I've heard the smart valve positioner vendors claim up to a 10x better mechanical output resolution than that provided by their former analog positioners, meaning the ability to move the valve's position with smaller incremental changes-of-position. None has provided any evidence to my challenge to prove such a statement.

Sounds nice, but really? ? ? Says who?

Are any of the forum's participants aware of any factual evidence higher resolution, or is it typical sales puffery?

 

You have a good question, but I am not sure that anyone at Fisher Controls/Emerson or at Masoneilan/Dresser monitors this list. They both manufacture "Smart Control Valves" with both HART and Foundation Fieldbus positioners. When we were writing the "User Layer" for ISA 50.02, that was input to IEC 61804, and Foundation Fieldbus, we expected the control valve positioners for Fieldbus would have the PID final control element function block, but we were not sure that the positioner manufacturers would actually do the valve stem positioning using that same PID function. In fact, many of the early Smart Valve positioners were smart only in using HART or Foundation Fieldbus function blocks. The valve stem positioning was done in the OLD WAY using mechanical or pneumatic proportional-only feedback control. This was satisfactory only because it was the way it had always been done using positioners.

There was always the option for digital valve positioners to actually apply the PI or PID feedback control algorithm using the microprocessor already in the smart positioner. If they did, then the position would have been a primary in a cascade with its setpoint (the desired position) taken from the Analog Output block in the same positioner. As we know, the main reason for using cascade control is to improve the performance of the inner loop and the whole cascade. Then you would be able to claim that 10x better response for Smart Valves. However, I do not have any evidence that the digital control valve positioner manufacturers are actually using an inner loop digital cascade for Foundation Fieldbus. I do not see this in their literature either.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,

E-mail: RCaro [at] CMC.us
Subscribe to the CMC Wireless Report <http://www.CMC.us>
Web: http://www.CMC.us
Buy my books:
http://www.isa.org/books
Automation Network Selection
Wireless Networks for Industrial Automation
http://www.spitzerandboyes.com/Product/fbus.htm
The Consumer's Guide to Fieldbus Network Equipment for Process Control
Buy this book and save 50% or more on your next control system!!!
===========================================

 

In most cases, higher positioner resolution is a moot point. Many control valves are simply incapable of highly accurate stem positioning (0.5% of travel). This is largely due to the frictional forces around the valve stem, primarily from the valve packing. Environmental concerns about fugitive emissions have exacerbated the problem, since many personnel will simply tighten the valve packing to try to reduce emissions.

The resulting "stiction" means that the valve stick and slip, stick and slip. A positioner can help to reduce the size of the stiction cycle, but not eliminate it. But any positioner improvement is contingent on several other factors: valve packing, air supply pressure, air vent, positioner size, sticky fluids, flow forces, etc. Only in the most optimistic situation would more resolution of the positioner output be likely to improve the performance of the valve itself.

I would love to be proven wrong! Does anyone have data from a real valve in a real plant that shows this sort of improvement?

If you are not sure how your control valves are performing, you may want to consider on-line performance monitoring. To see how you can measure stiction and other aspects of valve performance, visit:
http://www.expertune.com/r2.asp?f=AList&t=stiction&l=PTValveAssessments.html

-George

 

Thank you, gentlemen, for your responses. Mr. Buckbee's response made me wonder if he is telepathic.

Behind my original abbreviated question were precisely those issues he raised. I'm not really convinced that a digital positioner can, in harsh service, provide better position resolution than its analog predecessor for the reasons Mr. Buckbee mentions.

Digital positioners might very well provide 10x better positioning performance in 'easy' service with a new valve on clean water on a flow stand. But wouldn't you think that someone in the manufacturer's domain could pull up the graph where they actually ran a positioning test with clean water through a new valve on a flow stand to actually see what it could do compared to an analog pneumatic version?

What mystifies me is how difficult can support for such a claim be, when it's the business you're in? The people selling the positioners are the same people making the valves. They have access to a new valve for a test. They've got a flow stand. All the pieces of the puzzle in the box, but only a verbal description of what it should look when it's all put together. I guess they don't call Missouri the "Show Me" state for nothing.

I don't doubt the claims of the available diagnostic data in a digital positioner. I'm sure that's doable and I can see its value in predictive maintenance. But I am skeptical of the claims of higher resolution positioning.

 

Has anyone reviewed the current smart positioners on the market and made a comparison to the Rotork CVA as an alternate means? Am curious to hear comments.