; it not only boasts excellent corrosion resistance but also has a wide temperature range, allowing it to be used over extended periods at temperatures between -180°C and +200°C. (4) PTFE-coated washers: To fully utilize the advantages of PTFE while compensating for its poor elasticity, washers are made with PTFE covering rubber or asbestos rubber. In this way, it possesses the same corrosion resistance as polytetrafluoroethylene flat gaskets, along with good elasticity, which enhances the sealing effect and reduces the compression 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, and it is available in large quantities at low prices. When in use, the pressing force does not need to be very strong. Since it can adhere to metal, it is best to coat its surface with a layer of graphite powder to avoid difficulty during removal. Asbestos rubber sheets are available in four colors: gray, which is used for low-pressure applications (grade XB-200, with a pressure resistance of ≤16 kilograms per square centimeter and a temperature resistance of 200°C) ； Red, used for medium voltage (grade XB-350; can withstand a pressure of 40 kilograms per square centimeter and temperatures up to 350°C) ； Purple-red, used for high pressure (grade XB-450; withstanding pressure of 100 kg/cm² and temperature of 450°C) ； Green, used for oils, and it also has excellent pressure resistance. (6) Metal constant temperature rings: lead, with a temperature resistance of 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 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: 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 self-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 sheath washers: Metals possess excellent temperature and pressure resistance, as well as good elasticity. The materials for the sheath 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) Winded gasket: It consists of very thin metal strips and non-metallic strips that are pressed together and wound into multiple layers in a circular shape; its cross-section is wavy, giving it excellent elasticity and sealing properties. Metal strips can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Materials for non-metallic belts include asbestos, polytetrafluoroethylene, etc. Some of the figures cited above regarding the performance of sealing gaskets are closely related to factors such as the type of flange, the nature of the medium, and installation and maintenance techniques; these values may sometimes exceed the specified limits or fail to meet them. Moreover, pressure resistance and temperature resistance are interrelated – for example, as temperature rises, the pressure resistance tends to decrease. Such subtle issues can only be understood through practical experience.