VSDs payback in two years

By fitting VACON 100 FLOW ac variable speed drives (VSDs) completely eliminated ragging issues  at a sewage treatment works.
By fitting VACON 100 FLOW ac variable speed drives (VSDs) completely eliminated ragging issues at a sewage treatment works.

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 achieve large energy savings complemented by significant reductions in downtime. The energy savings alone will cover the cost of the VSDs and their installation in less than two years.

Thames Water continually assesses all areas of its operations to find opportunities for saving energy. As part of this exercise, the company determined that replacing the fixed-speed soft start used to control the main inlet pumps at Newbury Sewage Treatment Works (STW) with three separate VSDs would provide important energy saving benefits.

For assistance in carrying out this project, Ricky O’Sullivan, Technical Coordinator, Thames Water Swindon/Newbury Area Waste Operations, approached Varicon (3D) Drives, the company that provides support and repairs for all of Thames Water’s VSD and soft start installations. Varicon (3D) Drives responded by proposing a competitive solution based on three of the latest Vacon 100 Flow VSDs, each rated at 55 kW.

High operating efficiency

Developed specifically for pump applications, Vacon 100 Flow VSDs combine high operating efficiency with film-capacitor technology for long life and lowest 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 the Newbury STW application, Thames Water had 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.

Varicon (3D) Drives undertook the supply and installation of the Vacon VSDs. 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 completely eliminated ragging issues with the pumps at Newbury STW, and overall process control has been significantly 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.

“The project was completed with a minimum of disruption to our operations,” said Ricky O’Sullivan, “and we were extremely impressed both with the specification of the Vacon drives and the quality of the work provided by Varicon (3D) Drives. We’ll certainly hit our anticipated payback period of two years based on energy cost savings, but we’re actually getting much more in the way of benefits, and these benefits are sure to increase further over the traditionally challenging winter period.”