FS (FKM spring-energized) seals are the most common API 6A elastomeric seal for gate valves and tubing heads. Featuring a U-shaped PTFE or modified PTFE jacket with Inconel or stainless spring, they provide reliable sealing at 20,000 psi and temperatures to 345°C while resisting explosive decompression in gas wells https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/