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Scottish Water loses its 'rag'

04 May 2011

The fouling of waste water pump impellers - or 'ragging' as it is widely termed - is one of the water industry's more costly asset management issues. But with the introduction of innovative drive software, this costly phenomenon and its attendant dirty and potentially hazardous maintenance requirements might just become a thing of the past

At a time when water companies are under increasing pressure to improve services, against a background of increasing operating costs and rising energy bills, extreme weather events and rising ecological expectations, optimisation of asset performance has become critical. One costly and disruptive problem - 'ragging' - has been identified as one particular area in need of such a review.

A frequently occurring problem, with almost all types of impellers, is that fibres and other particles tend to build up on the impeller vanes. Generally, this build-up of rags (ragging) causes a decrease in the pump's hydraulic efficiency, increasing power consumption. Ragging can cause a total pump blockage if no interim remedial action is taken.

This long standing pumping problem necessitates manual clearing - a costly and unpleasant task requiring a maintenance team and, more often than not, specialist lifting equipment. Downtime arising from ragging may extend to several days, during which time the 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.

Various solutions claiming to overcome this serious problem have been introduced, some more effective than others. However, one company - Control Techniques (CT) - has developed a software package for pump drives that is already proven effective for a number of water companies in the UK and Ireland. Scottish Water, for one, is happy to endorse the technology, as we shall see later in this article. But first, how does CT's software work? The package, Intelligent Pump Control (IPC) is described by CT's UK water manager, Brian Redpath:

"IPC is unique in that it measures 'live' active current in real time, on-board the drive. Unlike competitors' systems that measure the motor's nominal current - a measurement that can give an error of up to 30-40% for required effective monitoring of impending ragging. Our system measures active current (impeller true torque component) every millisecond.  As soon as IPC sees a change in the active current set profile, an automated cleansing cycle is instantly initiated to clear the pump impeller."

IPC accurately monitors pump performance, including flow and pump speed where appropriate. The live data is compared with measured values taken during commissioning, plus expected pump profile characteristics. Any 'out of profile' performance is instantly detected, giving an early warning indication of ragging.  Should the pump 'out of profile' performance extend the profile settings, an automatic drive based cleansing cycle is initiated to clear the pump impeller. The system is totally embedded within the drive itself, so no external PLC programming, monitoring or control equipment is needed. Brian Redpath again:

"Every site is different. IPC is totally flexible, and can be configured to meet customer's applications, working equally effectively on wet and dry well installations and also treatment works pumps. As well as detecting and preventing ragging, IPC systems also provide several other intelligent performance optimisation functions, such as surge prevention to protect rising mains, and wear monitoring of pumps. And in the unusual event of a major blockage that cannot be automatically cleared, successive cleansing routines are prevented to protect the pump by performing a system lock-out and raising a telemetry alarm."

The Scottish Water experience
Scottish Water's Levenhall Sewage Pumping Station had significant issues with pump blockages and was considered the worst site for blockages in its south-east operating area, with blockages and ragging often needing to be dealt with two to three times a week.  It was thus identified as an ideal site to trial CT's IPC in conjunction with an 'off the shelf' CT variable speed drive (VSD).

While pump designs to deal with rags have improved over the years, no centrifugal pump currently on the market can be guaranteed to deal with all rags under all conditions, and increased frequency of blockage has been a persistent problem for VSD controlled pumps, often offsetting the clear energy-saving benefits.

Because the increased risk of blockages can be a significant factor when using VSDs for (unscreened) wastewater pumping, the 'tried and tested' fixed speed pumping solution has remained the favoured design approach over the years. Good design and pump selection generally ensures reliable operation with few blockages.  Hence, blockage avoidance through a traditional design approach, and selection of blockage resistant pumps, has generally been considered to be more important than energy savings. Scottish Water's senior project manager, Innovation & Technology, Graeme Moore takes up the story:

"At Scottish Water, we have tried PLC-based solutions, with success. However, this type of bespoke control package has to be provided over and above the VSDs - a costly solution that is only viable where it is not cost-effective to replace the drives. One interesting alternative has been put forward by Control Techniques - their IPC software. This is integral with the VSD, so doesn't require any bespoke control package and can be provided as a standard VSD option. The drive monitors active current to determine very small variations in torque, which then triggers a reversing cycle to break up rags as they begin to form on the impeller."

A pilot project commencing mid-2010 at Levenhall initially saw the installation of a single CT VSD on one of the existing pumps, essentially to investigate whether the energy usage and the occurrence of blockages could be significantly reduced. Improving efficiency and reducing the frequency of blockages and partial blockage was the main aim of this pilot project.

Positive results were achieved within the first week of the trial. All pumps at Levenhall, with the exception of that controlled by the VSD equipped with IPC, became blocked and had to be lifted to clear blockages on two occasions during the week. The VSP/IPC controlled pump, meanwhile, remained clear of blockages. Moreover, the average running current of the latter was between 15 to 30% less than the other pumps at the plant. Indeed, a monthly log of operating currents clearly indicated that the VSD controlled pump was operating much more efficiently than the other three pumps at the site.

Running currents were observed to be up to 40% more on the pumps that did not have any blockage detection and control associated with them, although operating currents between 10 and 25% higher was more typical. From the available data, Mr Moore believes that the average energy consumption at the site could be reduced by up to 15% per annum, if all pumps were controlled by drives equipped with IPC.

Within four months of the initial trial, all four pumps at Levenhall were fitted with CT VSDs equipped with IPC. Based on his observations at Levenhall, Mr Moore has subsequently recommended that all VSDs used for raw sewage pumping should be specified to have pump blockage detection and control as standard.

IPC - two levels to suit your needs
Control Techniques has developed two levels of IPC functionality: IPC Lite and IPC Pro, to meet the needs of the broadest range of pump applications. IPC Lite provides economical ragging detection and cleansing on pumps rated at less than 132kW, while IPC Pro is a fully-featured, multi-pump control system for pumps rated to 1.9MW.

IPC Pro offers anti-ragging program, duty control, fault status and back-up, 'dry run' protection and cleansing routines. It is compatible with telemetry systems and includes a watchdog alarm feature. Options include: HMI to provide customers with a clear window on their pumping control operations, plus an instant data logging facility for pump efficiency and energy usage; Ethernet TCP/IP for remote monitoring and operation, and drive based remote telemetry station capability.

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