This doubt may be silly but PLEASE REPLY.

Imagine a Flow Control Valve without positioner. My I/P Converter gives a 12 psi output now my valve suppose to open up to 50% but it opens only 46%. Finally flow transmitter in that particular closed loop gives a more signal to I/P converter to achieve 50% opening of that control Valve. then WHAT IS THE PURPOSE OF POSITIONER?

 

This is an old argument - Do you need a positioner on a control valve? For accurate flow control the answer is yes. For general applications where the exact regulation of the flow is not important, then no. In most cases, the control of flow is critical to the operation of the unit process. This is why positioners are used most of the time.

First, you have observed the non-linear nature of control valves. When the controller calls for 50% output, and the valve opens to that amount, it does not mean that you will get 50% of the valve capacity unless the valve is corrected to linear operation by a mechanical cam in the valve positioner. Therefore, the controller must respond to add additional output to make up for the gap. This is slow compared to a feedback local loop in a valve positioner. However, pneumatic valve positioners typically are very simple and do not implement integral control. That is why a further argument persists that positioners are not necessary - but applies only to pneumatic valve positioners. Most electronic valve positioners and all digital valve positioners do implement integral control and do improve performance of flow control loops.

Dick Caro
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I would like to add a few things.

Since the control valve and actuator are mechanical elements, they experience frictional forces. The frictional forces affect the valve in that they don't allow it to be positioned at the desired opening (%). That is why you might observe that a command from the controller for 50% open might not result in a 50% open valve but rather a 46% or higher than 50% depending on the direction from which the valve is approaching the 50% target.

Without the positioner, the error in valve position would impact the process loop, resulting in an error in the process variable (flow / pressure/level as the case might be). Then the process variable error would be sensed by the controller and subsequently the controller would send out a higher output to the valve. It is important to note that the controller output is not the problem here but it is the valve position error. But since the controller doesn't know anything about the valve position, it is "made" to think that the output it is sending out isn't proper.

A controller driving a valve without a positioner is like driving a car blind-folded. Unless the car hits something on the way, the driver would never know whether he is on the road or moving away. Similiarly only the process variable errors would tell the controller that there is something wrong with the valve position.

A positioner in the valve comes in like an eye sight for the blind-folded car driver mentioned earlier. The positioner constantly monitors the valve position by the feedback mechanism. This feedback to the positioner is an internal loop (cascade) and doesn't require the controller intervention to correct valve position errors. If the controller sends in a 50% open command, the positioner ensures that the 50% target is achieved overcoming all the valve forces esp. friction. This way, the valve position error does not result in a error in the process variable in the loop, as in the earlier case.

Additionally, smart E/P positioners have much more to offer. They can feedback the valve position and other diagnostic data to the DCS via. HART/Fieldbus communication and several other added on features like Partial stroke test in a Emergency shut-down (ESD) application. Features like valve cycle counter (how many cycles the valve has taken since installation) will help us initiate a valve maintenance when the valve has operated for a large number of cycles. Today's digital positioners have much more advanced features than just valve positioning accuracy, that have made them powerful and indispensable.

Ronald Deepak
jronalddeepak [at] gmail.com

 

I personally agree that advanced valve diagnostics is an important aspect of a valve positioner. You can read more about it in the white paper found here:

http://www.eddl.org/DeviceManagement/Pages/DeviceDiagnostics.aspx

Cheers,
Jonas

 

You can't forget about stick / slip of the valve. If you have no positioner, you will have to try to overcome this with tuning. (I have done this by using an enormous amount of added rate and then made the controller not overshoot by programming a hi level cutback.) Now for a non critical control of process that does not need to be precise, no positioner should be needed. I also find the control is very inconsistent without the positioner.

regards
MAV