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Velocity of water in Return Loop

Why should we maintain a velocity of 1m/s in th return loop? If we will not maintain that velocity what will be the impact on the distribution system? This velocity is similar to all pipings which have different diameters or it will change when we use different diameters?

i thik it is the same 1 m/s whatever the pipe diameter

The 1 m/s is an empirical value, which minimises biofilm buildup guaranteeing turbulent flow in the distribution loop. If you use a lower value you may have to increase sanitation frequency and the maximum accepted deadleg length should be shorter. This velocity changes with the diameter because one of the parameters that make up the Reynolds number (which is used to evaluate whether a flow is turbulent or not) is the “characteristic length”, which for a pipe with circular cross section is the diameter. Nevertheless, the 1 m/s figure is a value to be on the safe side for currently used pipe diameters, and it is well achievable with normally sized industrial sanitary pumps. Take into account that the flow velocity in the return piping should be measured with a reasonable number of points of use (normally around 30% of the sum of the cross section from all the user points) simultaneously open, because with peaks of water consumption the flow velocity in the return drops drastically.