Air Bubble and Cavitation Vibration Signatures

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Air Bubble and Cavitation Vibration Signatures of aCentrifugal Pump

SpectraQuest Inc.8205 Hermitage RoadRichmond, VA 23228

(804)261-3300www.spectraquest.com

May, 2006

Abstract: In this work, a centrifugal pump was tested for its vibration signatures underdifferent operational conditions. The two abnormal operating conditions studied are airbubble and cavitation. A transparent plastic cover was used in the experiments to observethe cavitation. It was found that the pump has higher vibration amplitude in the axialdirection than in the radial direction. From the experiments, it was also determined thatsignificant amount of air bubbles will increase vibration component associated withimpeller vane pass frequency significantly. Cavitation might excite high frequencystructural resonance. It may also reduce the impeller vane pass frequency vibration.Although cavitation is less likely to happen on a slow speed pump, it will develop veryfast if it happens.

1. IntroductionA pump is a mechanical device used to move liquids. Mechanical energy is transformedinto hydraulic energy at the pump. Pumps can be classified into two categories:

displacement pumps and centrifugal pumps. In this work, a centrifugal pump was testedand studied.

The essential elements of a centrifugal pump are:1. the rotating element, consisting of the shaft and the impeller2. the stationary element, consisting of the casing, stuffing boxes, and bearings.

Figure 1 illustrates the single stage bronze centrifugal pump used in this work. This pumphas a single rotating metal impeller. Liquid enters at the center and is thrown outwardradially by centrifugal force. The five impeller vanes can be identified in Fig.1 clearly.


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Figure 1. Centrifugal Pump

In a centrifugal pump, a rotating element called an impeller is enclosed in a case. Thefluid to be pumped enters into the case through the suction piping into the impeller and isforced out the discharge side of the pump by means of centrifugal force pushing the fluidout radially through the impeller. The fluid is discharged at a higher pressure and a highervelocity. The major portion of the velocity energy is then converted into pressure energyby means of a volute or by a set of stationary diffusion vanes surrounding the impellerperiphery. In a pump, vibration is caused by the interaction between the moving pumpimpeller and the stationary parts of the pump such as the volute and the diffuser vanes.Also, vibration is caused by the interaction between the impeller blades and the fluidbeing pumped.

One of the important phenomenon in pump is cavitation. Cavitation occurs when thepressure of the fluid drops below the vapor pressure for the temperature of the fluid.When this pressure drop occurs, whether it is a system pressure drop or a localizedpressure drop, voids or cavities (bubbles) will form in the liquid. These bubbles implode

or collapse when the fluid moves through impeller to the high pressure side of the pump,causing the impeller to erode. These implosions tear out tiny pieces of the metallicsurface near which they implode. This can be very damaging and eventually the impellerwill fail. Fig. 2 shows a schematic representation of the cavitation process.

Figure 2. Cavitation

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There are three common causes of vapor formation in a liquid:1. Flow separation of a viscous fluid from its guiding surface due to a surface

discontinuity.2. The addition of heat to the fluid, raising its vapor pressure (boiling point).3. Reducing the pressure of the fluid to below its vapor pressure.

One important terminology in pump theory is net positive suction head (NPSH). NPSH isa measure of the difference between the total suction head and the fluid vapor pressure.The concept of NPSH is related with cavitation closely. For a specific pump, there are therequired NPSH and available NPSH. The required NPSH is the factory suggested valuewhich must be maintained to prevent the happening of cavitation. The available NPSH isthe real pressure difference between the suction head and the fluid vapor pressure.

2. Experimental SetupThe pump was installed on the machinery fault simulator (MFS) which is shown in Fig.2.

Figure 3. Machinery Fault Simulator (MFS)

The pump was mounted at the lower right hand corner of the MFS base plate and coupledto the rotor shaft by two drive belts. The transmission ratio of the belt drive is 1:1. Waterwas used as the liquid in the experiment.

The pump and tank configurations during the experiment are illustrated in Figs. 3 and 4.The suction and discharge sides of the pump are fitted with pressure gauges. The pumpdischarge is directed through a manual modulating valve and then a flowmeter back intothe head tank. Two single axis accelerometers were glued on the pump in the radial and

axial directions respectively.

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Figure 4. Pump during test

Figure 5. Tank

The vibration data were collected by using a SpectraQuest software/hardware system.

3. Experimental ProcedureThe experiments are categorized into two groups. In the first group, the original brasspump was tested. In the second group, the original brass pump cover was replaced with atransparent plastic cover to observe the liquid motion inside the pump.

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3.1 Brass Cover Pump ExperimentsFirst, the pump was running around 3600 rpm to check the integrity of the system.Through the transparent hose connected with the pump suction end, it was noticed thatsignificant amount of air bubbles were sucked into the pump. The pointer on the pumpdischarge pressure gauge was vibrating. It was found that the air bubbles were created by

the returned water hitting the water inside the tank. Before the air bubbles were explodingand disappearing, they were sucked into the pump. By directing the water returning hoseto the wall of the tank, the returning water would return to the tank smoothly withoutcreating too much air bubble. The vibration data were collected for both the cases withand without air bubbles.

Next, the tank discharge valve was turned 45 degrees to restrict the flow rate into thepump. This caused the pressure on the pump suction end to drop. This might cause thewater to cavitate as discussed earlier. The water vapor pressure under room temperatureis 0.935 inHg. The atmosphere pressure is about 29 inHg. In order to prevent thehappening of water vaporization, the pressure of the pump suction end has to be higher

than NPSHrequired. We did not have the exact NPSHrequired data for this specific pump.Generally, the NPSHrequired is decreasing with flow rate or pump speed. On the otherhand, the NPSHavailableis increasing with flow rate and pump speed. As a consequence, itcan be argued that the possibility of cavitation is much smaller for low speed pump thanhigh speed pump.As the supply to the pump was restricted, the flow rate dropped. Because of the lowerflow rate and smaller impact force as the water returning to the tank, no significantamount of air bubble appears. Vibration data were collected and used for latercomparison.

The speed of pump was then decreased to around 2400 rpm. It was found that under thisspeed, the air bubbles did not appear anymore. The water flow is proportional to thepump speed-the higher the speed the greater the flow. Therefore, the flow rate under 2400rpm pump speed is lower than that of under 3600 rpm. The smaller impact force causedby the slower flow rate is not large enough to create the air bubble. Vibration data for thenormal operating status and cavitation status were collected.

Finally, the pump was running at around 1200 rpm. As the case for speed 2400 rpm, nosignificant amount of air bubbles were created. Vibration data for the normal operatingstatus and cavitation status were collected.

In the data acquisition process, the frequency limit was set at 2 KHz. Four seconds ofdata were collected for each case.

3.2 Plastic Cover Pump ExperimentsThe purpose of installing the plastic cover is to observe the cavitation phenomenon. Withthe brass cover, we have no definite answer as to whether there is cavitation or not. Wecan just give the best estimation as we can. However, with the transparent cover, we candetermine the cavitation formation with full certainty. Therefore, we can correlate thevibration signatures with the cavitation situation without uncertainty.


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The procedures for experiments with plastic cover are similar with those of brass cover.The pump was running at different speeds and at every speed, the tank discharge valvewas closed slowly to determine the threshold of the emerging of cavitation. The valvewas closed continuously until severe cavitation was observed. The pump suction headpressure, the pump discharge pressure and the pump vibration in the radial as well as

axial directions were measured and recorded.

In the data acquisition process, the frequency limit was set at 20 KHz. Twenty seconds ofdata were collected for each case.

4. Experimental Observations and Results4.1 Brass Cover ExperimentsThe acceleration spectra are presented in Fig. 6 for pump speed of 3588 rpm without airbubble and cavitation. Figs. 6 (a) and (b) display the acceleration spectrum in the pumpredial and axial directions respectively. The fundamental 1X component and itsharmonics can be identified. The fifth harmonic which corresponds to the impeller vane

pass frequency (Because there are five vanes on the impeller) has the highest amplitude.Moreover, two impeller vane pass frequency harmonics also have high amplitude. Acomparison of the amplitude of Figs. 6 (a) and (b) indicates that the pump has highervibration in the axial direction.

(a) radial acceleration

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(b) axial accelerationFigure 6. Vibration Spectrum for Pump Speed 3588 RPM

(without air bubble and extreme low suction head pressure)

The acceleration in the radial and axial directions is presented in Fig. 7 for pump speed of3590 rpm with significant amount of air bubble formed in the tank. A careful inspectionof Fig. 6 (a) and Fig. 7 (a) indicates that with formation of air bubble, the vibrationcomponent associated with impeller vane pass frequency increase significantly. Thevibration amplitudes of 1X and its other harmonics components do not change too much.A comparison of Fig. 6 (b) and Fig. 7 (b) suggests similar trend. An examination of Figs7 (a) and (b) indicates higher vibration level on the pump in the axial direction.


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(b) axial acceleration

Figure 7. Vibration Spectrum for Pump Speed 3590 RPM

(with air bubble )

The suction head pressure was dropped below atmosphere pressure by approximately 20inHg in the cavitation test for pump speed 3595 rpm. There is a great possibility thatcavitation will appear under this condition.

The acceleration in the radial and axial directions is presented in Fig. 8 for pump speed of3595 rpm with cavitation formed in the pump. Because there is no significant amount ofair bubbles formed during the cavitation test, a comparison of Figs 8 and 6 is appropriate.A careful inspection of Figs. 6 (a) and 8 (a) indicates that there is a frequency componentaround 1600 Hz emerging in the cavitation signal. In Fig. 6 (a), the background noise has

a almost constant level which does not show in Fig. 8 (a). The 1X and its harmonicscomponents have similar amplitude levels in Fig. 6 (a) and Fig. 8 (a). A comparison ofFig. 6 (b) and Fig. 8 (b) has the same conclusions.


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(b) axial acceleration

Figure 8. Vibration Spectrum for Pump Speed 3595 RPM(with cavitation )

The pump speed was reduced to around 2400 rpm. The suction head pressure wasdropped below atmosphere pressure by approximately 15 inHg in the cavitation test.There is a possibility that cavitation will appear under this condition.

The acceleration in the radial direction is presented in Fig. 9 for pump speed around 2400rpm. Fig. 9 (a) presents the data spectrum for pump speed 2355 rpm without cavitation.

Fig. 9 (b) presents the data spectrum for pump speed 2360 rpm with a possibility ofcavitation. Similar with the cavitation case with pump speed around 3600 rpm, there is avibration component around 1700 Hz for the case with cavitation possibilities.

(a)radial acceleration (without cavitation)9


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(b) radial acceleration (possible cavitation)

Figure 9. Vibration Spectrum for Pump Speed around 2400 RPM

The pump speed was then reduced further to around 1200 rpm. By turning the tankdischarge valve to restrict the flow rate, the suction head pressure could be droppedbelow atmosphere pressure by approximately 5 inHg in the cavitation test. The pressuredrop could not be increased further because of the low pump speed. It is not likely thatcavitation will happen.

The acceleration in the radial direction is presented in Fig. 10 for pump speed around1200 rpm. Fig. 10 (a) presents the data spectrum for pump speed 1166 rpm without flowrate restriction. Fig. 10 (b) presents the data spectrum for the same pump speed with aflow rate restriction. As expected, there is no significant difference between Figs. 10 (a)

and (b).

(a)radial acceleration (without flow rate restriction)

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(b) radial acceleration (with flow rate restriction)

Figure 10. Vibration Spectrum for Pump Speed around 1200 RPM

4.2 Brass Cover ExperimentsAs discussed earlier, the pump was running at different speeds. At each individual speed,the pump suction head and discharge pressures were read from the two pressure gaugesconnected with the pump for three tank discharge valve position: 1) the valve is full open2) the valve is closed somewhat until the appearance of cavitation 3) the valve is closedcontinuously until severe cativation is observed. The pressures are shown in Table 1.

Pump RPM Valve position Pump HeadPressure (inHg)

PumpDischarge

Pressure (psi)3600 Full open -5 14~15

(depends onair bubble)

Cavitation appear -13 13Severe cavitation -20 9

3000 Full open -4 11Cavitation appear -20 5.5Severe cavitaiton -21 3.5

2400 Full open -2.5 8Cavitation appear -18 3

Further valveclosing will cut

water off1800 Full open -1.5 5

No cavitation can begenerated

Table 1. Pump Pressure

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From Table 1, it can be found that the NPSHavaible and pump discharge pressure are allincreasing with pump speed. Another observation from Table 1 and the experiments isthat the NPSH range from the cavitation appearance to fully developed severe cavitationis also increasing with a increase in pump speed. For example, for pump speed 3600 rpm,the NPSH for cavitation appearance is -13 inHg. While the NPSH for severe cavitation is

-20 inHg. It has a 7 inHg pressure difference. For pump speed of 3000 rpm, the NPSH forcavitation appearance is -20 inHg, the NPSH for severe cavitation is -21 inHg. Thepressure difference is only 1 inHg. Moreover, for pump speed 2400 rpm, the cavitationappears at -18 inHg NPSH. And the cavitation develops into severe cavitation veryquickly. This observation indicates that although it is less likely for a slow speed pump tohave the problem of cavitation, however, the cavitation will develop quickly into severecondition if it happens.

Figure 11 illustrates the vibration spectra in radial and axial directions respectively forpump speed 3619 rpm with the tank discharge valve full open and without air bubble.Figures 11 (a) and (b) present the spectrum of pump radial and axial vibration with 20

KHz frequency limit respectively. Figures 11 (c) and (d) display the same spectra in the 1KHz frequency range.

(a) Radial Vibration (20 KHz)

(b) Axial Vibration (20 KHz)

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(c) Radial Vibration ( 1 KHz)

(d) Axial Vibration (1 KHz)

Figure 11. Pump Vibration with Tank Discharge Valve Full Open (3619 RPM)

Figure 12 illustrates the vibration spectra in radial and axial directions respectively forpump speed 3616 rpm with the appearance of cavitation. Figures 12 (a) and (b) presentthe spectrum of pump radial and axial vibration with 20 KHz frequency limitrespectively. Figures 12 (c) and (d) display the same spectra in the 1 KHz frequencyrange.

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Figure 12. Pump Vibration with Appearance of Cavitaiton (3616 RPM)

Comparing the corresponding subfigures in Fig. 12 and Fig. 11, Fig. 12 (a) and Fig. 11(a) have the largest difference. In Fig. 12 (a), there are several peaks emerging around 6KHz with the characteristics of structural resonance.

Figure 13 illustrates the vibration spectra in radial and axial directions respectively forpump speed 3617 rpm with severe cavitation. Figures 13 (a) and (b) present the spectrumof pump radial and axial vibration with 20 KHz frequency limit respectively. Figures 13(c) and (d) display the same spectra in the 1 KHz frequency range.


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Figure 13. Pump Vibration with Severe Cavitaiton (3617 RPM)

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In Fig. 13 (a), the peaks emerging around 6 KHz with the characteristics of structuralresonance are clearer. An inspection of Fig. 13 (c) indicates that the amplitude of thevibration component with impeller vane pass frequency (the fifth harmonic of 1X) hasdecreased significantly. However, this phenomenon does not appear for the pump axialvibration. The vane pass frequency vibration is still strong as illustrated in Fig. 13 (d).

Figure 14 illustrates the vibration spectra in radial and axial directions respectively forpump speed 3007 rpm with the tank discharge valve full open and without air bubble.Figures 14 (a) and (b) present the spectrum of pump radial and axial vibration with 20KHz frequency limit respectively. Figures 14 (c) and (d) display the same spectra in the 1KHz frequency range.



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Figure 14. Pump Vibration with Tank Discharge Valve Full Open (3007 RPM)

Figure 15 illustrates the vibration spectra in radial and axial directions respectively forpump speed 3010 rpm with the appearance of cavitation. Figures 15 (a) and (b) presentthe spectrum of pump radial and axial vibration with 20 KHz frequency limitrespectively. Figures 15 (c) and (d) display the same spectra in the 1 KHz frequencyrange.

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Figure 15. Pump Vibration with the Appearance of Cavitation (3010 RPM)

Figure 16 illustrates the vibration spectra in radial and axial directions respectively forpump speed 3010 rpm with severe cavitation. Figures 16 (a) and (b) present the spectrumof pump radial and axial vibration with 20 KHz frequency limit respectively. Figures 16(c) and (d) display the same spectra in the 1 KHz frequency range.


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Figure 16. Pump Vibration with Severe Cavitation (3010 RPM)

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Air Bubble and Cavitation Vibration Signatures