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1. Definition of a valve: A “valve” is a device used in fluid systems to control the direction, pressure, and flow rate of fluids. A valve is a device that allows or stops the flow of media (liquids, gases, powders) within pipes and equipment, and can also control its flow rate. Second, depending on the different functions of valves in opening and closing, there are many ways to classify them; here, several of these methods are introduced. 1. Classification by function and purpose: (1) Shut-off valves: Also known as isolation valves, their role is to connect or disconnect the medium in a pipeline. Cut-off valves include gate valves, globe valves, plug valves, ball valves, butterfly valves, and diaphragm valves. (2) Check valve: A check valve, also known as a one-way valve or non-return valve, serves to prevent the medium in the pipeline from flowing backward. The bottom valve at the water pump’s suction side also belongs to the category of check valves. (3) Safety valve: The function of safety valves is to prevent the pressure of the medium in pipelines or devices from exceeding specified values, thereby achieving safety protection. (4) Control valves: Control valves include regulating valves, throttle valves, and pressure reducing valves; their function is to regulate parameters such as the pressure and flow rate of the medium. (5) Diversion valves: Diversion valves include various distribution valves and steam traps, etc., whose function is to distribute, separate, or mix the fluids in the pipelines. 2. Classification by actuation method: (1) A automatic valve is a valve that does not require external force for operation; instead, it relies on the energy of the medium itself to move the valve. Such as safety valves, pressure relief valves, steam traps, check valves, automatic control valves, etc. (2) Power-driven valve: A power-driven valve can be driven by various power sources. Electric valve: A valve that is driven by electricity. Pneumatic valve: A valve driven by compressed air. Hydraulic valve: A valve driven by the pressure of liquids such as oil. In addition, there are combinations of the above driving methods, such as pneumatic-electric valves. (3) Manual valve: A manual valve operates the valve through human effort using a handwheel, handle, lever, or sprocket. When the torque required to open and close the valve is high, a gear or worm gear reducer can be installed between the handwheel and the valve stem. If necessary, universal joints and drive shafts can also be used for remote operation. In summary, there are many methods for classifying valves, but they are primarily classified based on the function they perform in the pipeline. General-purpose valves used in industrial and civil engineering can be divided into 11 categories, namely gate valves, globe valves, plug valves, ball valves, butterfly valves, diaphragm valves, check valves, throttle valves, safety valves, pressure relief valves, and steam traps. Other special valves, such as valves for instruments, valves for hydraulic control systems, and valves used in various chemical machinery and equipment, are not covered in this book.