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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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Are changes in pipeline pressure affecting your loop?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The tenth question in the ISA Mentor program is from Danaca Jordan (USA):

“How can you tell if changing pressure in a pipeline is affecting your process control loop?”

While it is well known changes in pressure change the control valve flow, less recognized is the changes in the installed characteristic and the prevalence of pressure upsets. Liquid flow (unless there is flashing) through a control valve is proportional to the square root of the pressure drop across the valve. The installed characteristic changes as the ratio of the valve to total system drop changes. You can track down disturbances by finding what flow changed first (see “Tracking Down Disturbances”). If you don’t have a flow measurement for each control valve, it gets difficult. If there are multiple users of the stream, then a pressure change in the stream will cause coincident changes in the controller outputs of the users. If there is only one user of the stream with the pressure change, the first process controller output that starts to change was first affected by the stream pressure change either as a disturbance flow or manipulated flow. Different loop deadtimes can mess up this analysis depending upon disturbance type and path. Data analytics packages can determine correlations between process variables and flows but multivariate statistical process control assumes linear relationships and synchronization of inputs with outputs for continuous processes. Unmeasured disturbances, inputs that are valve positions (nonlinear installed characteristics) rather than flows, and process deadtime are problematic for applications in continuous operations. If you had wireless pressure transmitters, you could move them around to track down the source pressure changes. If you had secondary flow loops (see “Secondary Flow Loops Offer a Primary Advantage”, the question would go away because the flow loop would correct for the pressure change before it affected the primary process loop. Changes in stream temperature, composition, and density are also disturbances. If you don’t have measurements of these stream variables, you are relegated to identifying the first flow to be affected. Often a process controller changes a manipulated flow to counteract the stream changes. For ratioed flows (flow feedforward), the primary process controller adds a feedback correction for stream changes. Secondary flow loops are essential for flow feedforward.

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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