Water System Consolidation Brings Challenges

Grundfos pumping system pays for itself by efficiently matching municipal water pressure with demand.
Water System Consolidation Brings Challenges
Speed control and pump staging allow the Grundfos variable-speed motors and advanced control software to maintain high efficiency. Top: The pressure booster system helps deliver reclaimed water over a range of elevations within the utility’s service area.

Interested in Infrastructure?

Get Infrastructure articles, news and videos right in your inbox! Sign up now.

Infrastructure + Get Alerts

Between 1990 and 2010, the population of Cottonwood, Arizona, located in the state’s second fastest growing county, nearly doubled. 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 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. And since the systems weren’t connected, they lacked the efficiency of a single, integrated water utility.

“If a municipality does not own the water system within its boundaries, it does not control its own destiny,” says 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, this necessity to address water conservation was especially crucial: Averaging a mere 12 inches of rain annually, the city faces some serious water supply challenges.

The situation was further complicated by variations in the length of the waterlines and elevations of the rural community’s water distribution network. For example, one booster station had to pump water nearly 6,300 linear feet with a 200-foot rise in 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 — a development made possible by intelligent, demand-based municipal pump technology from Grundfos. 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 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.

“We had to link these stand-alone, independent networks to one another 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.”

Prematurely aging pipes

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 pressure surges, 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 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 many as five to seven leaks per week in the mainline, 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 presented a new water boosting option for the reconfigured system.

Packaged solution

“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. “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 up-front design and labor costs.”

Ryan chose the Grundfos Hydro MPC BoosterpaQ, an integrated pressure boosting system that offers up to six vertical multistage CR pumps in parallel operation, designed to optimize pumping efficiency over a range of flow rates. Ideal for water supply systems, these integrated pumping systems utilize an advanced controller that adjusts pump speed and stages additional pumps as necessary to meet fluctuating system demand.

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 setpoint 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 and speed to exactly match any duty condition.”

Matching output with demand

With the fixed-pressure design, according to Ryan, one or more of the pumps were 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. 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 setpoint, since there is less frictional headloss.

“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 setpoint.”

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. The water utility also retrofitted seven of its water extraction stations with Grundfos’ SP submersible pumps, which feature energy-efficient, variable-speed drives.

“Not all of our systems have Hydro MPC BoosterpaQs in them,” Lueder says. “Because of space limitations or other issues, some existing pumps were retrofitted with variable-speed drives in order to align output with demand.”

Liquid gold

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 infrastructure, as well as less water lost to leaks.

From 2010 to 2014, the city saw a 30 percent reduction in the number of leaks and saved more than $38,500, over and above the electrical cost savings, Lueder says. And in 2014, the city drew less water from the ground than the combined volume used by the private water companies in 2000.

“These savings have not been lost on the community,” reports Cottonwood Mayor Diane Joens. “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.”
The level of unaccounted water has dropped from 40 percent to 11 percent, 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,” Lueder says. “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.

Although the initial investment in a packaged system such as the Hydro MPC BoosterpaQ is higher than some alternatives, Lueder says you can’t put a price on reliability. “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.”


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.