void high velocities in the suction pipes. Higher velocities increases the pressure loss in the pipe and there may be a chance that the pump start to cavitating. As a rule of thumb the suction velocity in front of a pump should be below:
Pipe bore Viscous Liquids
inches mm m/s ft/s
1 25 0.3 1
2 50 0.33 1.1
3 75 0.36 1.2
4 100 0.4 1.3
6 150 0.43 1.4
8 200 0.45 1.5
10 250 0.5 1.7
12 300 0.5 1.7

These values depends a lot of the fluid pumped. More about viscosity and viscosities of different fluids can be read here: