Butterfly Valve for Seawater Desalination: Design & Material

In seawater desalination systems, the performance of valves is directly related to the overall operational reliability and efficiency of the installation. As one of the most widely used key components, the structural design and material selection of butterfly valves are particularly important. Seawater desalination butterfly valves must not only withstand complex pressure and temperature conditions but also possess excellent seawater corrosion resistance to ensure long-term stable operation. Therefore, a deep understanding of the structure, material selection, process requirements, and practical application of seawater desalination butterfly valves is of great significance for improving system safety and operational economy.

Seawater desalination butterfly valves are composed of multiple key components, each bearing a specific function. The material selection also varies depending on its role and operating environment.

The valve body is the main framework of the butterfly valve, much like the foundation of a building; its quality and performance directly affect the overall stability of the butterfly valve. Depending on different pressure ratings, the manufacturing process of the valve body also varies. For medium- and low-pressure specifications, the valve body is usually manufactured by casting, which allows molten metal to fully flow in the mold to form complex shapes, meeting usage requirements in medium- and low-pressure environments. For medium- and high-pressure specifications, the valve body is manufactured by forging. The forging process applies high pressure to deform the metal, resulting in higher strength and toughness to withstand higher pressure challenges.

The material selection of the valve body is extensive, mainly including cast iron, carbon steel, and stainless steel. Cast iron has good castability and reasonable strength, suitable for applications where strength requirements are not particularly high. Carbon steel provides better strength and toughness, allowing it to withstand greater pressure. Stainless steel offers high strength and excellent corrosion resistance, making it particularly suitable for seawater desalination environments where corrosion resistance is critical, as it effectively resists seawater erosion and extends the service life of the butterfly valve.

The valve disc is the core component of a butterfly valve and is responsible for controlling the flow of fluid in the pipeline. Shaped like a circular plate, it rotates around its axis within the valve body to open, close, or modulate flow. The main materials for valve discs are ductile iron and stainless steel. Ductile iron has good castability and adequate strength to meet general operating conditions. Stainless steel valve discs provide superior corrosion resistance and strength, making them ideal for complex seawater desalination environments, as they effectively prevent seawater corrosion and ensure reliable operation of the butterfly valve.

The valve shaft, also known as the valve stem, is an important part of the butterfly valve's transmission system. It connects to the actuator or handle at the top and to the valve disc at the bottom. Through its rotation, it drives the disc to open or close the valve. The materials used for valve shafts are typically stainless steel and carbon steel. Stainless steel shafts offer strong corrosion resistance, ensuring good performance in highly corrosive seawater desalination environments. Carbon steel shafts provide high strength and toughness, allowing them to withstand large torque and ensure operational stability and reliability.

The valve seat is the key sealing component of the butterfly valve. Depending on the material, it can be classified into soft-seal and metal-seal types. Soft-seal butterfly valves typically use sealing materials such as nitrile rubber (NBR), ethylene propylene diene monomer (EPDM), and polytetrafluoroethylene (PTFE). These materials offer excellent elasticity, wear resistance, and corrosion resistance, allowing effective sealing under low pressure and making them suitable for applications requiring high sealing performance. Metal-sealed butterfly valves use metallic materials as the sealing surface, offering higher strength and wear resistance, allowing stable sealing performance under higher pressure, and making them suitable for conditions requiring stringent sealing and higher working pressure.

Material selection for seawater desalination butterfly valves is critical because it directly affects service life and reliability during desalination processes. When selecting materials, factors such as seawater corrosion characteristics, operating temperature, pressure, and the chemical nature of the medium must be comprehensively considered.

Materials used for valve bodies and valve discs include gray cast iron (HT200, HT250, HT300), ductile iron (QT400, QT450), carbon steel (WC1, WCB), alloy steel (WC6), and stainless steel (304, 316, 316L). In practical applications, suitable materials should be selected according to the specific working conditions of the desalination system. For environments requiring higher corrosion resistance, stainless steel may be preferred; for conditions requiring higher strength, alloy steel may be used.

Material selection for the valve shaft is equally important. Common materials include stainless steel (2Cr13, 304, 316, 316L) and carbon steel (35, 45, Q235). Stainless steel shafts resist seawater corrosion effectively, while carbon steel shafts provide high strength and toughness to withstand high torque, ensuring stable operation.

Sealing performance is a primary indicator of butterfly valve quality. Sealing material selection depends on medium temperature, pressure, concentration, and other factors. Soft-seal butterfly valves commonly use NBR, EPDM, and PTFE, offering elasticity, wear resistance, and corrosion resistance for effective low-pressure sealing. For metal-seal butterfly valves, suitable metallic sealing materials must be selected based on specific operating conditions to ensure stable sealing under high pressure.

Manufacturing seawater desalination butterfly valves must meet several process requirements. First, the valve body's pressure rating, temperature resistance, and corrosion resistance must meet or exceed the requirements of the process piping and should preferably use standardized products from manufacturers. Second, cast iron valves should not be used for steam, moisture-rich gas, or flammable/explosive media; cast iron valves should also be avoided when ambient temperatures fall below -20°C. For media with severe cavitation or erosion, wear-resistant materials such as cobalt-based alloys or Stellite surfacing should be used. For strongly corrosive media, corrosion-resistant alloys must be selected based on specific medium types, concentrations, temperatures, and pressures.

Seawater desalination butterfly valves are widely used to control air, water, steam, corrosive media, slurry, oils, liquid metals, and radioactive media in various operations. In pipelines, butterfly valves primarily serve shut-off and throttling functions. Their structural design and material configuration offer numerous advantages in desalination processes.

The typical size range for seawater desalination butterfly valves is DN300–1600, exceeding the usable size range of ordinary ball valves and globe valves. They are widely used not only in industries such as petroleum, gas, chemical processing, and water treatment but also in cooling water systems of thermal power plants, making them indispensable in desalination systems.

The valve disc adopts a dual flat-plate flow-through design, maintaining high strength under hydrodynamic impact and offering low head loss. The main body material, such as nickel-chromium alloy cast iron or nickel-chromium alloy ductile iron, provides excellent seawater corrosion resistance, improving service life. The double-eccentric structure ensures a tighter seal as the valve closes and avoids contact between the disc and sealing surface during opening, protecting the sealing surface from damage.

Valve seats are available in both soft-seal and metal-seal types, each with distinct advantages. Soft seals provide elasticity and corrosion resistance for superior sealing under low pressure. Metal seals offer high strength and wear resistance, maintaining sealing under high pressure. Additionally, butterfly valves offer bidirectional sealing and are not limited by flow direction during installation.

To further enhance corrosion resistance, internal and external surfaces are typically coated with epoxy resin at a thickness of ≥250 μm to prevent fouling and microbial growth. Heavy-duty anti-corrosion coatings such as ZF101 may also be applied upon request to improve resistance to seawater corrosion.

Special Structural Design: Special structural features are adopted to accommodate harsh desalination environments. For example, valve stems use triple O-ring seals and pass vacuum and packing leak tests to prevent atmospheric intrusion. The valve body is fully encapsulated by the liner to eliminate seawater infiltration, protecting the stem from corrosion. The connection between the valve stem and disc adopts a square tenon structure to ensure strong engagement and stable operation.

Special Material Applications: Certain advanced materials are also used to improve performance and service life. Powdered Nylon 11 is applied as a disc coating for wear and corrosion resistance, suitable for seawater with chloride content below 0.03% (wt). Halar (ECTFE), a copolymer of ethylene and chlorotrifluoroethylene, offers excellent chemical resistance and insulation properties, resisting a wide range of corrosive agents and providing high cost-performance for seawater corrosion protection. Aluminum bronze has good mechanical properties, formability, thermal conductivity, and excellent seawater corrosion resistance under chloride content below 0.03% (wt). Stainless steel offers strong corrosion resistance but is susceptible to pitting and crevice corrosion in seawater, requiring careful material selection. Super austenitic stainless steels such as 254SMO, 114529, and AL6XN offer exceptional resistance to chloride-induced corrosion for the most demanding applications.

Seawater desalination butterfly valves are specially optimized in structural design, material selection, sealing configurations, and corrosion-resistant treatments to address the multiple challenges of desalination operations. From the material standards of valve bodies, discs, and shafts, to the diverse options of soft and metal seals, and the application of anti-corrosion coatings and special alloys, every aspect reflects a design philosophy focused on stability, durability, and efficiency under harsh seawater conditions.