Experimental investigation of pressure instabilities affected by cavitation for a double-suction centrifugal pump

Transcription

1 IOP Conference Series: Earth and Environmental Science Experimental investigation of pressure instabilities affected by cavitation for a double-suction centrifugal pump To cite this article: Z F Yao et al 212 IOP Conf. Ser.: Earth Environ. Sci Related content - Internal flow numerical simulation of double-suction centrifugal pump using DES model P J Zhou, F J Wang and M Yang - Blade design loads on the flow exciting force in centrifugal pump Y Xu, A L Yang, D P Langand et al. - Numerical investigation of the effects of splitter blades on the cavitation performance of a centrifugal pump Y L Zhang, S Q Yuan, J F Zhang et al. View the article online for updates and enhancements. This content was downloaded from IP address on 25/8/218 at :4

2 Experimental investigation of pressure instabilities affected by cavitation for a double-suction centrifugal pump Z F Yao 1, F J Wang 1, R F Xiao 1 and C L He 2 1 College of Water Resources and Civil Engineering, China Agricultural University, Beijing, 83, P. R. China 2 China Water Resources Beifang Investigation Design and Research Co. Ltd, Tianjin, 3222, P. R. China yaozhifengchina@gmail.com Abstract. Cavitation is a common fault which has a close relationship with pressure instabilities in centrifugal pumps. In order to investigate the hydroacoustic response to cavitation phenomena, a pressure fluctuation experimental is carried out on a double-suction centrifugal pump. Frequency spectrum analysis and time-frequency joint analysis of pressure fluctuations in semi-spiral suction chamber and volute casing are presented based on fast Fourier transform (FFT) and adaptive optimal-kernel time-frequency representation (AOK TFR) methods. The results show that in semi-spiral suction chamber, as the available NPSH value continuously decreases, the frequencies affected by cavitation transfer to a lower frequency range and become relative higher concentrate in time-frequency representation. In volute casing, blade passage frequency (BPF), specific frequency at 1/2 of blade passage frequency (147 Hz) and some frequency bands are well captured. The amplitudes of BPF and BPF/2 are shown to have a reasonable dependence on available NPSH value, though the measuring location relative to volute tongue should be taken into account. Strong pulsation characteristics are observed on relative low available NPSH value conditions. Pressure fluctuation at impeller rotating frequency is shown to be sensitive with cavitation process, of which the amplitude sharply increases at first and then decreases. 1. Introduction Cavitation can occur within the entire range of operating conditions in centrifugal pumps, when the absolute static pressure at a certain point falls below the saturated vapor pressure of the fluid at the prevailing temperature conditions [1]. The bubbles condense when they reach a region of higher pressure and finally collapse or implosion. Cavitation may restrict the flow capacity of a system, generate unacceptable noise, cause erosion and failure, and in extreme cases cause instabilities which may damage or destroy a system [2]. Once cavitation commences, pressure fluctuations propagate upstream and downstream, and deteriorate the unsteady pressure field. Along with the construction of many large projects, such as Water Diversion Project from South to North in China, many double suction centrifugal pumps are employed with the size and capacity becoming larger and larger [3]. From the operation of large pump stations, there are general phenomena such as vibration and cavitation [4]. From most engineering methods, a pump is deemed to be suffering from a lack of NPSH available when the NPSH drops below a required figure, which is typically identified by a 3% drop in head or 1% drop in the efficiency for a given flow rate. However, small amounts of cavitation exist even when there is no measurable drop in performance [5]. To date, there is Published under licence by Ltd 1

3 limited published experimental information related to the pressure fluctuations influenced by cavitaion within double suction centrifugal pumps. Čudina [6-7] conducted a study concentrated on indirect acoustic monitoring of the centrifugal pump rather than monitoring pressure waves in the fluid directly. The results showed that the differences in noise level of the discrete frequency tone at BPF/2 before the incipient of cavitation and after it was fully developed is between 12 and 2 db. They came to conclusions that the noise signal at BPF/2 can be used to determine the NPSH critical value and for control operation of the centrifugal pump. But it is still not clear whether this BPF/2 tone can be applied as a magic bullet solution between all pumps, or the frequency just happened in the case of the pump used for the test to coincide with BPF/2. Building on the work of Čudina, clarification was obtained through the work of Chini, Rahimzadeh and Badrami [8]. They considered a number of frequencies at which the acoustic signals were affected by cavitation. In discussing the tonal noise at BPF/2, it was considered an imperfect measure due to the inherent variation of this frequency should the pump speed be varied. Dazin [9] measured pressure fluctuations in the suction and delivery pipes and in the volute of a single suction centrifugal pump in non-cavitating and cavitating operating conditions. A low frequency range (-8Hz) was analysed for the hydroacoustic purpose. The results showed that the frequency range affected became larger as the NPSH decrease in volute wall. Levels of pressure fluctuations at harmonics of the impeller rotating frequency are continuously decreasing, while these levels increase in the delivery pipe from incipient cavitation to developed cavitation conditions. The above listed investigations have given us a comprehensive understanding on the mechanisms of the pressure fluctuations influenced by cavitation. However, nearly all the analytical and experimental studies employed FFT method, which supposes the frequency and amplitude of each sinusoidal contribution are constant over the whole length of the signal. While spectral analysis was a very strong analysis tool, it can t provide the time information about when frequencies happen or disappear. And FFT method may be inappropriate to analyze the pressure fluctuation signals including cavitation effect, because cavitation is often only a transient and unavoidable phenomenon. Timefrequency techniques were quit useful for non-stationary and transient signal analysis. Time-frequency analysis could be implemented using windowed FFT algorithms (STFT, Gabor, Vigner-Ville), lately (since 198) wavelet analysis was available [1, 11]. A relative new (sine 1991) time-frequency analysis called adaptive optimal-kernel time-frequency representation (AOK TFR) was proposed to reach a higher time-frequency solution [12, 13]. This paper presents a case study and uses AOK TFR method to uncover the hydroacoustic response and time-frequency characteristics influenced by cavitation in a double suction centrifugal pump, and ascertains the applicability of the pressure measurement for detecting incipient cavitation. 2. Experimental setup and analysis methods The test pump has a double-suction impeller whose trailing edge is parallel with the pump rotation shaft, and the blades in the left and right sides are arranged in a non-staggered layout. In order to smooth the blade surface, sand blast was employed. The main geometrical and hydraulic data are given in Table.1. More details can be seen in reference [1]. The pump was tested in the closed test rig schematized in Fig. 1. The test pump was driven by a variable speed electric AC-motor controlled by a frequency converter. The shaft torque and rotational speed were measured by a torque and speed sensor with errors under ±.1%. Static pressure values were measured at the inlet and outlet of the pump by a differential pressure transfer, and the uncertainty was within ±.1%. The flow rate was measured by using a magnetic flow meter with the uncertainty less than ±.14%. The measurement accuracy of pump efficiency was quantified as ±.3%. Piezoresistive pressure transducers with high eigenfrequency were flush mounted in the semispiral suction chamber and the volute casing to measure the pressure fluctuations, and the uncertainty was equal to ±.25%. 2

4 Table 1. Main parameters of the investigated pump Main geometric data Inlet diameter of impeller D1 13 mm Outlet diameter of impeller D2 24 mm Volute tongue diameter D3 21 mm Blades number N 6 Specific speed Nominal operating condition Ns 12 (m3/s, m, r/min) Nominal rotational speed n 295 r/min Pump flow rate Qn 278 m3/h Pump head Hn 45.7 m Pump efficiency ηn 8.% 1. Pressurizer tank; 2. Regulating valve; 3. Differential pressure transfer; 4. Tested pump; 5. Torque and speed sensor; 6. AC-motor; 7. Storage tank; 8. Vacuum pump; 9. Magnetic flow meter Figure 1. Scheme of the test rig The investigation focuses on the effects on pressure fluctuations in semi-spiral suction chamber and volute casing caused by cavitation. In semi-spiral suction chamber, four monitoring points are proposed as is shown in figure 2 (Left). Location S1 is nearest to the divider of suction chamber among these four measuring locations. In volute casing, in order to obtain the specific frequencies of the pressure fluctuations caused by cavitating flow, monitoring points at six circumferential locations in middle section of volute casing are presented, and can also be seen in figure 2 (Right). In this paper, results of the pressure fluctuations on locations V 2, V 3 and V 4 are presented. The pressure transducers output a 4-2mA signal over their operating range. And these signals run across a high accuracy resistor, generate an output voltage which could then be synchronously passed into a data acquisition unit (a multi-channel analog-to-digital conversion card) and on into a specialty software (DASP3), for signal storage and analysis. The signals passed the pressure transducers were acquired with a sampling rate of 4 khz and a sampling time of 1 seconds. To avoid the phenomenon of signals aliasing, a low pass filter was intercalated with a cutoff frequency of 1 khz [14]. FFT method was used to transform pressure fluctuation signals from time domain to frequency domain. The changes of amplitudes of pressure fluctuations at different frequencies were analyzed. AOK TFR was employed to unveil the time evolution of the pressure fluctuations. 3

5 Figure 2. The arrangement of measuring locations in wall of semi-spiral suction chamber (left) and volute casing (right) 3. Experimental results and discussions The test pump took fluids from a closed vessel in which the air pressure above the liquid level was varied by a vacuum pump. To detect the onset of the cavitation of the pump, the pump efficiency and pressure fluctuations were measured at a constant speed and flow rate with varying NPSHa conditions. The same procedure was carried out at different flow conditions by varying the opening of the discharge valve. Curves of efficiency versus NPSHa at three flow rates are shown in Fig. 3. NPSHa is shown as the abscissa, and as this inlet head is reduced, the condition finally reached where pump efficiency breakdown occurs. According to ISO 996 [15] and SL 14 standards [16], the criterion of 1% efficiency drop at a given flow rate is used. A black dot on each curve denotes the critical point, where the efficiency drops by 1%, which means that there is a fully develop cavitation achieved. And on each hollow mark, transient static pressure signals are recorded. Efficiency(%) Q bep.87q bep 1.2Qbep For 1% efficiency drop NPSH a (m) Figure 3. Curves of efficiency versus NPSHa at three flow rates 3.1. Pressure fluctuations in semi-spiral suction chamber A frequency analysis between and 1 khz is presented for each transducer with results at different NPSHa values. Fig. 4 shows the frequency domain of pressure fluctuations on location S 1 at.87 Q bep in non-cavitating and cavitating operation conditions. Rotating frequency and some other low frequencies are clearly identified in the frequency domain. In non-cavitating operation, non-uniform impeller inlet static pressure is the main resource of the pressure fluctuation at rotating frequency. While the specific low frequencies of the pressure fluctuations in semi-spiral chamber have a close relationship with the rotating of impeller due to the viscosity of water. The appearance of these frequencies suggests the existence of some large scale vortex stripes in the semi-spiral suction 4

6 chamber. The spatially asymmetric because of the convoluted suction passage also contributes to unsteady pressure variations within suction chamber. The largest amplitude typically happens at 1/3 of rotating frequency in non-cavitating condition. When the NPSHa value continuously decreases, amplitudes of rotating frequency and the specific low frequencies sharply increase at first and then decreases. The peak value of the amplitude at rotating frequency is two times more than the value in non-caviting operation. These phenomena can be explained that the incipient cavitation does not happen synchronously in every impeller passage, and usually first occurs on suction surface near shroud side, especially the zone is vertically higher than pump axis. In this way, the occurrence of incipient cavitation enlarges the asymmetry of impeller inlet flow field. And this circumferential asymmetry turns out to be ameliorative, when the cavitation is fully developed. Figure 4. Frequency domain of pressure fluctuations on Location S 1 Figure 5. Time-frequency representation of pressure fluctuations on location S 1 Fig. 5 presents time-frequency presentations of pressure fluctuations on location S 1 corresponding to three NPSHa at.87 Q bep operating condition. Both Figs. 4 and 5 illustrate a phenomenon that frequencies affected by cavitation transfer to a lower frequency range. The distributions of pressure fluctuations mainly concentrate at some specific low frequencies below 4 Hz at point a. The pattern of the pressure fluctuations at the main frequencies seems to be unsteady during the time series process. At these frequencies the amplitudes are not constant in the time domain, but are of a pulsating nature. The time-frequency representation shows that the spectral components are slightly changing as 5

7 time processes. The range of high pressure fluctuations turns to be below 2 Hz at point f, when there is full developed cavitation formed Pressure fluctuations in volute casing In volute casing, the rotor-stator interaction can cause a pronounced pressure fluctuation pattern. Blade passage frequencies are well captured on locations V 2 and V 3. These spectrum lines are mainly the consequences of interactions between the flow at the impeller outlet and the volute tongue. The differences in the amplitudes measured on the transducers can be explained by the effect of the transducer location in reference to the volute tongue: higher amplitudes on location V 2, which is relatively nearest from the tongue. While the amplitude of the blade passage frequency is almost to zero on location V 4. Some specific frequencies are also clearly identified on location V 2 and V 3, especially the frequency at 147 Hz which is equal to 1/2 of blade passage frequency, Hz and Hz which are equal to 2 and 5 times the rotating frequency, respectively. Fig.6 presents frequency analysis of pressure fluctuations on location V 3 at.87 Q bep. A band frequency around Hz is identified in non-cavitating operating condition. The fluctuation energies at these frequencies are much higher than the one at the blade passing frequency as the frequency ranges are much larger than the peak at 3 Hz. Significant pressure fluctuation amplitude increase is observed at the BPF/2 on location V 3. This discrete frequency at 147Hz was reported to be in close correlation with the development of cavitation phenomenon [5-7]. The result in this case shows that the reflections of this specific frequency versus NPSHa values are greatly depend on the location relative to volute tongue zone. On location V 2, which is relatively nearest from the tongue, the amplitude of pressure fluctuation at this frequency has slight variation. While on location V 4, no frequency at this discrete frequency is discovered. Fig.7 presents frequency analysis of pressure fluctuations on location V 4. As the NPSHa value continuously decreases, the amplitude of pressure fluctuation at rotating frequency has the same trend as that on suction chamber monitoring location S 1. The result shows that during incipient cavitation period the amplitude of pressure fluctuation at rotating frequency has sensitively increase without efficiency drop. The amplitude on location V 4 has already started to respond when the efficiency has no change. Figure 6. Frequency domain of pressure fluctuations on Location V 3 6

8 Figure 7. Frequency domain of pressure fluctuations on Location V 4 Figs. 8 and 9 show time-frequency presentations of pressure fluctuations on locations V 3 and V 4 corresponding to three NPSHa values at.87 Q bep, respectively. For location V 3 as shown in Fig. 8, the main energy of pressure fluctuations concentrate around the Hz, and only some instantaneous high amplitudes are discovered at point a. While at point d when the efficiency had already fallen down, BPF/2 becomes to be the main frequency of the pressure fluctuations in the flow field below 1 khz. Hot pots which mean high amplitude at BPF/2 appear frequently in the time process. The amplitude of pressure fluctuation at BPF/2 sustains a high level nearly all the time as seen at point f when the cavitation is fully developed. For location V 4, the changes of the pressure fluctuation pattern in timefrequency domain are clearly unveiled. The highest pulsation characteristic is observed at point a, while the highest time-frequency concentrate amplitude at rotating frequency is found at point f. Frequency (Hz) Point-a Cp Frequency (Hz) Point-d Cp Cp Time (s) 1-2 Cp Time (s) Point-f Cp Frequency (Hz) Cp Time (s) 2 Figure 8. Time-frequency representation of pressure fluctuations on location V 3 7

9 Figure 9. Time-frequency representation of pressure fluctuations on location V 4 4. Conclusions Pressure fluctuation experiments were performed in semi-spiral suction chamber and volute casing of a double-suction centrifugal pump in non-cavitating and cavitating operating conditions. The pressure signals were processed by the spectral analysis in both frequency and time-frequency domains. The experimental results show a clear hydroacoustic response to cavitation. The pressure fluctuations at rotating frequency and some lower specific frequencies increase then decrease during the cavitation development. And these amplitudes have a sensitive response to the onset of cavitation before the efficiency begins to drop. The frequencies influenced transfer to a lower range and become relative higher concentrate in time-frequency domain. The amplitude of BPF/2 is proved to have a reasonable relationship with NPSHa value, strong pulsation characteristics are observed on relative low available NPSH value conditions, while the measuring location to capture this specific frequency should be taken into account. Adaptive optimal-kernel time-frequency representation is successfully employed to uncover the nature pattern and local frequency content of pressure fluctuation signals. The pressure fluctuations at BPF/2 becomes much more frequent and high amplitude persistent during the period from incipient cavitation to its full developed. Acknowledgments The authors would like to acknowledge the financial supports given by the National Natural Science Foundation of China (No , ), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 8112), and China Agricultural University Innovation Fund of Graduated student (No.KYCX21184). Nomenclature D 1 D 2 D 3 n η r u 2 Inlet diameter of impeller [mm] Outlet diameter of impeller [mm] Volute tongue diameter [mm] Nominal rotational speed [r/min] Pump efficiency [-] Fluid density [kg/m 3 ] Impeller outlet circumferential velocity [m/s] Q Q n P P C p Flow rate [m 3 /s] Nominal flow rate [m 3 /s] Transient pressure [Pa] Average value of transient pressure [Pa] Pressure coefficient [-] 8

10 N s N H Specific speed at best efficiency point [-] 3.65 n Q / 2 Ns = 3/ 4 H Blades number [-] Pump head [m] BPF NPSH NPSHa C = P- P p 2.5ru2 Blade passage frequency [Hz] Net positive suction head [m] Available net positive suction head [m] References [1] Wang F J 5 Pump and Pumping Station (Beijing: China Agricultural Press) [2] Miller D S 1978 Internal Flow Systems (Cranfield: Fluid Engineering) [3] Yang M, Wang F J and Qu L X 8 Performance Improvement of Double-suction Centrifugal Pump by Using CFD Proc. of 24th IAHR Symp. on Hydraulic Machinery and Systems (Fozdo Iguassu, Brazil, 8) [4] Nilsson H and Davison L 3 International Journal for Numerical Methods in Fluids [5] Rose P 7 Detecting NPSH available & cavitation in pumps through high frequency pressure measurement Master Thesis ( UK: Cranfield University) [6] Čudia M and Prezelj J 9 Applied Acoustics [7] Čudia M 3 Mechanical Systems and Signal Processings 17(6) [8] Chini S, Rahimzadelh H and Bahrami M 5 Cavitation detection of a centrifugal pump using Noise Spectrum ASME, International Design Engineering Technical Conferences & Computers and Information in Engineering Conference (California, USA, 5) [9] Dazin A, Charley J, Bois G and Caignaert G 1 Pressure fluctuations in the suction and delivery pipes and in the volute of a radial flow pump in non-cavitating and cavitating operating conditions ASME Fluids Engineering Division Summer Meeting (New Orleans, USA, 1) [1] Pavesi G, Gavazzini G and Ardizzon G 8 International Journal of Rotating Machinery 8 Paper [11] Pavesi G, Gavazzini G and Ardizzon G 8 Int. J. Heat Fluid Flow 29(8) [12] Yao Z F, Wang F J, Qu L X and Xiao R F et al. 211 ASME J. Fluids Eng. 133(1) 1133 [13] Douglas L J and Richard G B 1995 IEEE Transactions on Signal Processing 43(1) [14] Pan L P 4 Large Electric Machine and Hydraulic Turbine (2)