The sealing performance of a valve refers to the ability of each sealing part of the valve to prevent medium leakage, which is a very important technical performance indicator of the valve. There are three sealing parts of the valve: the contact between the opening and closing parts and the two sealing surfaces of the valve seat; The fitting between the packing, valve stem, and packing box; The connection between the valve body and the valve cover. The previous leakage is called internal leakage, which is commonly known as not being tightly closed, and it will affect the valve's ability to cut off the medium. For shut-off valves, internal leakage is not allowed. The last two leaks are called external leaks, which means that the medium leaks from inside the valve to outside the valve. Leakage can cause material loss, pollute the environment, and in severe cases, even cause accidents. For flammable, explosive, toxic, or radioactive media, external leakage is not allowed, so valves must have reliable sealing performance.