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Keeping waste water pumps clean24 October 2011Waste water from households and industry contains particles and fibres that are not removed by screen filters. This contamination will build up on the impeller vanes of wastewater pumps, leading to poor efficiency and extensive pump wear. The maintenance of these pumps can be expensive and time consuming, often requiring special lifting equipment. So-called ‘ragging’ is a serious ongoing issueRagging and its deleterious effect on pump impellers is a problem that affects many waste water treatment facilities. It is maintenance intensive and the maintenance work is labour-intensive, dirty and potentially hazardous. Fortunately, new developments in drive software technology are set to assign these problems to the past. One of these is a recent drive software module from Danfoss.
This so-called ‘Deragging’ software is now featured as standard on the Danfoss VLT AQUA drive, which ensures that highest pump efficiency is maintained and impeller wear is minimised, while allowing for programmable ramps and time delays. This feature optimises the efficiency of the pump by constantly monitoring the motor shaft power consumption relative to flow and reacts with a reverse spinning cleaning cycle when a deviation from the power consumption of the system design is recorded. So, how does it work?
Proactive cleaning - This intelligent system is fully flexible and can be optimised to meet application needs. Using the Danfoss ‘Deragging’ feature you can perform proactive cleaning – in other words, the user can determine when the system should react, be it at start command, stop command, timer based via a digital input (every morning at 08.00 for example), via serial communications or using a number of logic rules that can be programmed. All parameters are grouped together so programming is easy.
Reactive cleaning - During operation, a power monitoring system will ensure that a cleaning cycle starts when the pump takes more energy from the drive than it normally should, indicating that it is already starting to clog. Ideally, clogging should be prevented by adopting proactive cleaning. However, in the Danfoss VLT AQUA drive, both reactive and proactive cleaning regimes can be combined.
It is recommended that proactive cleaning is programmed by the user and that automatic reactive cleaning takes place as and when necessary between proactive cleaning cycles.
Cleaning pattern
The impeller is cleaned by running the pump quickly in the opposite direction. The user can define how many times the drive should spin the impeller in reverse, at what speed and for how long. The drive display gives a clear indication of the status of the ‘Deragging’ function.
A note of caution here: users should be aware that not every pump can be run in reverse and the operator should always check with the pump manufacturer before specifying this type of control.
Modern drives technology not only reduce lifecycle (running) costs, thanks to their energy saving capabilities, but now with dedicated software features they can tackle everyday problems such as ragging. This avoids the need for additional hardware that might otherwise be required to protect pump motors, and significantly reduces the frequency of an unpleasant and costly maintenance procedure.
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