The respondents are all ...

The respondents are all quite right. There is no set maximum other than what the constraints are for the system, it's use and engineering specifications. I have managed a long trunk main for potable water that is pumped at up to 1.5m/s. The power costs of the very large pumps are too great for any higher velocity. 

In the UK with many very old water distribution pipes the concern is for the shear force that has conditioned the film inside each pipe. We have prescribed limits (low ones!) for any discolouration. Therefore velocity changes outside of a diurnal norm are very carefully managed. Unless a dedicated pipeline is designed most water distribution is typically up to 0.5m/s.