Propeller pumps have never been high on the list of pump choices for WwTP managers. Such pumps were not considered to be up to the task of pumping sewage, with its high rag and fibre content. Now all that has changed, thanks to ITT Flygt’s self-cleaning design, which cuts the risk of clogging without sacrificing hydraulic performance.

Propeller pumps, with their capacity for pumping large volumes of water, have long been used for drainage works and stormwater and floodwater control. “Now, with this new technology, we’ve opened up many new applications,” explains Göran Bruske, ITT Flygt’s marketing manager for pump applications for waste and process water pumps.

The company launched the revamped propeller pumps, along with the new N-pump at the end of 1998. Since then, the pump has proved itself in several new applications. “We are presenting a new way of thinking when it comes to wastewater pumps,” says Bruske. “With this technology, propeller pumps become suitable for return sludge pumping in treatment plants where you don’t need a high pump head.”

The nine models in the P-pump range offer a broad power output up to 477kW. Drive units can be matched to a selection of hydraulic ends to provide the optimum choice of pump for any specific application. “We offer a series of modular drive units so that we can cover a wide power range. That opens up a broader range of applications, not only pumping stormwater, raw water and flood control, but handling sludge, industrial effluent and wastewater,” says Bruske.

The new propeller-pump series was launched in North America in 1998, and in 1999 ITT Flygt launched the new product in Europe and the rest of the world.

There are three key elements to the new propeller-pump design with the relief groove as the most important. First, self-cleaning guide vanes enable the fibres to loosen up from the guide vanes to avoid clogging. Then, the engineers further backswept the blades to allow the fibres to slide out to the periphery of the blade. As the fibres slide away, they come in contact with the ring with its relief groove, which releases the debris from the blades.

Ulf Arbeus is the ITT Flygt engineer behind the design of the new propeller pump. His work on the propeller pump ran parallel with his design of the N-pump, an open type of impeller. Both pumps were launched simultaneously. But the redesign of the propeller pump posed a tough challenge, says Arbeus.

“Propellers have been of interest to us for a long time because of the problems with objects getting stuck on the leading edges of the blades. That situation is much more disastrous for a propeller pump than, say, a centrifugal pump, because the power output jumps sharply when the objects get caught on the blades. If only this fundamental drawback could be addressed, the P-pump would be the perfect pump for low-head applications,” he explains.

While backswept propeller blades have been around for some time, no satisfactory solution had been found for the problem of contaminants accumulating at the periphery of the blade. ITT Flygt’s answer was the relief groove, which smoothly handled the problem without sacrificing efficiency.

Arbeus and his team put their theories to the test by building a prototype and installing it at a WwTP near Stockholm. It ran continuously for 10 weeks with steady and efficient power consumption. “With this new design, we can dispel the notion that propeller pumps just can’t handle tough clogging problems at treatment plants,” says Arbeus.

Bruske stresses that while clog resistance and efficiency are certainly important advantages to the new design, reliability is the major feature. “Maintenance and operation of pumps is the highest cost at pump stations. With the new propeller pump, we can offer the lowest total cost for the entire life cycle of the system.”

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