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Between 1990 and 2010, the city of Cottonwood, Ariz., located in the state’s second fastest growing county, nearly doubled its residential service area. This growth put significant strain on the community’s aging and disjointed water delivery systems, which served a customer base of 30,000.
Residents were faced with frequent water outages that would last a day or two, as well as inconsistent pressure and continual water hammer noises in their homes.
Due to the community’s rapid growth, the city’s water supply was managed through a patchwork of four separate and privately-owned water systems, each controlling individual, and sometimes overlapping, service areas. Worse, the smaller water supply operations were not interconnected, eliminating the normal efficiencies enjoyed by a single, integrated water utility.
“If a municipality does not own the water system within its boundaries, it does not control its own destiny,” asserted Cottonwood Development Services General Manager Dan Lueder. “A private water company is worried about the bottom line and making money; the city is more concerned about providing and conserving water.”
For Cottonwood, the necessity to address water conservation was especially crucial. Averaging a mere 12 inches of rain annually, the city faced some serious water supply challenges.
The situation was further complicated by variations in the length of the water lines and elevations of the rural community’s water distribution network. For example, one booster pump had to pump water across nearly 6,300 linear feet, as well as up a 200-foot elevation.
Controlling its own destiny
In 2005, the city devised a water management strategy that included modernizing its water system with effective monitoring, control and pumping technology. The strategy was developed subsequent to the city acquiring the private water companies, which presented a number of piping and pumping challenges.
One of Lueder’s first tasks was integrating the separate water systems — representing roughly 10,000 primarily residential service connections — into a single municipal utility department. In 2004, the city began incorporating the private water systems and establishing a Water Division, which was responsible for supplying and distributing water through storage tanks, 28 wells, fire hydrants, pumps and water meters.
The city’s plan also called for the Wastewater Division, which was operated as a separate entity, to be incorporated into the utility department. This division manages a 1.5-million-gallons-per-day wastewater treatment facility, five sewage lift stations, 48 miles of collection system, and about 80 acres of effluent reuse area.
“We had to link these stand-alone, independent networks to one another and merge them into one interconnected system,” Lueder explained. “We did a lot of work identifying pressure zones. It has been an interesting experience to basically take us from the 20th to the 21st century in water supply and water delivery.”
Pressure surges prematurely age pipe
The majority of pumps in the city’s existing distribution system were constant speed, across-the-line pumps that would pump well water into a holding reservoir, and then to service stations and individual communities. In addition to providing surges of pressure, the fixed-speed pumps could not provide incremental pressure; the pumps were either off or running at top speed.
For example, if the pressure in the hydro-pneumatic holding tank dropped below 50 pounds per square inch (psi), a pump would activate to replenish the tank to about 75 psi. This pressure swing subjected the pipes to a 25 psi pressure change, which stressed and prematurely aged the lines, many of which dated back to 1930.
With roughly 100 miles of pipe in the system, the constant pressure surge and water hammer caused significant leakage and capital repair costs for the city — as much as five to seven leaks per week in the main line, in addition to one or two leaks in the service lines.
The issue came to a head in 2006, when two 11-year-old booster pumps showed signs of failure, recalled Doug Ryan, of Grand Canyon Pump and Supply’s Phoenix location, which provides equipment sales and application support to the city. Ryan is a sales engineer with more than 28 years of experience working with pumps and pumping systems. Instead of replacing like-with-like, Ryan seized the opportunity to present a new water boosting option for the reconfigured system.
“The city’s contracted civil and sanitary engineer originally advocated a custom-designed pumping station that, in addition to being costly, was a problem for the city’s long-term plan to relocate that station in seven months,” Ryan explained. “As an alternative, we recommended a ‘plug-and-play,’ packaged boosting solution that the Cottonwood municipal staff could pipe and install on its own, saving considerable upfront design and labor costs.”
The Grundfos Hydro MPC BoosterpaQ was selected as the solution. The integrated pressure boosting system offers up to six vertical multi-stage CR pumps in parallel operation, designed to optimize pumping efficiency over a range of flow rates. Ideal for water supply systems, as well as industrial and irrigation applications, these integrated pumping systems utilize an advanced controller that adjusts pump speed and stages additional pumps as necessary to meet fluctuating system demand.
The project’s lead engineer, Ken Knickerbocker, of Pineview Consulting, LLC, explained that coming to the decision of selecting the system was not a rushed one.
“I looked into the system and what I looked for specifically was the integration of the controller with the VFD, motor efficiency ratings, and the ability for the system to manage water usage, which thereby ensures maximum system efficiency. I was impressed with the pump system itself. It offered vertical, multi-stage in-line centrifugal pumps,” Knickerbocker explained. “What I typically do, even with the systems that have a good history behind them, is go through and check the engineering, including on packaged systems. You have to check the hydraulics, power, drives, controls, etc.”
Knickerbocker continued, “I’m still careful, and recommend that engineers still be careful, about specifying systems. Don’t just take for granted that you’re going to get a good package. You need to look into it and make sure that you do your due diligence.”
A highlight of the packaged unit for the project’s engineering team was that it could be easily moved from one location to another, an important feature for Cottonwood, as it reconfigured its pumping network. Ryan noted that one of the city’s booster systems is now operating in its third application.
“The engineered system’s flexibility was a real advantage for us,” Lueder said. “You just hook up a water inlet and outlet, establish power, key in the set point and turn it on. The integrated pump system does the rest.”
Lueder noted that ease of installation was critical to the city’s decision to purchase the product.
“The advanced controller determines the most efficient combination of pumps to run and speed of pumps to exactly match any duty condition,” Lueder added.
Matching output with demand
With the fixed-pressure design, according to Ryan, one or more of the pumps was online all the time to keep the system pressurized.
“Rather than running at full speed and peak horsepower to reach the desired pressure, the more energy-efficient option is to design a system that starts from zero and ramps up to the desired speed to maintain a constant pressure and stages additional pumps as necessary to meet the specific flow demand,” Ryan said. “Since demand for municipal water delivery varies throughout the day, why not vary output?”
With variable speed motors and advanced controllers, high efficiency is maintained with both speed control and pump staging. According to Ryan, another advantage of the new pump is the ability to further reduce pipe fatigue and energy use by switching from constant to proportional pressure. He explained that at lower flow rates, such as overnight, the pump controller will automatically lower the pressure set point, since there is less friction head loss.
“By loading pump curve data directly into the controller, the Hydro MPC BoosterpaQ system can continuously estimate proper flow rates,” Ryan explained. “The controller uses the flow rate calculation to determine how to adjust the proportional pressure set point.”
Engineers utilized this proportional pump control setting in some of the city’s installations to further optimize energy consumption and minimize water loss during low flow rates because of the reduced pressure requirement. Ryan anticipates that additional applications will leverage the proportional pump control feature as the community and its Water Division matures.
Since the program began in 2005, the city has installed nine of the pressure boosting systems, seven for potable water and two for reclaimed water. The water utility also retrofitted seven of its water extraction stations with Grundfos’ SP submersible pumps that feature energy-efficient, variable-speed drives.
Eliminating pressure surges in the system has slashed the number of pipe breaks and leaks requiring repair.
“From 2010 to 2014, we saw a 30 percent reduction in the number of leaks,” Lueder asserted. “Based on an average $500 to repair each leak, we’ve saved more than $38,500, over and above the electrical cost savings.”
“These savings have not been lost on the community,” reported Cottonwood Mayor Diane Joens, who represents the city on several water conservation organizations, including the Verde Front and String of Pearls committees, and is a board member of Verde Valley River Nature Organization.
Mayor Joens continued, “Water conservation is critical to communities like Cottonwood. We’re a model for how rural water systems can adopt new technologies to meet the demands of growth while conserving water and saving energy.”
Knickerbocker explained that there are multiple scenarios in which the Grundfos system could aid in efficiency.
“The compatibility of the drive and controller, matched with high efficiency motors on the multi-stage centrifugal pumps, is an innovative design that provides for better compatibility and a more efficient system approach,” Knickerbocker noted. “So, in the situation of let’s say a drought you want to make sure that you minimize leaks in the system. You can do that by varying the pressure. If you lower your set point, you will also minimize water loss through any leaks in your system, as there will be less pressure driving water to those leaks. Also, with the variable speed systems not only do you gain efficiency, that is in power, but you also are providing a continuous flow of water to areas that are always demanding water. It controls the pressure such that you maximize the use of your water.”
The sizeable reduction in leaks was a major component in Cottonwood drawing less water from the ground in 2014 than the combined volume used by the private water companies in 2000.
The level of “unaccounted water” — which refers to the difference in the amount of water extracted versus the amount of water billed to customers — has dropped from 40 percent to 11 percent. Lueder attributed the reduction of water loss through leaks to the packaged booster systems.
“In addition to the energy savings from the more efficient pumps, our overall water usage level has dropped,” Lueder said. “We’re drawing less water from the aquifer today than five years ago, which is imperative if we are to bridge the water crisis facing the Southwest.”
Also, since the new booster systems were installed, the city has experienced a reduction in complaints about pressure swings and water hammer noises.
The city’s energy savings were bolstered by a utility rebate from Arizona Public Service, which offers financial incentives for the use of energy efficient motors over 15 horsepower. In 2012, with roughly half of the Cottonwood utility's pump stations retrofitted with some type of VFD pump technology, Arizona Public Service paid the city a one-time rebate of $24,702. These funds helped offset the Cottonwood utility department's operating budget.
Although the initial investment in a packaged system is higher than some alternatives, Lueder said reliability is something that can’t be given a price tag.
Lueder said, “Even for a small community like ours, the premium performance and energy savings are worth the investment.”