Flow Rate

Flow rate is the density of the substance flowing through the pipe times the velocity of the substance times the cross sectional area of the pipe (which would be pi times the radius of the pipe squared.)

m=pvA

http://en.wikipedia.org/wiki/Mass_flow_rate

you should make a separation between "volume flow" and "mass flow" even if is usually not made, what you give is the equation for the "mass flow" but if some body is interested only in the flow as fluid the density is not playing a role. For instance for the Reynolds number only velocity counts. Density is implicit in the cinematic viscosity index.