stevo,
I assume that the quoted class 800# should be a missprint, either class 600# or 900#, in accordance with API 6D. Please find below an "extract" from the API6D, ed. April 2008, Clause 7.2:
"Pressure-temperature ratings for class-rated valves shall be in accordance with the applicable rating table for the appropriate material group in ASME B16.34.
Pressure-temperature ratings for PN-rated valves shall be in accordance with the applicable rating table for the appropriate material group in EN 1092-1.
If intermediate design pressures and temperatures are specified by the purchaser, the pressure-temperature rating shall be determined by linear interpolation.
Pressure-temperature ratings for valves made from materials not covered by ASME B16.34 and EN 1092-1 shall be determined from the material properties in accordance with the applicable design standard."
Since the 1975 psi is exceeding the class 600# alltogether, then the temperature rating for 1975 psi in class 900# would be some 235-236 F, group material 1.1, as per ASME B16.34.
cheers,
gr2vessels