Look up what ever valve standard you are using (ASM B 16.34, API 6A, 6D etc) and look in there for your class rating and material and design temperature.
Note that not all valves are designed able to handle their full class rating, so the real check is the valve data sheet or the valve nameplate stamped on it.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way