Hi,
This is depending on the applicable design code for the piping systems, like ASME B31.3. This code gives you the formula and the parameters such as the design stress which you are required to use for wall thickness assessment.
Off course you also need to consider additions such as tolerances and corrosion allowances. After that you can determine the minimum required thickness and hence round it up to the nearest commercial standard wall thickness as per ASME B36.10 / ASME B36.19, depending on the grade of material.
FKE is correct as to the minimum thickness required per applicable Code based on system design temperature and pressure. This result is minimum wall thickness based on pressure and temperature.
However, you asked about "pratical" minimum thickness. Just be aware that all wall thicknesses are not necessary the same. All piping charts list nominal wall thickness. The pipe manufacturer has +/- allowances based on Code.
For example, The manufacturer minimum wall thickness shall not be more than 12.5 % under the nominal thickness as specified (hence, 6 in std wt pipe nominal wall 0.280 can be furnished by pipe manufacturer with 0.245 wall and be acceptable by Code.
Hope this helps
In that case you can (after wall thickness measurements) use the same formula as presented in the relevant design code.
You can calculate the MAWP of the piping system by filling in the thinnest measured wall thickness.
If the MAWP as a result is higher than the design pressure of the system the integrity with respect to internal pressure is sufficient.
There have been good/many responses to this inquiry. About the only thing I'd add is that while I don't know exactly how/if all codes address these issues, I think minimum practical thicknesses particularly of some very thin, large or long etc. pipes? can conceivably in some cases be influenced also by handling or other issues, due to the ring or beam stiffnesses of whatever lengths of pipe are involved, as well as whatever coating or lining system is specified for the piping (perhaps analogous to to special limitations of span deflection e.g. when a steel beam is used to support a plaster ceiling).
