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Welcome to the official blog of the International Society of Automation (ISA).

This blog covers numerous topics on industrial automation such as operations & management, continuous & batch processing, connectivity, manufacturing & machine control, and Industry 4.0.

The material and information contained on this website is for general information purposes only. ISA blog posts may be authored by ISA staff and guest authors from the automation community. Views and opinions expressed by a guest author are solely their own, and do not necessarily represent those of ISA. Posts made by guest authors have been subject to peer review.

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Top 10 Mistakes in pH System Design and How to Avoid Them

We learn best by reviewing examples. Seeing other people’s mistakes and understanding what they did wrong is the easiest way to learn; making the mistakes ourselves is the hard way. Let’s look at someone else’s mistakes in pH system design and the issues that caused them.  

This article covers the top 10 major mistakes I have seen in pH design and installation in a neutralizer tank in a waste treatment system. Each mistake is the result of a fundamental lack of understanding of the heightened sensitivity of pH control systems to dead time, process and measurement noise, insufficient process attenuation (filtering), and valve backlash and stiction. By researching the issues that caused each mistake, you can gain the necessary understanding, expand your skill set, and design successful pH systems.  

  1. Insufficient number of stages of neutralization (inadequate rangeability and sensitivity) 
  2. Improper vessel geometry and agitation patterns (excessive equipment dead time) 
  3. Backfilled reagent dip tube (excessive reagent delivery delay) 
  4. Incorrect location of reagent injection point (excessive reagent delivery delay) 
  5. Gravity flow reagent (excessive reagent delivery delay) 
  6. Incorrect location of the reagent control valve (excessive reagent delivery delay) 
  7. Control valve with excessive stick-slip (poor sensitivity and excessive variability)   
  8. Electrodes submersed in a vessel (coating and maintainability problems) 
  9. Electrodes located in pump suction (bubbles, clumps, and wrenches) 
  10. Electrodes located downstream in the recirculation line (excessive measurement delay)  

There are many underlying principles and guidance required to avoid mistakes, meet the extreme challenges and achieve the extraordinary opportunities for concentration control offered by pH. The following overview helps get the practitioner started on the right path for mistake-free pH control:  

  • Reagent piping and injection must minimize transportation delays.  
  • Volumes must have proper geometry and agitation to maximize the process time constant that filters pH oscillations.  
  • Several volumes may be needed to provide multiple process filters in series.  
  • Valves must be able to make extremely small changes in reagent flow precisely.  
  • Electrodes must be installed to minimize noise and response time and maximize reliability. 
  • The Digital Twin plays a critical role in avoiding mistakes and successfully developing, prototyping, testing, training, maintaining, and continuously improving pH systems. 

Book cover Advanced pH"Advanced pH Measurement and Control: Digital Twin Synergy and Advances in Technology," Fourth Edition, by Greg McMillan, Christopher Stuart, Dean Cook and Zachary Sample provides a clear, concise and comprehensive view of how to select, install and maintain electrodes and control valves and how to develop control strategies for pH applications critical for product and water quality in process industries.

The book covers every aspect of system design, including the mixing and reagent piping requirements that are important for a successful application. Digital twin use is detailed to show how to avoid mistakes and provide a reliable and cost-effective design and installation to meet process performance objectives. Each chapter concludes with an extensive list of best practices to provide the detailed guidance and focus needed.

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.


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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