Why do we need to install any flow control valves at return pipe lines. Further how to size these valves?

valves on the return pipe line are installed due to control the surge and also suction pressure of the pump. and valve such as dampers in air flow duct are installed in order to control the amount of return air and fresh air in the mixing chamber. valves are size with the help of following equation

Cv =  Q/fp*sqrt(SG/delta P)

where:

Cv= valve flow coefficient

Q= volumetric flow

Fp= piping geometry

SG= Specific gravity of liquid relative with water

Delta P= effective pressure drop across valve