Minimum Efficiency Index: testing its performance

The new European standards and the Minimum Efficiency Index

According to Directive 2005/32/EC and Directive 2009/125/EC for Energy-using and Energy-related Products, the European Commission published Regulation 547/2012 EC and at the beginning of 2016, the European Committee for Standardization (CEN/TC 197) published the standard EN 16480. The minimum efficiency required for a pump to be marketable in Europe was defined by specifying a threshold value of a new parameter: the Minimum Efficiency Index (MEI).

Regulation 547/2012 EC specifies the procedure to calculate the MEI for each type of pump and establishes a specific threshold value to be achieved: from 01-01-2015 the value MEI>0.4 has been imposed, representing the minimum value of the MEI for pumps. The value MEI=0.4 has a clear meaning in terms of product selection: by imposing this target MEI value, 40% of pumps present on the market before 2015 (the year of the market survey) can no longer be sold commercially. Most pump manufacturers were involved in a new design process of their products to provide them with an MEI>0.4.

The procedure for qualification is reported in the Standard EN 16480 that specifies how pumps need to be tested to verify their MEI value. Standard EN 16480 covers a large number of aspects involved in the experimental measurements, ranging from the test methodology to the propagation of errors related to measurement uncertainty and industrial tolerances.

The Hydro Energy Laboratory

CeSMA, (Figure 1), is the new department of Federico II for Advanced Metrological Services and one of the most advanced laboratories of CeSMA is the Hydro Energy Laboratory (HELab) ^[1]. HELab is a third party laboratory for the qualification and verification of pump performance as required by Directives 2005/32/EC and 2009/125/EC. The laboratory is structured with a number of measuring lines with a fully automated signal acquisition. Each measuring line is devoted to a specific pump family and range of pump parameters, according to the pump classification included in Regulation 547/2012 EC. In addition, a specific measuring line is dedicated to submersible pumps which are going to be included in future regulations. The laboratory is equipped with flow meters, pressure cells, level gauges and temperature gauges. The mechanical pump parameters include rotational speed and mechanical torques. The electrical parameters consist in absorbed power, circulating currents and power factor. All measurements are of Level 1 accuracy according to ISO 9906. Figure 2 gives an overall view of the laboratory and of one of the experimental rigs.

HELab qualification tests

According to Standard EN 16480 qualification tests have to be performed according to ISO 9906, close to the pump Best Efficiency Point (BEP), and the test results must be interpreted by comparing the pump efficiency curve with the so called “house of efficiency”, representing a measurement of the minimum required efficiency to obtain the pump qualification. Because of the lack of an independent institution assigned to verify the MEI values specified by the pump industry, many pump manufacturers are sending their products to the HELab to have them tested by a third party laboratory.

Figure 2. The Hydro-Energy Laboratory. 

In addition, we are now performing a research program on the new methodology for the Extended Product Approach, as described in the prEN17038/1 and the prEN17038/2 standards still being discussed bytechnical committee CEN/TC197 in the framework of Directive 2009/125/EC^[2]. Research activity is also undertaken about the effect of sediment erosion on the performances of wastewater pumps.

Test reports include pump data, measured data, characteristic and performance curves, and MEI evaluation. In Figure 3 a data report is shown. In Figure 4 an example of the comparison between the performance curve and the “house of efficiency” for MEI evaluation is given.

Figure 3. HELab test report.

Figure 4. MEI evaluation as a result of a HELab test.

Limits of MEI application

All pump families and pump sizes included in Regulation 547/2012 EC are considered in a unique easy to use qualification method. In the unified procedure for the assessment of MEI provided by Standard EN 16480, MEI calculations for pumps belonging to different families are quite straightforward, but a few products have been found to be penalized. These products, designed for specific uses but very popular on the market, cannot be qualified according to the present standard, in particular because the method by which the MEI threshold level has been defined did not consider specifically these products.

A typical example is given by well pumps (submersible pumps), namely the MSS of 4 in and 6 in size. It is clear that well size is a geometric constraint in the design of these kinds of pumps. In particular, for the high cost of drilling in rocky soils small well diameters are preferred and the market requires some pump models that maximize the flow rate and the head available given a specific well size. These pumps behave differently from traditional multistage pumps.

In Table I the performances of a series of Vertical Multistage pumps (MS-V) of different BEP flow rates are listed, D being the hydraulic overall diameter: as an effect of the geometric similarity the BEP efficiency, first rapidly and then more gradually, increases and then almost stabilizes for the medium and big model sizes. This trend is similar to the surface shape from which derives the equation at the base of the MEI calculation. 

Table I: Performances of MS-V models

On the contrary, let us examine the technical documentation of a 4 in and 6 in MSS as reported in Table II and Table III: the maximum efficiency BEP in both series is not attained for the bigger model sizes. Comparing 4 inch and 6 inch MSS for a similar BEP value, models with greater diametrical dimensions always present the highest efficiency values. The same effect can be observed comparing the technical documentation of a 6 in and 8 in submersible pump, confirming that MSS behave differently from other pump families included in the 547/2012.  

Table II: Performances of 4 inch MSS models

Table III: Performances of 6 inch MSS models

Much caution will be necessary for the future standard on some families of waste water  pumps in extending the MEI based qualification procedure. We took from the market a sewage electric pump with shredder manufactured by a well-known Italian pump factory. We tested it in our HELab and test results are plotted in Figure 5. The BEP was obtained for the flow rate Q_BEP=4,35 l/s. 

Figure 5. Characteristic curve of a waste water electric submersible pump.

If Standard EN 16480 were used, the pump efficiency curves might stand over the "house of efficiency" for three flow rates Q_PL=0.85Q_BEP, Q_BEP and Q_OL=1.1Q_BEP, with an efficiency in the Q_OL point larger than 0.985 ?_BEP. It is evident from Figure 5 that the efficiency collapses immediately after the BEP point because BEP flow rate is in incipient cavitation regime. This situation is not so rare among the sewage pumps family: there are pumps with cut performance curves where Q_BEP is outside the manufacturer's recommended operating range.

Conclusions

The new laboratory for pump testing, HELab of the University of Naples Federico II, is now performing qualification tests to assess conformity to Regulation 547/2012 EC. In addition to standard tests for industry and other end users, advanced research is in progress to verify the consistency of future standards as a support activity for Italian pump, motor and control manufacturers.

The application of Standard EN 16480 for the assessment of pump MEI, and the threshold values proposed by Regulation 547/2012 EC, has been found to be adequate for most of the pumps on the market when a proper technical design has been performed. On the contrary, at present, a few pump families with technical constraints due to specific working conditions have been found to be penalized. Caution will be necessary in extending the existing approaches to the pump families not included in Regulation 547/2012 EC.

Figure 6. Two centrifugal pumps and two motors to be tested.

References

[1] Carravetta A., (2016) New test facility in Naples, Italy, World Pumps, 10.

[2] Carravetta A., Conte M.C., Antipodi L. (2015) Energy efficiency index for water supply systems, AEIT International Annual Conference.

Contacts

Armando Carravetta, PhD Oreste Fecarotta, PhD Maria Chiara Conte 

CeSMA Centro Servizi Metrologici Avanzati, University of Naples Federico II, Naples, ITALY armando.carravetta@unina.it