Spring-energized PTFE seals (Helicoflex® or Variseal®) are standard for seat inserts in metal-seated valves requiring API 6A PR2F plus fire-test compliance. Inconel or Elgiloy springs provide constant load while virgin or filled PTFE jackets resist chemicals and extrusion up to 30 ksi. https://globalsealstech.com/product-category/oil-gas-seals/api-6a-gate-valve-seals/