Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | NOVEMBER 2015 | 33 In the know Digging deeper: wastewater pumps Stopping the Block Free passage remains the key design element for wastewater pumps to avoid blockages, but there is much more for specifiers to consider when choosing the perfect pump By JameS Brockett EDITOR WATER & WASTEWATER TREATMENT F or wastewater pumps, which are required to deal with all kinds of effluent, reliability is key: the ability to keep running without clogging or blocking is the central demand for the equipment. Increased levels of fibre in today's sewage, as products such as wet wipes are flushed down the toilet, have added to the challenge that has always been posed from solid objects accidentally entering the waste stream. With wastewater pump blockages a frequent factor behind pollution incidents that have led to six-figure fines for water companies in recent times, it's a crucial issue for utilities' bottom line. So what are the design features that can prevent blockages – and given the AMP6 emphasis on Totex, how can they be weighed up against other considerations such as energy use? Free passage – the size of the largest spherical object that could pass through the pump impeller without touching the sides – is traditionally the measure of how clog-resistant a pump is. The concept, which has been around for at least a century, is that if the space available through the impeller is larger than anything that can be flushed down a toilet, the pump should not block. Of the three most common impeller types – single vane, multi- vane and vortex – vortex pumps provide the largest free passage, followed by single vane, with multi- vane impellers the lowest. However, there is a trade-off between free passage and hydraulic efficiency: vortex impellers are most inefficient and use the most energy, while by contrast, multi-vane impellers are most efficient, but usually have the lowest proportional free passage. "Raw sewage is unpredictable and you never know what is going to be in it, so when it comes to free passage, bigger is always better - providing the geometry is right on the impeller," says Andy Wilson, Hidrostal. "You only get parity, 100mm free passage out of 100mm, on a vortex pump, and they are not efficient at all. If you could have that with a high efficiency, that would be the ultimate pump, but nobody makes it – yet." Efficiency considerations Specifiers o•en play safe and opt for the largest possible free passage. Hidrostal offers single-vane screw impeller pumps with a free passage of 300mm – bigger than a basketball – which are an appropriate choice for asset-critical, inlet pumps close to the treatment works. Since a degree of compromise is always involved with pump choice, the size of the pump is a key variable when making decisions about efficiency and design, explains Wilson. "Sometimes on smaller pumping stations you can obsess about saving half a kilowatt on a small pump, but if it blocks every week, you'll just lose that money spending three or four hundred pounds for every site visit or rag out," says Wilson. "So for those smaller pumps, yes, energy efficiency is a nice-to-have but the main thing is you don't want to block the pump. For larger pumps, when you start getting over 20 or 30 kilowatts and beyond, energy starts to play a bigger part if it is utilised quite a lot." While it used to be common for pump buyers to specify 100mm minimum free passage, some pump manufacturers, notably Xylem, have marketed pumps with adaptive ends that can expand when they Free passage is defined by the size of spherical object that could pass through the impeller. This Hidrostal pump has enough free passage to pass a football

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