For Piping spec of class Cl 125FF sometimes we take flange & valve ratings as 150# , 300# & sometimes as Cl 125. Which is mandatory requirement & why?

Regards
pme

for your specified rating 125FF , flange and valve rating 125 is required for your service . but in some case due to unavailability of material designer used to select higher one according to commercial Availability.

mkhan

There is never a problem with using materials that EXCEED the design requirements, especially if they are cheaper and/or more readily available.