Branch Reinforcement (Carbon Steel Pipe)

Insufficient information. Needed are:

What wall thickness of each pipe?
What type of steel?
What internal pressure?
What local codes apply?

Almost surely you will need reinforcement, unless one or both pipes have quite a bit of surplus thickness.

Steel is C.S api 5l x52 internal pressure is 10 bars Both pipe is about 9.54 mm applied code is awwa m11

I haven't done calculations, but it doesn't look as though you have much (if any) surplus wall thickness. That almost definitely will require a reinforcement pad. If AWWA doesn't have a calculation method for this, API probably does. Also ASME 31.x would have this (I forget which subchapter x applies to your situation).

I think AWWA M11 contains reinforcement requirements. If not, maybe AWWA C200 or C208. Since this is an AWWA job, I wouldn't use B31.1 or B31.3 unless the client agrees to deviate.

Are you constructing a pipeline without any governing code or specification ?

How do you know if your materials are acceptable ?

How do you know if your welding is acceptable ?

At 10 bar design pressure, this is a serious pipeline.

Rules for area reinforcement of fabricated tees have been around for 50 years.

Acceptable methods can be found in ASME B31.1/B31.3..... but you must know your code of record