Require a 6" 1500# ball valve the current datasheet specification gives the seat material as REINFORCED TEFLON(PTFE + 15% FIBER GLASS).
Reinforced PTFE (RPTFE) typically is not used in Class 1500 valves because when it is subjected to constant tensile stress or pressure loads, PTFE will escape, i.e. flow, at room temperatures. This behavior is called "cold flow" and is contingent upon the pressure or tensile stress exerted, the duration of this stress as well as temperature.
There are several grades of Devlon thermoplastics.  The most popular being Devlon V, which is a proprietary blend of Nylon 6 and Nylon 12 manufactured by the James Walker/Devol Engineering company in Scotland.
Specifying "nylon" on a data sheet is very common, and it permits the valve manufacturer the flexibility of selecting the specific grade of nylon for both the service and the seat design.
Nylon is generally inert to most chemicals, but is hydroscopic (that is, will absorb water) so it may not be a good selection for wet gas or water/seawater service.

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