Case Study 2: VSD in a wastewater pumping scheme
Case study summary
Energy cost and noise were reduced by replacing fixed speed equipment with a VSD so that pump rate was at the minimum necessary therefore reducing pipe friction losses. Whilst actual monetary figures are not available it is believed the payback period was less than two years.
The station was of the traditional separate wet/dry well design. Wastewater was pumped to the treatment works some 2.5 km (1.5 miles) away. Control was by simple on/off switching of the motors, with the level monitored with a mechanical float.
Installation of VSDs
To monitor the wet well, ultrasonic level instrumentation was installed. This generated a speed reference signal, which increased or decreased the pump speed and also allowed the start and stop levels to be selected. This resulted in the motors running between 73% and 88% duty, which was the optimum speed range for energy efficient operation of the pumps.
Energy efficient motors were also introduced to the system to further improve energy efficiency, as the benefit of high efficiency motors over standard becomes even greater in variable speed operation.
Economic analysis
The old pumps were estimated to transfer 14 m3 of liquid per kWh. After fitting new pumps and drives, but still running at the same speed as the old pumps, this increased to 21 m3 per kWh. With the drives interfaced to the analogue reference signal, this was further improved to 30 m3 per kWh. Monitoring of this value can indicate pump wear or blockage, enabling the service intervals to be optimized.
The efficiency of the wastewater pumping station has more than doubled since the new pumps and VSDs were installed. It is predicted that the savings will be at least €/$ 131 000 in electricity costs over a 20-year life span.
The total electricity consumption per year fell by 53% with 33% coming from the change of pumps and motors and 20% from the introduction of the VSDs.
Other benefits
The installation has reduced noise, which is of importance in the residential area where the station is located.