O-rings provide excellent sealing performance in the bonnet assembly at a relatively low cost. Their mechanical properties depend on temperature, pressure, and chemical exposure. The service conditions can affect o-ring strength, flexibility and resilience, which needs to be maintained for proper sealing. At low temperatures, elastomers become harder, less resilient and lose their memory. At elevated temperatures the softness lowers the strength which in turn limits o-ring effectiveness under pressure. The media can have a significant impact on the suitability of o-ring performance. Therefore, proper selection of o-ring material is very important.
NOSHOK also offers optional packing bonnet assemblies for our needle & manifold valves.
To overcome the service limits of elastomers, two of the most common packing materials, PTFE and Graphite, are used to extend the range of temperature, pressure and fluid exposure.
PTFE has a wide range of chemical resistance and may be used in applications which are incompatible with elastomers. PTFE is thermally stable across a wide range of temperatures and has a service range of -100 °F to +400 °F. PTFE can temporarily withstand temperatures of -240 °C and has been successfully use in cryogenic applications.
Graphite is a flexible graphite made from pure, natural graphite flake and is resistant to heat, fire, corrosion and certain aggressive chemicals. Graphite packing is chemically resistant to nearly all organic and inorganic fluids, except for highly oxidizing acids and chemicals. This outstanding chemical compatibility generally exists across Graphite’s entire temperature range of -400 °F to 850 °F.
It is important to note that all materials are susceptible to accelerated chemical attack by at least one chemical compound, and increasing the service temperature may also weaken a material’s ability to withstand reactive chemicals.
Packed valves rely upon the mechanical compression of a soft packing material against the valve stem. This compression of the packing material eliminates all but extremely small leak paths between the packing material and valve stem.