Double Block & Bleed (DBB) design in API 6D trunnion LNG valves uses independent seat seals on both sides of the ball. Each seat is self-relieving and spring-loaded with PCTFE or filled PTFE inserts, ensuring zero leakage even during thermal cycling between ambient and -196°C operating conditions https://globalsealstech.com/product-category/oil-gas-seals/api-6d-ball-valve-and-lng-seals/