One of the most costly problems facing water companies is that of 'ragging', the fouling of pump impellers, a long-standing nuisance that eats up 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 drives manufacturer Control Techniques, a solution has been developed which automatically reduces the problem and which requires no human intervention.
"In most instances, this will stop rag build up developing to a point where the pump jams completely," says Andy Laundon, M&E General Manager at Byzak Limited. "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 nine months since installation, where previously regular blockages were experienced -- often once a week
Andy Laundon 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 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 Unidrive SP's attributes matched Byzak's specification, two, in particular, proving crucial to meet the speed and accuracy needed. Firstly, unlike others, the Unidrive SP measures true load torque in real time. Secondly, because Unidrive SP has a powerful internal PLC, which has 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 SPs on-board PLC, the SM Applications module.
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%, 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.
