The type of control valve action requirement on a process line depends on the protection required on the upstream or downstream of the control valve in case of an air failure. Depending on the control valve action, the controller action has to be set to control the process.
For example: Station back pressure control valve is reverse acting and its controller is set for reverse acting. If there is an air failure the control valve opens fully and prevents the separator from high pressure. If the separator gas pressure goes below the set point the controller output goes high and keeps the control valve closed.
Separator level control valve is direct action and its controller is set for direct action. If there is an air failure, the control valve closes fully and prevents the surge tank from high pressure. If the separator level goes below the set point the controller output goes low and keeps the control valve closed.