1. Determine the Application Criteria
Flow type required:

Two-way, equal % or modified linear.

Three-way, mixing or diverting.

Fluid Media (Oil,Gas,Chemicals,Edible product,syrups,Hot water, steam,etc.)

Process Temperature.

Inlet Pressure required.

Output Pressure required.

Type of Valve.

Material of Construction based on flowing media and temperature.

Existing piping or tubing size.

2. Determine the Valve Control Form Required

Manual.

Pneumatic.

Electrically Actuated.

Hydraulically Actuated.

3. Determine the Required Cv
In case of only flow Rate availability ,Cv calculation is required.
If the size of the valve is predetermined,then Cv caculation is not required.

Paramenters for Cv Calculaions:

Inlet pressure

Fluid temperature

Fluid specific gravity

Fluid flow rate in GPM (liquid) or Fluid flow rate in lbs./hr (steam)