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The dynamic and static sealing types of valves, as well as the packing and gasket seals, are important considerations. The sealing issues associated with valves cannot be ignored, as most cases of leakage from valves occur due to problems in these sealing elements. In this article on Valve Network, we will discuss issues related to dynamic and static sealing of valves. 1. Dynamic sealing: The dynamic sealing of a valve mainly refers to the stem seal. Preventing the medium inside the valve from leaking as the valve stem moves is key to solving the problem of dynamic sealing in valves. 1.1 Packing box type: At present, for the dynamic sealing of valves, packing boxes are the primary method used. The basic forms of stuffing boxes are: (1) Gland type. This is the most commonly used form. The same form can have many differences in details. For example, in terms of compression bolts, they can be divided into T-bolts (used for low-pressure valves with a pressure ≤ 16 kg/cm²), double-headed bolts, and swivel bolts, among others. In terms of gland covers, they can be divided into integral and modular types. (2) Compression nut type: The compression nut type has small external dimensions, but its compressive force is limited, so it is suitable only for small valves. 1.2 Packing: Inside the packing box, the packing is in direct contact with the valve stem and fills the packing box, preventing the medium from leaking out. The filler must meet the following requirements: (1) Good sealing performance ； (2) Corrosion resistance ； (3) Low friction coefficient ； (4) Adapt to medium temperature and pressure. 2. Common fillers include: 2.1 Flexible graphite: a new type of sealing material. 2.2 Asbestos packing: Asbestos packing has excellent resistance to both temperature and corrosion, but when used on its own it does not provide good sealing performance; therefore, it is always impregnated or combined with other materials. 2.3 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 shapes. 2.4 Polytetrafluoroethylene braided packing: Polytetrafluoroethylene strips are woven into packing, offering excellent corrosion resistance and being suitable for use with cryogenic media. 2.5 Rubber O-rings: Provide good sealing performance at low pressures. The operating temperature is limited; for example, natural rubber can only be used at 60°C. 2.6 Plastic molding fillers: Generally made in a three-piece design, but can also be shaped in other forms. Polytetrafluoroethylene is the most commonly used plastic, with nylon 66 and nylon 1010 also being used. 2.7 Other types of packing: In steam valves at 250°C, alternating the use of asbestos packing and lead rings reduces steam leakage ； In some valves where the medium changes frequently, using both asbestos packing and PTFE tape together can improve the sealing effect. To reduce friction on the valve stem, molybdenum disulfide (MoS2) or other lubricants can be used in some cases. 3. 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 more stringent requirements for valve sealing. In some applications, packing seals can no longer be used; hence, a new type of seal was developed – bellows sealing. This type of seal does not require a filler, which is why it is also called a fillerless 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. 4. Static seals: Static seals generally refer to seals between two stationary surfaces. The main methods for static sealing are the use of gaskets and washers. 4.1 Gasket materials (1) Non-metallic gasket materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc. Materials such as paper, hemp, and cowhide have capillaries and are prone to permeation; therefore, they must be impregnated with oil, wax, or other anti-permeation substances before use. They are rarely used in ordinary valves. 4.2 Asbestos products, including 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 both the valves themselves and in the flange connections between valves and pipes. 4.3 Plastic products have excellent corrosion resistance and are widely used. The types include polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010, etc. 4.4 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. 4.5 Material of metal washers: Generally speaking, metal materials have high strength and excellent temperature resistance. But lead is not like this; 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. 4.6 Materials for composite washers: such as metal-clad (asbestos-filled) washers, combined wave washers, wound washers, etc. 4.7 Performance of commonly used gaskets Depending on the specific circumstances, gaskets are often replaced when using valves. Commonly used 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 washers: They deform easily and can be compressed without much effort; however, their pressure and temperature resistance are relatively poor. They are only used in applications with low pressure and moderate temperatures. Natural rubber has a certain degree of resistance to acids and alkalis, and its operating temperature should not exceed 60℃ ； Neoprene can also resist certain acids and bases, with a usable temperature of 80℃ ； Nitrile rubber is oil-resistant and can be used up to 80℃ ； Fluororubber has excellent corrosion resistance, and its temperature resistance is also higher than that of ordinary rubbers; it can be used in media 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 compression 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 long 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 made of PTFE coated with rubber or asbestos rubber are used. 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 gaskets: Made by cutting asbestos rubber sheets, they possess excellent heat resistance, cold resistance, and chemical stability; in addition, they are readily available at low prices. Its composition consists of 60–80% asbestos and 10–20% rubber, along with fillers, vulcanizing agents, etc. When using asbestos rubber washers, a high compressive force is not necessary. 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 come in four colors: gray, which is used for low-pressure applications (grade XB-200; withstanding pressure ≤16 kg/cm² and temperatures up to 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) ； Purplish-red, used for high pressure (grade XB-450, with a pressure resistance of 100 kg/cm² and a temperature resistance of 450°C) ； Green, used for oils, and it also has excellent pressure resistance. (6) Metal flat washer: made of lead, with a temperature resistance of 100℃ ； Aluminum 430℃ ； Copper 315℃ ； Low-carbon steel category 550℃ ； Silver 650℃ ； Nickel series 810℃ ； Monel alloy (nickel-copper): 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 asymmetric 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 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: 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. Metals possess both good temperature and pressure resistance as well as good elasticity. (9) Wave washer: It features low compressive force and good sealing performance. A combination of metals and non-metals is commonly used. (10) Spiral 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 wave-shaped, 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. Non-metallic strip materials include asbestos, polytetrafluoroethylene, etc. 5. New sealing materials and technologies (1) Liquid sealing: With the rapid development of the synthetic organic polymer industry, liquid sealants have emerged in recent years and are used for static sealing ； This new technology is commonly known as liquid sealing. The principle of liquid sealing relies on the adhesiveness, fluidity of the liquid sealant, and the monolayer effect (the thinner the film, the greater its tendency to return to its original state). Under appropriate pressure, it functions like a gasket. Therefore, the sealant in use is also known as a liquid gasket. (2) PTFE green material seal: PTFE is also a high-molecular organic compound; before it is sintered into a finished product, it is in its green state, is soft in texture, and exhibits a monomolecular film effect. A tape made from raw material is called raw material tape, and it can be rolled up for long-term storage. It can be freely shaped during use, with any type of joint; once pressure is applied, it forms a ring-shaped membrane that provides uniform sealing. As a gasket between the valve body and the valve cover in valves, it is possible to create a gap without removing the valve disc or gate, and then simply insert tape there. It has low clamping force, does not stick to the hands nor to the flange surface, making it very easy to replace. It is most suitable for tenon-and-socket flanges. PTFE raw material can also be made into tubular and rod forms for sealing purposes. (3) Metal hollow O-rings: They have good elasticity, require less compressive force, and possess a self-tightening effect. Various metal materials can be used, enabling them to function in low-temperature, high-temperature, and highly corrosive environments. (4) Graphite plate seal ring: In people’s minds, graphite is a brittle material lacking elasticity and toughness, but graphite that has undergone special treatment is soft in texture and possesses good elasticity. In this way, the heat resistance and chemical stability of graphite can be utilized in gasket materials ； Moreover, this type of washer has low compressive force, resulting in an exceptionally excellent sealing effect. This type of graphite can also be made into strips that, when combined with metal strips, form winding washers with excellent performance. The emergence of graphite plate seals and graphite-metal wound gaskets represents a major breakthrough in high-temperature corrosion-resistant sealing.