The installation of two ABB Ultra Low Harmonic (ULH) drives, along with changes to the control philosophy on the facility’s inlet screws, is helping Anglian Water’s Bedford Sewage Treatment Works to save upwards of £1,500 per week in reduced energy costs.
Anglian Water was facing challenges with the inlet Archimedes screws at its Bedford wastewater treatment plant. Due to reliability issues, the screws were routinely consuming between 4,000 and 5,500 kWh per day. As a result, the screws’ soft starters were operated manually, however this pushed energy consumption even higher.
Archimedes screws are constant torque loads, and are often run at a fixed speed. As a result, variable speed drives were thought for many years to have little to offer in the way of energy savings. Furthermore, introducing variable speed control to these applications requires a careful balancing act. Running the screw too quickly will only succeed in stirring the sump rather than lifting material out of it, while running too slowly is also inefficient.
The two screws at Bedford Treatment Works previously used soft starters. Because the screws were run at a continuous fixed speed, this meant that during periods of low flow or dry weather the system was oversized for what it needed to handle, resulting in wasted energy. When the sump was full, the submerged portion of the turning screw would stir the sump rather than lift it, again reducing efficiency. Archimedes screws also need to be primed on start-up by filling them with liquid. When the screw is stopped, this liquid runs back down to the sump and will need to be lifted again once the screw is running again.
Anglian Water approached ABB Value Provider Inverter Drive Solutions Ltd (IDS) to see if a change in control philosophy could help matters. IDS were confident that using drives to control the screws would allow them to be kept at an optimal speed at all times, achieving substantial energy savings while significantly improving reliability and reducing failures.
After running the calculations, IDS estimated that installing two drives and adapting the screws’ control philosophy could save £1,500 per week in energy costs (at 0.19p per kWh), with no adverse effect on the operation of the screws or any other plant processes.
Two 200 kW ABB ACS880-37 ULH were delivered to site and installed. ULH drives were chosen to keep harmonics down. As well as permitting the use of smaller cables, the drives’ lower harmonic distortion reduces wear on electrical equipment, and provided additional headroom for the site’s G5/5 compliance. New mains and motor cables were installed, and the existing panels were modified by IDS to accommodate the new equipment. A level controller was also installed to optimise the control of the screw pumps depending on the sump level. In practice this means that the screws achieve fewer revolutions, which reduces energy usage while prolonging the life of mechanical components.
Following the installation, energy usage dropped from 4,000 to 5,500 kWh per day down to around 1,000 kWh. Additionally, the application’s operating hours were reduced from 48 hours per day across the two screws to around 20 hours per day, with no adverse effect on the process. As well as reducing mechanical stress on components and improving reliability, this also enabled Anglian Water to identify which of the two screws was the more efficient and allocate more duty hours to it. Regreasing intervals and bearing replacement intervals have both been significantly increased as a result.
According to Jake Lawrence, Project Manager at IDS, the project met all its goals and more: “A lot of customers with Archimedes screws on soft starter control end up with frequent gearbox issues, and find that they have to do more maintenance than they’d like. For many years drives weren’t thought to be a good match for Archimedes screws, yet this is one of several projects we’ve carried out in recent years where we’ve shown that drives can offer significant improvements in both efficiency and reliability.”
“Anglian Water already suspected that the application had energy-saving potential, hence calling us in,” continues Lawrence, “But the actual savings surpassed all expectations. As well as substantially reducing maintenance costs, the drive also provides a smoother flow into the works. The project was completed on time and well within budget, and the site was even able to continue operating while the work was carried out.”
