Abstract: A valve seal mechanism for use in pressurized fluid systems subject to fire risk, the seal having a soft seat to provide essentially zero leakage at normal operating temperatures and a backup hard metal seating surface capable of holding leakage to extremely low rates in the event of temperatures high enough to destroy the soft seat material. The seal mechanism is in the form of a hard metal seal ring adapted to fit within an undercut circumferential groove in a valve seat. The metal seal ring has a central inner circumferential rib adapted to extend through the groove opening for mating engagement with a sealing surface on the circumference of a valve disc. The rib of the metal seal ring itself has a circumferential undercut groove into which is tightly fitted a seal ring of material softer than the metal of the seal ring, the soft seal ring protruding beyond the adjacent inner surface of the rib to provide a primary valve seat surface.