1. Introduction

 Pump efficiency is usually measured by two methods. They are called traditional technique and the thermodynamic method(Kwon et al., 2004). In traditional technique, pump efficiency is calculated from the pump equation, flow rate, head, and electrical power. Of these parameters, flow rate is the most difficult to determine accurately. Many pumps do not have installed the accurate and individual flow meters because they are high in cost and more expensive for larger diameter pipes especially. Also they can be difficult or impossible to install, calibrate, and maintain in some system. The accuracy of flow meter is variable depending on the pump's operating point and other factors such as build-up of debris in pipes or on sensor's surface. The advent of the thermodynamic method(Routleyand Baxter, 2005) has provided a solution to this problem.

2. The principle of the thermodynamic method

 The thermodynamic method results from the application of the principle of energy conservation which transfers the energy between water and pump rotor(Noah, 2005). Measurement of pump efficiency is given below

 where
E_H  : hydraulic energy per unit mass of fluid
E_M : mechanical energy per unit mass of fluid

 and

where
dp : differential pressure
P_D : discharge pressure
P_S : suction pressure
dt : differential temperature
T_D : discharge temperature
T_S: suction temperature

 Above 6 values are measured by the temperature and pressure probes.
c_p : specific heat capacity
a : isothermal coefficient
p : fluid density

 And above 3 parameters are specified in fluid properties table(ISO 5198).

 The critical parameter is the differential temperature across the pump, which must be measured to an accuracy of typically 1mK[℃]. This is especially important for fixed installations and is achieved with this method.

 From classical pump efficiency formula (4), flow rate (Q) will be calculated if Watt transducer is available or electrical power to motor  (P_M) will be determined if flow rate is given in advance(PumpCalcs, 2012).

 In case of Watt transducer and flow rate are available, energy losses transmission of electrical power to the motor are effectively being measured.

 The thermodynamic method is the simplified methodology to measure pump efficiency. It is simple and possible to measure pump efficiency on installed pumps to evaluate the condition of the pump. Only two parameters, temperature and pressure are required to determine pump efficiency(Berge, 2009; Teodor and Alexandru, 2004). Furthermore temperature and pressure transmitters are installed easily with low cost. They are reasons why it has been widely used in all over the world with great success.

3. Measuring instruments based on the thermo-dynamic method

3.1 Calculation software

 LabVIEW software is used in this study. It is the most suitable for this application(Zangeneh, 2007). It is a graphical programming environment used by millions of engineers and scientists to develop sophisticated measurement, test, and control systems using intuitive graphical icons and wires that are resemble a flowchart. With LabVIEW, real time operating becomes easier and helps to find out a friendly interface for users.

3.2 Hardware

 Fig. 1 shows the data acquisition diagram composed of 8 sensors and data acquisition devices.

Fig. 1. Data acquisition diagram.

 The NI 9208 current input C Series module has 16 channels of ± 21mA input with built-in 50/60 Hz noise rejection. Resolution is 24 bits, operating system/target with Real-Time. The NI cDAQ-9174 is a 4-slot NI CompactDAQ USB 2.0 Hi-Speed chassis designed for small, portable, mixed-measurement test systems(National Instrument, 2012).

 Several sensor data were collected from temperature probe (T), pressure transmitter(P) and flow meter(FM). These sensors have all current outputs with 4-20[mA] range. They are connected as Fig. 2.

Fig. 2. The wiring diagram for input signals.

3.3 Programming

 Data received from hardware has actual value in mA. These signals are converted to temperature in ℃ or pressure in bar, etc..

 It has output current signal for temperature probe from 4-20mA which is corresponded to the original 0-40℃ so that the temperature is as follows

 And also pressure transmitter has current output from 4-20mA which is corresponded to the original 0-20 kg/cm²  so the pressure in bar(1bar = 1.02 kg/cm²) is as follows

 The temperature probes do not measure a absolute temperature accurately but they are calibrated in pairs to read the differential temperature to one milli-Kelvin(One thousandths of a degree Celsius). The system acquires multiple sampling as an averaged reading(Typically at one second intervals for ten seconds) to take a test as accurate as possible and then take 8 or more readings at 15 to 30 second intervals(Teodor and Alexandru, 2004). This averaging of multiple readings is used to smooth out the temperature differential fluctuations and average the instrument variability. Flow meter is similar.

 Fig. 3 shows the monitoring program for pump efficiency measurement(Robertson Technology, 2012). Parameters are monitored continuously with time. Graphs can be plotted from measured parameters and compared with manufacturer’s curves to illustrate any changes in pump efficiency. In addition, these parameters are saved in special type of file which is useful information to operator.

Fig. 3. Monitoring program for pump efficiency measurement.

4. Experimental instruments

 Description of the experimental installation and of the measuring instruments are shown Fig. 4.

Fig. 4. Experiment installation in closed loop.

 A 2000 liters water tank is set to reserve of liquid at out side. Pump A(Model DRL8-50) and pump B(Model DRL16-50) are a vertical multistage pump and were manufactured by DOOCH. Table 1 shows the detail specification of each motor.

Table 1. Description of pump

 This method requires high temperature probe and pressure transmitter as accurate as possible. It is important to choice a temperature probe which it’s range closes to the temperature of the liquid that the pump operates. The temperature difference between suction and delivery of pump changes very small when the pump is running and the pressure transmitter range is also similar. Temperature probes(ATT2100-S1K0-M1BA) made by AUTROL and pressure transmitter(TPS-G17F8) made by KONICS are used. Product details are given in Table 2.

Table 2. Description of temperature and pressure sensor

 A flow meter(T-W1000) made by WinTEC is used. It has LCD indicator and current signal output with 4-20[mA] range.

5. Results and discussions

5.1 Water temperature

 Fig. 5 is the graph of suction and delivery water temperature after operating of pump A in 7 hours at 6.2 bar.

Fig. 5. Water temperature of pump A.

Temperature of water increases gradually because the water is just circulated in closed loop. 

 The delivery temperature is lager than suction side and its difference is close to 0.182[℃].

 Fig. 6 is the graph of suction and delivery water temperature after operating of pump B in 7 hours at 9 bar.

Fig. 6. Water temperature of pump B.

 The decline of temperature graph is similar to the Fig. 5 but the difference of temperature is close to 0.105[℃]. Pump B’s temperature values between suction and delivery is smaller than pump A. So if the pump has a lager power, the temperature change between suction and delivery will be smaller.

5.2 Water pressure

 The pump efficiency will vary when difference pressure values are changed. We obtained the best efficient point from the repeated experiment when the pressure difference was about 6.2 bar at pump A. And also it was about 9 bar in case of pump B. Pump efficiency curve offered by pump maker is usually showed as a graph related to pump efficiency and flow rate, so it is convenient to compare and evaluate the accuracy of thermodynamic method. We used flow rate parameter instead of the difference pressure values because the pressure relates to flow rate closely.

5.3 Efficiency of pump

Firstly, pump A’s efficiency was measured. In this case, the output valves of pump B(V_B) and pump A(V_A) are fully closed. After running of pump A and then valve(V_A) is opened slowly. Its efficiency curve is shown in Fig. 7. It is clear that pump efficiency curve measured by traditional technique has the almost same curve offered pump maker but it has a little lower values. 

Fig. 7. Comparison of pump efficiency curve for pump A.

There were unstable values when the flow rate varies from 0 bar to 1 bar(15 to 19[m³/h]). Pump efficiency measured is not exact at full flow(18.5-19[m³/h]). However, pump is not usually operated at these flow rate values. Working points of pumps are in range of -20[%] to +10[%] flow rate in which pump efficiency is maximum as Fig. 8. The best efficiency point(BEP) is found and it reaches 60[%] when the flow rate is close to 6.2 bar(11[m³/h]). 

Fig. 8. Better practice of pumps.

 Fig. 9 presents efficiency of pump A in over 20 operating hours at 11[m³/h] flow rate. There have been fluctuations from near 58[%] to 61[%], but about 3[%] fluctuation is acceptable to determine the pump efficiency. There are a lot of parameters to make the fluctuations of efficiency, however the critical parameter is the differential temperature across the pump. The fluctuations will be reduced if we use the more accuracy temperature transducer.

Fig. 9. Efficiency of pump A at Q = 11[m³/h].

 Secondly, pump B’s efficiency was measured. Now the output valves of pump B(V_B) and pump A(V_A) are fully closed. After running of pump B and then valve(V_B) is opened slowly.

 Their efficiency characteristic is shown in Fig. 10. It is similar to pump A like Fig. 7. However, it has greater values. The best efficiency point of pump B is 63[%] when delivery pressure is around 9 bar(22[m³/h]).

Fig. 10. Comparison of pump efficiency curve for pump B.

 Fig. 11 shows efficiency values of pump B in over 20 operating hours at flow rate Q = 22[m³/h]. Its efficiency is from 62[%] to 64[%].

Fig. 11. Efficiency of pump B at Q = 22[m³/h].

6. Conclusion

 This paper introduced the thermodynamic method as the simplified methodology to measure a pump efficiency. Following conclusions are given from this research.

1) The measured BEPs from two pumps were the almost same values compare to be offered pump maker. 

2) The efficiency fluctuated between 2 and 3[%] when pumps operate continuously, but those values are enough to determine the efficiency. Also the fluctuations will be reduced if we install the more accuracy temperature transducers.

3) Only two parameters, temperature and pressure are required to determine a pump efficiency, thus it makes simple and possible to measure a pump efficiency on installed pumps with low cost.

4) Making pump efficiency instrumentation based on this method is not difficult. This instrumentation can be applied in practice to find a BEP and evaluate on real pump systems. It provides the information required for management of energy. 

Acknowledgment

 This study was supported by the Small & Medium Business Administration(SMBA), Korea. This financial support is gratefully acknowledged.

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