03 Teflon (PTFE)
Polytetrafluoroethylene (PTFE) is prone to cold flow and creep under pressure and high temperature, so it is generally used in low pressure, medium temperature, strong corrosion and non-polluting media, such as strong acid, strong alkali, halogen, drugs, etc. The safe use temperature is 150 ℃ and the pressure is less than 1MPa. Although filled PTFE has higher strength, its service temperature is not more than 200 ℃, and its corrosion resistance is reduced. The maximum service pressure of polytetrafluoroethylene gasket is generally not more than 2MPa.
As the temperature rises, the material creeps, resulting in a significant decrease in sealing pressure. Even if the temperature is not raised, the compression stress of the sealing surface will decrease with the time, resulting in “stress relaxation” phenomenon. This phenomenon can occur in all kinds of gaskets, but the stress relaxation of the PTFE gasket is relatively serious, which should be noted.
The friction coefficient of polytetrafluoroethylene is small (the compression stress is greater than 4MPa, and the friction coefficient is 0.035-0.04), and the gasket is easy to slide outward during preloading, so it is better to use concave and convex flange face. When the flat flange is used, the outer diameter of the gasket can be contacted with the bolt, and the bolt can be used to prevent the gasket from sliding outward.
Because the glass-lined equipment is sintered after spraying a layer of enamel on the metal surface, the glaze layer is very brittle, coupled with uneven spraying and glaze flow, the flatness of the flange surface is poor. The metal composite gasket is easy to damage the glaze layer, so it is recommended to use polytetrafluoroethylene gasket with asbestos board and rubber as the core material. The packing pad is easy to fit with the flange surface and corrosion resistant, with good use effect.
Many factories use asbestos rubber plate wrapped with polytetrafluoroethylene raw material belt in strong corrosive medium with low temperature and pressure, which is used on manhole and pipeline that are often disassembled. It is very popular because of its convenient production and use.
Unfinished (to be continued) ……
Post time: Mar-13-2023