Pump clogging solutions for wastewater

Published:  06 March, 2015

It appears that once more the issue of throughlet size has come to prominence, as pump users are again searching for solutions to the problem of pump clogging in wastewater applications. Mike Hutchinson, systems engineering manager at Xylem, explains.

The most important facet of a pumping system is its ability to pump wastewater without clogging. In recent times there has been an increasing number of queries from pump users questioning throughlet size as a method of ensuring clog-free pumping in wastewater applications. In spite of this heightened level of enquiry, throughlet size is, in reality, not an accurate measurement for the clog-free operation, particularly in today’s modern wastewater systems.

Many in the industry believe that the myth of throughlet size had been put to bed years ago, but it is clear that there is still some outdated and non-factual information in the marketplace. Arguably, there is a danger of end-users and specifiers being misled.

Throughlet size is determined by the largest diameter of a solid object that can pass through the smallest section of the pump, the passage through the impeller.

There have traditionally been several types of impeller-design options available for wastewater pumping, from these the singe-vane impellers and the vortex impeller designs have historically been favoured on raw sewage applications.

Both of these designs have some negative qualities. Single-vane impellers can have difficulty with balancing causing significant rotating radial forces which cause high shaft and bearing loads. This can result in increased noise and vibrations levels with relatively low levels of efficiency especially with larger throughlets.

Vortex impellers, also known as recessed or torque-flow impellers, are known to be very inefficient.

The logic behind throughlet size being integral to a pumping system’s efficiency dates back to around 1915 when pump manufacturers believed that clogging could simply be avoided by having a throughlet large enough for most normal sized objects to pass through.

The expectation back then was that a large throughlet would increase the reliability of a pumping system and reduce unplanned service calls. However, decades of research has proven that this simplistic logic is false, confirming that the myth of throughlet size being the key criteria for clog-free pumping is flawed. In spite of this, it is still prevalent in wastewater pump procurement specifications.

More to the point, modern wastewater rarely contains hard spherical objects that are as large as the inner diameter of the piping system, such as stray pieces of brick or metal. What’s more, if such materials were to appear within a pumping system, it would be unlikely for them to reach the pump itself as they should become trapped on a flat horizontal surface where the liquid is stagnant or the carrying velocity is low.

The build-up of raggy material can also significantly decrease the flow rate of the pump or, if an excessive amount of raggy material is accumulated, cause the pump to completely stop altogether due to the motor being overloaded. Such occurrences will significantly affect the overall pumping system efficiency and if left untreated, will result in increased energy bills for the end-user as the pump is required to work much harder in an attempt to complete the same operation.

In short, the ability of a pump’s impeller to effectively negotiate the build-up of long fibrous materials is arguably much more important to pump station efficiency than a pumping system’s throughlet size.

The success of advanced wastewater pumps which use a hydraulic self-cleaning design again confirms that throughlet size is irrelevant to the efficiency of a modern day wastewater pumping station.

That said, it’s important to bear in mind that whilst some in the industry claim that hydraulic designs which use vortex impellers are self-cleaning, it is not strictly accurate.

Without a specific self-cleaning design the raggy material can reappear and continue to block the impeller vanes even if back flushing the pump has initially cleared the impeller.

Hydraulic self-cleaning pumps are far more effective at negotiating the build-up of raggy material and solids which land on the impeller as they incorporate backswept leading edges in a radial (as opposed to axial) orientation.

These materials are instead pushed towards the periphery and out of the pump through designed relief mechanisms and unique impeller geometry.

Ultimately, the biggest threat to efficiency for modern pumping systems is the build-up of raggy material on the impeller, not throughlet size. The installation of pumps with hydraulic self-cleaning mechanisms can reduce both the number of blockages and noticeably improve the overall efficiency of a pumping system. As such, throughlet size should not qualify as a measure for pump users and suppliers when appraising inefficient pumping stations.

For further information please visit: www.xyleminc.com

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