Machining of buttweld fittings

Please where does it state in the standards that you can machine bevell of ends of buttweld fittings from for example Sch 160 down to Sch 80 (?) or is it just an industry accepted standard (?)

Its called “back beveling” …where you machine a back taper in the thicker schedule fitting to match the thinner schedule. To be honest I don’t know of any standards that “OKs” this, BUT I have done this a time or two. Normally it’s done as an economic move …larger scheduled fittings are already purchased, but the requirement is for a lesser pressure scheduled fitting. More simply stated it’s cheaper to machine these fitting as apposed to buying the correct fittings! One other thing I know about this, you should try to get a minimum of 4 to 1 taper when you do this! Good Luck!