xiaxiuqing,

The stainless pipe standard that you refer to is probably ASME B 36.19 and it is a dimensional standard - not a design standard. The dimensional standards enable a group of compatible pipe, fittings, and valves to be assembled together using off-the-shelf components. A custom thickness of pipe might be economical for long transmission pipeline of miles - Km. The pipe fittings in custom wall thickness would be 2X the cost of custom pipe.

The CaesarII calculations are implemented from the ASME B31 pping codes, as pointed in other queries of the forum. It is good to try to understand the design code more, so that the CaesarII results are understood and can be appreciated to then make design changes or adjustments.

The 1.86 MpaG converts to a litle less than 270 psig, and in the smaller diameters the wall thickness could be very thin. From your 2.1 mm wall thickness your pipe diameter could be Nps-12 (nominal 300 mm) and it is a moderate diameter. The 2.1 mm wall thickness is nearly thin enough to be difficult to weld without blowing through weld puddle, or handling the thin 2.1 mm wall pipe without denting or deforming it out of round.