Efficient Pumps And VFDs Bring Fast Payback On A Rhode Island Plant Upgrade

A Rhode Island town expects payback of 3 1/2 years on an extensive project to upgrade inefficient pumps and install variable-frequency drives.
Efficient Pumps And VFDs Bring Fast Payback On A Rhode Island Plant Upgrade
New motors and pump at the Marshall Avenue Booster Pump Station. New motors with variable-frequency drives were placed on all three pumps at this station, and one pump (painted blue) was replaced with a smaller pump suitable for lower flows.

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Easing the ratepayers’ burden and shrinking the carbon footprint were the twin objectives of the Cumberland Water Department’s modernization and upgrade of four pumping stations, including the main water treatment plant.

Improvements to date have earned an incentive payment from the local utility and slashed electricity consumption to the point where the town expects to recoup its $235,000 investment in 3 1/2 years.

In summer of 2012, Cumberland collaborated with the Rhode Island Public Energy Partnership, whose mission is “to achieve deep energy savings in state and municipal facilities.” That summer, the town signed on with National Grid’s Commercial and Industrial Energy Efficiency Program. National Grid, a multinational electric and gas utility headquartered in the United Kingdom, supplies electricity to some 470,000 customers in Rhode Island.

More efficiency, less consumption

Located in the northeast corner of the nation’s smallest state, due north of the capital city of Providence, Cumberland has about 35,000 residents. In early 2012, the Water Department set out to improve water-pumping efficiency and issued a task order to determine the best opportunities for energy savings, with planning and engineering assistance from the Woodard & Curran engineering firm. The task order encompassed:

  • Reviewing information about the efficiency improvement grant program to identify potential upgrade funding.
  • Collecting available pump and motor information, including nameplate pump head and flow, motor speed, voltage, horsepower and drive operation.
  • Collecting and reviewing station operational data, including flow, pump head, suction and discharge pressure under various conditions, on-off sequence, valve throttling, electrical consumption and impact on upstream and downstream facilities.
  • Developing recommendations for pump and motor replacement and drive installation to improve operational efficiency at each station.
  • Developing cost estimates for various potential improvements.

Among other improvements, the team recommended that the town replace its 125 hp finished water pump at the Sneech Pond Water Treatment Plant with a 100 hp pump system controlled by a variable-frequency drive (VFD). The plant, at the north end of town, treats Sneech Pond source water and pumps both raw and finished water.

With confirmation from National Grid that the proposed upgrades at the plant and three other facilities would satisfy the requirements of the utility’s energy efficiency program, construction began in the second week of October 2012. The new equipment was installed and operational two months later. The town and the utility then tracked energy consumption throughout 2013.

Multiple upgrades

Sneech Pond, the town’s main water treatment plant, is a 1.0 mgd facility built about 50 years ago. With substantial modernization in 2006-07, when a SCADA system was installed, the facility was up to date with modern process equipment by 2012. The raw water pumping and flocculation, sedimentation and filtration processes were all upgraded.

As a result of the 2006-07 upgrade, the raw water pump and VFD combination is controlled by the SCADA system. The SCADA system and VFD are optimized to ramp pump speed and corresponding flow up and down efficiently, maintaining clearwell levels while accounting for filter backwash demands without hindering filter operation or turbidity removal.

Despite these earlier upgrades, at the Sneech Pond plant and at several locations around the town, water was still pumped inefficiently, using constant-speed motors through flow-control valves. Budget limits at the time precluded replacement of this equipment.

With a renewed focus on energy efficiency, with the support of its engineering consultant, and with incentive-based funding from the utility, the town in 2012 substituted energy-efficient motors governed by VFDs at four facilities. The new motors and VFDs, which operate at a speed appropriate for real-time water demand, replaced pumps that had worked equally hard in low-flow and high-flow periods.

The town replaced one of the old Sneech Pond plant’s finished water pumps with a new, smaller pump, motor and VFD to pump more efficiently to the distribution system at the required flow rate. To maintain redundancy and further improve efficiency, a 100 hp finished water pump was refurbished and a new, high-efficiency motor and second VFD were installed.

A similar upgrade was made at the Marshall Avenue Booster Pump Station, Cumberland’s water purchase point from the neighboring Pawtucket (R.I.) Water Supply Board. Based on historical water demand, two 100 hp pumps at this station were typically throttled back 40 to 80 percent. After the 2012 upgrade, the station can pump a maximum of 4.5 mgd with two new 75 hp motors governed by VFDs. Its current peak demand is 1.0 mgd.

These pumping systems replaced the oversized 100 hp motors whose excess capacity made them energy-inefficient. In addition, a third pump at the Marshall Avenue facility was replaced with a smaller pumping system, also equipped with an energy-efficient motor and VFD, to work efficiently in low-flow winter periods.

The Manville Well No. 1 and No. 2 pumping stations, about 100 yards apart, supply a combined 800,000 gpd to the system. Two new 60 hp motors with VFDs operate both pumps at peak efficiency. The new equipment replaced outdated, inefficient pumps and significantly reduced energy consumption.

Replacing constant-speed motors and flow-control valves with VFDs on energy-efficient motors yielded three benefits:

  • A one-time $109,000 incentive payment from National Grid
  • Lower monthly electric bills
  • Smaller carbon footprint

Tracking power usage

With energy-efficient motors and VFDs in place in December 2012, the town and the utility tracked electricity usage throughout 2013, and National Grid paid the $109,000 energy efficiency incentive in January 2014.

Electrical savings were 41 percent (about $17,500 per year) at the Sneech Pond plant. Based on lower flow rates and pressure head requirements, energy savings at the three other pumping stations were lower than at Sneech Pond, but still measurable and significant.

The combined savings will continue year after year and, along with the utility incentive payment, will allow the town to recoup its total investment in 3 1/2 years. And the efficiency enhancements to date are not the end of the story: The Water Department continues to seek efficiencies that will deliver potable water to residents reliably and at the lowest cost.

About the authors

Chris Champi (cchampi@cumberlandri.org) is water superintendent in Cumberland, R.I. Rob Little, P.E., (rlittle@woodardcurran.com) is a vice president and senior project manager with the Woodard & Curran consulting firm in Providence. Mark Donovan is senior commercial energy consultant for National Grid in Providence.


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