A valve's sealing performance refers to the ability of each sealing part of the valve to prevent leakage of the medium. It is the most important technical performance indicator of a valve. A valve seals at three locations: the contact between the opening and closing parts and the sealing surfaces of the valve seat; the mating between the packing, the valve stem, and the stuffing box; and the connection between the valve body and the bonnet. Leakage at the first location is called internal leakage, commonly known as a loose seal, which will affect the valve's ability to cut off the medium. For cut-off valves, internal leakage is unacceptable. Leakage at the second and third locations is called external leakage, meaning the medium leaks from the valve interior to the exterior. External leakage can cause material loss, environmental pollution, and, in severe cases, accidents. External leakage is particularly unacceptable for flammable, explosive, toxic, or radioactive media, making valves absolutely essential for reliable sealing.