VSDs eliminate ragging problems in waste water pumps

17 December 2013

By fitting VACON 100 FLOW ac variable speed drives (VSDs) to the three main inlet pumps at Newbury sewage treatment works (STW), Varicon (3D) Drives has made it possible for Thames Water to achieve energy savings, complemented by significant reductions in downtime. The energy savings alone will cover the cost of the VSDs and their installation within two years.

Thames Water is always looking ways to make energy savings, in all areas of its operations. As part of this exercise, it identified that replacing the fixed-speed soft start used to control the main inlet pumps at Newbury STW with three separate VSDs would offer energy saving benefits.

Ricky O’Sullivan, technical coordinator at Thames Water Swindon/Newbury Area Waste Operations approached Varicon (3D) Drives to discuss possible solutions. Varicon (3D) Drives proposed a solution based on three of the latest VACON 100 FLOW VSDs, each rated at 55 kW.

Developed specifically for pump applications, VACON 100 FLOW VSDs combine high operating efficiency with the film-capacitor technology to ensure long life and low total cost of ownership. The drives also incorporate a range of pump-specific features as standard, including anti-ragging and cleaning options, provision for multi-pump control and a real time clock that facilitates time-stamped logging of operating parameters and fault information.

In this particular application, Thames Water found that using soft start control for the main inlet pumps caused large surges to the inlet screen system and, because of the small inlet size, this would weir over to storm. This resulted in a significant amount of rag moving through the plant, causing pump blockages throughout the site.

The inlet pumps were, in fact, subject to rag blockages on an almost daily basis, and the recent replacement of the pumps did not completely resolve this situation. Varicon (3D) Drives was, however, confident that the anti-ragging and cleaning features provided as standard by the VARICON 100 FLOW VSDs would be the key to an effective solution.

Because of the lack of available space and difficult cable access with the existing motor control centre (MCC) the new drives were fitted adjacent to the MCC. This presented no problems as the version of the drive chosen has an IP54 ingress protection rating.

The new drives have eliminated ragging issues with the pumps at Newbury STW, and overall process control has been improved. There are no longer surges when the pumps start as their acceleration is closely controlled and this has resulted in significant downtime reductions across the whole site.

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