case study: Nebraska City Utilities
problem: Hydraulic transient pressures created by stopping and starting the four existing WTP pumps made it difficult to control supply pressure, particularly during certain conditions like low well water levels when the tower was down for maintenance.
solution: After the installation of the new pump and motor control, much tighter system pressure control was obtained, even during an extended period when the water tower was removed from service for annual maintenance.
After recently installing a new high service pump outfitted with variable frequency drive (VFD) control, the Nebraska City Utilities Water Supply plant dramatically improved the efficiency and performance of its water delivery system, lowered turbidity, achieved tighter pressure set-point control, cut non-revenue water costs and energy costs.
The upgrade also enabled more efficient use of the plant’s clearwell storage capacity and enabled the utility to maintain more constant chlorine levels to meet the latest Department of Health regulations.
The recent upgrade included installing a new 400 hp, 3,500 gpm high service pump over the facility’s newer clearwell, outfitted with Danfoss VLT 8000 AQUA drives and AHF 010 harmonic filters. A second pump and VFD control system were installed on the smaller backup pump to utilize during high demand periods. The plant also needed to maintain low turbidity as a state regulation.
“Turbidity monitoring is what initiated all of the major changes at the water plant,” said Dave Messing, electronics specialist at the plant. “Each time the plant was stopped and then restarted during the day, the turbidity levels spiked at the start.”
Nebraska City’s lime-softening treatment plant operates a well field with nine wells north of the plant at Riverview Marina State Park. The facility services about 7,000 residents living in the city and surrounding rural areas, with a peak demand for distribution of about 3,200 gpm. The Nebraska City area gets its water through the wells from groundwater sources, and several factors impact water supply pressure in and around the city, including periods of drought, an aging water supply facility, and the considerable distance of the water tower from the water supply plant.
Due to pressure fluctuations, the plant would regularly receive a high number of resident complaints due to supply pressure swings.
Hydraulic transient pressures created by stopping and starting the four existing WTP pumps made it difficult to control supply pressure, particularly during certain conditions like low well water levels. When the tower was down for maintenance and water supply relied on the clearwells, some residents experienced extremely low or no pressure in their water supply.
In addition, water hammer was a problem because as soon as the pump was needed, it would start directly across the line and quickly jump to full speed, and when power was removed, the pump would quickly stop.
Clear as A & B
At the plant, transmission mains transport raw well water to the treatment plant, and the water flows by gravity into two clearwells: Clearwell A was built with the original construction has a nominal storage capacity of 225,000 gal.
The newer Clearwell B has nominal storage capacity of 1.3 million gal. The water tower steel structure has a capacity of about 750,000 gal.
Prior to the upgrade, the water tower level was maintained exclusively by on/off control. When the level dropped below a certain point, the pump would turn on and pump water into the system, and when the level increased to a certain point the pump would switch off.
Capacity was underutilized in the aging system as well. Due to dynamic head conditions, maximum flow capacity previously equaled about 3,500 gpm. The maximum delivery was provided by using the two high service pumps over Clearwell A. Flow rate from the pump over Clearwell B was 2,260 gpm. Total maximum flow using one pump from each Clearwell was 3,340 gpm. Prior to the new pump system, the plant used four high service pumps—three within the treatment plant—three over Clearwell A, and one over Clearwell B inside a separate building structure.
When pressure in the system went too high, pipes were prone to breaking, and as in all older infrastructures, there are inherent leaks within the system—higher pressures cause more net revenue water losses through these leaks.
When the system pressure went too low, health concerns came into play due to the possibilities of higher pressure groundwater leaking into the piping system. Prior to the upgrade, Nebraska City Utilities was required to staff the facility 24 hours a day during water tower maintenance periods.
The water tower location also affected supply pressure in the water system. With the pumps on, the pressure at the pumps is higher than the pressure at the water tower. As soon as the pumps turned off, the pressure at the water tower was higher than the pressure at the pumps. Therefore, there was inconsistent pressure due to reversal of water flow from the water tower, through town, toward the pumps.
Extensive analysis and computer modeling indicated that flow direction reversal at the pressure problem areas correlated to the high service pump status. Without pump motor control for gradual startup and stopping, there were hydraulic transient effects in the system. When the pumps were on, flow went in one direction and when they were off, it went in another direction fed by the water tower.
After the installation of the new pump and motor control, much tighter system pressure control was obtained, even during an extended period when the water tower was removed from service for annual maintenance. Clearwell capacity is now efficiently utilized and water is pumped into the system using one high service pump instead of four. As a result, the plant was able to maintain the low turbidity required by the state.
“Following the upgrade, running an even pace through the entire day has solved the level changes,” said Messing. With VFD pump motor control, the water level in the tower can now fluctuate without affecting system pressure.
Pressure measurements taken before and after the installation of the new VFD controlled pumps show that the supply pressure fluctuations improved from major swings to virtually constant pressure, greatly improving the overall reliability of its daily water delivery to residents throughout the Nebraska City area.
Regulating water pressure is commonly done using a control valve. However, VFD control is preferable. A control valve in the system increases the backpressure to the pump, and although it can reduce pressure in the system, it absorbs a great deal of the energy used by the pump resulting in high energy losses. A VFD has the ability to match the speed of the pump to the pressure requirements in the system, thus keeping the pump operating at it’s best efficiency point resulting in much less energy usage.
In the new pumping system, the Danfoss VFD technology, designed for water systems, controls the start-up, shut-down, and maintains the speed of the pump based on pressure in the system.
The city also requested that the VFD have a soft start bypass should the drives be taken out of service.
“With the controlled start and no high in-rush current, the electrical design for supplying power is less costly,” said Messing. When the VFD is in service it also prevents water hammer and its associated problems because it slowly ramps the motor up to speed on starting and slowly ramps the motor down to a stop when the pump is stopped.
One of the secondary goals of the upgrade was to ensure that the new VFD did not cause any harmonic distortion issues with the electrical system.
Harmonic testing after the system was installed showed that the Danfoss AHF 010 filters were able to meet the requirements of Harmonic Guidelines set forth in IEEE 519-1992.