"Fire Safe" ball valve, particularly in accordance with API 607. My understanding is that the construction of the valve seat has both a resilient component for better sealing, as well as a metallic component in for secondary sealing in the case of a fire.
I used to test valves acc to API 607 for a living.  Polymeric seals melt during the fire and a secondary metallic seal takes over.
In the case of the valves that I tested the valves were designed with a "Fire Lip", a sharp edge in the seat seating area.  The ball would seat on the fire lip once the seal failed.
API 607 is hard

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