Changing three 180 kW, fixed-speed re-circulation pumps to variable speed inverter drive control has saved Yorkshire water over £60k pa in energy use.

The filter re-circulation pumps at Yorkshire Water’s Esholt Waste water Treatment Works are used to pump final effluent to the filter distribution chamber, diluting the effluent and providing the required wetting rates for the filters.

Three 180 kW, fixed-speed re-circulation pumps operate on a continuous duty/duty/standby cycle. Normal operation is for two pumps to be operated continuously providing a re-circulation flow of approximately 1,000 litres per second. Operating the two pumps this way costs £235,000 per year.

Recent operational changes at the site have meant that the load to the filters is substantially reduced and some of the filters have been de-commissioned. As part of its plan to reduce electricity consumption by seven percent by 2015, Yorkshire Water wanted to review the flow rates of these pumps to determine if there was any scope for savings.

Better energy savings than expected

Investigations showed that the pumps were efficient, but that flows could be reduced to save £23,500 per annum. More detailed process calculations indicated flow reduction should be possible, giving a calculated saving of £50,800 per annum.

The project involved installing 200 kW ABB low harmonic variable-speed drives (VSDs) on the three re-circulation pumps. The project included building and supply of the drives in panels to replace the existing soft start units and installation of insulated bearings on the existing motors.

Once the installation was complete, a series of trials reduced the speed of the VSD driven pumps every two weeks and monitored the effect. Power readings were taken to assess the savings that would be achieved on an annual basis if the pumps were continually operated at that speed.

It was found that running the drives at 45 Hz produced the optimal flow performance and this would  generate savings of over £6o,000 p.a. In addition, the ability to vary the flow rate according to demand provided Yorkshire Water more operational control than with fixed speed operation.

Go to 999 Inverters for more information on ACS800 and ACS880 inverter drives