Loves Park, Ill. — A new energy-efficient well system, including a pump with a variable frequency drive, providing accurate pressure control 24/7 and even when there is a fire emergency, was added to the water distribution system here, in 2008, to help serve the demands placed on the system because of population growth along the city's eastern edge.
Five water wells and three 500,000-gal. water towers maintained adequate system pressure for a number of years, until the Loves Park community started growing. Without the introduction of additional capacity into the system, the increased flow would eventually cause a decrease in pressure, so a well was installed to provide additional capacity.
“When you're fighting a fire, a large amount of water is being drawn from the supply system at a very rapid rate,” explained Susan Dimond, marketing communications manager at Danfoss Drives, Loves Park, Ill. “Some cities use a water tower (gravity), but this means building and maintaining a large structure (higher cost and inconsistent water pressure). By using a variable frequency drive (VFD)/pump in the supply system, you can monitor the system pressure (with a pressure transmitter) and speed-up or slow-down a pump that is delivering the water to the system. The VFD will provide an accurate and stable pressure within the system, while saving energy by running the pump at the optimum speed necessary to keep-up with the system demand.”
Strand Associates, Madison, Wis., an engineering firm with experience in municipal water systems, designed the new well system and building that houses it.
The well is 1,400-ft. deep and fitted with a submersible pump approximately 800-ft. below the surface. Strand Associates worked with Jerry Janzen at Instrument Control Systems, Plymouth, Minn., to develop the system required to efficiently operate the pump motor.
In order to maximize the control and energy efficiency of the well and pumping station, a 350 HP Danfoss VLT AQUA variable frequency drive was used, according to Janzen.
The drive is connected to the city's Supervisory Control and Data Acquisition (SCADA) system, allowing it to run the pump motor at whatever speed is necessary to maintain the appropriate system pressure. By matching the actual demands of the water distribution system and running the pump at less than full speed, the drive provides energy savings and reduces water losses. It also gives the city the ability to plan for future expansion. As new developments are built further out and water flow increases, the drive increases pump speed to meet the demands and maintain the pressure set-point.
An advantage of using a variable frequency drive is the capability to provide constant pressure at variable flow conditions. The VLT AQUA drive at the well provides between 200 and 1,600 GPM at a pressure of 70 psi.
However, this project had challenges due to the depth of the well and the size of the pump motor. Since the pump is a submersible pump, it had to be placed at the bottom of the well, 506-ft. below grade.
“The first issue is that the voltage typically used for motors is 480 volts,” said Janzen. “If we had used a 480 volt motor, the cable sizes would have been quite large and expensive and heavy for powering the motor at the bottom of the well. Instead, a 2,400 volt motor was used and the cables therefore only need to be 1/5 the size.
“The second challenge is with 500+ ft. cables and using a variable frequency drive, we would experience vicious harmonics at the motor unless we incorporated a low harmonic drive,” added Janzen.
There are no low harmonic drives rated for 2,400 volts, but there are low harmonic drives rated for 480 volts, one being the Danfoss VLT. To use a 2,400 volt motor with a 480 volt drive, a step-up transformer was used to increase the voltage to the motor along with a sine wave filter connected between the drive and the transformer, further mitigating any harmonics before they were allowed to pass into the transformer.
It was also determined that using a medium voltage motor would further reduce overall expenses due to the smaller diameter motor cables that it required.
“At 460 VAC we would need very thick cables to operate the pump,” explained Brent Studnicka, engineer at Strand Associates. “But at 2,400 VAC the current required is much lower, meaning that a smaller gauge of wire could be used. With 800 feet of cable, this resulted in a significant savings.”
Equipment stress is also minimized, using the motor. By reducing the number of starts required of the well pump, the drive maximizes the pump motor's longevity. Controlling the acceleration and deceleration when starting and stopping further lowers stress to the motor and reduces water hammer in the pipe system. The drive will also allow the city to work on their water towers without impact to water distribution.
“Right now, this pumping station is filling the gap,” said Craig McDonald, water department supervisor at Loves Park. “But it will also allow us to bypass a tower in order to maintain it while still providing water to the people who rely on it.”