Hi Vishnu, Valves types ...

Hi Vishnu,

Valves types depend on the fluid application and several physical factors (pressure, allowable pressure drop, flow regulation/open-close application, turbulence, throttling, and flowrate). The ball/butterfly/gate valve/solenoid is intended for open-close only application and to cope with high pressure while needle valve is to regulate flow. There are many others types of valves (an example is within this link: https://megadepot.com/resource/a-guide-to-types-of-valves)

So to answer your question, particular application/physical condition in your STP/ETP need to be considered and which part of STP/ETP process.

Meanwhile pneumatic control is used for open-closed application  and motorized is used when you need to control the openness of the valve chamber to regulate flow/pressure.