Instrumentation Question:

Answer #1
Control valve can not be without positioner. The purpose of the positioner is to control the control valve stroke so as to keep the valve in desired position. The positioner receives signal from the controller, and send the boosted signal to the actuator to reach the desired position as and when the valve reaches the desired position the positioner cuts the boosted signal to the actuator and keeps the position.

Answer #2
Yes we can use but it works like on/off valves .Take a solenoid valve before solenoid valve put a air regulator & set pressure according to air required for 100% opening of c.v.

Answer #3
Yes we can use control valve without positioner directly we can use i/p or e/p transducer.

Answer #4
Yes... generally in steam lines we use control valves with on off application i.e. either 0 or 100%. Until or unless we need it to control flow using it then only we require positioner.

Answer #5
No

Answer #6
Yes, In a small process, for example an oil lube station which requires a small valve. The valve operates with a 20-100 kPa signal which is directly send from 43 controller. I would not say it is ideal. But it is possible

Answer #7
Yes, for small size of control valve which require less torque/ force to operate , is can be operated without Positioner also.