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Applications of Eight Different Gaskets (Part Three)
Posted: 09/27/2017 04:36:42  Hits: 12
3. PTFE (polytetrafluoroethylene) gaskets
Cold flows and creeps will easily happen in PTFE gaskets at pressure and high temperature. Therefore, PTFE gaskets are usually applied to low-pressure, medium temperature, strong corrosive and uncontaminated media such as strong acid, alkali, halogen and drugs. Operating temperatures of PTFE gaskets are 150℃, and pressure of PTFE gaskets is below 1MPa. Gaskets filled with PTFE have high strength high, but their operating temperatures do not exceed 200℃and corrosion resistance is worse. The maximum working pressure of the PTFE gasket is generally not higher than 2MPa. 

Creeps of materials will happen due to rises of temperatures, resulting in sealing pressure obviously becoming low. Even if the temperature doesn't rise, the compressive stress of the sealing surface will decrease, resulting in the stress relaxation. The stress relaxation will occur in various gaskets, but the stress relaxation of the PTFE gasket is more serious, which should be paid attention to.

Friction coefficients of PTFE gaskets are relatively small; compressive stresses of PTFE gaskets are over 4MPa; friction coefficients of PTFE gaskets are from 0.035 to 0.04. PTFE gaskets will easily slide outward when PTFE gaskets are preloaded. Therefore, we had better adopt flanges with concave and convex surfaces. When flat flanges are adopted, areas of PTFE gaskets' outer diameters should be in contact with flanges' bolts so as to use flanges' bolts to prevent outward sliding of PTFE gaskets.

Because surfaces of flanges are coated with a layer of ceramic glaze and then are sintered by glass lining equipment, very brittle glaze layers coupled with uneven coatings and flowing glaze layers will cause poor flatness of flanges' surfaces. Adopting metal composite gaskets will easily break glaze layers; therefore, it is recommended using PTFE gaskets whose core materials are asbestos sheets and rubber. This kind of gasket not only is easy to fit the flange's surface, but also is corrosion resistant, which can have the good using effect.

In strong corrosive media without high temperature and pressure, many factories use asbestos rubber sheets wrapped with PTFE Teflon tapes for manholes and pipes which are often disassembled. Asbestos rubber sheets wrapped with PTFE Teflon tapes are very popular with people due to their easy production and use. 

4. Asbestos resin and impregnated asbestos gaskets
Asbestos resin and impregnated asbestos gaskets are mainly used for pipes, pumps, valves and import and export flanges which are used for various acid media. The operating temperature of the asbestos resin and impregnated asbestos gasket is 80℃, and the pressure is below 0.6MPa.

Asbestos resin and impregnated asbestos gaskets can be used for low pressure and high temperature working conditions whose temperatures are lower than 800℃. Asbestos resin and impregnated asbestos gaskets can be made with different width, thickness and diameters based on the specific requirements of the equipment. Cut the asbestos tape, and then put it on the surface of the flange. The asbestos tape is applied to the connecting part of large-scale sulfuric acid, nitric acid oxidation furnace and some unprocessed equipment, and its effect is far more better than the original asbestos rope.

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