S-tube by Grundfos

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The S-tube is the only impeller available in the wastewater market that does not compromise hydraulic efficiency or free passage through the pump.

The key to the design is simplicity, with no cutting or moving functions that can get worn over time and hereby ensuring a constant, superior efficiency over time. The S-tube impeller is a tube-shaped impeller placed in a pump housing that matches the smooth tube shape through the entire pump with no obstructions or dead zones through the hydraulic.


The S-tube impeller and the new tube hydraulic through the pump provide:

  • World-class hydraulic efficiency up to 84% without compromising free passage.
  • Spherical free passage up to 160mm, meaning better solids handling and greater non-clogging capabilities.
  • A design as simple and robust as a tube results in longer lifetime and lower maintenance costs.


The difficulties in designing high-efficiency impellers have traditionally been challenges with sealing against backflow, vibrations, abrasive wear and clogging. The Grundfos S-tube impeller meets and surpasses all these challenges.

Furthermore, we have tested the S-tube in the field many hundreds of wastewater installations worldwide prior to release with very good results. What our customers experienced was substantially lower noise and vibration levels, reduced maintenance, and far better non-clogging capabilities than ever experienced previously.


A history of wastewater

Traditionally, wastewater pump owners have faced a trade-off between larger free passage and high hydraulic efficiency. If optimised for free passage for trouble-free operation, the efficiency of the pumps has had to be compromised. Or if optimised for efficiency to lower energy consumption, it has been necessary to narrow the free passage in the pump hydraulics.

Many attempts have been made to increase the reliability level of wastewater pumps, some more successful than others. Various kinds of impellers have been developed over the years, such as vortex, semi-open to single- and multi-channel impellers, and other more sophisticated creations.

Furthermore, these pumps have often been fitted with advanced and costly add-on assemblies that in a static or an adaptive way are designed to lead, cut or treat the wastewater as it passes through the pump. To this we can add the fact that the very nature of wastewater is challenging, with dry solids content varying over time. Water use has not remained static either.

All of these hydraulic layouts have been designed in good faith – but have they resolved the fundamental trade-off facing pump owners?

The answer must be ‘no’. With the S-tube impeller we have resolved these issues:

  • We have resolved sealing issues:
    The unique and patented sealing system provides the highest possible reliability, as the hydraulic parts are kept clean during operation.


  • We keep vibration levels as low as possible:
    A new and unique static and dynamic balancing of S-tube impellers provides the lowest possible vibration level. Compared to earlier pump generations, the vibrations are now reduced to less than half. This means less noise and longer lifetime for bearings and shaft seals, helping reduce maintenance costs.


  • Abrasive wear is considerably lower:
    The smooth design of the S-tube impeller with no sharp edges and bends gives the most optimised flow and the lowest abrasive wear.


  • Non-clogging capabilities:
    The large free passage combined with the smooth design of the channel in the S-tube impeller ensures clogging-free operation in the most demanding applications.


With hydraulic efficiency up to 84% and spherical free passage up to 160mm for small and medium sized pumps, the S-tube is rewriting the history of wastewater!

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