Alternatives to PCTFE for Cryogenic Valve Seats
High Performance Thermoplastic Valve Seat Materials
Fluoropolymers like PTFE and PCTFE are commonly specified for valve seats due to their chemical inertness and conformability to mating metals across temperature extremes.
Sealing performance at cryogenic temperatures as low as 4 Kelvin (-453°F) is of particular interest due to the increasing usage of liquefied natural gas (LNG - 113K), liquid hydrogen (LH2 – 19K) and liquid helium (LHe – 4K) cryogens.
The physical properties of polymeric materials are strongly dependent on temperature. At low temperatures, plastic materials generally become stronger and stiffer. However, ductility is compromised – sometimes leaving components vulnerable to brittle fracture.
Polyaryletherketones (PAEK) and polyimides (PI) are often specified for high-temperature valve seats. These materials also have attractive mechanical, thermal, and tribological properties at cryogenic temperatures. Certain grades offer reliable sealing performance for high-pressure applications across both temperature extremes.
Thermoplastic materials that retain ductile characteristics at cryogenic temperatures include PTFE, PCTFE, PEEK, and DuPont™ Vespel® polyimide shapes. Engineered reinforcements are commonly used to enhance the mechanical and tribological characteristics of the base polymer.
Increasing Lead Times for PCTFE
In 2022, supply chain disruptions have severely affected fluoropolymers, especially PCTFE. Extended lead times have prompted design engineers to evaluate high-performance polymers like PEEK and DuPont™ Vespel® for applications which have traditionally specified fluoropolymer and fluoropolymer composite materials including PCTFE, PVDF, ETFE, ECTFE, PFA, and FEP.
High Performance Thermoplastic Sealing Components
Fluoropolymers, polyaryletherketones, and polyimides have broad chemical and corrosion resistance, extreme-temperature capabilities, as well as excellent flammability characteristics. Fluoropolymers couple these properties with softness and flexibility which facilitates easy conformability to metal counter-face surfaces, thereby achieving robust seals at low pressures.
Key limitations of fluoroplastics include poor resistance to cold-flow and a susceptibility to extrusion when subject to high temperature or pressure. PEEK is a strong, stiff, high-temperature material in the polyaryletherketone family of thermoplastics that resists deformation in extreme temperature-pressure conditions. It is used where superior mechanical properties, dimensional stability, resistance to abrasion, or improved friction and wear characteristics are required. PEEK is commonly used for piston rings, valve seats, and various sealing components in demanding valve and compressor applications.
In addition to primary sealing components, PEEK is also widely used as a mechanical back-up ring for softer fluoroplastics and elastomers in the oil and gas industry.
Bearing grade PEEK formulations such as TECAPEEK® PVX are engineered to achieve exceptional tribological performance in dynamic sealing applications, boasting a limiting PV of approximately 500,000 lb-ft/in2-min. The reduced static and dynamic coefficients of friction also reduce actuation torque requirements.
Where conditions exceed the limitations of fluoropolymers, polyamideimides, and polyaryletherketones, certain grades of DuPont™ Vespel® polyimide are frequently specified. DuPont™ Vespel® shapes are strong, ductile, and creep-resistant thermoplastic materials that offer relatively consistent mechanical and thermal properties from cryogenic temperatures up to 500°F+. DuPont™ Vespel® has been used for decades in demanding applications, some utilizing the material for excursions approaching 900°F. Owing to outstanding tribological properties and resistance to creep and stress-relaxation, DuPont™ Vespel® is widely used for valve seats and seals in aerospace applications that require extreme reliability.
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