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How to solve the sealing problem can not be ignored, because the valve running, running, dripping, leakage phenomenon, the vast majority occur here. Dynamic seal sealing of the valve, mainly refers to the stem seal. Do not let the valve medium with the stem movement and leakage, the valve is the center of the subject of dynamic sealing. Stuffing box form At present, the valve dynamic seal to stuffing box based. The basic form of stuffing box is: (1) gland type This is the most used form. The same form can distinguish many details. For example, from the compression bolts, can be divided T-bolts (for pressure ≤ 16 kg / cm2 of low-pressure valve), studs and bolts and so on. From the gland, can be divided into the overall and combined. (2) compression nut type This form, small size, but the pressure is limited, only used for small valves. Packing stuffing box, with the filler and the stem direct contact and filled with stuffing box, to prevent media leakage. The following requirements on the packing: (1) good sealing; (2) corrosion resistance; (3) small coefficient of friction; (4) to adapt to the media temperature and pressure. Commonly used fillers are: (1) Asbestos packing: Asbestos packing, temperature and corrosion resistance are good, but when used alone, the sealing effect is poor, so always impregnated or attached to other materials. Oil impregnated asbestos packing: Its basic structure in two forms, one is twisted, and the other is braided. Can be divided into round and square. (2) Teflon braided packing: Teflon braided packing, has excellent corrosion resistance, but also can be used for cryogenic medium. (3) Rubber O-ring: Under low pressure, the sealing effect is good. Use temperature is limited, such as natural rubber can only be used at 60 ℃. (4) plastic molding filler: generally made of three-piece, can also be made into other shapes. Polytetrafluoroethylene plastic used as much, but also with nylon 66 and nylon 1010. In addition, the use of units according to their own needs, often exploring a variety of effective filler forms. For example, in the steam valve at 250 ℃, asbestos packing and lead ring alternately overlap, leakage will be reduced; some valves, the media often change, such as asbestos packing and PTFE tape together , The sealing effect will be better. In order to reduce the friction on the stem, some occasions, you can add molybdenum disulfide (M0S2) or other lubricants. At present, new stuffing is being explored. For example, polyacrylonitrile fiber impregnated with PTFE emulsion, and after pre-oxidation, sintering in the mold compression, can be sealed with excellent performance of the molding filler; Another example is the use of stainless steel sheet and asbestos made of corrugated filler, high temperature resistant High pressure and corrosion. Bellows seal With the rapid development of chemical industry and atomic energy industry, flammable, explosive, highly toxic and radioactive substances increased, the valve seal has been more stringent requirements, in some cases have been unable to use the packing seal, resulting New form of seal - bellows seal. This seal without filler, it is also called no packing seal. Bellows ends, welded with other parts. When the stem lifts, bellows telescopic, as long as the bellows itself does not leak, the media will not be able to escape. To be on the safe side, double seals of bellows and packing are often used. Static seal Static seal is usually the seal between two stationary surfaces. The main method of sealing is the use of gaskets. Washers materials (1) non-metallic materials: such as paper, linen, leather, asbestos products, plastics, rubber and so on. Paper, hemp, leather and the like, with pores, easy to infiltrate, use must be impregnated with oil, wax or other impermeable material. Valve is rarely used. Asbestos products, there are asbestos belt, rope, board and asbestos rubber sheet. Asbestos rubber sheet which compact structure, pressure performance, temperature performance is also very good, the valve itself and the valve and the pipe flange connection, the use of a very wide range. Plastic products, have good corrosion resistance, use is also more common. Varieties of polyethylene, polypropylene, soft PVC, PTFE, nylon 66, nylon 1010 and so on. Rubber products, soft texture, all kinds of rubber, respectively, a certain acid, alkali, oil, sea water resistance. Varieties are natural rubber, styrene butadiene rubber, nitrile rubber, neoprene, butyl rubber, polyurethane rubber, fluorine rubber. (2) metal materials: In general, metal materials, high strength, high temperature resistance. But lead does not, just take it against the characteristics of dilute sulfuric acid. Common varieties are brass, copper, aluminum, low carbon steel, stainless steel, Monel, silver, nickel and so on. (3) composite materials: such as metal foreskin (internal asbestos) washer, combined wave washer, wound washer and so on. Common gaskets performance When using the valve, often according to the specific circumstances, the replacement of the original gaskets. Often have gaskets are: rubber flat washers, rubber O-ring, plastic flat washers, PTFE packing gaskets, asbestos rubber gaskets, metal flat washers, metal washers, metal foreskin washers, wave washers, wound washers and so on. (1) Flat rubber washer: It is easy to deform and work hard without any pressure. However, its pressure resistance and temperature resistance are poor. It is only used for places with low pressure and low temperature. Natural rubber has certain acid and alkali resistance, the use of temperature should not exceed 60 ℃; neoprene can also be resistant to some acid, temperature 80 ℃; nitrile rubber oil, available to 80 ℃; fluorine rubber corrosion resistance is good, resistance Temperature performance than the average rubber, can be used in 150 ℃ medium. (2) Rubber O-shaped washer: the cross-sectional shape is a perfect circle with a certain self-tightening effect, the sealing effect is better than the flat washer and the pressing force is smaller. (3) plastic flat washers: The most prominent feature of plastic is corrosion resistance, most of the plastic temperature performance is not good. PTFE plastic crown, not only excellent corrosion resistance, but also a wide temperature range, can be long-term use within -180 ℃ ~ +200 ℃. (4) Teflon gasket: In order to give full play to the advantages of PTFE, while making up for the shortcomings of its poor flexibility, made of PTFE coated rubber or asbestos rubber gasket. In this way, both with PTFE flat washer corrosion-resistant, but also good flexibility, and enhance the sealing effect, reducing the pressure. The cross-sectional shape is shown in Figure 4-20. (5) Asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60 to 80% asbestos and 10 to 20% rubber, as well as fillers, vulcanizing agents and the like. It has good heat resistance, cold resistance, chemical stability, and rich sources, the price is cheap. When used, the pressing force does not have to be great. Because it can adhere to metal, it is best coated with a layer of graphite powder, so as to avoid strenuous demolition. Asbestos rubber sheet has four colors: gray, for low pressure (grade XB-200, pressure ≤ 16 kg / cm2, temperature 200 ℃); red for medium voltage (grade XB-350, 40 kg / cm², temperature 350 ℃); purple, for high pressure (grade XB-450, pressure 100 kg / cm² temperature 450 ℃); green, for oil, pressure performance is also very good . (6) Metal flat ring: lead, temperature 100 ℃; aluminum 430 ℃; copper 315 ℃; low carbon steel 550 ℃; silver 650 ℃; nickel 810 ℃; Monel (nickel copper) alloy 810 ℃, stainless steel 870 ℃ . Among them, the lead pressure capacity is poor, aluminum can withstand 64 kg / cm, other materials can withstand high pressure. (7) Heterogeneous metal washers: Lens washers: Self-tightening effect, used in high pressure valves. Oval washers: also belong to high-pressure self-tight washers. Conical double washer: for high-pressure self-tight seal. In addition, there are square, diamond, triangle, toothed, dovetail, B-shaped, C-shaped, generally only used in high pressure in the valve. (8) metal foreskin washers: metal both good temperature and pressure performance, but also good flexibility. Leather materials are aluminum, copper, low carbon steel, stainless steel, Monel and so on. Filling material inside asbestos, PTFE, glass fiber and so on. (9) Wave Washer: with a small clamping force, sealing effect is good. Often used in the form of metal and non-metallic combination. (10) Wound gasket: It is a thin metal and non-metallic belt close together, wound into a multi-layer circular, section was wavy, with good elasticity and sealing. Available with 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy production. Non-metallic belt materials are asbestos, PTFE and so on. Above, when it comes to gasket performance, a few numbers are listed. It must be stated that these figures are closely related to the form of flanges, the condition of the medium and the technique of installation and repair, etc., and sometimes can exceed or sometimes can not be achieved. Moreover, the withstand voltage and temperature resistance are also interchanged, for example, the temperature is high, Pressure capacity is often reduced, these subtle problems can only be realized in practice. New materials and new technologies The gaskets described above are still far from comprehensive, and the sealing technology is in rapid development. Here are some examples of new materials and new technologies. (1) liquid seal: With the rapid development of polymer organic synthesis industry, liquid sealant appeared in recent years, used in static sealing; this new technology, usually called liquid seal. The principle of liquid sealing is to utilize the liquid sealant's adhesion, fluidity and monolayer effects (the thinner the natural tendency of the film to recoat is), under the proper pressure to make it behave like a washer. Therefore, the use of sealant, also known as liquid gasket. (2) Teflon raw material sealing: Teflon is also a polymer organic compound. Before it is sintered into a product, it is called raw material. It is soft in texture and also has a monolayer effect. Made of raw materials called raw meal with a tape, you can roll into a long-term preservation. When used to freeform, any joints, as long as there is pressure, they will form a uniform seal ring membrane. As the valve body between the valve cover and the gasket, without removing the valve flap or gate case, pry open a gap, stuffed into the raw material to go on the line. Small pressure, no sticky hands, nor sticky flange, the replacement is very convenient. The most suitable for tongue and groove flange. Teflon raw material, but also made of tubular and rod, for sealing. (3) hollow metal O-ring: good elasticity, small clamping force, self-tightening effect, the choice of a variety of metal materials, which can be adapted to low temperature, high temperature and highly corrosive medium. (4) graphite plate ring: In people's minds, graphite is a brittle material, the lack of flexibility and toughness, but after special treatment of graphite, it is soft texture, good elasticity. In this way, the heat resistance and chemical stability of graphite can be displayed in the gasket material; and the gaskets have a small pressing force, and the sealing effect is extremely superior. This graphite can also be made into a band, with the metal band with the composition of the performance of the spiral wound gasket. Graphite plate seals and graphite ---- metal spiral wound gasket appears, is a major breakthrough in high temperature anti-corrosion seal. Such gaskets, foreign countries have mass production and use. More Keywords Search: Valve