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Case Studies - Variable Frequency Drives

Close pump control doubles efficiency for West of Scotland Water

The efficiency of West of Scotland Water’s Lomond Street Wastewater Pumping Station in Helensburgh, West Dunbartonshire, has more than doubled since two ACS 600 AC drives from ABB were installed. The company predicts the drives – supplied and installed by ABB Drives Alliance partner EDC (Scotland) Ltd – will save at least $80,000 in electricity costs over a 20-year life span whilst minimising the risk of overflowing, as pump throughput has been increased from 14 to 30m3 per kWh consumed.

“Indications are that the electricity consumption has been reduced by 48% compared to the same period in the previous year”, says Charles McCaig, Electrical Design Engineer at West of Scotland Water’s Planning & Capital Procurement Department.
The station, pumping waste water to the Ardmore treatment works some 2� km away, is of the traditional separate wet/dry well design. The wet well is monitored with ultrasonic level instrumentation, generating a speed reference signal, which increases or decreases the pump speed and allows the start and stop levels to be selected. This results in the motors running between 73% and 88% duty, which is the optimum speed range for energy efficient operation of the pumps.

This arrangement replaces simple on/off control of the motors, with the level monitored with a mechanical float. Flow meters now monitor the performance of the pumps. Combined with the kilowatt-hour readings from the drives, this gives a pumping index for each pump. A decreasing index indicates wear in the pump, enabling West of Scotland Water to optimise the pumps’ service intervals.

The old pumps were estimated to have a pumping index of 14m3 per kWh. After fitting new pumps and drives, but still running at the same speed as the old pumps, this increased to 21m3 per kWh. With the drives interfaced to the analogue reference signal, this was further improved to 30m3 per kWh.

“I estimate that 44% of the savings can be put down to maintenance and 56% to the drives,” says McCaig.

With the number of pumping cycles being increased, the holding capacity of the wet well and combined sewer are able to cope with a greater flow than the old system would allow, decreasing the risk of overflow.

The installation has also reduced noise. The ACS 600 drives feature motor flux optimisation, making the pumps quieter when in operation, which is of importance in the residential area where the station is located.

“Already small decreases in pump speed give large energy savings, when a variable speed drive is used,” explains Allan Murray, Managing Director of EDC, Ayrshire, the company that supplied, programmed and commissioned the system.

“This is because decreasing a centrifugal load, such as a pump, gives a power decrease based on the cube of the load.

“We also supplied energy efficient motors to the system to further improve energy efficiency, as the gap between standard and high efficiency motors becomes even greater in variable speed operation.

“The use of the reference signal from the level transmitter also shows what can be achieved when using drives in conjunction with level monitoring equipment.

“If this type of system can be used for the majority of wastewater pumping stations then this can ultimately be of great benefit to West of Scotland Water’s customers as well as the local environment.”

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