Temperature ratings under API 6A range from K (-75°F) to X (650°F). Seal materials must maintain mechanical properties across the entire range. For example, an X-rated valve using FKM stem seals must still seal at -20°F after thermal cycling. Manufacturers qualify seal compounds by performing aged tensile, compression set, and volume swell tests at the upper and lower temperature extr... https://globalsealstech.com/product-category/oil-gas-seals/api-6a-gate-valve-seals/