Start-up valves, also known as discharge valves, are critical components in extrusion lines, polymerization lines, and underwater pelletizing lines; in underwater pelletizing lines, they are standard core equipment. Their primary functions include precisely controlling the flow of melt, rapidly discharging material and impurities, protecting upstream equipment, and ensuring the continuous, stable, and efficient operation of the entire production line. I. Operating Principle and Structural Featur