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Butterfly and Ball Valves with Positioners Are Not Control Valves

 

The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #10, and was written by Greg.

 

One of the more heated arguments I had with a client involved his extreme desire to use line size butterfly valves for controlling his process. The entire concept of using control valves for the available pressure drop and for controllability at low flow rates was utterly lost on him. We went round and round until I finally got the client to install spool pieces at each valve location so we could easily replace the butterfly valves with a different valve if we encountered control problems in the future.

A lot of those spool pieces and butterfly valves got replaced.

 

Concept: Despite what the on/off valve sales person will tell you, an on/off valve with a positioner is NOT a control valve. Such an arrangement can have its uses, but you should understand the limitations of on-off valves and the differences between on-off valves and control valves.

Details: In an attempt to reduce cost, a young engineer may resort to using ball and/or butterfly valves with positioners to control a process. These valves CAN be used in throttling applications, but certain limitations can greatly impact their performance. An automation engineer must understand these limitations and specify this type of valve only where appropriate.

PROS:

  1. This arrangement is much cheaper than a standard control valve.
  2. A ball valve’s flow characteristic can be “characterized” to provide a range of CVs and provide a more linear response if required. (Butterfly valves are not so easily modified.)
  3. On/off valves tend to have less pressure drop when fully open.

CONS:

  1. A butterfly valve’s flow characteristic is essentially closed until about 40 degrees, then flow increases from 20% to 90+% of capacity between 40 and 80 degrees. Such a narrow range of control and poor turndown rarely suits most control valve applications.
  2. The recovery factor for a butterfly or ball valve is generally poor compared to a control valve. Therefore, these valves tend to have more problems with cavitation and permanent pressure drop when they are throttling flow.
  3. Precise, repeatable control is difficult to achieve – especially at low flow rates.

One application that IS well suited to a throttling on/off ball valve is a dribble application in batch raw material charging. The typical dribble valve uses a second solenoid to “lock” the valve position in a half-closed position, which slows the charge rate at the end of a charge and helps improve the accuracy of liquid charges. However, the dribble valve solenoid arrangement is prone to maintenance problems and the final charge rate and accuracy tend to drift as the actuator ages. For nearly the same money, a small positioner can be installed on a characterized ball valve and the unit can be continuously throttled at the end to provide very accurate charging. Typical control logic would look something like this:

  • If remaining charge amount > 100 lbs, output = 100 percent
  • If remaining charge is <=100 lbs, output = MAX (remaining charge/100, 25)

Such an algorithm charges at maximum speed until the end, then continuously throttles the valve down to 25 percent as the end of charge approaches. Finally, it trickles in the last few pounds to nail the total charge amount accurately.

 

Watch-Outs: Beware of using butterfly valves with class VI shutoff in any kind of a throttling application unless the valve is normally at least one-third open. These valves usually torque into the seat very hard and take a great deal of force to crack open from a fully closed position. If the valve is used to throttle in this regime, it will wear quickly and the flow rates will be wildly erratic.

Exceptions: If accurate control at low flow rates and cavitation is not a concern, then ball and butterfly valves can be successfully used in certain applications. However, you must understand the limitations of these valves types and not misapply them.

Insight: Premature valve failure usually occurs due to seat erosion under low flow/high pressure drop conditions or under conditions where the valve is cavitating or flashing. True control valves can be designed to handle and/or mitigate these conditions. Throttling on/off valves are a poor choice for these applications.

Rule of Thumb: Carefully evaluate the application before selecting a valve type. Take time to understand the strengths and weaknesses of the various valve offerings, and choose the valve type that fits the application. Do not skimp on the design to save a few dollars; over the life of the valve, the wrong valve will cost much more than the original savings.

 

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

 

Connect with Greg
LinkedIn

 

Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.

 

Connect with Hunter
LinkedIn

 

Greg McMillan
Greg McMillan
Greg McMillan has more than 50 years of experience in industrial process automation, with an emphasis on the synergy of dynamic modeling and process control. He retired as a Senior Fellow from Solutia and a senior principal software engineer from Emerson Process Systems and Solutions. He was also an adjunct professor in the Washington University Saint Louis Chemical Engineering department from 2001 to 2004. Greg is the author of numerous ISA books and columns on process control, and he has been the monthly Control Talk columnist for Control magazine since 2002. He is the leader of the monthly ISA “Ask the Automation Pros” Q&A posts that began as a series of Mentor Program Q&A posts in 2014. He started and guided the ISA Standards and Practices committee on ISA-TR5.9-2023, PID Algorithms and Performance Technical Report, and he wrote “Annex A - Valve Response and Control Loop Performance, Sources, Consequences, Fixes, and Specifications” in ISA-TR75.25.02-2000 (R2023), Control Valve Response Measurement from Step Inputs. Greg’s achievements include the ISA Kermit Fischer Environmental Award for pH control in 1991, appointment to ISA Fellow in 1991, the Control magazine Engineer of the Year Award for the Process Industry in 1994, induction into the Control magazine Process Automation Hall of Fame in 2001, selection as one of InTech magazine’s 50 Most Influential Innovators in 2003, several ISA Raymond D. Molloy awards for bestselling books of the year, the ISA Life Achievement Award in 2010, the ISA Mentoring Excellence award in 2020, and the ISA Standards Achievement Award in 2023. He has a BS in engineering physics from Kansas University and an MS in control theory from Missouri University of Science and Technology, both with emphasis on industrial processes.

Books:

Advances in Reactor Measurement and Control
Good Tuning: A Pocket Guide, Fourth Edition
New Directions in Bioprocess Modeling and Control: Maximizing Process Analytical Technology Benefits, Second Edition
Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance
101 Tips for a Successful Automation Career
Advanced pH Measurement and Control: Digital Twin Synergy and Advances in Technology, Fourth Edition
The Funnier Side of Retirement for Engineers and People of the Technical Persuasion
The Life and Times of an Automation Professional - An Illustrated Guide
Advanced Temperature Measurement and Control, Second Edition
Models Unleashed: Virtual Plant and Model Predictive Control Applications

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