The sealing performance of a valve refers to the ability of each sealing part of the valve to prevent medium leakage, and it is the most 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 sealing surface of the valve seat; The fitting between the packing and the valve stem and packing box; The connection between the valve body and the valve cover. The leakage in the first part is called internal leakage, which is commonly referred to as not being tightly closed, and it will affect the valve's ability to cut off the medium.

For block valves, internal leakage is not allowed. The leakage in the last two places is called external leakage, which means that the medium leaks from inside the valve to outside. External leakage can cause material loss, environmental pollution, and in severe cases, accidents. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so valves must have reliable sealing performance.