cavitation in centrifugal pumps connected in series and parallel

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

CAVITATION IN CENTRIFUGAL PUMPS CONNECTED IN SERIES AND PARALLEL Ahmed A. B. Alarabi

and Ali M. Alsalmi

[email protected] [email protected] Department of mechanical Engineering, College of Engineering Technology Hoon, Libya Abstract Design, operation and refurbishment of centrifugal pumps are strongly related to cavitation flow phenomena, which may occur in either the rotating runner-impeller or the stationary parts of the centrifugal pumps. The current research concerning with cavitation detection ( inception, developed and breakdown ) degrees. A special test rig was designed and constructed in fluid mechanical laboratory at College of Engineering Technology – Hoon, Libya. The flow system has the ability to test the pumps as a single unit or as a two pumps connected in series or parallel. The results show that connecting the pumps in parallel gave a better resistance to cavitation than series connection and single pump operation. Key words: Centrifugal pumps, cavitation, impeller, NPSH, cavitation inception.

‫ظاهرة التكهف فً مضخات الطرد المركسي المىصلة على التىالً وعلى التىازي‬ ‫الملخص‬ ‫ وانخً قذ ححذد‬،ٌ‫ٌؼخبش حصًٍى وحشغٍم وحجذٌذ يضخاث انطشد انًشكضي رو اسحباط قىي بظاهشة انخكهف فً أَظًت انسشٌا‬ ‫ انبحذ انحانً ٌخطشق إنى إٌجاد ظاهشة‬.‫إيا فً األجضاء انذواسة يزم انذفاػت أو األجضاء انزابخت نًضخاث انطشد انًشكضي‬ ‫ انًُظىيت انًؼًهٍت حى حصًًٍها وحُفٍزها بًؼًم‬.‫انخكهف بذسجاحه انزالد حكهف االسخهالل وانخكهف انًخطىس وحكهف االَهٍاس‬ ٍٍ‫ انًُظىيت ًٌكٍ اسخخذيها الخخباس يضخت يفشدة أو يضخخٍٍ يىصهخ‬.‫ نٍبٍا‬،ٌ‫يٍكاٍَكا انًىائغ بكهٍت انخقٍُت انهُذسٍت – هى‬ ‫ وقذ أربخج انُخائج أٌ حىصٍم انًضخاث ػهى انخىاصي أػطى يقذسة أكبش نًقاويت ظاهشة انخكهف‬.‫ػهى انخىانً أو انخىاصي‬ .ً‫يقاسَت بانًضخت انًفشدة أو بانًضخخٍٍ فً حال انخىصٍم ػهى انخىان‬ .‫ اسخهالل انخكهف‬،ً‫ اسحفاع انسحب انًىجب انصاف‬،‫ انذفاػت‬،‫ انخكهف‬،‫ يضخاث انطشد انًشكضي‬:‫الكلمات المفتاحية‬

increased vibration and noise. The pump is said to be cavitating. Depending on its severity and duration, cavitation can cause pump damage, failure, or impairment of performance. Cavitation can occur at both the suction and discharge sides of an impeller. Most suction side cavitation is caused by insufficient NPSHA, whereas most discharge side cavitation is caused by

Introduction Cavitation is the localized formation and subsequent collapse of cavities, or bubbles, in a liquid. The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing that result in localized low pressure points. As the flow moves from a low pressure point to a high pressure point inside the casing, the cavities collapse resulting in 10

‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

discharge flow recirculation when the pump operates at low flow whereby a higher amount of liquid recirculates back to suction. The flow recirculation creates localized low pressure points causing the formation of cavities at the discharge side of the impeller. This can cause what is referred to as discharge cavitation. M. Cudina (2003), stated that to prevent the onset of the cavitation, the beginning of cavitation phenomenon should be known. The detection process was carried using noise technique. Experiments showed that there was a discrete frequency tone within the audible noise spectra, at 147 Hz or BPF=2; which is strongly dependent on the cavitation process and its development. The discrete frequency tone at 147 Hz was separated from the noise spectra of cavitating pumps and then used to detect the incipient of cavitation and its development, and to determine required the net positive suction head. An alarm was connected to the system which will give a signal at the beginning of cavitation. Al-Arabi and Selim, (2007) presented a theoretical model for the prediction of the incipient of cavitation in centrifugal pumps. The model includes the physical fluid parameters and the real working phenomena at the offdesign condition. The parameters considered in the model were, flow rate ratio, pump rotational speed, water temperature, thermodynamic properties of water, nuclei and gas content, relative velocity and incidence angle. The model showed that the thermodynamic effects were more complex than other parameters. The model has been tested against extensive published experimental results for centrifugal pumps over a wide operating condition. The comparison of the predicted net positive suction head at inception with the published data showed a good agreement. Al-Arabi and Selim, (2009) studied the reality of cavitation occurrence in centrifugal pumps. They proved that the first appearance of vapour bubbles was observed earlier than 3 % drop in the head. Therefore

using 3 % drop in head, as indication parameter for cavitation inception has always resulted in damage in pumps during operation. The cavitation inception point was decided visually and a special correlation between the visual net positive suction head and net positive suction head corresponding to 3%drop in head was predicted. Huan Xu et. Al (2016), have carried an experimental work concerning cavitation dose in an ultrasonic cleaner and its dependence on experimental parameters. A higher-sensitivity method for quantifying cavitation activity as a cavitation dose based on passive cavitation detection was presented. The influences of various factors (e.g., insonation duration, driving power, gas content, temperature and cleaning agents) were studied for this system. The results showed that cavitation becomes unstable at long insonation times, and that this instability is more pronounced at high power. They concluded that increasing the power, gas content, and adding a cleaning agent increases the probability of cavitation occurrence. Bram Verhaagen, et al, (2016) , reviewed several techniques for measuring the presence and amount of cavitation, and for the quantification of cleaning. They reported that after reviewing chemical, physical, and biological studies, a universal cause for the cleaning effects of bubbles cannot yet be concluded. They proposed an ‘‘ideal sensor’’ with high spatial and temporal resolution to investigate bubble jetting, shockwaves, streaming, and even chemical effects, by correlating cleaning processes with cavitation effects, generated by hydrodynamics, lasers or ultrasound. Fanxiao Meng et. Al., (2017) constructed a test bench of a multistage centrifugal pump using R123 as working fluid in simulative organic Rankine cycle (ORC) conditions. Experimental results of the pump under various operating conditions were obtained based on controlled working frequency and 11

‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

mass flow rate. Results show that increasing the working frequency of the pump can improve pump efficiency, evaporating pressure, thermal efficiency and net power of the ORC system. Also the back work ratio BWR and specific pumping work increase substantially with the evaporating temperature. They stated that the superheat degree has limited effects on BWR and thermal efficiency.

diagram shown in figure 1. The flow system consists of two centrifugal pump using AC current and maximum rotational speed of 3000 rpm with power of 1 hp each, the flow orifice meter, pressure measuring devices, suction and delivery pipelines, and valves. The temperature has been measured by a thermometer, ranging from -50ºC to 110ºC with an error of ± 1ºC. The temperature has been checked continuously at different interval times and at different points in the system such as at the tank suction point, at the pump inlet, and at the end of the delivery line, in order to avoid any deviation in temperature measurements. The maximum allowed deviation in temperature was ± 2°C. The accuracy of suction and delivery pressure gauges was ± 0.02 bar.

B.K. Sreedhar et. al. (2017), presented a review study to understand cavitation damage. The study covered the theoretical formulation of cavitation bubble collapse and the estimate of bubble collapse pressure, the techniques for measurement of cavitation damage in the laboratory and the special facilities for measurement of cavitation damage in sodium, the instrumentation for measurement of collapse pressure during cavitation as well as the work done in predicting damage from material properties. The limited success in achieving good damage prediction was discussed. They reported that the major reason for the limited success of cavitation erosion models based on bulk mechanical properties may be due to the inability to account for the behavior of materials under the very high strain rates (104– 106 /s), as occurs under cavitation ,as opposed to the almost static loading conditions under which bulk mechanical properties are measured. Another reason for the limited success in damage prediction may be attributed to the change in the behavior of materials (for e.g. strain hardening) under cavitation loading

The test rig was designed and constructed in such away that four flowing lines can be used two for each pump, the third is two series pumps line and the fourth is the two parallel pumps line. The number of elbows, the pipe fitting, valves and the pipes are considered to be as less as possible to the minimize the major and minor friction losses. After assembling all the parts of the system the pumps was operated with gradually increase in pressure in order to check out for any leakage or blockage in flowing lines. The operating point of each condition has been obtained experimentally, and good performance curves was established for every condition. In parallel connection flow in suction was designed to be controlled by two ways, in the first the control is carried by one main suction valve (2), in the second way the control is carried selves (3). The parallel pump performance has been carried using the two ways of control in the suction line and no differences in the results has been achieved.

EXPERIMENTAL WORK All parts of the test rig were designed, to withstand the working stress levels. The pump was specified to overcome the head losses in the rig. The general arrangement of the test rig is indicated in a schematic

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

Fig. 1 Experimental test rig

B. and comparing them with series and parallel connection. Table 1 shows the B.E.P. in flowing system and the corresponding points to the B.E.P.

EXPERIMENTAL RESULTS AND DISCUSSION The pump performance in each connection has been obtained in order to check out the best efficiency point (B.E.P). of pumps A &

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

Table 1: Performance data of pumping systems Item

Unit

B.E.P. HB.E.P. Q B.E.P. Qmax

% m Lit/sec Lit/sec

Pump A 24.455 23.97 1.3 1.72

Pump B 26.3562 26 1.24 1.66

Pumps A&B Series 29.7283 47.75 1.561215 2.149351

The experiments were carried out at almost equal room temperature and atmospheric pressure in order to obtain more accurate results. The experimental readings were taken with more care, tabulated and then plotted graphically. Figures 2 and 3 show the H-Q curves for pumps A and B respectively. Q-H curves were taken at

Pumps A&B Parallel 32.61876 26.826 3.093763 3.57237

different flow ratios varied from 0.5 to 1.25. By comparing the two figures it can be noticed that high similarity between the results were obtained. At each curve with increasing the flow rate the pump head tends to decrease until reaching break down point due to high flow velocity.

Fig. 3 H-Q curve for pump B at various flow rate ratios

Fig. 2 H-Q curve for pump A at various flow rate ratios

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

Figures 4 and 5 show the H-Q curves for pumps A and B connected in series and parallel respectively. The curves were taken at same flow ratios that varied from 0.5 to 1.25. It can be seen in series connection the pump head increased to about the twice at

each flow rate ratio compared with the head of pump A and B. In parallel connection it can be seen that the flow rate values increase to about the twice. Both figures are agreed with logic of series and parallel connection.

Fig. 4 H-Q curve of pumps A&B connected in parallel at various flow rates

Fig. 5 H-Q curve of pumps A&B connected in parallel at various flow rates

Figures 6 and 7 show the η-Q curves for pumps A and B respectively. η-H curves were taken at different flow ratios varied from 0.5 to 1.25. By comparing the two figures it can be noticed that the difference between the B.E.P. pumps A and B was about 2%, which is not big value. Both figures show that the pump efficiency increases with the increase of flow rate ratio.

Figures 8 and 9 show the η-Q curves for pumps A and B connected in series and parallel respectively at flow ratios varied from 0.5 to 1.25. It can be seen that using series and parallel connection the B.E.P. increases by 5% compared with B.E.P. of pumps A and B. It can be stated that in parallel connection highest B.E.P. and highest flow rate value were achieved.

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

.

Fig. 7 η-Q curve for pump A at various flow rate ratios

Fig. 6 η-Q curve for pump A at various flow rate ratios

Fig. 8 η-Q curve of pumps A&B connected in series at various flow rates

Fig. 9 η-Q curve of pumps A&B connected in parallel at various flow rates

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

Cavitation performance test of pumps A and B was shown in figures 10 and 11 respectively. The results indicate that for all tested conditions the NPSH depends strongly on the flow rate ratio. It can be seen that the pump head at the beginning of each test tends to be almost constant. This range contains the cavitation inception which the first degree of cavitation. Cavitation in this range appears as small nucleus of gas bubbles some times can not be seen. With farther reduction in NPSH developed cavitation starts and cloud of gas bubbles appears. Continuous reduction in NPSH breakdown cavitation starts and the cavities cover the suction side of pump and transferred with the flow to the delivery side. Breakdown cavitation denoted by the constant vertical head line, where the pump head drops suddenly at constant NPSH. Figures 12 and 13 show the cavitation performance of pumps A and B connected in series and parallel respectively.

The figures have the same trend of figures 10 and 11 with some differences in the values head due to the series and parallel connection. Figure 14 (a,b,c and d) shows the effect of the type of connection on the cavitation performance curves. At low flow rate ratios overlapping between the results occurred to presence of flow circulation. This tends to entrain nuclei within their low-pressure core where gaseous cavitation can occur. In the circulation core the value of NPSH is fluctuating due to instability of the flow at the suction side of the pump. At higher flow rate ratio specially at the optimum condition ( Q/Qopt = 1.0 ) and above the pumps connected in parallel got the best results in cavitation tests, where the breakdown cavitation occurs later compared with other pumping system.

(a)

(b)

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

(c)

(d)

Fig. 14 Variation of pump head with NPSH at different flow rates and various pumping system

[2] Al-Arabi and Selim, " A theoretical model to predict cavitation in centrifugal pumps" 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics - Sun City, South Africa, July 2007

CONCLUSION 1) At any flow rate the pump head and pump efficiency increase with increasing the flow rate ratio. 2) At high flow rate ratio, breakdown cavitation occurred earlier when using a single pump and pumps in series, while at low flow rate ratios, breakdown cavitation occurred earlier when using pumps in parallel connection.

[3] Al-Arabi and Selim, " Reality of cavitation in centrifugal pumps" SET2008 – 8th Internal Conference on Sustainable Energy Technologies; Aachen, Germany. 31th August – 3rd September, 2009 [4] Huan Xu, Juan Tu , Feng Niu, Ping Yang, " Cavitation dose in an ultrasonic cleaner and its dependence on experimental parameters", Applied Acoustics 101 (2016) 179–184, 2016 Elsevier Ltd. [5] Bram Verhaagen a, David Fernandez Rivas, "Measuring cavitation and its cleaning effect" Ultrasonics Sonochemistry 29 (2016) 619–628, Elsevier Ltd [6] Fanxiao Meng, Hongguang Zhang, Fubin Yang, Xiaochen Hou, Biao Lei,

3) At optimum flow condition which is the best efficiency point it was obtained that operating pumps in parallel gave good resist to cavitation compared with operating the pumps in series or separately. REFERENCES [1] M. Cudina, " Detection of Cavitation Phenomenon In a Centrifugal Pump using audible Sound", Mechanical Systems and Signal Processing (2003) 17(6), 1335–1347, 2003 Elsevier Science Ltd 18

‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬

JOURNAL OF ENGINEERING RESEARCH AND APPLIED SCIENCES (JERAS) 4th Edition –Volume (1) - December 2017, ( Hoon – Libya ), ISSN 2414-6515

Lei Zhang, Yuting Wu, Jingfu Wang, Zhicheng Shi, " Study of efficiency

[7] B.K. Sreedhar, S.K.Albert, A.B.Pandit, "Cavitation damage: Theory and measurements – A review", Wear 372373 (2017) 177–196, Elsevier Ltd.

of a multistage centrifugal pump used in engine waste heat recovery application" Applied Thermal Engineering 110 Elsevier Ltd.

(2017)

779–786,

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‫يجهت انبحىد انهُذسٍت وانؼهىو انخطبٍقٍت‬ ) ‫ ( هىٌ – نٍبٍا‬2017 ‫) – دٌسًبش‬1( ‫انؼذد انشابغ – انًجهذ‬