The sealing performance of the valve refers to the ability of the sealing parts of the valve to prevent medium leakage. It is the most important technical performance index 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 fit between the filler and the stem and packing box; and the connection between the valve body and the cover.
The former leakage is called internal leakage, which is commonly referred to as lax closure, and it will affect the ability of the valve to cut off the medium. Internal leakage is not allowed for truncation valves. Leakage at the last two places is called leakage, i.e. medium leaks from inside to outside of the valve. Leakage can cause material loss, pollution of the environment, and serious accidents. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.