NW goes from rags to no hitches

Northumbrian Water had been suffering pump blockages at a sluice due to ragging - but an automated solution was found, writes Katie Williams of drives manufacturer Control Techniques

One of the most costly problems facing water companies is that of ragging of the pump impellers. It is a long-standing nuisance, which eats up thousands of hours of maintenance time in sewage pumping stations and wastewater treatment plants around the world. Clearing a blocked pump can be a dirty, unpleasant task - and the cost of can easily run into hundreds of pounds or more, involving a maintenance team and sometimes a crane.

Downtime may extend to several days, during which back-up systems are under additional pressure. A total system failure can result in effluent leakage with implications for the environment, health, clean-up costs and breaches of legislation.

Co-operation between Northumbrian Water (NW) framework contractor Byzak and variable-speed-drives manufacturer Control Techniques, part of the Emerson group, has created a solution which automatically reduces the problem and which requires no human intervention.

Andy Laundon, M&E general manager at Byzak, explains how the new system works. "We were approached by Northumbrian Water to offer a solution to the problem of pump blockages at Seaton Sluice," he says. "We developed a control philosophy which included different pump-operating routines for freeing the impeller as soon as any load change is detected within the pump system to prevent a potential blockage.

"When we looked at the pump drives market for a suitable product to meet our demanding list of requirements, the one with the closest match was Control Techniques' Unidrive SP."

Many of Unidrive SP's attributes matched Byzak's specification, two, in particular, proving crucial to meet the speed and accuracy needed:
  • True load torque is measured in real time
  • The powerful internal PLC has a reaction time that is measured in microseconds
The project team approached Control Techniques to develop a software package to satisfy the specification and to run in the Unidrive SPs on-board PLC, the SM Applications module. Its key feature is detection of potential ragging at an early stage, before it can become a problem. But this is just part of the solution.

The project requirements include the software taking into account static and dynamic heads in the pump installation and factoring in pump characteristics, water condition and other parameters. The new system is capable of detecting load torque changes as small as 1-2%, indicative of potential ragging. But the torque change trigger value is also user definable to accommodate specific pump characteristics relating to pump size and impeller.

Further features include trend analysis of changes over a long period, indicative of small build-ups and early diagnosis of drive or pump problems. When the PLC within the drive recognises a change in the pump preset torque profile, it initiates a set of procedures designed to clear the impeller. These procedures are multi-programmable, and are tailored to suit client specification and individual pumping station operational requirements. If this procedure fails to clear the problem and, for example, total blockage occurs, then an alarm is initiated.

The engineer can remotely access the drive by ethernet using software tools to assess the situation and perform manual operations. In addition, the system includes programs for routine pipe scouring, which involves running of the pump at full speed to flush through pipe work.

At Seaton Sluice, near Whitley Bay, communication between drives using Control Techniques' drive-to-drive network gives 100% redundancy in the event of a blockage or failure.

Pump manufacturers have tried to solve ragging problems with different impeller designs with limited success. Others have previously attempted monitoring of motor current for other applications. This solution is encompassed totally within the Control Techniques' Unidrive SP AC drive. The same programming is equally applicable for any pump up to 1.9MW.

The particular application at Seaton Sluice comprises two 160kW Unidrive SP drives, both fitted with SM Application modules and communicating with each other via Control Techniques' own high-speed network CT-Net. Each drive controls a single pump, and these are configured in duty and standby mode. The drives integrate with NW's existing telemetry system so that performance can be monitored remotely.

"It's working very well," says Bob Dixon, framework manager at NW. "Seaton Sluice is a critical pumping station and any blockage means immediate call-out. Before refurbishment, regular blockages were experienced- often once a week - and operating expenditure was becoming unacceptably high. The Byzak/Northumbrian Water project team, working with Control Techniques, came up with an extremely effective solution. Since completion of the project, no pump blockages have occurred and the station is operating to the satisfaction of everyone at Northumbrian Water."

Andy Laundon says: "Only time will tell its long-term effectiveness - but trials to date are very encouraging, and we believe it will significantly reduce the number of pump blockages and lead to lower maintenance costs."

The first installation at Seaton Sluice has suffered no blockages in the nine months since installation, where regular blockages were experienced - often once a week.

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