Pipe Sizer is based on two articles that appeared in Chemical Engineering Magazine (McGraw Hill) - June 16th 1980 and May 1999 - together with the relevant section in Perry 5th Ed. (6th Ed seems to have dropped the detail).

Basically the constants are based on yr 2000 values. In the latter Chem Eng article the constants are updated and you can see how they have changed since 1971. The yr 2000 values are still relevant today as the constants that change yearly only have a very small effect on the calculated economic size. Last updated 2007 in FluidFlow.

Please note that the calculated size is only an estimate and assumes turbulent flow, the equations do not hold for laminar or Non-Newtonian flow conditions. Being conservative most (all ?) engineers would normally go to the next standard size greater than the exact calculated diameter.

Over the years the economic pipe velocity is reducing in line with a larger increase in energy costs over other costs in the Generaux equation.