I have started work on a CCGT plant where the HRSG drain valves have electric motor actuators, but the steam mains and steam turbine drain valves have pneumatic piston type actuators. These selections seem sort of traditional.

Does anyone know why different types are used, and which criteria are used to select one type or another?

 

In steam circuit, pneumatic or hydraulic actuators used due to their option of spring return for fail safe in case of power failure or safety shut off (fail open / fail close)