Air in the empty pipe will first get displaced by incoming water. Gradually the water occupies that space before moving out, thereby losing some momentum (and velocity)! Once the larger is area is full same amount of water will continue to flow but with leass velocity so that the quantity...
You should calculate the thickness of the pipe for the specified service. Then add manufacturing tolerance on thickness. Then you check the standard schedule numbers of pipes available in the market. Go for the schedule giving the next higher thickness of the pipe you want.
That is why...
Backing rings are used while welding Pressure Pipes or tubes. They will avoid irregular penetration of weld metal inside the pipe or tube, creating extra friction losses.
