Pump manufacturers have traditionally believed that pump clogging can be avoided by making the internal pump throughlet equal to or larger than what can be flushed down a toilet. A new white paper, however, states that the impeller type can play a significant role both in preventing clogging and saving energy. Compared to a single-vane impeller and a vortex impeller, the non-clogging Flygt N-impeller can cut energy usage by 25 percent to 64 percent, respectively.
Clogging is a critical and highly undesirable operational problem in wastewater pumping. It drastically reduces pump efficiency and causes pump tripping, which results in increased costs and more emergency calls from the end user. Throughlet size – that is, the diameter of the free passage through the impeller of the pump – is a parameter that is frequently used to specify the clog resistance of a wastewater pump.
“A long time ago, the concept of throughlet size was invented as a measurement of clog resistance,” says Mats Karlén, Hydraulic End Design Manager at Xylem. “Manufacturers sought to maximize the size of the throughlet in order to improve clog resistance. However, decades of experience have shown that these designs still suffer from substantial drawbacks.”
White paper on clog resistance
Mats Karlén and his colleague Stefan Abelin recently published a report on clog resistance and throughlet size in order to address what they believe is a common misperception about wastewater pump design. Their report states that research for several years has found that in wastewater there is “a very low probability of finding large, hard objects compared to small, hard particles and various small and large soft and stringy organic objects.” The low probability of large, hard objects, according to the report, reduces the importance of a large throughlet size.
In addition, the report finds that stringy objects, which are much more common, “tend to get caught in traditional single-vane or vortex impeller types even if the throughlet size is large.” Fibers tend to wrap around the leading edge of the impeller vanes and the debris will not dislodge, instead continuing to build up.”
This build up results in soft or partial clogging, meaning a decreased flow rate and decreased efficiency. As a result, input power increases and once the running current exceeds the trip current, the pump is shut off. This is called hard clogging.
A design that prevents clogging
In the late 1990s, Xylem – then ITT Corporation – introduced a new impeller design solution, known as Flygt N-technology, with a self-cleaning design. The impeller has substantially backswept leading edges so that as solids land on them they are pushed to the periphery and out through a relief groove.
According to Karlén and Abelin’s report, the self-cleaning design increases the efficiency and clog resistance of the pump, which “results in minimized costs for operation, service, and maintenance.” During Karlén’s 14 years at Xylem, he and his colleagues have performed hundreds of clog tests – both at Xylem’s laboratory and in field installations – to compare different pump designs.
Both Xylem and its competitors still sell pumps with conventional large-throughlet designs, such as single-vane impeller and vortex impeller pumps. However, Karlén hopes that the report will lead the industry beyond energy-consuming and unnecessarily large throughlets towards more efficient and reliable solutions.
“The global need for solutions to transport and clean water is constantly growing,” says Karlén. “Helping to find ways of doing this more effectively and with improved energy efficiency is both a massive challenge and extremely rewarding. It’s clear that the amount of energy required to transport water could be significantly reduced by delivering improved sustained efficiency and higher clog resistance.”
Findings from Xylem’s clog tests
The N-technology offers energy savings* of:
– 25 percent, compared with a single-vane impeller pump
– 64 percent, compared with a vortex impeller pump
* Based on Xylem’s research, which takes into account these three key parameters to overall pump efficiency:
1. Clean water efficiency
2. “Real” wastewater efficiency, measured in terms of the presence of partial clogging and the effect this has on overall operational efficiency
3. The cost of emergency call-outs due to a clogging-related pump stop.