Valves are pipeline fittings used to open and close pipelines, control flow direction, and regulate and manage the parameters of the conveyed medium—such as temperature, pressure, and flow rate. Based on their functions, they can be classified into shut-off valves, check valves, control valves, and others.

　　Valves are control components in fluid‑transport systems, performing functions such as shut‑off, regulation, flow diversion, prevention of backflow, pressure stabilization, and分流 or overflow‑relief. Valves used in fluid‑control systems range from the simplest shut‑off valves to highly complex types employed in automated control systems, with an extensive variety of types and specifications.

　　Valves can be used to control the flow of various types of fluids, including air, water, steam, a wide range of corrosive media, slurry, petroleum products, liquid metals, and radioactive media. Depending on the material, valves are further classified into cast‑iron valves, cast‑steel valves, stainless‑steel valves, chromium‑molybdenum steel valves, chromium‑molybdenum‑vanadium steel valves, duplex‑steel valves, plastic valves, and custom‑made non‑standard valves, among others.