If you are talking about ASME B31.3 code for solving urproble, the following steps you need to follow:

1. Get the FluidList(Fluid Index)from Process Dept. for categorizing the Fluid in Category D/ Category M or Normal Fluid Service

2. Define the Piping class Say for ss & cs piping items(Basic MoC is defined in Fluid Index & temp, Pressure & service conditions)

3. Go through the Thickness calculations(Tmin=pd/2sew+py)+c

4.Consider Manufacturing tolerance & get the Nominal thickness as per the market availability considering ASME B36.10 & B36.19 standards


5. This is about thickness cals. based on internal pressure.


6. If piping system is under Vacuum then follow ASME section VIII div. 1 UG28 through 30

7. If you get the thickness in the first place itself then apply pipe thickness calculatrion formula & get the Maximum allowable working Pressure

Hope the above will suffice


Thanks