Variable speed drives reduce STW energy costs

Severn Trent Water (STW) is said to be saving around GBP100,000 per year on energy costs following the installation of ABB variable speed drives (VSD) at its Wanlip sewage treatment works.

STW was suffering low flow rates on its dry well flow pumps, with typical rates being less than 400 litres per second against a design of 550 litres per second.

This meant that under storm conditions the works was unable to achieve the rates specified under the Environmental Agency's consents without the need for supplementary pumping - further adding to the company's operating costs.

Sentridge Control, a member of the ABB Drives Alliance, had solved a similar problem at another of STW's treatment works and was asked to provide a solution at Wanlip.

Glen Hickman from Sentridge, said: 'We suspected the problem was due to ragging of the pumps and reversed the flow of selected pumps to prove this.

'Ragging is where rags foul the pump inlet and prevent the pump from operating normally,' he added.

The original installation had four direct-on-line pumps and two pumps controlled by VSDs.

One of the VSD driven pumps was a duty pump while the other was used to assist as pumping requirements demanded.

Historically, reversing the pumps had allowed them to achieve higher flow rates for short periods of time.

One of the direct-on-line pumps achieved 550 litres per second compared with 325 litres per second previously, while one of the VSD-powered pumps achieved 575 litres per second compared with 390 litres per second.

To solve the problem permanently, Sentridge suggested installing 75kW ABB industrial drives on all the pumps, each equipped with ABB Anti-Jam software, part of its Intelligent Pump Control (IPC) software.

An add-on to ABB industrial drives, IPC contains all the common functions needed by water and waste utilities, industrial plants and other pump users.

The Anti-Jam software module performs a number of cleaning cycles every time the pump starts.

Each cycle consists of a series of rapid ramp-ups in both forward and reverse directions.

Taking one to two minutes to complete, the cleaning cycle removes the debris from around the pump volute, preventing it from entering the pump and blocking it when the pump ramps up from zero to its normal operating speed.

The cleaning cycle is also started when the VSD detects a drop in pump efficiency.

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