Valve actuators are of various types namely electrically operated , pneumatically operated.
- Pneumatically operated actuators can be diaphragm operated for globe , gate valves.
- Pneumatic actuators can be quarter turn type for ball or butterfly valves.
- Pneumatic actuators can be cylinder operated for say Knife gate valves.
- Actuators are again single acting spring return type or double acting.
- Again electric actuators can be classified as on-off type or control ( modulating type).
On- off pneumatic actuators can be operated with solenoid valves . As you mention the fluid to be natural gas, you have to select solenoid valves which are intrinsically safe at the same time these valves may be placed at a safe distance from the valves and piping. These valves are again actuated by controllers( on -off , PID) connected to the level switches, pressure sensors or temp sensors as the case may be.
To control flow rates, you need to use a pneumatic control valve with a pneumatic positioner operating on (0.2-1 kg/cm2) with current to pressure convertor or an electroneumatic positioner ( 4-20mA control signal). These can be used to control flow rate or downstream pressure. Other elements required are pressure or flow sensors, PID controllers.
The control again depends on how the plant is operated ie DCS or PLC controlled.
If it a simple requirement then the actuatar can be made to operate on a system of limit switches. While selecting an actuator the fail safe mode ( in case of power, signal or air failure) should also be taken into consideration , which is specified by words like normally open or normally closed etc.