@ragul_ravi_ext
Welcome to the Community!
Deciding whether or not to create a new Pipe Run depends on how the branch will be fabricated, and where it is in your process flow. If the branch needs to be a part of the same pipe line based on your P&ID or process flow, I would not create a new Pipe Run, but rather a new Route within the existing Run. If the branch line is part of a different process line, then I might make a new Pipe Run starting right after the branch.... in either case, when you create the Route that comes off of that tee, you would need to select a Tube & Pipe Style that is the correct spec and size for the new Route. In my experience, keeping the 3" Style going after the tee, and manually changing the size of the pipe does not work well. If you add fittings or make any other changes to the 1" pipe, it will revert back to 3" because it is obeying the Style associated with the route.
For more tips and help getting started with Tube & Pipe, you may refer back to this old class I did at Autodesk University (WOW!!) 11 years ago:
Going With the Flow
Good luck!
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Chris Benner
Community Manager - NAMER / D&M