Because to prevent media leaks, valve shims become use to the main seal, used to fill the two binding surfaces (such as the valve body and the sealing surface between the cover) among all the asperities to prevent the media leakage from between the joint surface.

Usually with a spacer can be divided into soft and hard two kinds. Generally soft non-metallic, such as cardboard, rubber, asbestos rubber sheet, PTFE ; generally rigid metal or metal package asbestos, asbestos and metal wound gasket made. Metal gasket material generally higher with high-quality carbon steel and stainless steel, machining accuracy and finish requirements, power strong bolt is also larger, it is used for high temperature and pressure valves.

Valve requirements gasket material is resistant to corrosion, but also has a certain degree of elasticity and plasticity as well as sufficient strength at operating temperature, in order to ensure the seal. Reasonable choice of gasket material and type should be determined according to the working conditions of media, such as temperature, pressure and corrosion.