Home » How Valve Positioners act as Cascade Control Systems in a Loop?
Control Systems Control Valves

How Valve Positioners act as Cascade Control Systems in a Loop?

Explain how all valve positioners essentially act as cascade control systems in a loop, and why valve positioners generally improve control quality as a result.

Valve Positioners

How Valve Positioners act as Cascade Control Systems in a Loop?

Furthermore, examine this block diagram of an electronic (“smart”) valve positioner and comment on its own internal “control strategy” for achieving optimum positioning:

Answer:

Electronic valve positioners act as controllers of a sort by sensing stem position (PV), comparing that PV against the command value coming from the loop controller (the positioner’s SP), and finally generating its own pneumatic output signal to drive the actuator and move the valve to where it should be.

A potential problem with electronic positioners is the potential to mis-tune its internal control(lers), resulting in oscillations if the internal control is too aggressive and sluggishness if the internal control is not aggressive enough.

Read Next:

Questions for you:

1. Comment on the “+” and “−” symbols shown in this diagram. Based on these symbols, determine the control action (direct or reverse) for each of the controllers.

2. Certainly, modern “smart” valve positioners promise superior control valve performance than mechanical valve positioners did. Can you think of any potential disadvantages of a smart valve positioner, especially after examining the block diagram in detail?

3. How exactly does a “smart” positioner sense the valve stem’s position?

4. For those who have studied control valve positioners, identify some of the special features offered by a “smart” positioner.

5. Predict the effect(s) of the nozzle plugging inside the positioner’s relay.

6. Predict the effect(s) of the travel sensor failing with a 0% (fully-closed) signal.

7. Predict the effect(s) of the pressure sensor failing with a 100% (full-pressure) signal.

Join the Discussion! Share your answers with us through below comments section.

Share With Your Friends

Related Articles

DCS Program to Maintain Draft in Furnace

S Bharadwaj Reddy

ON-OFF Controller Principle

S Bharadwaj Reddy

What is Globe Valve ?

S Bharadwaj Reddy

What is Loop Checking?

S Bharadwaj Reddy

Difference between RTJ and T&G Flange Facings

S Bharadwaj Reddy

Proportional-only Control Theory

S Bharadwaj Reddy

Leave a Comment

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More

How Valve Positioners act as Cascade Control Systems in a Loop?

WordPress Image Lightbox
Send this to a friend