Solutions to the problems of dynamic and static sealing in valves

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With the continuous development of the valve industry, a variety of valves have been developed. Leaks in valves are caused by problems with the sealing mechanisms. Below, Biaoguang Valves will explain to you the issues related to dynamic sealing and static sealing in valves. 1. Dynamic sealing: The dynamic sealing of a valve refers mainly to the stem seal. 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: At present, for the dynamic sealing of valves, packing boxes are the primary choice. The basic forms of stuffing boxes are: (1) gland type, which is the most commonly used form. The same form can have many differences in details. 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 glands, they can be categorized as integral and modular. (2) Compression nut type: This type has small external dimensions, but its compressive force is limited; it is used only for small valves. 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. Common packings include: (1) Asbestos packing: Asbestos packing has excellent heat and corrosion resistance. However, when used alone, its sealing performance is poor; therefore, it is always impregnated 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 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: Provides good sealing performance at low pressures. The operating temperature is limited; for example, natural rubber can only be used at 60°C. (4) Plastic molding filler: It is 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 addition, depending on their own needs, users often explore various effective forms of fillers. For example, in steam valves at 250°C, alternating layers of asbestos packing and lead rings can reduce 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 applied in some cases. Currently, research is being conducted on novel fillers. For example, by impregnating polyacrylonitrile fibers with a polytetrafluoroethylene emulsion and then subjecting them to pre-oxidation, they can be sintered and pressed in a mold to produce molded packings with excellent sealing properties ； For example, wave-shaped packing made of stainless steel sheets and asbestos can withstand high temperatures, high pressures, and corrosion. 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 stricter 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. 2. Static seal: A static seal generally refers to a seal between two stationary surfaces. The sealing method mainly involves using gaskets. 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 coated with oil, wax, or other impermeabilizing materials when in use. Generally, such valves are rarely used. Asbestos products include asbestos tape, rope, sheets, and asbestos rubber sheets, etc. Among them, asbestos rubber sheets have a dense structure, excellent pressure resistance, and good temperature resistance. They are widely used both in valves themselves and in the flange connections between valves and pipes. Plastic products have excellent corrosion resistance and are widely used. The varieties 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, and others. (2) Metal materials: 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. (3) Composite materials: such as metal-clad (asbestos-filled) washers, composite wave washers, spiral washers, etc. 2) Performance of commonly used gaskets: When using valves, it is often necessary to replace the original gaskets according to specific circumstances. Common types of 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 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 better temperature resistance compared to 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 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 regarded as the king of plastics; it not only exhibits excellent corrosion resistance, but also has 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: To fully utilize the advantages of PTFE while compensating for its poor elasticity, these gaskets are made by coating rubber or asbestos rubber with PTFE. 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. Its cross-sectional shape is shown in Figure 4-20. (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 widely available at low prices. When in use, the clamping 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) ； Purplish-red, used for high pressure (grade XB-450, with a pressure resistance of 100 kilograms per square centimeter and a temperature resistance of 450°C) ； Green, used for oils, and it also has excellent pressure resistance. (6) Metal constant temperature ring: lead, with a temperature resistance of 100℃ ； Aluminum 430℃ ； Copper 315℃ ； Low-carbon steel 550℃ ； Silver 650℃ ； Nickel 810℃ ； Monel (nickel-copper) alloy at 810°C, stainless steel at 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 square, diamond, triangular, toothed, dovetail, B-shaped, C-shaped, etc., which are generally used only in high- and medium-pressure valves. (8) Metal sheathed washer: Metals possess excellent temperature and pressure resistance as well as good elasticity. Cladding materials 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) Spiral gasket: It consists of very thin metal strips and non-metallic strips that are pressed together and wrapped 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. Above, when describing the performance of sealing gaskets, some figures were cited. 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; sometimes they may be exceeded, and other times they may not be reached. Moreover, pressure resistance and temperature resistance are interrelated – for example, as temperature rises, pressure resistance tends to decrease. Such subtle issues can only be understood through practical experience. 3) New materials and new technologies: The gasket types mentioned above are far from comprehensive, and sealing technology is in rapid development. The following are examples of several new materials and technologies. (1) Liquid sealing: With the rapid development of the polymer organic synthesis industry, liquid sealants have emerged in recent years and are used for static sealing ； This new technology is commonly referred to as liquid sealing. The principle of liquid sealing relies on the adhesiveness, fluidity of 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 single-molecule film effect. A tape made from raw material is called raw material tape, and it can be rolled into coils 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 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 use in sealing. (3) Metal hollow O-rings: They have good elasticity, require low 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 sealing ring: In people’s minds, graphite is a brittle material lacking elasticity and toughness. However, graphite that has undergone special treatment is soft in texture and highly elastic. In this way, the heat resistance and chemical stability of graphite can be utilized in gasket materials ； Moreover, this washer has low clamping force, yet its sealing effect is exceptionally superior. This type of graphite can also be made into strips that, when combined with metal strips, form winding washers with excellent properties. The emergence of graphite plate seals and graphite-metal wound gaskets represents a major breakthrough in high-temperature corrosion-resistant sealing. Such washers are already produced and used in large quantities abroad.