To select the control valve, the first is to gather complete the physical characteristics of the process fluid parameters and control valve operating conditions, including the main fluid composition, temperature, density, viscosity, normal flow, maximum flow, minimum flow, maximum flow and minimum flow under the inlet and outlet pressure and the maximum pressure difference. 

In the technical aspects, you should know well the structure of the control valve itself, flow characteristics, rated flow coefficient Kv value, size, process to allow the pressure difference calculation and the implementation of the agency, materials and installation and other aspects.

Generally, the selection of control valve should follow the principles of the following points.
First, the structure of a control valve should be able to meet the media temperature, pressure, mobility, flow, adjustment scope and tightness requirements.
Second, the flow characteristics of the control valve should be able to meet the system characteristics for reasonable compensation.
Third, control valve's size should be able to meet the process of the flow requirements.