VSD installation and commissioning Borehole pump
Installation of the borehole pump VSD allowed the flow rate to be controlled by changing the pump speed, thus removing the need for the throttle valve, and thereby reducing the power consumption of the motor.
The VSD was rated at 27 kW output. This unit was selected as it was envisaged that the motor loadings would not exceed 27 kW when operating under the new regime.
Speed control of the pump was achieved by manual adjustment of a potentiometer which provided the VSD with a signal representing the desired pump speed, and hence giving the required flow rate. Automatic control was considered and then rejected as being unnecessary; changes in pump speed were only required to compensate for pump wear or to meet new abstraction allowances.
Raw water pump
The VSD installed was rated at 15 kW, which was also the power rating of each of the raw water pumps.
Control of the ring main water pressure was achieved by varying the speed of the pumps in response to a signal from a pressure transducer, which was fitted in the ring main. This pressure transducer was fitted along with the VSD as a complete package to enable automatic control to be achieved.
Energy saving
Monitoring was initially performed with the system operating under the original conditions and then again under VSD control.
Recordings of energy consumption from the monitoring programme revealed that substantial energy savings had been achieved by each of the installations.
The borehole pump showed a reduction in unit consumption of 79 300 kWh/year, which was equivalent to a 30% saving.
The raw water pump produced even larger savings of 111 750 kWh/year corresponding to an 88% reduction in consumption.
A combined saving from the two installations of 191 050 kWh/year was therefore achieved. This was equivalent to an overall reduction in consumption of 48%.
Economic analysis
Savings achieved by the installation of the two VSDs have been substantial, resulting in short payback periods. Installation of the borehole VSD cost €/$ 6750. The energy saving achieved was worth €/$ 4000 per year, resulting in a simple payback for this installation of 19 months.
The raw water pump VSD cost was €/$ 4600. Comparing this capital cost against savings of €/$ 5700 achieved by the installation gives a simple payback of 10 months.
Combined project savings were €/$ 9700 per year, giving an overall payback period of 14 months.