Control valve response is a critical factor in successful control strategies and process variability. In a ControlGlobal.com article, Responsible control valve response, Emerson’s Greg McMillan and James Beall discussed the efforts underway to include valve responsiveness into the ISA 75.25 Control Valve Dynamic Testing standard.
With Greg’s many decades of experience working with pH control, and shared in his ISA book, Advanced pH Measurement and Control, Third Edition, he is very:
…sensitized to the need for control valves to respond to small signal changes… A resolution limit of 0.1% for even the best control valves could translate to a 2 to 12 pH oscillation in a strong acid and strong base neutralization.
Also his work with compressor surge control applications made Greg:
…keenly aware of the need for control valves to quickly respond to large signal changes and the first-hand-experienced dangers of replacing a positioner with a volume booster, instead of putting the booster on the positioner output, as documented in my Momentum Press book, Centrifugal and Axial Compressor Control.
Greg notes that currently, control valve:
…specifications have an entry for capacity and leakage, but nothing so far about precision and speed of response.
Recognizing these issues, Greg join the ANSI/ISA-S75.25.01 standard committee which James chairs working on the standard and ANSI/ISA-TR75.25.02 technical report on how to do control-valve response testing. Greg prepared a Valve Response and Control Loop Performance, TR75.25.02 annex proposal:
…which considers valve nonlinearities such as backlash, shaft windup, slew rate, dead time and valve flow gain from the installed flow characteristic. This is in addition to the existing nonlinearities noted and associated with dead band resolution and changes in 86% response time and travel gain.
In James’ time with Eastman Chemicals:
…he discovered the tremendous impact of small step sizes on valve response due sensitivity limitations in positioners. He went to work for Emerson in the then newly acquired EnTech Control organization that had a mission of helping all realize that more than 30% of the variability in control loops could be traced back to deficiencies in valve response.
…it would be quite appropriate to elaborate about the impact of valve nonlinearities on process/loop performance, and help customers translate process performance requirements into valve-performance requirements.
Read the exchange between Greg and James as they discuss plans to add to the technical report on response nonlinearities, positioner integral action, external-reset feedback, secondary flow loops, tuning, response resolution, backlash, shaft windup, excessive error, and more.
Greg noted in a LinkedIn comment to my post sharing this article:
We are trying to make a difference where the perception of cheap, big, and tight is good and the lack of any requirement for valve to respond on specification sheet leads to bad process control. Anyone who wants to join in the ISA-75.25.02 committee effort to turn this around, is more than welcome. Just send me a message. There may be two virtual meetings a year but nearly everything is done by email.