Bonnet seals in API 6A valves isolate the body cavity from the stem area. Typically FS or S-seals made from filled PTFE with stainless springs, they accommodate thermal expansion and high differential pressure. Parallel or tapered designs ensure consistent energization across temperature cycles in subsea and surface trees https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/