Nowadays, end users just specify if a pneumatic operator for the valves is required or not based on the number of turns required to open/close the valve. Generally valve operation requiring more than fifty turns is the threshold for the selection of such device.
• Other factors such as valve availability, overall valve size, valve suitability for expected temperatures, pressure drop across the valve etc are quite secondary.
It is recommended that the reader should look at the end user philosophy for some big operators like Shell, Total and Exxon Mobil to understand the basis of valve selection. See Figure 2.
Fig. 2: A typical plant isolation philosophy