Control Valve Positioner Working
Control Valve Positioner Working:- A control valve is a device which is used to vary the flowrate of fluid by varying the size of passage through which fluid passes. A bigger size of passage means that more quantity of fluid will pass and a smaller passage means that lesser quantity of liquid will pass. Most of the control valves are commonly operated by pneumatic actuation action. The air flow makes the stem move in correspondence with what is dictated by the controller in the control system.
The actuator applies a force on the spring using air and the spring provides a reaction force. This causes the forces to get balanced and the valve stem attains a particular relation pertaining to the achieved equilibrium. If these where the only two forces acting in a control valve then the relationship between the air flow intensity and stem position would have been linear. It would have been easily possible to set the stem at exact position we want. But this is an ideal case, in reality there are more forces acting on it such as frictional forces, reaction forces, etc which disrupts the linear relationship.
A control valve positioner is a device which is used to aid the control valve in order to increase its performance. Control valve positioners actively compare the control signal and stem position and adjust the pressure accordingly so that the stem reaches and settles at the correct location. The positioner has its own pressure line.
The primary process controller will provide the valve stem positioner with the desired location at which the stem should settle, this is a set point. The positioner then manipulates the pressure in order to control the stem position so that its location is equal to the set point. This makes the valve stem positioner a feed back control system in itself.
Thus, there is a feedback loop inside a feedback loop. This makes it an example of a cascade loop control system.
The pneumatic signal sent by the controller is calculated assuming a linear relationship between the signal and the stem position but in reality they are not linear due to existence of additional forces. Valve stem positioner help in overcoming the additional forces by acting in such a way so as to apply as much pressure to bring the stem to the exact position where it would be if situation was ideal. Thus positioners can be considered devices which help in making the relationship appear linear in nature.
The control valve positioner has to be chosen and installed properly. If there is some defect in the positioner then it can cause overshoot. If the controller is also not tuned properly then it will also cause additional overshoot. The total overshoot can even cause the system to highly oscillate and make the process unstable and one may not be able to find the faulty positioner or tuning so easily. Overshoots can be especially problematic in processes which have catalysts because if temperature overshoots then it may disintegrate the catalyst.