As a rule of thumb the following velocities can be used in design of piping and pumping systems for water:
Pipe Dimension Water
inches mm m/s ft/s
1 25 1 3.5
2 50 1.1 3.6
3 75 1.15 3.8
4 100 1.25 4
6 150 1.5 4.7
8 200 1.75 5.5
10 250 2 6.5
12 300 2.65 8.5