RETRACTED ARTICLE: Experimental study of cavitation criterion in centrifugal pumps

Abstract

Pumps are designed and manufactured to operate far from the cavitation condition. Pump designers define the cavitation inception point as the point where the head drops by 3 %. This definition is incorrect, because the first appearance of vapour bubbles is 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. This paper describes visual studies conducted to determine the variation of net positive suction head with flow rate, rotational speed and water temperature. The experiments showed that the visual incipient of cavitation in the pump was existed long before drop in pump head occurs. A special correlation between the visual inception cavitation net positive suction head and net positive suction head corresponding to 3 % drop in head was developed at various operating conditions.

Graphical Abstract

1 Introduction

The present study was intended to investigate visually the changes in flow rate, suction pressure, rotational speed and temperature of pumped water on the cavitation performance of centrifugal pumps. These factors are generally termed thermodynamic effects. Practical difficulties in cavitation testing and obtaining data from the vicinity of a rotating impeller have reduced the availability of consistent information on cavitation effects. The state of knowledge is such that quite fundamental effects may still be overlooked. As the pump is throttled up, its characteristic, the pressure in the suction peak region may drop below the vapor pressure resulting in strong local cavitation. The studies on these aspects are numerous and comprehensive.

For pump designers, knowledge of the cavitation phenomenon is very important for proper design. Some pump designers have defined the critical cavitation coefficient as the point where the head drops by 3 %. This is because, it was believed that if the performance was not affected no cavitation occurred and that the head first started to drop was the point of cavitation inception. This definition does not show the real state of cavitation inception in the pump, and that incipient of cavitation may exist long before the performance is affected (Pearsall 1982). At this point where the head drops by 3 % cavitation erosion was noticed. Reliance on this definition can cause errors in the prediction of efficiency and performance at the design stage.

Therefore, there is clearly need for reliable data on the variation of the visual net positive suction head corresponding to visual incipient of cavitation through the perspex cover of the pump (NPSH_iv) and NPSH_3% corresponding to 3 % drop in head at various operating conditions, with a view to develop possible relationships between them. Visual incipient of cavitation refers to the first appearance of a limited cavitation zone on the leading edge of the blades of the impeller. This achieves visually by eye using stroboscopic light through the transparent cover of the pump.

El-Kadi (2001) has studied the cavitation in a centrifugal pump handling hot water from (28 to 58 °C). The suction side of the pump was made of transparent perspex material to notice out the cavitation degree at the corresponding flow condition, but no results between visual net positive suction head and net positive suction head corresponding to 3 %drop in head were mentioned. Hirschi et al. (1998) have carried out a numerical prediction in the performance drop of centrifugal pumps due to leading edge cavitation. They used a new 3-D numerical method based on 2-D profile done by Yamaguchi and Kato (1983). They compared the numerical prediction results with the test results for the behavior of cavitating flow in centrifugal pumps. The comparison was very good.

Sloteman (2007) has studied the cavitation in high energy centrifugal pump for detecting and assessing the damage potential. The detection of cavitation was based on the noise and visualization technique measurements. He reported sufficient information about the acoustic technique used by the previous researchers. For noise level measurements, a waveform of six different sensors positioned very nearly in phase has been used, and a special transparent window was made on the pump for flow visualization. It was observed that changes in cavitation level can be measured at many locations on the pump and piping. He concluded that there still is a need for detection tools that can serve aftermarket concerns of pumps already in the field and that also can be applied to new design optimization efforts of manufacturers while on the test stand.

Wulff (2006) introduced an overview on the basic principles of digital particle imaging velocimetry (DPIV) and the main components of standard systems. Cameras and laser head were mounted separately on two stepper motor controlled 3-axis petitioners. The stereoscopic DPIV was set-up at the pump test rig and directed to the transparent window for visualization process. Using DPIV permits measurement of three velocity components, and recording different shapes of nuclei. Special and accurate photos have been taken at inlet section of the pump impeller, and the local values of circumferential velocities on the impeller inlet with and without casing treatment were measured.

Alfayez et al. (2004) investigated the applications of acoustic emission for detecting incipient cavitation. They presented results from an on-going program to ascertain the applicability of the acoustic emission technique for determining the best efficiency point of an operating system. A large-scale pump rig (2.2 MW) formed the basis of this investigation that included a series of NPSH and performance tests. They concluded that the results obtained demonstrate a successful use of acoustic emission technique for detecting incipient cavitation and identifying the best efficiency point a system employing pumps.

1.1 Experimental work

The flow system was designed and constructed according to the pump specifications in such a way that the pump must overcome the head losses in the system. The general arrangement of the test rig is indicated in a schematic diagram shown in Fig. 1. The pump specifications are indicated in Table 1. The pump inlet section was made of transparent material “Perspex”. The perspex face is connected with a perspex pipe of 5.08 cm inside diameter and 43 cm long to visualize any extension of swirling waves in the suction line. The impeller front shroud was manufactured from transparent perspex to permit visual observation of cavitation inception. The outer surface of original impeller was removed gently to replace it by a perspex cover. There was a special cooling coil to remove the dissipated energy in water from frictional losses through the circuit. An electromagnetic flow meter was used to measure the volume flow rate. For more accurate flow rate measurements, a special orifice was connected to a mercury manometer. Heat is supplied to the working fluid by three glass wool insulated Nichrome heater taps of 2 kW each wrapped around the lower end of the main tank. To control the rotational speed a speed inverter of type (Hitachi LI00) has been used.

Fig. 1

Cavitation test circuit

Table 1 Pump specifications

1.2 Test conditions

The visual observation of inception, developed and breakdown cavitation conditions were conducted by eye using stroboscopic light through the perspex cover of the pump. The suction static lift of 0.67 m was kept constant, and it was checked repeatedly during the tests. The flow rate ratio (Q/Q_opt) was varied from 0.3 to 1.1 with step of 0.1. The pump was operated at different rotational speeds of 2,600, 2,700, 2,800, 2,900 and 3,000 rpm. Water temperature was varied from 20 to 90 °C with a step of 10 °C. The experiments were carried out at fixed room temperature of 25 ± 1 °C to obtain more accurate results.

2 Results and discussion

2.1 Visual observation of cavitation

The purpose of this section is detailed photographic investigation of cavitation patterns to elucidate physical phenomena not previously observed or understood. The observation and photograph of cavitation through the transparent perspex casing of the pump were achieved stroboscopically by eye and using Sony digital still camera (Model DSC-S70). Photographs of cavitation behavior were obtained under a wide range of discharge throttle settings from wide open up to recirculatory condition.

Figure 2 shows a series of photographs taken at various flow rates with constant rotational speed of 3,000 rpm and temperature of 28 °C. At point (1) in which the delivery valve is fully open the breakdown of cavitation occurred and the cavity causing “shocking” of the channel between blades. A reduction in flow rate point (2) produces a large milky cavity moves on the surface of the blades. Then the cavity moves with vortex towards the middle of the channel until it reaches a point adjacent to the surface of the opposite blade. The liquid adjacent to the large-cavity surface has been observed to contain a multitude of small traveling transient cavities. When the flow rate was further reduced, point (3), the cavity appears to decrease in its volume. The cavity observed is of milky appearance and a significant fluctuation in the cavity length takes place.

Fig. 2

Photographs of cavitation occurrence at various flow rate ratios with constant temperature and rotational speed (T = 28 °C, N = 3,000 rpm)

Further reduction in flow rate point (4) the cavity appears to be a fixed sheet attached to the surface of the pressure side of the blade. At point (4) the state of alternative cavitation is observed at 0.7 flow rate ratio. The photograph at point (4) shows the appearance of two fully developed cavity attached to the pressure sides of the two blades within the impeller separated by channel without cavity.

This observation can attributed to the impeller stability problem, which is driven by the start of recirculatory flow at the inlet of the impeller. The onset of the inlet circulation changes the flow pattern in the pump inlet. In this type of cavitation, two adjacent impeller channels are operating at different points of the impeller characteristic. The cavitating channel operates at a point with a lower flow rate ratio leading to an increase in the local incidence angle. While, the un-cavitating channel operates at a higher flow rate ratio leading to a decrease in the local incidence angle. Consequently, the lower flow rate ratio means a stronger pressure rise in the channel between blades. While the higher flow rate ratio means a weaker pressure rise. The stronger pressure gradient suppresses the development of cavitation further into the channel. While the depression of static pressure rise in the next channel allows cavity to grow in it.

This finding may confirmed by the Hofmann et al. (2001) experimental results which indicated the occurrence of a periodic cavitation state called “rotating cavitation” in a wide range of part load conditions. Their results were obtained using a high technique photographic process with a 12-bit CCD-camera and a digital video camera operating in frame integrating mode with a frame-grabber device and both a stroboscopic light source and a laser-light-sheet-technique used for illumination of the optical observation.

For further reduction on the flow rate ratio point (5) and (6) the cavity appears as a cloud of small glassy appearance bubbles, the cloud grew and extended into the impeller passages. This glassy appearance is due to the growth of gas bubbles in the water by diffusion with cores of free vortex, which is established outside the boundary layer surrounding the blade surface. Vortex type flows characteristically have high velocities near the vortex core with corresponding low pressure.

Comparisons of photographs in Fig. 2 and that in Fig. 3 show that the intensity of cavitation increases with the increase of rotational speed at constant temperature. This is mainly occurred due to the recirculatory flow, which increases with increasing the rotational speed.

Fig. 3

Pliofojrapbs of cavitation occurrence at various flow rate ratios with constant temperature and rotational speed (T = 28 °C, N = 2,700 rpm)

Increasing the water temperature from 30 to 50 °C at constant speed of 3,000 rpm leads to a change in the appearance of the cavity color as shown in Figs. 2 and 4. The photographs in Fig. 4 show that the color of the cavities changed gradually and becomes white with high brightness in color. Moreover, the cavities appear to be a cloud of continues linked bubbles as shown in Fig. 4. With further increase in the water temperature to T = 80 °C at 3,000 rpm, the color of the cavities becomes intermediate between the white and the glassy color as shown in Fig. 5. Also at T = 80 °C, the bubbles will be elongated in the shape and discontinuous clouds of cavities will be observed as shown in positions 2 and 3 in Fig. 5.

Fig. 4

Photographs of cavitation occurrence at various flow rate ratios with constant temperature and rotational speed (T = 50 °C, N = 3,000 rpm)

Fig. 5

Photographs of cavitation occurrence at various flow rate ratios with constant temperature and rotational speed (T = 80 °C, N = 3,000 rpm)

2.2 Visual incipient net positive suction head

In this paper, the major goal is to develop possible relationship between visual NPSH_iv and the NPSH corresponding to 3 % drop in head. This is to provide the pump designers with more accurate criterion of cavitation inception, and will be very convenient for practical purposes. To achieve this purpose, extensive series of experiments were carried out at various operating conditions.

The NPSH of a pump can be obtained from the following equation:

$$ {\text{NPSH}} = \frac{{P_{\text{atm}} }}{\gamma } - \frac{{P_{s} }}{\gamma } - \frac{{P_{v} }}{\gamma } $$

(1)

Where: $P_{\text{atm}} ,P_{s} ,P_{v}$ are the atmospheric pressure, measured suction pressure, vapour pressure at water temperature and $\gamma$ is the water specific weight.

At the first appearance of a limited cavitation zone at the leading edge of the blade of the impeller the suction pressure and temperature are recorded and NPSH_iv is calculated from the previous equation. A typical variation of the pump NPSH and head (H) at constant flow ratio and changing the suction resistance are shown in Fig. 6. Both the NPSH_iv and NPSH_3% are shown in Fig. 6. The results of the extensive series of experiments for NPSH_iv and NPSH_3% at various operating conditions are shown in appendix A.

Fig. 6

Variation of NPSH with pump head at constant flow rate ratio

2.3 Relation between visual inception NPSH_iv and NPSH_3% corresponding to 3 % drop in head

Table 2 shows the results of the reality of cavitation at 3 % head drop at constant Q/Q_opt = 0.4 and various water temperatures and different rotational speeds. This table indicates that the visual inception condition commences long before the 3 % drop in head and developed cavitation is observed. Visual observation at 3 % drop in head shows that the length of cavity on the blade is varied from 25 to 70 % of the blade length according to the test condition. Figure 7 shows photographs of the cavitation at a 3 % drop in head. It is appeared that at 3 % drop in head the pump impeller is subjected to severe erosion that is detrimental to the life of the blades.

Table 2 Reality of cavitation according to observation

Fig. 7

Photographs of developed cavitation corresponding to 3 % drop in head at various conditions

Therefore, the experiments results indicated that the reliance on the definition of critical cavitation coefficient at the point where the head drops by 3 % can cause errors in the prediction of efficiency and performance at design stage.

In analyzing the results of cavitation tests, the relations between NPSH_iv and NPSH_3% were examined. The plots of NPSH_iv against NPSH_3% is shown in Fig. 8. This figure illustrates that the relationship between visual NPSH_iv and NPSH_3% corresponding to 3 % drop in head is independent of flow rate ratio and rotational speed. Nevertheless, it is dependent of the water temperature. In addition, this figure indicates that there is a very good relation between NPSH_iv against NPSH_3% for all the data. Equations for the regression lines were derived for Fig. 8, using the method of least squares. Values of the correlation coefficients and the regression lines are presented in Table 3. It can be seen from Table 3 that for this data set, which quite comprehensive, that the correlation coefficient for the relationship with account for temperature is better than that with all data. On the basis of these results the best correlation is in terms of water temperature. These findings are the interesting feature of this correlation. This would seem to be a most useful tool in the prediction of the actual condition of the incipient of cavitation for a pump from performance tests as are easy to conduct.

Fig. 8

NPSH_3% in head versus visual cavitation inception (NPSHiv)

Table 3 The correlation coefficients and regression lines derived from Fig. 8

3 Conclusions

The most important conclusions which can be drawn from the foregoing extensive experimental investigations are:

• 1) The visual observation by eye of the state of cavitation indicated that at the point where the head drops by 3 %, developed cavitation was appeared and the incipient of cavitation existed long before the performance is affected. At this point where the head drops by 3 % severe cavitation erosion is most likely to occur in the pump impeller. Therefore, reliance on the definition of critical cavitation coefficient at the point where head drops by 3 % drop can cause errors in the prediction performance of pump at design stage.

• 2) One of the interesting features of this investigation is that the visual net positive suction head (NPSH_iv) is well correlated with the net positive suction head corresponding to 3 % drop in head (NPSH_3%) and the water flow temperature. This relation could be defined by an equation of the type: NPSH_iv = 1.125 NPSH_3%−0.05235 T + 6.3575 The above equation can be used for predicting the actual condition of the incipient of cavitation for a centrifugal pump from performance tests as are easy to conduct.

• 3) Visual observation and photographs of the state of cavitation at various flow rate ratios and constant values of temperatures and rotational speeds showed that:

• i. At higher flow rate ratios, a milky appearance cavity was found to exist. However, at lower flow rate ratios gaseous cavity was observed.

• i. At developed and breakdown conditions, the color of the appearance of cavities changed with changing the water temperature.

• ii. Alternative cavitation commenced for low flow rate ratios (Q/Q_opt < 0.5) (i.e., near the start of circulatory flow at the pump inlet).

Appendix A

See Tables 4 and 5

Table 4 Experimental values of NPSH corresponding to 3 % drop in head

Table 5 Experimental values of visual inception NPSHiv