TO STOP CLOGGING, THINK SMALLER, NOT BIGGER

These objects can cause damage, often jamming the mechanism or slowing it down. The results are never good, ranging from reduced performance to total shutdown, pump tripping, and motor burnout.

Engineers are very aware of clogging. Yet there is a lingering misconception on how to prevent clogging based on the size of large objects in wastewater streams. The real culprits behind clogging are persistently small ones, namely synthetic fibres and threads.

A Century-old rule

The pump world uses a century-old rule of thumb to determine clogging risks, namely that the pump’s throughlet (the passage containing the pump’s impeller) must be equal to or larger than any object that can be flushed down a toilet. Yet researchers have demonstrated that this “impeller passage diameter” rule has become less accurate in preventing clog situations.

“The biggest issue with the throughlet rule is that it focuses on large and hard objects as the main culprits behind clogs and impeller damage,” says Chetan Mistry, Xylem Africa’s strategy and marketing manager. “But a lot of research, including work by Xylem, shows that this idea is misdirected. Hard and heavy solids tend to settle in low-flow parts of pipe systems and rarely reach pumps. They are not the ultimate source of clogging problems.”

However, the rule is not redundant, says Mistry: it does help to have a generous throughlet size. Such designs have sustained relatively good pump practices for decades.

Yet enlarging throughlet sizes create a considerable drawback. Such designs need different impeller designs that can operate with the larger area and avoid damage from large objects. Single-vane impellers and vortex impellers are the best options, but are also inefficient, harder on components such as bearings, and prone to becoming unbalanced.

The synthetic fibre problem

These performance sacrifices may be in vain because they still do not address the primary cause for clogging, namely cumulative synthetic fibres. Today, nearly two-thirds of produced fibres are synthetic. Wet wipes, cleaning rags, fibres such as nylon, vinyl, polyester, or rayon: many consumers flush these products down the toilet, or the fibres detach during washing cycles, entering the wastewater stream.

“Synthetic fibres are a pump’s version of death by a thousand cuts,” says Mistry. “Small bits of fibre snag onto the impeller. At first, there isn’t an obvious impact. But as the fibres collect on the impeller and its shaft, they start to slow it down and eventually create a tangle that damages the pump.”

The solution

Fortunately, there is a solution. Impellers with substantially backswept leading edges push solids to the periphery, where a relief groove in the insert ring ejects the matter into the pump discharge channel. This selfcleaning system – first pioneered in Flygt N-Technology pump solutions –actively removes the different types of solids that otherwise contribute to clogs.

Depending on the environment, these newer impeller designs are between 25 and 64 percent more efficient than alternatives such as single-vane or vortex impellers. Xylem’s engineers have tested the concept repeatedly, resulting in a whitepaper that explores the new challenges behind clogging.

“The global need for solutions to transport and clean water is constantly growing,” says Mats Karlén, the paper’s co-author. “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.”

De-clogging modern pumps

As Mistry concludes, to avoid clogs don’t focus on the throughlet size. Large objects are not the problem; buildups of fibres are the leading causes behind clogs. The right impeller design makes all the difference, keeping wastewater pumps running at their best for longer.