Fluid velocity is determined by the economics of pumping, not some arbitrary factor. However, you can be sure that high fluid velocities and long pipe runs will not be economical.
Reasonable pipe velocities depend on the application. There is no correct velocity for all applications. Here is a general guideline.
Reasonable Velocities for the Flow of Water through Pipe (from Cranes Technical Paper 410):
Boiler Feed.............8 to 15 ft/sec
Pump Suction ............4 to 7 ft/sec
General Building Service.........4 to 10 ft/sec
City.......................to 7 ft/sec
Transmission Pipelines...3 to 5 ft/sec
Go to a basic hydraulics book. Try Cranes Technical Paper 410 as a reference for the above velocities.
