Between 1990 and 2010, the city of Cottonwood, Arizona, USA, located in the state's second fastest growing county, nearly doubled its population. 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,” asserts 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 in 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 ft elevation.
Water management strategy
In 2005, the city devised a water management strategy that included modernizing its water system with effective monitoring, control and pumping technology; a development made possible by intelligent, demand-based municipal pump technology from Grundfos. The strategy was developed after the city acquired the private water companies, which presented a number of piping and pumping challenges.
One of Lueder's first tasks was integrating these 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, 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 had 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 together and merge them into one interconnected system,” explains Lueder. “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.”
Constant speed system
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, recalls Doug Ryan of Grand Canyon Pump and Supply's Phoenix location, which provides equipment sales and application support to the city. 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,” explains Ryan, a sales engineer with more than 28 years' experience working with pumps and pumping systems. “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.”
Integrated pressure boosting
Ryan chose the Grundfos Hydro MPC BoosterpaQ, an integrated pressure boosting system that 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 Hydro MPC BoosterpaQ is an integrated pressure boosting system for water supply systems; as well as municipal boosting, fire flow, water transfer, and industrial applications. This pressure booster system delivers domestic water and fire flow to residents of Cottonwood, Arizona.
Moreover, the packaged unit could easily be moved from one location to another, an important feature for Cottonwood, as it reconfigured its pumping network. In fact, Ryan notes 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,” says Lueder. “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 notes 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.”
With the fixed-pressure design, according to Ryan, one or more of the pumps was online all the time to keep the system pressurized. “But the new packaged system would deliver the exact pressure necessary to achieve optimal performance — all without direct human intervention,” he notes.
“Rather than running flat out at top 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,” he continues. “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 explains 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,” explains Ryan. “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.
Six vertical multi-stage CR pumps in parallel operation are offered, designed to optimize pumping efficiency over a range of flow rates.
The ease of installation and performance of the pump solutions was so successful, even turning the city's contracted engineer into an advocate, that Ryan and Lueder developed a plan to replace more pumping systems with the Hydro MPC BoosterpaQ.
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, according to Lueder. The water utility also retrofitted seven of its water extraction stations with Grundfos' SP submersible pumps that feature energy-efficient, variable-speed drives.
“Not all of our systems have Hydro MPC BoosterpaQs in them,” admits Lueder. “Because of space limitations or other issues, some existing pumps were retrofitted with variable-speed drives in order to align output with demand.”
Another site that was a candidate for the pressure boosting system helped support the community's fire protection system. When he learned that the emergency pumps did not start when fire trucks flushed the hydrants, Lueder's initial response was to install a complete boosting station. After investigating the pumping station, Grand Canyon Pump's Ryan advised against the additional investment, instead recommending fine-tuning the current system performance. The project helped Ryan win Lueder's trust, which has led to more sales and a stronger partnership.
Eliminating pressure surges in the system has slashed the number of pipe breaks and leaks requiring repair. Moreover, because some of the booster pumps leverage Grundfos' proportional pressure setting, which lowers water pressure/volume during off-peak demand cycles, the reduced flow volumes translate into less wear-and-tear on the community's pipe infrastructure, as well as less water lost to leaks. “From 2010 to 2014, we saw a 30% reduction in the number of leaks,” Lueder asserts. “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,” reports Cottonwood Mayor Diane Joens. “Water conservation is critical to communities like Cottonwood,” notes 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. “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.”
Ironically, the sizeable reduction in leaks has been a major component in the city 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% to 11%, which Lueder also attributes to the reduction of leaks since installing the packaged booster systems. “In addition to the energy savings from the more efficient pumps, our overall water usage level has dropped,” says Lueder. “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.”
Just as importantly, 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 hp. 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.
According to Grundfos, power consumption accounts for 85% of all costs incurred during the life cycle of a pump. The initial pump purchase price and the cost of regular maintenance account for the remainder. Therefore, even the smallest improvement in energy efficiency can translate to sizeable savings. In addition, system designers can squeeze an additional 10 percent to 35 percent in energy savings by transitioning from constant-speed pumps to those that leverage variable-speed drives, depending on the system parameters.
Although the initial investment in a packaged system such as the Hydro MPC BoosterpaQ is higher than some alternatives, “you can't put a price on reliability,” concludes Lueder. “We've had no sustained system outages since we installed the Grundfos solutions. It is extremely reliable and has built-in redundancies; if one pump kicks off, another compensates. Even for a small community like ours, the premium performance and energy savings are worth the investment.”