Control.com - Nerds in Control

I want to know the difference between the Valve postioner (like Neles Jamesbury Positioners) and Controllers (like Level-Trol and CVS 4160 Pressure Controllers).

Can positioners replace controllers? I work with 3 phase well test separators.

The way it works is that you will have say a pressure transmitter in the field somewhere measuring the pressure inside a vessel. Now if you want to control that pressure inside the vessel to a specific pressure you need to build a control loop. That means that the signal from the pressure transmitter is first transmitted to a controller where it will be compared to a setpoint that was set by yourself on the controller. The controller will then give an output proportional to the difference between the measured signal and the setpoint to a positioner on a valve. The positioner on the valve is there to increase the response time of the valve. The valve will now be driven automatically open or close by the controller in order to control the pressure inside the vessel to the setpoint you have specified on the controller. That is the way the controller will control the vessel pressure in auto control. If you switch the controller to manual you can drive the valve open and close yourself from the controller thereby manually increasing or decreasing the vessel pressure with the control valve.

This is the way it works for pneumatic, electro-mechanical, electronic and latest technology HART and SMART instrumentation and control loops.

The equipment you have specified seems like old pneumatic instruments but that they will work in the same way so no, the one needs the other in order to do proportional control and cannot replace each other.

It is like asking if a car's engine can replace the gearbox.
With latest technology HART and SMART field transmitters there are however some of them that have local PID controllers build into them so that they do the measurement of the process and at the same time act as a local PID controller so yes things like that is available but still each component is needed in the control loop.