Hello BhavsarRC
I suggest you phrase your question with full details for the sake of other forum users. I presume you are referring to API 650 Figure 5-21.
Figure 5-21 is the erection or fabrication procedure. If a hole is cut into the shell to accommodate the nozzle, the outside diameter plus some clearance would be needed for the hole size. That figure does not make specific reference to any pipe size. In the case of a 100 DN pipe, the hole diameter would be in the region of 120 mm (or thereabouts).
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Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM