Size pipes so water flows between about 1 and 2.5 m/s.
Slower and sediment settles; faster and you get erosion, noise and water hammer. Pump suction lines sit at the low end.
Velocity is the wrong control on a long run: 60 m of pipe at 2.5 m/s can lose more head than the pump can supply, even though the velocity looks textbook. And in copper hot-water lines anything above about 1.5 m/s risks erosion-corrosion regardless of what the pressure drop says.
Estimate with the rule, then check it against the calculator that models it properly.
Open Pipe Flow & Pressure Drop Calculator →Size pipes so water flows between about 1 and 2.5 m/s. Slower and sediment settles; faster and you get erosion, noise and water hammer.