Bonnet seals are typically BX or RX ring gaskets in 6A valves, but extreme service uses metal end-cap elastomer seals or spiral-wound Inconel with FFKM core. These maintain 20,000 psi integrity after repeated make-and-break and full PR2F temperature/pressure cycling. https://globalsealstech.com/product-category/oil-gas-seals/api-6a-gate-valve-seals/