Double FS-seal stacks with Vented intermediate chambers allow block-and-bleed verification of seal integrity. Widely used in frac trees and manifold valves, this configuration detects primary seal failure before hydrocarbons reach the environment, meeting API 6A DD and EE material class requirements for sour service https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/