What kind of valve is a dynamic seal valve?

Dynamic sealing: The dynamic sealing of a valve refers mainly to the stem seal. Prevents the medium inside the valve from leaking as the valve stem moves. 1. Packing box type: At present, for the dynamic sealing of valves, packing boxes are the most commonly used. 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, in terms of compression bolts, they can be divided into T-bolts (used for low-pressure valves with a pressure ≤ 16 kilograms per square centimeter), double-headed bolts, and swivel bolts, among others. 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 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 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 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 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. 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. Furthermore, the using units often explore various effective forms of fillers according to their own needs. For example, in steam valves at 250°C, alternating layers of asbestos gasket and lead rings can reduce steam leakage ； For some valves where the medium changes frequently, using an 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 used in some cases. Currently, research is being conducted on novel fillers. For example, by impregnating polyacrylonitrile fibers with polytetrafluoroethylene emulsion, followed by pre-oxidation and then sintering and pressing them in a mold, molded fillers with excellent sealing properties can be obtained ； For example, wave-shaped fillers 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. This post was last edited by guoba1985 on 2009-3-11 13:48.]