Pump study reveals the cost-cutting choices
The water industry needs to tackle its energy bill. UKWIR's Gordon Wheale and Black & Veatch's Malcolm Brandt and Roger Middleton outline the findings of an international study into how this can be achieved
After manpower, energy is the highest operating cost item for most water and wastewater companies. During the last decade the sector’s energy consumption has increased considerably as a consequence of the need to meet new potable water and effluent treatment quality standards. This trend is likely to continue.
On behalf of UK Water Industry Research (UKWIR), Black & Veatch (B&V) has undertaken an international research project to identify current energy efficient best practices and technologies in the water industry.
The project covered the efficient design and operation of water industry assets for the whole water cycle from abstraction to discharge. The result is a compendium of best practices covering the water cycle matrix and includes variations between regions and continents, large urban and small rural systems and complex high and simple low technical solutions.
International case studies are used to illustrate best practices. This article presents on overview of the study’s findings in the field of pumping.
Pumping represents upwards of 80% of clean water and at least 30% for wastewater energy demand. Until recently some utilities have focused on reducing energy costs rather than energy demand through tariff management, but this approach may have distracted from saving energy, however, national Carbon Reduction Commitments and targets should redress the balance.
Variable speed drives (VSDs) have been installed in some pumping applications to realise energy savings. VSDs enable turn down of machinery to match operating conditions.
Where only one pump is expected to cope with a wide duty range or seasonal or diurnal variations, VSDs are an economical solution. Modern VSDs include power factor management and one case study showed an 83% saving. However, VSDs use power to drive their electronics and typically take 4-5% of the rated motor power. One case study described the removal of an energy wasting throttling valve linked to the replacement of a fixed speed pump with a VSD.
There are also examples of pumps being replaced to improve the efficiency of fixed speed operation thus dispensing with VSDs. If pumps can be accurately sized for their duties, the 4-5% power savings can be added to significant capital cost savings.
One example confirms that for high static head and low friction head system curve, a VSD will have little influence over the pump system efficiency. Furthermore operating one VSD pump with other fixed speed pumps leads to inefficient pump performance due to the best efficiency points not being matched.
For a number of parallel duty pumps, VSDs should be fitted to all, and their speeds should be controlled as one in order to match the pumps’ best efficiency point (BEP) to the duty point. A few studies of borehole pumps have shown that attention to the aquifer draw-down can save energy. One case shows that running the most efficient pump of a group of boreholes in an aquifer gives good results.
A second case study demonstrates that using multiple pumps, each at low flow, instead of running a single pump at the required total flows results in less aquifer draw-down, lower pumping head and thereby energy demand. In other applications sump level adjustments have yielded similar benefits, and most cases demonstrate the value of accurate real time data for aquifer management.
However borehole pumps and their long, small diameter motors are not very efficient. One case study showed that replacement by line-shaft pumps with conventional surface mounted motors can be beneficial.
This may act as an incentive to manufacturers to look at raising the efficiency of their multi-stage pumps and submersible motors. However where speed control is important for aquifer management, modern VSDs can be installed. Historically the older types of VSDs generated significant harmonics and on long cable runs from VSD to borehole pump motor there would be unacceptable losses, but these are now much reduced.
There are few examples of pumps being replaced on energy efficiency grounds. Most are replaced for other reasons including reduced blockages, changed designs, and an incorrect original selection.
The small number of examples reflects the relatively high cost of plant replacement and the payback time involved. Utilities have reported a number of instances where pump refurbishment or replacement had been proposed but shelved due to uncertainty and risk. This approach is likely to change as the cost of energy rises and cost benefit analyses suggest reduced financial return periods.
A very simple modification to belt driven pumps is to replace the Vee belts and pulleys with toothed timing belts which do not wear or lose tension and there is no variation in pump speed through an extended working life. Belt drives can be used with synchronous motors to change the pump speed and align its best efficiency point with its required duty point if fixed speed operation is required.
Scheduling control of water abstraction and fresh water distribution can bring benefits. One case study of a large distribution (supply) area describes the use of real-time software model incorporating electrical costs of delivering different water sources to schedule the use of the least costly sources first and only run more costly resources if necessary.
While this technique incorporates some tariff management, the same principles are applicable to energy saving by optimising the supply-demand balance rather than maximising storage.
Other examples show benefits from optimising the control philosophy of single, albeit complex, stations.
Performance testing has been used to assess the potential for savings, and in most cases, worthwhile savings were demonstrated, however, there is a risk that for some installations there are limited opportunities for efficiency savings and that the testing costs are not recovered.
However one case demonstrated a more cost effective approach was to test a complete zone, thereby identifying those pumps offering the best potential for energy gains. Applying internal coatings to pumps is an accepted practice and is often included as part of a routine or major maintenance overhaul in addition to, for example, replacing packed glands with mechanical seals or sleeve bearings with roller elements on older pumps.
However the consequence of multiple interventions is that the economics of a single intervention becomes masked by the combined effects of all the changes and their costs. However, the converse is that an economically viable payback may not be viable from one intervention alone and that a broader multiple intervention approach may be needed to justify expenditure.
Case studies demonstrate a range of methods for saving energy in pumping across different areas of the industry; some showing significant improvements. The ranges represent a risk of uncertainty not least because of the unique characteristics of each case study, including operational, regional and environmental.
There is a risk therefore in generalising the above numbers and it is recognised that many pumps are operating close to their best efficiency points. In addition, the case studies illustrate the general level of awareness of operational and engineering efficiency which tends to indicate that the potential for further savings may be reducing.
If a pump or impeller change cannot be justified on the basis of energy efficiency alone, it may be worth considering other factors as in the table below. The individual effect is small but the cumulative effect of all these factors could be significant.
Notwithstanding these provisos, it is likely that the industry’s pumping energy efficiency could be improved by between 5 and 10%. This could occur in different areas, for example incremental changes may get close to the best for some pumps, particularly in clean water, whereas other pumps may need significant efforts.
The other possibility, probably more applicable to wastewater, is that minimal investment, for example on portable instrumentation, may lead to operational improvements and indicate where further gains could be achieved through more expenditure for example on maintenance, refurbishment, or replacement with a better suited impeller or complete pump.
To find out more about the Energy Efficiency in the Water Industry report, or to obtain a copy, contact UK Water Industry Research on 0207 344 1807 or firstname.lastname@example.org. UKWIR reports are also available online at www.ukir.org. nnn
Gordon Wheale is UKWIR’s programme manager; Malcolm Brandt is B&V’s global practice technology leader, water supply; Roger Middleton is a technical director at B&V