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Drives prove big saver for Irish wastewater plant10 November 2010The Leixlip water treatment plant in County Kildare is saving more than half a million Euros a year on the electrical consumption of its pumps, thanks to the installation of variable speed drivesLeixlip is the second largest water treatment facility in Ireland. With a capacity of 175,000 cubic metres per day, it supplies over 30 percent of the drinking water requirements of the Greater Dublin Region, serving Fingal, Kildare and the northern part of Dublin City.
There are six pumps at the site, in two sets of three - one duty and the other standby. In each set of three motors, two were controlled by soft starts and one was controlled by a variable speed drive (VSD). The control strategy was to vary the speed of the duty pump via the VSD and bring in the others as required. In fact, all three motors were running constantly to keep up with demand. Moreover, the pumps were throttled by butterfly valves to maintain the required head. Leixlip plant supervisor Pat Nolan was prompted to find a more energy efficient approach because costs had risen substantially.
Contractor AECOM (formerly Earthtech) and ABB were asked to investigate and come up with a solution to reduce the plant’s energy consumption. The investigation found that the three motors had an absorbed power of 1,690kW, a total made up of two soft start motors drawing 620kW and the VSD driven motor drawing 450kW. The design team concluded that major savings could be achieved by opening the valves and employing four more VSDs.
Four 710kW ABB cabinet built drives were subsequently installed to replace the existing soft starts and these now work in conjunction with the existing drives installation. Prior to installation of each new drive, its associated motor was sent off-site to be fitted with insulated bearings in order to protect them from damaging bearing currents generated by the drives. In addition the soft start control logic and safety relays had to be transferred to the new VSDs. Like the drives installation and motor upgrade before it, all this additional work was executed in a way that ensured the Leixlip plant remained fully operational at all times.
Following the drives installation, all three motors run at the same speed, providing a flow of 5,700 cubic metres per hour; the balance of the plant’s capacity is routed via other methods, such as gravity feed. The new absorbed power is 1,326kW, a saving of 364kW. With 24 hour, 365 day operation, this aggregates to 3,179,904 kWh savings per year. With energy costing 0.16c per kWh, the total saving is Euro 508,784 per year, assuring a payback time for Leixlip of less than a year. Pat Nolan is more than happy with the outcome:
“The savings were very significant and even more than we expected,” he says. “Another benefit was the reduced wear and tear on the discharge valve through using the variable speed system.” Meanwhile, John Conboy, ABB’s drives and motors manager says: “The initial control logic at Leixlip made sense when the plant was built; however, with energy costs increasing all the time, it is certainly worth investigating further opportunities for efficiency. The savings here are incredible”
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