The adjustment of the optimum operating point of a pump depending on the requirements of the water supply.

Milos, Teodor

1. INTRODUCTION

Economical and social development of the cities of Romania leads to capital changes in the daily and the annual water consumption. The water supply is assured by one or more pumping stations. The adaptation of the operating parameters of the pumping stations to the dynamic of the consumption and the most efficient exploitation is a serious challenge for the decision factors of water delivery (Sanks, 1998).

2. CHARACTERISTICS OF THE TESTED PUMP

For this study in a first phase there were reproduced the energetic parameters Q, H, [P.sub.sh] with the reference data given by the pump producer. The curves from the catalogue were made at 3 different rotational speeds. The rotational speeds were: [n.sub.1] = 2900 rot / min as a maximum reference speed (100 %), [n.sub.2] = 2320 rot / min (80 %) and [n.sub.3] = 1740 rot / min (60 %). Reading the values of head and power from the catalogue for flow rate between 0 and 11 [l/s], the analytical curves were interpolated using polynomial functions (third degree for head and second degree for power). In figure 1 and 2 is shown pumping head vs. flow rate and pump efficiency vs. flow rate.

With these two categories of curves we can build the pump operation characteristic which gives us information about the optimum operating point of the pump (Gandhi et all, 2001). The identification of the equal efficiency curves was made analytical, using a developed program and the graphic postprocessor program used was developed in AutoCAD under AutoLisp. The results obtained for the pump operation characteristic are shown in figure 3.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The pipe network operation characteristic, [H.sub.pn], represents the amount of the losses from the hydraulic loop exterior to the pump and it is calculated with the formula, (Anton et all, 2002):

[H.sub.pn] = [H.sub.g] + [C.sub.hl] x [Q.sup.2] (1)

The hydraulic losses constant, [C.sub.hl], is calculated for an operating point measured from the pump characteristic knowing that [H.sub.pn]=H and from equation (1) results, (Alexandrescu, 2003):

[C.sub.hl] = [H.sub.pn] - [H.sub.g]/[Q.sup.2] (2)

For the installation being a closed hydraulic circuit, [H.sub.g] = 0. The other points of the pipe network operation characteristic are calculated giving values to flow rate from zero to the maximum value. Having an installation with variable rotational speed, the pipe network operation characteristic can be obtained adjusting the flow rate through the modification of the rotational speed.

3. A CASE STUDY ON ENERGY SAVING

The pumping station behavior results is obtained by overlapping the pump operation characteristic at variable rotating speed on the pipe network operation characteristic. A minimum flow rate [Q.sub.min], a medium flow rate [Q.sub.med] and a maximum flow rate [Q.sub.max] can be estimated by taking into account the requirements of the consumers from a network supplied by the pumping station. The distribution of these flow rates in the exploitation is expressed in percents through a coefficient, [C.sub.p], which results from a statistical study. Also, for the closed hydraulic loop from this case we assume that it has to ensure a constant pressure in conformation with a constant pumping head, [H.sub.rq] = 50 m, for the following domains of flow rate, (Kudo, 1994):

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[DELTA][Q.sub.min] = 4/5 l/s with a coefficient of [C.sub.pmin] = 20 %

[DELTA][Q.sub.med] = 5/8 l/s with a coefficient of [C.sub.pmed] = 65 %

[DELTA][Q.sub.max] = 8/9 l/s with a coefficient of [C.sub.pmax] = 15 %

The corresponding flow rates for the three domains are: [Q.sub.1] = 4 l/s; [Q.sub.2] = 5 l/s; [Q.sub.3] = 8 l/s; [Q.sub.4] = 9 l/s. To these flow rates are corresponding the pumping heads H1, H2, H3, and H4 according to the pumping head characteristic for the maximum rotational speed. If the pumping station would not have the possibility to adjust the rotational speed then the flow rate adjustment on the consumers would be realized only through the outlet vane and on the given domain will be provided a covering pumping head between 75 m and 53 m, although in the pipe network would be sufficient a 50 m pumping head. Even if the pump efficiency is relatively good on this domain it can be proved that a big part of the consumed energy is unnecessarily wasted. If it is to take into consideration the possibility of adjusting the rotational speed then it becomes possible the adjusting of the installation so that the pumping head to be constant, [H.sub.rq] = 50 m, and provides to any consumer from the network optimum operating conditions. On the industrial installation the adjusting through outlet vane is realized on the consumer and the adjustment through rotational speed is performed in the pumping station. The operating of the pump on the three domains of flow rate it observes that the efficiencies and the pumping heads are sensible different from an operating point to another. That is way in the following section it is presented an averaging method of the power on the three exploitation domains. The shaft-power of pump result from the formula:

[P.sub.sh] = [p.sub.u]/[[eta].sub.p] = [rho]gQH/[[eta].sub.p] (3)

Important is the electrical power, Pem, consumed in the power station and which is affected by the efficiency of the electrical motor [[eta].sub.em] = 85 %. Two efficiency curves are used for the calculus. The first one corresponds to the operating pump at maximum rotational speed [[eta].sub.100%] = f(Q) and the second one [[eta].sub.50m] = f(Q) results from the characteristic curve for H = [H.sub.rq] = 50 m. For a good averaging of the powers on the considered domains it is computed the electrical consumed power in 25 points for each interval of 1 l/s. The medium power will be the average of the powers on each considered domain. The equation deduced is:

[DELTA][P.sub.em] =[rho]gQ/[[eta].sub.em] x ([H.sub.100%]/[[eta].sub.100%] - [H.sub.50m]/[[eta].sub.50m]) (4)

The averaging of the power on the three domains it is realized with the equation:

[FIGURE 5 OMITTED]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

The saving of consumed energy depends on the working time according to the equation:

[DELTA]E = [([DELTA][P.sub.em]).sub.med] x [N.sub.dy] x [N.sub.hd] x [C.sub.p] [kWh] (6)

where [N.sub.dy]=number of the days from a year (365), [N.sub.ha]=number of hours from a day (24). The results obtained for the three domains are presented in table 1.

The researches will be continued for establish Research will be continued for determining the proportion of optimal adjustment with control gate versus the speed.

4. CONCLUSIONS

This study show that flow rate adjustment through the variation of the rotational speed leads to a significant energy saving.

The modality how the automatic adjustment of a pump operating in a pumping station so that the pumping head remains constant indifferent of the operating flow rate determine first the economic advantage through the energy saving realized in time.

The hydraulic network is not overcharged anymore at extreme pressures, which could lead to damages in case of uncontrolled maneuvers.

The averaging method of the economic efficiency on the three operating domains allows a much precise appreciation than the estimation at the medium flow rate of each domain.

6. ACKNOWLEDGEMENTS

The present work has been supported by National Centre of Management Programs through Project No. 1365/21-041/2007 and Grant type "IDEI", CNCSIS Department, Contract No. 35-68/2007.

5. REFERENCES

Alexandrescu, A., (2003), Concerning optimization's working of the pumping station for water feedings, 18th International Conference on Hydraulics and Pneumatics, Prague, Czech Rep., ISBN 80-02-01567-3, pp. 263-268.

Anton L., Baya A., Milos T.& Resiga R., (2002), Experimental Fluid Mechanics, Vol. 1, Publishing house "University Horizons", Timisoara, Romania, ISBN 973-8391-72-5.

Gandhi B.K., Singh S.N.& Seshadri V., (2001), Performance Characteristic of Centrifugal Slurry Pumps, Journal of Fluid Engineering, June 2001, Volume 123, pp. 271-280

Kudo K., (1994), Japanese experience with a converter-fed variable speed pumped-storage system, The International Journal on Hydro-power & Dam, March 1994, pp 27-32

Sanks R.L., (1998), Pumping Station Design, Second Edition Publishing house "Butterworth-Heinemann", Woburn, MA-USA, ISBN 1-7506-9483-I.

Tab. 1. The results obtained for the three domains

 Flow rate [C.sub.p] [P.sub.sh100%]
 domains [%] [kW]

[Q.sub.1]/[Q.sub.2] 20 6,34
[Q.sub.2]/[Q.sub.3] 65 7,31
[Q.sub.3]/[Q.sub.4] 15 8,08

 Flow rate [P.sub.sh50m] [([DELTA][P.sub.em])
 domains [kW] .sub.med]
 [kW]

[Q.sub.1]/[Q.sub.2] 3,98 2,36
[Q.sub.2]/[Q.sub.3] 5,37 1,94
[Q.sub.3]/[Q.sub.4] 7,18 0,89

 Flow rate [DELTA]E
 domains [kWh]

[Q.sub.1]/[Q.sub.2] 4142
[Q.sub.2]/[Q.sub.3] 11052
[Q.sub.3]/[Q.sub.4] 1175