Model-predictive control reduces main breaks 21% at EnWin Utilities

Windsor, Ontario, is blessed with abundant water. The Windsor Utilities Commission (WUC) prides itself on being a good steward of it.

WUC, managed by EnWin Utilities, Ltd., distributes about 48,000 million liters (ML) of water annually to more than 72,000 customers. The utility maintains one of the lowest production and distribution costs in Canada.

EnWin has a continuous-improvement model for process changes and capital projects. In 2011, the utility installed a Rockwell Automation supervisory control and data acquisition (SCADA) solution to improve efficiencies and processes. Only two years later, EnWin looked to expand that system to help mitigate increasing numbers of water main breaks throughout its infrastructure.

By late 2012, EnWin was averaging 238 main breaks a year at a cost of about $5,000 each. While water main breaks can be attributed to many factors, EnWin determined that a significant number of the ones it experienced were caused by system pressure spikes and dips.

Way the breaks go

As is well known, because water is a non-compressible fluid, a change in pressure anywhere in the system is felt throughout the entire infrastructure. Under certain conditions, these pressure fluctuations can cause main breaks. Older water infrastructures, characterized by iron water mains, corroding pipes and soil erosion, are particularly vulnerable to pressure fluctuations. Cold weather conditions exacerbate the problem.

Although EnWin’s capital plan includes continuous maintenance and aggressive replacement of its aging infrastructure, the utility simply cannot replace the entire system fast enough.

"Because we have an older infrastructure, it’s very susceptible to main breaks," says Garry Rossi, director of water production, EnWin Utilities. "We needed a solution to help us improve the performance of our existing infrastructure until we have the opportunity to replace it."

The EnWin Utilities distribution system includes two treatment plants, thousands of kilometers of distribution piping, two pumping stations and a booster pumping station, used during high demand periods.

To the pump station

At the pumping stations, pump flow was controlled through simple proportional-integral-derivative (PID) logic based on outlet header pressure. Operators monitored elevated tower levels and made adjustments to compensate for demand fluctuations. Pumps were stopped and started manually to adjust the system flows. The booster station was also controlled with PID logic — and started and stopped manually based on system demand and operator judgment.

"PID logic has significant limitations," Rossi says. "It can only control a single input and generate a single output."

In this case, the high lift pumps were controlled by maintaining a flow set-point with fluctuating pressure. Multivariable elements — such as variable frequency drives, flow control valves and other incoming pressure data — could not be factored into the control scenario.

"We were basically doing what we could with the technology we had," says Rossi. "The result was inconsistent system pressure — and costly repairs."

A possible solution first presented itself in late 2012, when John Stuart, EnWin Utilities vice president of operations, saw a model predictive control (MPC) demonstration at a Rockwell Automation event – Automation Fair. This server-based solution collected data at 15- to 16-second intervals.

"The response time of this technology was particularly attractive," Rossi says. "The system could react to multiple variables simultaneously — and make adjustments accordingly."

Solution surmised

EnWin planned to leverage the capabilities of its existing SCADA system — and integrate the MPC controller into the overall solution. The SCADA system is based on a fully redundant, Allen-Bradley Control Logix programmable automation controller (PAC) platform.

"With MPC, we would have the ability to monitor and control the pumping stations based on multiple factors," Rossi says. "Therefore, we could focus on maintaining consistent pressure throughout the system while maintaining fluctuations in flow demand."

To minimize potential service disruptions, EnWin planned to implement the solution in two phases.

For the first phase, EnWin installed 17 remote pressure stations across the distribution service area. To maintain consistent pressure throughout, minimum pressure constraints were developed for all pressure stations. The remote stations were monitored by the MPC controller and programmed to meet fluctuating system demand throughout the day.

The system was set to maintain the lowest pressure possible for adequate service throughout the area. The MPC controller managed flow by attenuating two running high lift pumps — one at a pump station and one at the booster station. These pumps were controlled by Allen-Bradley PowerFlex 700 and PowerFlex 7000 variable frequency drives.

The EnWin team commissioned phase one in June 2013 and began to plan for phase two, which would focus on optimizing the main campus header pressure through the addition of modulating flow control valves (FCVs).

An onboard solution

While the initial results from the first phase implementation were impressive, the Rockwell Automation team hoped to incorporate additional functionality for the second phase.

The server-based MPC solution enabled multivariable control of the various pressure points in the system as well as variable-speed control of running pumps. However, pump start/stop control was not part of the first phase.

"We knew we could optimize by incorporating pump start/stop functionality and flow control valves," says Quin Dennis, application engineer, Rockwell Automation. "But given the existing interval speed, MPC would not be able to make system adjustments quickly enough to mitigate the rapid pressure spikes from pump starts or stops."

EnWin agreed to work with Rockwell Automation to test a new onboard MPC controller that could dramatically improve interval speed. The prototype solution features onboard MPC functionality in the controller. No separate server or software is required.

With the onboard MPC solution in place, interval speed is reduced from 15- to 16-seconds to the 0.5- to 1-second range. Integrated with PowerFlex 7000 medium voltage drives, this highly responsive system can now regulate speed on running pumps — plus offset any pressure spikes that result from starting or stopping them through the integration of adjustable flow control valves.

By setting energy cost factors on usage, the system’s embedded optimizer can define medium voltage drive usage explicitly without heuristics. Since energy costs are higher for flow control valve usage, the system ensures the valves remain as open as possible — until the drive’s low flow limit is reached. At that point, the operator is prompted to shut down the pump and the valves take over.

Opportunity delivers results

"We welcomed the opportunity to collaborate with Rockwell Automation on testing the onboard MPC solution," says Rossi. "Ultimately, we commissioned the second phase in January 2014 — and applied the improved functionality across our main campus and booster station." In the third phase, the utility plans to apply the onboard solution to the remaining pumping station in its system.

EnWin has been monitoring solution results since the first phase was applied in June 2013.

"Very early on, we noticed that pressure was no longer spiking up and down — it was consistent," says Rossi. "System reliability and performance continued to trend in the right direction, but we wanted to be sure we had a good data set before we made any claims."

In late December — backed with six months of first phase data — EnWin was confident the MPC solution was successfully controlling system pressure throughout the service area. EnWin had experienced on average 238 main breaks per year. In 2013, EnWin experienced 187 main breaks — a 21 percent improvement resulting in a cost savings of approximately $125,000.

The utility also reduced average system pressure by 2.8 psi and standard deviation by 29 percent. As a result, EnWin saved an estimated $125,000 attributed to lower electricity cost and system leakage.

Thanks to the enhancements implemented in phase two, pump start and stop pressure spikes have been virtually eliminated. "At the beginning of this project we were cautious. We wanted to be sure the technology would perform as stated," says Rossi. "But the proof is in the results."

In addition to improving performance, the onboard solution helps contain operational expenditures in other ways as well. "With onboard MPC, we have reduced the overall cost of our solution," says Rossi. "Of course, we still pay MPC licensing fees, but we eliminated the cost of an additional server and related licenses."

Rockwell Automation, Inc., is a global provider of industrial automation and information solutions. Brands include Allen-Bradley and Rockwell Software.