For engineers and plant managers in the water industry, pump selection has long been a complex and time-consuming task, usually involving the collection and analysis of significant volumes of information with a myriad number of parameters to take into account.
Accurate assessment of head and flow, identification of the most suitable generic type or types of pump based on fluid properties and process requirements are needed for the preliminary selection.
Coupled to this are pump efficiency, long-term energy, maintenance and service costs – key information to ensure that costly mistakes arising from inappropriate pumps for the job or poorly-performing pumps are avoided.
For many purchasers the sheer complexity of assessing such a wide range of variables has often meant that the focus has been on the most immediately identifiable and transparent area of energy costs.
However, with the advent of more sophisticated assessment tools which can cope with the input of a wide range of complex variables to generate more accurate analysis of whole life costs (WLC) for pumping technology, purchasers are increasingly aware that using energy consumption as the main determinant can significantly skew selection of the most appropriate, cost-effective pump.
Energy use, frequently used as the benchmark in the decision-making process, often masks hidden costs, which if taken into account, would render some choices uneconomic.
Accurate WLC analysis looks at all the components that make up the total cost of pump ownership in the context of how the pump operates as part of the overall system. This includes both the internal components of the pump itself and external ancillary equipment. As a result, conventional approaches to pump selection are changing significantly.
This is particularly relevant in the context of peristaltic pumping technology - reflected in its increasingly wide uptake across the water and wastewater industry.
The traditional approach of viewing WLC primarily in terms of energy efficiencies is now increasingly being replaced with a far more comprehensive analysis which includes other factors such as maintenance, reliability, failure rates and replacement parts of associated equipment. One of the key changes is that the industry is now making its assessment in a more systematic and informed way.
In recent years, water and wastewater companies have become more aware of the significant benefits peristaltic pumping technology offers over conventional solutions in a wide range of pumping applications and treatments. To state the obvious, low energy costs are irrelevant if pump components fail in the face of day-to-day operational requirements such as day-in, day-out delivery with minimal downtime, high reliability and low, straightforward maintenance needs.
Peristaltic pump technology undoubtedly lowers the total cost of ownership, both in terms of direct costs, including pump/motor assembly, all ancillary equipment and installation costs, and indirect costs, including spare parts, maintenance-related labour and associated energy consumption.
Compared with other positive displacement pumps, peristaltic pumps, with only one wearing part – the hose – are very inexpensive. Even the hose itself is built to last, consisting of a thick, reinforced wall to cushion abrasive or sharp materials which enable the pumps to cope both with high viscosities and handle up to 25percent soft solids.
Take into account the fact that they can run dry indefinitely, they are true ‘dry’ self-priming to 9.5metres, reversible and have 100percent volumetric efficiency, and the sums quickly start to add up. The pumps are seal-less with no seals to leak or wear out – often one of the main complaints from plant operators. Likewise they incorporate no check valves to clog or wear out – again one of the operators’ key complaints about unplanned maintenance, repair work and downtime where conventional pump solutions often fail to deliver.
The simple design of peristaltic pumps also plays a key role in significantly reduced downtimes and servicing/maintenance costs – replacing a single hose takes only minutes. Contrast that with hours or days spent on dealing with planned maintenance and unplanned pump failures, not to mention the added complications of craning or lifting the complete pump out.
A real application
To look at one specific application, sodium hypochlorite dosing, the benefits are immediately clear.
Peristaltics are impervious to vapour locking, which commonly occurs as this type of chemical dosing is prone to gassing-off. Mechanical diaphragm pumps are both complex and expensive to repair, requiring long periods of downtime. They are also inaccurate at lower turndown ratios.
In comparison, peristaltic tubing pumps are both simple and very inexpensive as only the tube needs to be replaced – and this can take as little as four seconds. In addition, they are impervious to vapour locking and have massive flexibility with up to eight tubes and six ‘gangable’ heads per pump.
Likewise solenoid diaphragm pumps, while inexpensive, are 'throw-away pumps' which typically do not allow for rebuilds. They are also still fairly complex pieces of equipment, incorporating diaphragms, seals, ball check valves which can clog and wear out and so forth.
Again, similarly inexpensive peristaltic pumps are emphatically not throw-away pumps – the only working part which needs to be changed is the tube. Add to this the significant differences in relative accuracy – solenoid pumps only have a 5percent±accuracy rate, compared to highly accurate peristaltic pumps with ±0.5per cent. Other key advantages include reduced footprint, with peristaltic solutions frequently taking up significantly less space over conventional routes, and the ability to integrate peristaltic technology with virtually every SCADA system.
Listing the type of ancillary equipment that is not incorporated into peristaltic pumps highlights both the intrinsic simplicity of the technology itself and, crucially, the numerous potential failure and reduced efficiency points which form part of other conventional solutions:
- No run dry sensors, switches, gauges.
- No check valves in piping.
- No sealing water flush systems.
- No flow meters because the pump itself is more accurate.
- No expensive metallurgy.
- No artificial back pressure valves.
So where are peristaltic pumps now being used in water and wastewater treatment processing? To sum up, typical applications include metering and transferring of a wide range of harsh or aggressive materials, including sodium hypochlorite, sodium bisulphite, ferric chloride, lime slurry, hydrofluorosilisic acid, carbon slurry, sodium hydroxide, potassium permanganate, aqueous ammonia, alum, methanol and a range of polymers.
The ability to demonstrate that this flexible and inherently simple technology represents the genuine lowest whole life cost option means that peristaltic pumps are increasingly being chosen over conventional solutions.
Looking ahead to the not too distant future and the likely requirements of the Water Framework Directive for significant levels of additional treatments, more and more companies will undoubtedly be choosing peristaltic technology.
Philip Boulton is Water Industry Specialist at
Watson-Marlow Bredel Pumps. For more information, visit www.watson-marlow.co.uk