sand and grit)
8.2 Best Available Technology (BAT)
The best available technology can be identified according to the best component technology available, meaning technology on:
• Impellers
• Casing
• Wear rings
• Bearings
• Motor and control (power drive system, extended product) 8.2.1 Clean water pumps
Clean water pumps are generally very similar in terms of design options. With clean water there is little risk of clogging or blockage, therefore there is no reason for having a high clearing between the blades. Since a low clearance gives the highest energy efficiency, clean water pumps are almost exclusively designed with a low clearance and a high number of channels. As such, there is no differentiation between most of the standard designs of clean water pumps between manufactures. However, minor improvements in design are archived with minor design modifications. Some of the most energy efficient designs come at a compromise with other parameters such as head, pressure and net positive suction head required (NPSHR) against flow. For these reason, it is possible to find similar water pumps where one pump has slightly higher energy efficiency than the other. In the preparatory study for the current regulation191 this was also observed:
“With many years of feedback, an established manufacturer should have arrived at close to the optimum impeller vane number, vane shape, impeller inlet diameter, impeller
191 Lot 11, page 134
sectional profile, and casing geometry. This should produce an effective compromise between the various curve shapes for head, power, efficiency, and NPSHR against flow.
However, in most cases efficiency could be improved by sacrificing one or both of the ideals of head stability at low flows (e.g. by using a smaller diameter impeller), or NPSHR at best efficiency flow (e.g. by using a smaller impeller inlet diameter).“
For clean water pumps, which are subject to the Regulation 547/2012, the product efficiency is ranked according to MEI (Minimum Efficiency Index). In Regulation 547/2012, a MEI = 0.7 is defined as a benchmark value, which means that the pumps that have a MEI > 0.7 are considered to have the best possible pump design. Several pump manufactures are marketing their high efficient water pumps as being MEI > 0.7 compliant192. The difference between MEI = 0.4 and MEI = 0.7 is about 3.5 %-points in energy efficiency193.
In order to arrive at even higher energy efficiencies, the surface roughness of the pumps has to be improved. The surface roughness of the pump depends on the casting method and if the surface is polishing or coated.
Standard pumps are often produced by sand casting of metal (cast iron, bronze, steel, etc.), which is a cost efficient production method and therefore widely used in pump production. Sand casting does, however, result in products with a higher roughness than products made using other types of casting. A reduced roughness of the impeller and the volute can increase the energy efficiency194; however most manufactures find that increased cost of investment casting does not outweigh the benefits.
Only in cases where hygiene is important (food or pharmaceutical industry), manufactures use investment casting to reduce the surface roughness, because smooth surfaces prevent the formation of deposits and thereby easy cleaning. When roughness is important, manufactures often include polishing in the final production stages of the pumps to further reduce the roughness, but polishing can further increase the cost. One manufacturer estimates that the increased cost coming from other types of casting than sand casting and hand-polishing is between 5% and 15% of the total cost of the pump.
For most manufactures, it is possible to increase the energy efficiency of the pumps, but any larger improvement requires a change in the production method and an increase in the total cost of the pump. Most manufactures choose not to do so, because they do not believe that benefits will be higher than the increase in the cost.
Corrosion and erosion are common problems for water pumps. Corrosion is occurring when there is direct contact between metal and water. Corrosion is most severe in cast iron impeller that pumps cold water. Stainless steel is often used instead of cast iron due to its resistance to corrosion. Stainless steel is protected against corrosion by a protective
192 For example the Wilo-Stratos GIGA and the new Sulzer SNS
http://productfinder.wilo.com/en/COM/product/00000026000219d40002003a/fc_product_datasheet
https://www.sulzer.com/de/Newsroom/Business-News/2015/150916-Sulzer-Launches-the-New-SNS-Process-Pump-Range?type=blank,
193 http://europump.net/uploads/Fingerprints.pdf
194 SAVE study on improving the efficiency of pumps, AEAT for European Commission, 2001, page 37-40.
passivation layer. Provided this passive film stays undamaged, corrosion rate will be very low. If the film is damaged, localised corrosion can still occur. 195,196
Erosion can occur when substrates in the water meet the surface with high velocity. In clean water the amount of substrates are in general low, but erosion can also occur as cavitation. Cavitation is a result of a pressure difference in the fluid and is most commonly observed on the impeller, in particular at the low pressure surfaces.197
A method to both reduce surface roughness and protect the metal against corrosion and erosion is to coat the surface of the impeller and the casing interior with a smooth resin.
There are several coating materials that can be used including PTFE, FBE, rubber linings, glass flakes, epoxy etc. One type of solvent-free epoxy coating, Belzona®1341 Supermetalglide, has been thoroughly test on water pumps.198 This is shown in Figure 22, where the difference on efficiency from coating is relatively small compared to the total efficiency, and where difference on performance (i.e. head) is observed minimal.
Figure 22. Influence of coating (Belzona®1341 Supermetalglide) on efficiency and performance199.
Pump efficiency could be improved by reducing the leakage at the wear rings when reducing the clearance. This would require most or all of the following, which would increase costs:
• Tighter manufacturing tolerances
• Increased shaft diameter to minimise contact and wear at reduced or increased flow, which would also require fitting of larger bearings and seals
• Very hard but compatible wear ring materials (e.g. Tungsten carbide).200
195 Lot 11, page 135
196 Coating technology increases pump performance. Maillard, J. (2008). Belzona Polymerics Ltd.
www.belzona.com.
197 Ibid.
198 Ibid.
199 Ibid.
200 Lot 11,
Specific options for multi-stage clean water pumps201
The efficiency of each impeller in a multistage pump tends to depend on the width of the stage, where a wider stage correlate with a higher efficiency. Furthermore, a higher number of stages will normally mean a higher efficiency for the same duty point. However, manufactures tend to limit both width and number of stages to reduce cost and size of the pumps.
Individual stage efficiency could be improved by using outward flow or outward/inward flow diffusers. This also means stage numbers would increase and therefore the size of the pump.
Extended Product Approach
Motor technologies and the application of power drive systems (i.e. motors + VSDs) are more important to increase the energy efficiency of the pumps when looking at the extended product. However, still most pump manufacturers only choose operating motors according to minimum requirements (i.e. IE3) and without VSDs. A few manufacturers are advancing to high efficient motors (i.e. IE4) for their pumps or using their IE2 or IE3 motors in combination with VSDs. Best available technology for motors can be considered to be IE4 motors such as the KSB “SuPremE” motor202. But when looking at BAT for clean water pumps, it is the use of VSDs for variable flow applications which could be already an advantage without having to buy more efficient motors. In spite this is not affected directly by the pump’s design, the effect the power drive systems have on clean water pumps provide great opportunities for energy savings.
The use of VSD with clean water pumps is still not a standard practice, even though about half of the clean water pumps could reduce their energy consumption significantly if applied with a VSD. But some manufacturers203 routinely sell clean water pumps with VSD and it is definitely possible to acquire a pump with VSD. As it could be seen in section 7.1, many clean water pumps are not yet taking advantage of using VSDs in variable flow applications.
8.2.2 Self-priming pumps
Self-priming pumps are able to operate when the pump case is not filled completely with fluid but contains some air, or air slugs, and they have the function of overcoming the air-bind problem (where air stops the pump from being able to pump the fluid). Three main types of self-priming pumps include liquid recirculation chamber types, compressed air and vacuum self-priming pumps204. The most common is liquid self-priming pumps. These pumps overcome the air-bind problem by creating a vacuum effect, using the impeller, in the chamber that sucks fluid through the suction line into the chamber of the pump case.
Once fully primed, and with no air in the chamber, the fluid is pumped 205. This is shown in Figure 23 where on the left side, the self-priming centrifugal pump is in priming mode with a mixture of air and fluid circulating and creating a vacuum which pulls fluid into the
201 Based on information provided in previous preparatory studies Lot 11, Lot 28 and Lot 29.
202 http://www.ksb.com/SuPremE
203 For example Grundfos CME and Xylem VFLO
http://www.grundfos.com/products/find-product/cm-cme.html
http://www.xylemflowcontrol.com/marine-and-rv/flojet-water-pressure-pumps/sensor-vsd-pumps/42755-series-vflo-50-gpm-19-lpm-water-pressure-pumps.htm
204 http://www.waterworld.com/articles/print/volume-28/issue-10/departments/pump-tips-techniques/considerations-for-centrifugal-pump-priming.html
205 http://www.gongol.net/knowledgebase/selfpriming/
chamber of the case. On the right, in pumping mode only fluid is pumped once the pump is fully primed and no air is in the circuit.
Figure 23. Self-priming centrifugal pump in self-priming mode (left) and pumping mode (right)206.
Self-priming pumps are designed slightly differently than non self-priming pumps. For example, in the most common liquid self-priming pumps they often have a priming chamber and air separation chamber in the casing (see Figure 23 at the top of the chamber) in order to make the self-priming work, and air must be able to be released from the pump, e.g. through a valve207. Furthermore, the self-priming pumps will have the suction centreline above the centreline of the impeller (see Figure 23 which shows the impeller at the bottom).
Self-priming pumps generally have a lower efficiency than non self-priming pumps due to doing more liquid turns during pumping and the close clearance between the impeller and the casing volute tongue in a water-primed self-priming pump208. The close clearance is required in order to achieve the self-priming function effectively. Furthermore, the recirculation self-priming pumps have a swan neck on the suction to retain liquid around the impeller which also introduces hydraulic losses and lowers the efficiency of the pump.
Overall, self-priming pumps are designed and purchased for the self-priming functionality and have a narrow application, e.g. emptying a water tank. They are often utilised for relatively short usage periods. Self-priming is the most important aspect, even if the efficiency of the pumping is lower, since they are used in situations where fluids are being pumped upwards where priming failures cannot be tolerated.
Self-priming pumps can pump many types of fluids in different location including clean water and swimming pool water but they can also pump fluids with solids and be primed with different methods209. Ideally one should subtract the priming function to establish the efficiency when it only pumps fluids however because the self-priming function is applicable to a wide range of centrifugal pump types and in situations where there is a diverse range of self-priming types, including liquid recirculation chamber types and compressed air and vacuum self-priming pump. Different self-priming pumps have different self-priming capabilities and efficiencies, and categorization by application or technology would lead to
206 http://www.fao.org/docrep/010/ah810e/AH810E07.htm
207 http://www.acdrive.in/difference-between-self-priming-and-centrifugal-pump-589158.html
208 http://www.waterworld.com/articles/print/volume-28/issue-10/departments/pump-tips-techniques/considerations-for-centrifugal-pump-priming.html
209 http://www.pacificliquid.com/selfprimer.pdf
multiple categories and this would lead to a complex analysis. It would be very difficult to determine an average fluid/air mixture or to determine the average pump flow time. This means that it would be very difficult to determine an average energy efficiency during priming and pumping. In addition, self-priming pumps are often utilised in short time periods for special purposes.
It can be concluded that there is no harmonised definition and design of self-priming pumps since they are often designed and purchased for specific usage and many manufacturers have their own designs which differ from each other. Furthermore, the pattern on how long and when the pump performs the priming function is widely diverse. This affects their efficiencies as well.
It is therefore difficult to categorise them into one category. To achieve this two main aspects would have to be harmonised: 1) types and designs and 2) how they operate in their self-priming and pumping function.
8.2.3 Wastewater pumps
• In order to fit wastewater application, industry has developed a variety of closed and open channel impellers, as well as vortex and special impellers which can fit the pumping needs. The most widely used are listed in the next paragraphs.
Multi-channel impellers
These impellers are used for two purposes210140:
• Collection and transport of wastewater from far areas when wastewater is not so polluted.
• Activated sludge where wastewater characteristics are carefully controlled for process optimisation where wastewater does not have big objects and its composition is quite homogeneous.
Multi-channel impellers can be open or closed and are usually for handling wastewater with no big objects nor highly abrasive solids and are usually very efficient being the most efficient closed multi-channel impellers. These impellers can manage high flow rates, and according to a simplified classification provided by industry stakeholders, they could be defined as for ‘Light duty wastewater applications’ (see Table 30). ’Light duty’, according to information provided by KSB, refers to handling wastewater with suspended solids but not big objects such as clothes, cans, plastic bottles, wood or metal parts. A concrete definition is missing, but according to information provided by KSB, the definition is an ongoing discussion by the Lot 28 working group at Europump. Some examples of multi-channel impellers are shown in Figure 24.
210 Communication at IFAT 2016 fair.
Figure 24. Examples of multi-channel impellers211. Source: presentation and exhibition at IFAT 2016212.
Single channel impellers
These impellers can be open or closed and are used for wastewater types containing big solids (depending on the pump’s passage) and/or abrasive materials such as fat or other solids containing corrosive substances. Furthermore, other impellers are designed to handle gases (in the liquid), which disturb the hydraulics of the impeller making the pump less efficient. These impellers can manage moderate to high flow rates, and according to a simplified classification provided by industry stakeholders, they could be defined as for
‘Heavy duty wastewater applications’ (see Table 30). Some examples of multi-channel impellers are shown in Figure 25.
Figure 25. Examples of single channel impellers213. Source: presentation and exhibition at IFAT 2016214212.
Vortex impellers
These impellers are used for lower flow rates but for wastewater that has high content of suspended solids (although not big objects). These impellers have a functionality that creates a vortex in the water avoiding direct contact with most of the abrasive or damaging materials to the impeller. Vortex impellers are usually less exposed to wear so they maintain their functionality (including set energy efficiency levels) longer without the need for replacement. The applications of these impellers is slightly different to that for ‘Heavy-duty applications’, as these impellers cannot manage high flow rates but prevent more clogging. Therefore, they could be defined for ‘Special heavy-duty applications’ (see Table 30). Some examples of vortex impellers are shown in Figure 26.
211 Impeller at the left is an example of a closed multichannel impeller for pre-filtered wastewater and for activated and digested sludge.
212 http://www.ifat.de/index-2.html
213 The second impeller from the left is an example of a single channel impeller for the same applications as multi-channel impellers, but with the possibility to handle raw wastewater (depending on the pump’s passage).
214 http://www.ifat.de/index-2.html
Figure 26. Examples of vortex impellers215. Source: presentation and exhibition at IFAT 2016216212.
Axial flow impellers
These impellers are designed for very high flow rates which most commonly have a poor ability avoid clogging. They are therefore mainly suitable for relatively clean wastewater at big treatment plants or rain water catchments before the water collects many solids. See an example in Figure 27.
Figure 27. Example of axial flow impeller. Source: presentation and exhibition at IFAT 2016217212.
Special impellers for high solids contents and/or big objects
These are typically channel impellers with special functionalities such as grinding and/or shredding, which can have this functionality already integrated in the impeller, and are therefore sold as one unit, or can be sold as separate unit. These impellers could be defined for ‘Special heavy-duty applications’ (see Table 30). The impeller itself will typically have the same energy efficiency of a normal channel impeller, but the grinding capability is very energy consuming and can be as high as twice the pump’s energy consumption and will therefore drastically reduce its overall efficiency. In some cases, the use of a vortex pump would be a more energy efficient solution.
Other parameters that influence pump performance for improving it to higher levels, in particular:
• Energy consumption
• Reliability
• Ease of maintenance
215 The last impeller to the right is an example of a vortex impeller for the same applications as channel impellers, but with the possibility to handle all types of raw wastewater , slop wastewater and wastewater with coarse particles.
216 http://www.ifat.de/index-2.html
217 Ibid.
• Clog resistance
• Wear resistance
Often the optimal design is a compromise between these factors, and a wastewater pump’s efficiency depends on its reliability and maintenance. Wear decreases the energy efficiency and reliability of the pump over time, while proper maintenance reduces the effect of wear.
Clogging and other failures have a high impact on the life cycle cost as they reduce the availability of the pumps and could potentially be dangerous in some systems. Therefore, reliability is always a fundamental design parameter for wastewater pumps, while energy efficiency is secondary. Reliability (seen as the degree on which the pump is able to stay fully functional) is important because of the cost of sending out a maintenance crew (including energy cost for transportation) and the cost of interruption of the operation.
In Lot 28 it was found that the best energy efficiency for wastewater pumps with channel impeller are 88.7 % and for pumps with vortex impeller are 63%. This however does not mean that the best available technology for wastewater pumps are pumps with an energy efficiency of 88.7%, since the best design depends on the application (see Table 30).
Other technologies for improving energy efficiency
Usually the choice of impeller type depends on the wastewater type. There is usually a
Usually the choice of impeller type depends on the wastewater type. There is usually a