Openings and branch connections on schedule pipes

Hi, I'm new here, I'm Italian and I'm a piping designer.

I just wanna know if there are some rules about drilling and branches on schedule pipes at medium pressure (160 bar).
In particular a client of mine asked me to study a collector formed by a 2"1/2 sch.80 pipe in which he wants to butt weld two 1"1/2 sch.80 branches in a cross form (so the opening in the main collector is a through hole).
I fear that this might weaken the pipe and I want to know if it is allowed by international standards.
An alternative, I'd use a drilled manifold with SAE flanges on sides or a hydraulic block with machined butt welding nipple on each side.

Thank you and sorry for my bad English

Your client wants to “stub-in” 2 ea. 1 ½” lines, 180 Deg.s apart on a 2 ½” main header!?? …In a 1500# service (160 bar = 2320 PSI). I would say that is a real BAD idea! Besides the welding nightmare, you’d end up with an over lapping heat effected zone at the welds which would now require a PWHT …adding to the delay and cost (I’m not even thinking about any excessive stress from the piping configuration).
I’d suggest cutting in a “reducing cross fitting”, this fitting may take a bit more time to get in, but from there it’s down hill! …No PWHT, No welding nightmares, and I believe the cost would be less …a win-win scenario as far as I can see!

Welcome to the Forum!

Thank you very much 11echo, I completely agree with you!
The problem is that I've got very narrow space, what do you mean with "cutting in a reducing cross fitting"?

This is how I've thought to resolve the problem (measures are in millimeters): machined b.w. couplings. What do you think about it?
www.dropbox.com/sh/tzslt6i8ufo8mb9/SwYEaaZlrm/01.JPG