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Model-Predictive Control Smoothes Water Distribution

Nov. 18, 2014
Windsor's Water Distribution System Slashes Water Main Breaks 21% by Using PlantPAx Embedded MPC
About Jim Montague
Jim Montague is the Executive Editor at Control, Control Design and Industrial Networking magazines. Jim has spent the last 13 years as an editor and brings a wealth of automation and controls knowledge to the position. For the past eight years, Jim worked at Reed Business Information as News Editor for Control Engineering magazine. Jim has a BA in English from Carleton College in Northfield, Minnesota, and lives in Skokie, Illinois.

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Cold winters and pressure spikes add up to popped pipes in municipal water distribution systems everywhere. But, while chronic water main breaks are an unavoidable, occupational hazard for most cities and towns, engineers and operators in Windsor, Ontario, have found a way to drastically reduce these costly and time–consuming incidents by using the soon–to–be–released PlantPAx Embedded MPC (model–predictive control) technology from Rockwell Automation.

"Cold winters increase the frost depth in the ground, and this stresses pumps and allows resulting pressure variations to cause breaks in the mains," said Garry Rossi, water production director at EnWin, which provides management, maintenance services and staff for Windsor Utilities' water distribution system. "We decided to investigate MPC because we were losing about 17% of our water, and so we weren't getting paid for it. This was one of the highest non–revenue water rates in Canada, and even reached 24% in the third year prior to our upgrade project. The cost of this lost water was compounded by the fact that our electricity bill was increasing by an average of 5–6% per year."

Rossi and Edward Scott, process optimization lead for Canada in the Information Systems and Process Business at Rockwell Automation, presented "EnWin Utilities Reduces Water Main Breaks by 21% with MPC" today at the Rockwell Automation Process Solutions User Group (PSUG) meeting in Anaheim, California. PSUG is being held right before Automation Fair 2014, hosted by Rockwell Automation this week at the Anaheim Convention Center.

"Our return on investment for the whole project was less than one year." EnWin's Garry Rossi explained how model-predictive control has enabled more stable pressure control in Windsor, Ontario's water supply mains—reducing electricity and chemicals costs as well as water main breaks in the process.

The city gets its water from the Detroit River and distributes about 92 million gallons per day to about 280,000 customers in Windsor and the nearby communities of LaSalle and Tecumseh. The utility also has 31 million gallons of reservoir storage, two treatment plants, three pump stations, two elevated storage tanks, and operates 690 miles of water mains with 66–psi average pressure at the pump. The average age of these pipelines is 44 years old, but some are as old as 80 years; their age and mineral build–ups make them brittle and susceptible to breaks. Rossi says Windsor experiences about 238 water main breaks per year, which Rossi reports is second only to Toronto.

To start reducing breaks and plugging leaks, Rossi reported that EnWin started out by installing 17 pressure control stations over the past four years that feed data back to the fully redundant SCADA system it also installed in 2010. This whole system is connected via a fiberoptic ring network. The utility also installed two variable–frequency drives (VFDs), including one at its second–largest pumping facility.

"We began researching what MPC could do for us because, while our 10 operators all produce excellent product, they all operate slightly differently, and these slight inconsistencies produce inefficiency," explained Rossi. "We also have fewer operators monitoring far more parameters than in the past, and many of them can influence each other. For instance, flow rates can have a direct effect on chemical requirements and turbidity. We also did lots of training and have volumes of procedures, but our former PID–based algorithms could only look at one input and output at a time."

To examine and analyze multiple parameters and variables, EnWin began by developing a definitive model for its MPC program, which allows its controllers to estimate expected future performance and adjust setpoints earlier. "PID loops look at deviations from setpoint, but they can only handle one input and output," explained Scott. "Users tell an MPC what target they want to maintain, and then the MPC has the controllers push adjusted setpoints down to the DCS. This means MPC can look at all relevant inputs and outputs and adjust back to the target before the overall system begins to move."

Consequently, EnWin adopted PlantPAx Embedded MPC to adjust the constraints in Windsor's distribution system sooner, and then smooth out and reduce its formerly chronic pressure spikes. Based on Pavilion8 technology, PlantPAx Embedded MPC will be formally launched by Rockwell Automation in 2015.

Rossi reported that one of Windsor's water towers used to run at 62–74 psi when it was operated manually, a relatively large 12–psi pressure range. However, adding the VFD in Phase 1 of the upgrade, which also integrated data flows between pressure station and pump station, reduced the variability to 5 psi. Further, implementing MPC in Phase 2 enabled the VFD to actually dampen the effects of valves opening and pumps starting and stopping, further reducing variability to just 1 psi. This is the source of many if not all of Windsor's greatly reduced water main breaks.

"We reduced water main breaks by 18% during January, February and March 2014 compared with the past eight years, even though 2014 was colder than almost all of them," added Rossi.

In the future, Rossi reported that EnWin is going to seek further gains by using sensors with 0.1–psi sensitivity to produce better setpoints; its present sensors are accurate to only 0.4–psi. Also, Windsor is going to add a VFD to its largest pumping station and probably enable MPC at its reservoir as well.

"This will allow us to run our production at a steady state and further improve our chemical and electrical efficiency," added Rossi. "So far, better pressure control has saved us 28% or $125,000 on chemicals and electricity. We reduced our standard deviation on water pressure by 29% overall, and this saved us another $125,000. As a result, our return on investment for the whole project was less than one year."

About the Author

Jim Montague | Executive Editor

Jim Montague is executive editor of Control. 

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