Abstract: A positive shut-off seal mechanism for use in pressurized fluid systems subject to high temperatures or fire risk. The seal mechanism includes a metallic valve seat in the form of an annular thin-walled skirt-like lip, preferably formed in a valve seat insert ring or retaining ring, the lip extending generally upstream toward the high pressure side of the valve. The edge of the lip engages a mating circumferential sealing surface of a valve disc, preferably at a location downstream from the plane of maximum disc diameter, with an interference fit to provide a preloaded radial sealing force between the edge of the lip and the sealing surface of the disc. The annular lip preferably forms the downstream side of an undercut groove for a resilient seal ring that provides a primary seal, the engagement of the lip edge with the sealing surface of the valve member providing a metal-to-metal backup seal in the event of damage to or destruction of the resilient seal ring.