Flow Velocity (min.) (Siphonic Roof Drainage System)

At the design flow rate:

In accordance to BS 8490:2007, the minimum flow velocity in tailpipes, or sections of horizontal pipe longer than 1m, should be 1.0 m/s, in order to aid the removal of air during priming and prevent the build-up of sediment or other debris in the pipes;The minimum flow velocity in downpipes should be 2.2 m/s, in order to promote efficient priming.
In accordance to ASPE ANSI-45 Standard,a fully primed flow (i.e., full-bore flow), the velocity in the stack shall be greater than 2.2 m/s for stack 150 mm and smaller. For pipes larger than 150 mm, refer to appropriate testing data from drain manufacturers. Minimum velocity is a function of pipe diameter.

References