Knowledge on selecting corrosion-resistant pipeline control valves

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　　The problem of valve corrosion remains a significant challenge in control instruments, with severe consequences; as a result, some automation systems are forced to switch to manual operation, and there is also a need to frequently replace the corroded instruments and equipment. Therefore, the issue of corrosion resistance remains a concern for manufacturers and design institutes both at home and abroad. I. Corrosion-resistant materials (1) Rubber has a certain degree of corrosion resistance and is inexpensive. Therefore, it is widely used in control valves for corrosion-resistant equipment at low temperatures; rubber-lined diaphragm valves were developed in the 1950s and 1960s. To handle high-volume corrosive media, rubber-lined butterfly valves were developed. However, rubber is not completely corrosion-resistant, and its operating temperature and pressure tolerance are subject to certain limitations. (2) Polytetrafluoroethylene (abbreviated as F4 or PTFE in foreign languages) is a relatively excellent corrosion-resistant material today, and is known as the \"king of corrosion resistance\". It possesses excellent corrosion and heat resistance, allowing it to be used over a long period at temperatures ranging from -250 to 250°C. Apart from being corroded by fluorine and molten alkali metals under high temperature and pressure, and experiencing slight contraction due to certain halides or aromatic hydrocarbons, other substances such as strong acids (including concentrated nitric acid and aqua regia), strong bases, strong oxidizing agents, oils, ketones, ethers, alcohols, etc., have no effect on it even at high temperatures. It is virtually capable of withstanding all chemical agents without being corroded by them. (3) Polytetrafluoroethylenepropylene is a new type of material with excellent corrosion resistance and heat resistance; it possesses high chemical stability as well as outstanding resistance to chemical corrosion, such as strong acids, strong bases, and strong oxidizing agents. It also boasts excellent heat and cold resistance, allowing it to be used over long periods in environments with temperatures ranging from -40 to 250°C ； At the same time, it also has properties such as non-sticking, water-resistant, and non-extinguishable. Furthermore, this material has good mobility and can be lined on the surface of components as a corrosion-resistant material. (4) As early as the 1920s and 1930s, corrosion-resistant alloys such as Hastelloy and Monel were available abroad, while development in China only began in the 1950s and 1960s. The use of corrosion-resistant alloys in control valves began in the 1980s, but their widespread adoption was not possible due to the high cost of these alloys. II. Corrosion-resistant control valve series (1) Fully PTFE-corrosion-resistant electric actuated single-seat control valve The fully PTFE-corrosion-resistant single-seat control valve features a structure with a metal casing and PTFE inlays. The metal casing is usually made of carbon steel or stainless steel, while the parts inside the valve that come into contact with the medium—such as chambers, channels, and throttling elements—are all made of polytetrafluoroethylene. This separates the valve material from corrosive media, thereby addressing corrosion issues effectively; its performance is particularly outstanding when used in media such as sulfuric acid, hydrochloric acid, and wet chlorine gas. (2) Electric/pneumatic fluorinated rubber O-ring ball valve The fluorinated rubber ball valve, also known as a ball valve specifically designed for water treatment, is mainly used in two-position control applications involving strong alkalis and water treatment (desalinated water). It uses a fluoroplastic lining method, in which a corrosion-resistant material is lined inside the valve body, ensuring that all areas in contact with various highly corrosive media are made of fluoroplastic, thereby separating the corrosive media from the valve body material ； In terms of structure, since the ball core of a ball valve is a component with high stiffness and reliability, it effectively addresses the issue of poor reliability associated with the diaphragm in traditional diaphragm valves, thereby improving the valve’s reliability and extending its service life. Therefore, the fluorine-lined ball valve is an ideal product to replace diaphragm valves. (3) Electric/pneumatic fluorinated V-ball valve: The fluorinated O-ring ball valve is suitable only for on/off control. In applications involving highly corrosive media containing impurities, the fluorinated V-ball valve is more appropriate. Its structure and installation dimensions are the same as those of the fluorinated O-ring ball valve; the only difference is that the ball core features a V-shaped aperture that provides regulatory capabilities. As a result, it not only has excellent anti-clogging properties but also good regulatory performance when operating at low opening degrees. In media containing particles, thin fluids, suspensions, etc., it is recommended to use this corrosion-resistant valve. (4) Electric/Pneumatic Fluorinated Butterfly Valve The fluorinated butterfly valve is suitable for use in applications with large diameters and high flow rates, where strong corrosion resistance is required. It uses the same manufacturing process as fluorinated lining ball valves, with a fluoroplastic lining inside the valve body to ensure that all materials in contact with highly corrosive media are fluoroplastics with excellent corrosion resistance. Therefore, it also possesses excellent corrosion resistance. The use of a butterfly valve design also provides excellent anti-clogging performance. (5) Corrosion-resistant alloy valves Corrosion-resistant alloy valves are suitable for applications with high pressures (above 2.5 Mpa) and extreme temperatures (above 180°C or below -40°C). However, their high cost is due to the use of extremely expensive materials. It is recommended that DN≤20, and a straight-through single-seat valve design should be used ； For 25≤DN≤150, a full-function valve design is selected ； For DN≥200, a butterfly valve design is selected, which can significantly reduce costs and improve the cost-performance ratio. III. Products Not Recommended for Use (1) Fluoroplastic Diaphragm Valves Before the 1980s, since no better corrosion-resistant valves were available at that time, fluoroplastic diaphragm valves were widely used by many manufacturers for a long period of time. However, feedback from on-site use indicates that this valve has two significant defects. First, it has poor reliability and a short lifespan. Thin diaphragms made of fluoroplastics are easily broken when forced to fold up and down during operation ； Secondly, they are bulky; while control valves are gradually evolving toward smaller size, lighter weight, and more integrated instrumentation, diaphragm valves have failed to see any reduction in weight. Due to the pressure of the media on both sides of the valve acting on the diaphragm, the uneven thrust generated is very large; consequently, the thrust required from the actuator must also be very high, making it impossible to reduce its weight. (2) Alloy two-seat valves and sleeve valves: Alloys are expensive, costing several hundred to over a thousand yuan per kilogram. Such designs consume a large amount of material, so they are not suitable; instead, fully functional ultra-lightweight valves or butterfly valves that use less material should be chosen.