Valve packing is a kind of dynamic sealing structure which is installed between the valve stem and the packing box of the valve cover to prevent outside leakage. Packing material, reasonable packing box structure and installation methods ensure the valve reliable sealing performance. There are various valve sealing packing materials are available and different packing suitable for different working conditions, including asbestos, graphite, PTFE, etc.
Flexible graphite packing
Flexible graphite packing is the most widely used material in the valve, which can be pressed molding, has been widely used in the field of petroleum, chemical industry, power generation, chemical fertilizer, medicine, paper, machinery, metallurgy, aerospace and atomic energy and other industries wherewith the nominal pressure ≤32MPa. It has the following excellent performance:
Good flexibility and resilience. The incision packing can be freely bent more than 90° in the axial direction and will leak-free due to the change of temperature/pressure/vibration, safe and reliable; Good temperature resistance. The wide range of -200℃-500℃ usages, even in non-oxidizing medium up to 2000℃ and maintain excellent sealing; Strong corrosion resistance. It has good corrosion resistance to acid, alkali, organic solvent, organic gas and steam. Low friction coefficient, good self – lubrication; Excellent impermeability to gases and liquids; Long service life, can be repeated use.
Polytetrafluoroethylene packing has good lubrication, weaving polytetrafluoroethylene packing has excellent corrosion resistance, and can be used for cryogenic medium, but its temperature resistance is poor generally only used in the temperature below 200℃ conditions, while can not be used for melting alkali metal and high temperature of fluorine, hydrogen fluoride medium.
Vegetable fiber packing
Made of hemp or cotton impregnated oil, wax or other anti-seepage materials, used for low-pressure valves below 100℃ and media like water, ammonia and etc.
Asbestos fiber has better heat resistance, absorption and strength can withstand weak acid, strong alkali. Inked asbestos, rubber asbestos and oil-impregnated asbestos are suitable for valves with the steam temperature of 450℃.
Rubber cloth, rubber rod, ring rubber packing for temperature ≤140℃ ammonia, concentrated sulfuric acid and other media.
Carbon fiber packing
The carbon fiber filler is made of polytetrafluoroethylene emulsion impregnated with carbon fiber is a woven rope. Carbon fiber packing has excellent elasticity, excellent self-moistening and high-temperature resistance. It can work stably in the air temperature range of -120 ~ 350℃, and the pressure resistance is less than 35MPa.
Metal + rubber packing
It can include metal-wrapped packing, metal laminated packing, metal corrugated packing, lead packing, etc. The metal wrapped packing and metal laminated packing is characterized by high-temperature resistance, erosion resistance, abrasion resistance, high strength, good thermal conductivity, but poor sealing performance must be used with plastic packing, its temperature, pressure, corrosion resistance that depends on metal material.
Stainless steel wire + flexible graphite woven packing
Generally, v-shaped packing is composed of upper packing, middle packing and lower packing. The upper and middle packing is made of PTFE or nylon, and the lower packing is made of 1Cr13, 1Cr18Ni9 and A3 steel. PTFE can withstand high temperature 232℃, nylon 93℃, general pressure 32MPa, often used in corrosive media.
Generally speaking, the valve packing materials are mainly PTFE and flexible graphite, be noted that the packing box dimension accuracy, roughness, stem surface dimension accuracy also affect packing seal performance.