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

Summary

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. 

"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.

_{This feature originally appeared on ISA Interchange.}