FS seals (spring-energized PTFE) are widely used in API 6A wellheads for low-friction, high-reliability sealing. U-shaped jackets with stainless or Inconel springs maintain contact pressure in grooves under thermal and pressure cycling in valves and connectors https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/