Process piping is probably something that every Chemical Process Engineer will always deal with.
In a new project, you need to design the pipe size that best fit the process. In existing plant you need to evaluate if the piping system is not bottlenecking the process.
From pipe size you calculate fluid velocity and pressure drop. These calculations are based on the pipe inside diameter (ID). Nominal pipe size (NPS) can have different ID according to its pipe schedule number.
“The manufacture of pipe NPS 1/8 (DN 6) through NPS 12 (DN 300), inclusive, is based on a standardized outside diameter (OD) while the manufacture of pipe NPS 14 (DN 350) and larger proceeds on the basis of an OD corresponding to the nominal size.” (ASME B36.19M-2004).
Observe that NPS and DN are dimensionless, while its OD is expressed in inches for NPS and in millimeters for DN. Schedule number (sch#) is related to pipe wall thickness. Higher the sch#, smaller is the ID. With this, for a constant flow, higher will be the fluid velocity and the pressure drop.
Based on that, during process design calculation, take a look at Pipe Specification Manual your Company should have or be in touch with the Piping Engineer or equivalent. The wrong assumption can increase power consumption and decrease plant performance.
