Novel pump control overcomes ragging problem

One of the most costly and longstanding problems facing water companies is that of "ragging', or the fouling of pump impellers, which consumes thousands of hours of maintenance time in sewage pumping stations and wastewater treatment plants throughout the world.

As well as it being a dirty, unpleasant task, the cost of clearing a blocked pump in a dirty water facility 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 time back-up systems are under additional pressure. A total system failure can result in effluent leakage with implications for the environment, human health, clean-up costs and breaches of legislation.

Now, thanks to co-operation between contractor Byzak Limited (Framework Contractor to Northumbrian Water) and variable-speed drive manufacturer Control Techniques, a solution has been developed which automatically reduces the problem and which requires no human intervention.

Andy Laundon, M&E General Manager at Byzak, explains: "In most instances, this will stop rag build-up developing to a point where the pump jams completely. 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!"

In fact the first installation at Seaton Sluice, near Whitley Bay, has suffered no blockages at all in the six months since installation, where previously regular blockages were experienced - as often as once a week

How it works

Laundon describes how the new system works: "We were approached by Northumbrian Water to offer a solution to the problem of pump blockages at Seaton Sluice and 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 the Unidrive SP's attributes matched Byzak's specification; two, in particular, proved crucial to meeting the speed and accuracy needed. First, unlike others, the Unidrive SP measures true load torque in real time. Second, because the Unidrive SP has a powerful internal PLC with a reaction time measured in microseconds.

Applying Byzak's control philosophy in conjunction with Northumbrian Water's operational knowledge, the Project Team approached Control Techniques to develop a software package to satisfy the specification and to be suitable for running in the Unidrive SP's on-board PLC, the SM Applications module.

Early detection

Its key feature is detection of potential "ragging' at a very early stage, before it can become a problem, but this is only part of the total 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 per cent, which can be 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. Trend analysis of changes over a long period, indicative of small build-ups and early diagnosis of drive or pump problems, are further features.

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 the above fails to clear the problem (for example, if a total blockage occurs), then an alarm is initiated. An engineer can then remotely access the drive by Ethernet using Control Techniques' software tools to assess the situation and perform manual operations.

In addition, the system includes programs for routine pipe scouring, which involves running the pump at full speed to flush through pipe work. At Seaton Sluice, communication between drives using Control Techniques' drive-to-drive network gives redundancy in the event of a blockage or failure.

Some pump manufacturers have tried to solve ragging problems with different impeller designs, but with limited success; others have previously attempted monitoring motor current for other applications. The new solution is claimed to be unique and is encompassed totally within the Control Techniques Unidrive SP AC drive. The same programming is equally applicable for any pump right up to 1.9MW.

Seaton Sluice system

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 Northumbrian Water's existing telemetry system so that performance can be monitored remotely.

Bob Dixon, Framework Manager at Northumbrian Water, comments: "It is working very well. 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 concludes: "This is an excellent example of modern technology providing a cost-effective solution to a longstanding water industry problem, giving significant improvements in performance, as well as cutting downtime and maintenance call-outs. Working with Control Techniques through a real teamwork approach has been a pleasure,"