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In fluid piping systems, valves are control elements whose primary functions are to isolate equipment from the piping system, regulate flow rate, prevent backflow, and regulate and release pressure. Valves can be used to control the flow of various types of fluids, including air, water, steam, various corrosive substances, mud, oils, liquid metals, and radioactive materials. Since it is very important to select the most suitable valves for pipeline systems, it is equally crucial to understand the characteristics of valves as well as the steps and criteria for choosing them. Classification of valves: I. Valves can be divided into two main categories: The first category is automatic valves – valves that operate automatically relying on the properties of the medium itself (liquid or gas). Such as check valves, safety valves, control valves, steam traps, pressure reducing valves, etc. Category 2 drive valves: Valves whose operation is controlled by manual, electric, hydraulic, or pneumatic means. Such as gate valves, globe valves, throttle valves, butterfly valves, ball valves, plug valves, etc. II. Based on structural characteristics, and according to the direction in which the closing element moves relative to the valve seat, they can be classified as follows: 1. Gate type: The closing element moves along the center of the valve seat ； 2. Gate type: The closing element moves along the vertical center of the valve seat ； 3. Plug and ball type: The closing element is a plunger or a ball that rotates around its own central axis ； 4. Swing type: The closing element rotates around an axis outside the valve seat ； 5. Disc type: The disc of the closing element, which rotates around an axis within the valve seat ； 6. Slide valve type: The closing element slides in a direction perpendicular to the channel. III. By purpose, valves can be classified according to their different functions: 1. Shut-off valves: Used to connect or disconnect the fluid in a pipeline, such as globe valves, gate valves, ball valves, butterfly valves, etc. 2. Check function: Used to prevent the medium from flowing back, such as check valves. 3. Control devices: Used to regulate the pressure and flow rate of the medium, such as control valves and pressure reducing valves. 4. Distribution use: Used to change the flow direction of the medium and distribute it, such as three-way stopcocks, distribution valves, slide valves, etc. 5. Safety valves: Used to release excess fluid when the pressure of the fluid exceeds the specified value, thereby ensuring the safety of the piping system and equipment; examples include safety valves and emergency valves. 6. Other special uses: such as steam traps, vent valves, drain valves, etc. IV. By driving method, it can be classified according to different driving approaches: 1. Manual: It relies on hand wheels, handles, levers, or sprockets for human-driven operation; reduction devices such as worm gears and gears are used when transmitting high torque. 2. Electric: Driven by a motor or other electrical device. 3. Hydraulic: Driven by (water, oil). 4. Pneumatic: Driven by compressed air. V. Pressure rating: Based on the nominal pressure of the valve, they can be classified as follows: 1. Vacuum valves: Absolute pressure