This equation is based on the law of conservation of
mass. For a fluid flowing through the pipe having
single inlet and single outlet, the flow rate of
fluid at all the cross sections is constant.
i.e. mass flow rate at inlet = mass flow rate at
(ρ A V)inlet = (ρ A V)outlet
the fluid is incompressible then
V)inlet = (A V)outlet
Where ‘A’ is Area of cross section, ‘V’ is Velocity of
fluid and ‘ρ’ is density of fluid.