ID Controlled Pipes vs. OD Controlled Pipes

Instead of having different fittings for each class of pipe on for example screw-in or press -in fittings one keep the OD constant and vary the ID to accommodate the different wall thicknesses required for the pressure'.

API 5L: less than 12 3/4" ID. Mare than 12 3/4" OD

This question is related to the same subject as many others that surface from time to time. Here is my answer to one of the previous questions.

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#1
You and any others who still are not clear about Pipe, Pipe sizes, Pipe outside diameters, Pipe Inside Diameters, pipe wall thicknesses, Pipe Schedule nomenclature/NPT to DN Cross Reference, etc should look at this web site. You should copy (Print out) the chart and study it in detail.
http://www.maselmon.com/Content.aspx?ContentID=24
#2
NPS = Nominal Pipe Size (US) For metric the term is "DN" = Diameter Nominal. "Nominal" means "in name only". For Imperial (inch) pipe sizes below NPS 12" the actual outside diameter is not equal to the NPS designation. For imperial (inch) sizes 14" and larger the NPS designation just happens to be the same as the actual outside diameter.
#3
You used the letters "NB". I can only assume that you mean "Nominal Bore". If I am correct in this assumption, then you should understand that there is no such thing as a nominal bore. The "Bore" of all pipe for a specific pipe size (NPS or DN) changes with the Schedule change or wall thickness change. So there is really no such thing as Nominal (Normal) Bore.
#4
The person responsible for setting (selecting) the pipe wall schedule is the Piping Material Engineer. The Schedule (or wall thickness) is determined by the system Design Pressure/Design Temperature/Commodity/Corrosion Allowance/Plant Life and other mitigating factors. Look at the chart. There are seven (7) listings for 4" (DN 100) pipe commercially produced and available. Each has a different wall thickness and multiple schedule listings. Some of the listings (5S, 10S, 40S, etc.) are for Stainless Steel pipe. The others are for Carbon Steel and Alloy Steels.
#5
As for why do we have one method for 12" (DN 300)and under sizes and a different method 14" (DN 350) and larger sizes. I do not know the answer to that and I don't care. I do not think anyone else knows the real reason either. I have seen some peoples thoughts on it but I suspect they are just guessing. It is like Gravity, we do not fully understand everything about it but we just learn to accept it.

<...DN...> can also mean "German standard".

You say

"#3 You used the letters "NB". I can only assume that you mean "Nominal Bore". If I am correct in this assumption, then you should understand that there is no such thing as a nominal bore. The "Bore" of all pipe for a specific pipe size (NPS or DN) changes with the Schedule change or wall thickness change. So there is really no such thing as Nominal (Normal) Bore."

I can't agree there is no such thing as Nominal Bore. The whole point of Nominal Bore is that it's nominal. The actual ID varies with wall thickness, as you say. It can be bigger than NB for the lower schedules, smaller for the higher ones. And NB is a recognised term in the industry.