Online Detection of Cavitation Phenomenon in a Centrifugal
Pump using Audible Sound
1
Shrikumar Gupta, 2Vimal Kumar Chouksey & 3Manish Srivastava
1
C.M.J. University-India, 2CSVTU Bhilai-India,
3
Chonbuk National University-South Korea.
E-mail : shrisscet@gmail.com, vimalkumarchouksey@gmail.com, 84.srivastava@gmail.com
occurs due to low vapour pressure and bubble
formation. These bubbles collapse together and also
impact on the impeller blades which generate noise,
vibration, also decrease the pump performance and
damage the pump if cavitation is present in the pump for
long time.
Abstract - Experimental study has been performed on a
centrifugal pump to analyze the level of sound due to
cavitation and the change in the level of sound with the
change in discharge level. The experimental study has been
carried out in the centrifugal pumps of different capacities
.When cavitation occurs then due to low vapour pressure
the bubbles collapse. These bubbles impact on the blades,
as a result, the level of sound changes. This level of sound
has been used to detect the cavitation in the centrifugal
pump. With the change in NPSH (Net positive suction
head) the sound level varies and at a critical point, when
cavitation gets fully developed, the level of sound goes at
peak.
The noise generated by a centrifugal pump depends
on its geometry (size and form) and on the operating
conditions (speed and load). Another factor that
increases noise is pump instability. Pump instability may
be caused by stall, surge and cavitation in the pump. Stall
and surge can occur when the pump operates below the
design flow rate or BEP (Q<Qdes), whereas cavitation can
occur within the entire operating regime. Cavitation can
occur without stall and surge, and vice versa [1, 2].
Cavitation occurs more easily at higher flow rates (Q
>Qdes), due to the increased velocity of flow and the
pressure drop; therefore this type of instability can be
avoided by pushing the operating point toward lower
flow rates. But at lower flow rates, the stall and surge can
cause pump operation instability, especially with pumps
with unstable characteristics, when the surge point
appears at the flow rate, which is higher than zero (Q >
0). The cavitation as a type of noise generation
mechanism is the scope of this paper. Cavitation occurs
when the absolute static pressure at some point within a
pump falls below the saturated vapour pressure of the
fluid. At the prevailing temperature conditions, the fluid
starts to flash and vaporization occurs. Vaporisation of
the flowing fluid is connected with the onset of bubbles.
The bubbles are caught up by the flowing liquid and
collapse within the impeller when they reach a region of
higher pressure, where they condense. This process is
accompanied by a violent collapse or implosion of the
The level of sound has been measured for a centrifugal
pump at different flow rates. Different parameters related
to sound has also been measured. This shows the group of
results with specific patterns. From these results it has
been obtained that when the discharge level decreases, the
NPSH critical value (where cavitation gets fully developed)
also decreases. When this experiment is repeated, each
time similar results are obtained. The different calculations
based on sound have been performed to analyze the result.
These results help to develop an online detection technique
for centrifugal pump using audible sound.
Keywords-Pump,
Cavitation,
Technique, Audible Sound.
I.
Cavitation
Detection
INTRODUCTION
Cavitation is the formation of vapour bubbles of a
flowing liquid in a region where the pressure of the
liquid falls below its vapour pressure. At this vapour
pressure, growth of bubbles takes place which collapse
together causes vibration, noise generation and finally
known as cavitation. At the suction side cavitation
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ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
bubbles and a tremendous increase in pressure, which
has the character of water hammer, blows [3]. This
process of cavitation and bombardment of the pump
surface by the bursting bubbles causes three different,
undesirable, effects:(a) Deterioration of the hydraulic
performance of the pump (total delivery head, capacity
and efficiency), (b) possible pitting and material erosion
of the impeller blades and shrouds(and volute casing) in
the vicinity of the collapsing bubbles and (c) vibration of
the pump walls excited by the pressure and flow
pulsations, and resultant noise. Therefore, cavitation in
the pump, as an unacceptable phenomenon, should by all
means be avoided.
a distance of 0.5m .software based on Real time
analyzer, was used for the spectral analysis. Since the
NPSH values vary with flow rates, the procedure was
repeated for different flow rates.
Microphone is required so that sound can be sensed
and analyzed with the help of software and computer.
The microphone should be sensible enough that
minimum level sound pressure can also be measured.
The sound pressure due to noise is sensed by the
sensible microphone .When the microphone is placed at
some distance from the setup; connect this microphone
through the computer or laptop. Then, due to the change
in pressure level of air, the variation in the level of
sound pressure takes place which is used as a parameter
to analyze through the software.
To do this, we have to know the moment of cavitation
inception within the pump and when it gets fully
developed, especially for pumps working in industrial
environments.
Cavitation within the water pumps
(and water turbines) has been the subject of much
research, numerous studies and almost all books
describing centrifugal pumps [3–9]. According to the
available literature, there are two different ways to detect
the onset of cavitation in a liquid: numerical or analytical
modeling and engineering methods.
The microphone is placed at a distance of 0.5 meter
from the experimental setup and the experiment is
performed. The level of sound is measured by
connecting the microphone with the computer. The
variation in the level of sound is measured by the
software.
The software is based on the frequency scale from
20Hz to 20 kHz in the X- axis and decibel sound
pressure level (SPL) in the Y- axis. During observation,
the range of decibel scale varies up to 100 dB.
Numerical and analytical modeling is often used to
predict the onset of cavitation of a single bubble [4, 7–
10] and rarely within a pump. There are some wellknown models, which can be used in describing the
phenomena and behaviour of cavitation cores [4, 7–9, 11,
12]. However, there is no exact algorithm to calculate the
noise due to cavitation at the different operating
conditions of a centrifugal pump. It is well known that
the noise due to cavitation increases approximately
linearly with the flow rate, the number of bubbles, the
maximum equivalent volume of the cavitation bubbles
and their concentration per unit volume [3], but due to
the chaotic rise of the bubbles in number and size
(volume), however, there is no possibility of predicting
an exact value of the emitted noise.
The software is a real time analyzer, so graph
through the software can be directly plotted. For using
the software, computer (in which software is running) is
connected with the microphone and the exact value of
the noise level is obtained at real time.
The experiment set up diagram has been shown in
figure 1. Pump, in which experiment has been
performed, is connected with the suction pipe and
discharge pipe. Manometer and Control valve are
connected towards suction side. Pressure guage, Control
valve, and Flow meter are connected towards discharge
side.
Cavitation causes, Reduction in pump capacity,
Reduction in the head of the pump, formation of bubbles
in a low pressure area of the pump volute, a noise that
can be heard when the pump is running, and Damage
which can be seen on the pump impeller and volute.
II. EXPERIMENTAL SETUP
A 550 kW pumping set is used for the experiment.
The performance and the cavitation characteristics, as
well as the noise characteristics were measured on a
special test stand in a closed loop, according to the valid
ISO 3555 standard (quality level B) [18].To detect the
onset of the cavitation of the pump, NPSH available is
measured at a constant speed 2900 rpm and constant
flow rate. At the same time, the spectra and total emitted
level of noise were measured by a microphone placed at
Figure -1 Experiment Set Up
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According to the need of this experiment, the
available NPSH is varied and checked for the incipient
cavitation point of cavitation and for the time at which
cavitation gets fully developed.
because of flow separation. The variation in the level of
sound has been shown in the Fig 3.
III. EXPERIMENTAL RESULTS AND
OBSERVATIONS
Table -1 Specification of Pump
Flow Rates
Head
n
Power
0.150 m3 /s
80 meter
2535 min-1
550kw
Discrete Frequency Method
According to this method, the noise spectra before the
cavitation incipient and after cavitation gets fully
developed are observed. By using the software and
microphone, the sound pressure level has been observed
corresponding to NPSH. The lower graph (Fig-2)
indicates the noise level before cavitation inception and
the upper graph (Fig-2) indicates the noise level when
cavitation gets fully developed. The curve has been
plotted between noise level dB (Sound pressure level
SPL) in the Y- axis and frequency (Hz) in X-axis.
Figure-3 Difference in the noise level at the discrete
frequency 175 Hz with respect to NPSH for pump
Again the same experiment has been done for
different flow rates then the following results for same
discrete frequency at 175 Hz have been obtained. This
clearly shows the similar pattern result for different flow
rates. It has also been observed, as the NPSH critical
value and sound pressure level decreases, flow rates also
get decreased.
The maximum difference of sound pressure level
between before the cavitation incipient and after
cavitation gets fully developed is at the discrete
frequency 175 Hz of 18 dB. This initial result clearly
shows that the maximum sound difference is at 175 Hz.
Again the observation is taken for 175 Hz with respect
to NPSH.
Observations have been taken for different flow
rates 0.120 m3/s to 0.075m3/s. The observation clearly
indicates that for the change in discharge rates
0.120m3/s to 0.075m3/s at NPSH critical point, when
cavitation gets fully developed, Noise level (Sound
pressure level) changes from 86 [dB] to 72 [dB] .
At NPSH critical point the level of sound is
maximum. At 175 Hz, before cavitation incipient the
sound level for 175 Hz frequency was 68[dB] and at
fully developed cavitation (NPSH critical) the sound
level is 86 [dB] for the discharge rates of 0.120m3/s.
NPSH critical point decreases with the decrease in
discharge rates. In the fig.-4 the noise level variation for
each flow rates has been shown. For each flow rate, the
sound pressure variation has been observed which can
be used as a detection parameter for different pumps.
Figure-2 Noise spectra before cavitation inception and
after it was fully developed for PUMP
In this Fig-3, noise level (sound pressure level)
corresponding to NPSH has been observed. At 5.2 m of
NPSH the noise level obtained is 68 dB before
cavitation and at 2.8 m of NPSH the noise level obtained
is 86 dB after fully developed cavitation. After this
NPSH value the sound level decreases, this decrease is
Figure- 4 Change in NPSH and level of sound with
different flow rates for pump
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ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
The Fig.-4 clearly shows the change in level of
sound and NPSH critical value for different flow rates.
The similar pattern curve has been obtained for each
flow rate .This figure clearly indicates that the sound
pressure level at fully developed cavitation decreases as
the flow rates decreases.
V. REFERENCES
The sound pressure level at discrete frequency tone
has been used as a detection parameter and this
technique is fully automated technique to determine
cavitation in the pump.
[1]
L.Seungbae and Kh.Jung, ―Cavitation mode
analysis of pump inducer, KSME International
Journal,‖ Vol. 16No. 11, pp. 1497—1510, 2002.
[2]
M.Cudina, ―Noise as an indicator of cavitation
and instability in centrifugal pump,‖ International
Journal of Mechanical Engineering 45, pp.134–
146, 1999.
[3]
L.Nelik, ―Centrifugal and Rotary Pumps:
Fundamentals with Applications,‖ LLC, FL: CRC
Press, Journal of Mechanical Engineering, 1999.
[4]
E.Grist, , PA Taylor & Francis ―Cavitation and
the Centrifugal Pump,‖Journal of Mechanical
Engineering, 1999.
[5]
T.Uchiyama,
―Numerical
simulation
of
Cavitation flow using the upstream finite element
method,‖ Journal of Applied Mathematical
Modeling 22, pp.235–250, 1998.
[6]
D.Japikse, D.W.Marscher and R.Furst , ―Design
and Performance,‖ Journal of Mechanical
Engineering Centrifugal Pump Vermont, U.S.A.,
Concepts ETI, Inc B, 1997.
[7]
M. Fanelli, ―Some Present Trends in Hydraulic
Machinery Research, Hydraulic Machinery and
Cavitation,‖
London:
Kluwer
Academic
Publishers, Journal of Mechanical Engineering ,
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[8]
N. Bidin, R. A. Hassan and M. M. Sanagi, ―The
behaviour of cavitation bubble on low impedance
material,‖ Journal of Mechanical engineering
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Conference on Pumps and Fans, Beijing, 1995.
[9]
L.Mongeau, ―Sound generation by rotating stall
in centrifugal turbo machines,‖ Journal of
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[10]
D.G.Newland, ―Spectral and Wavelet Analysis,‖
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[11]
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University, 1986.
Table-2 Method Analysis
Method used
Discrete frequency
lies between 100 to
200 Hz
Difference in noise
level [dB]
10-25[dB]
PUMP
18[dB]
IV. CONCLUSION AND FUTRE SCOPE
The decreases in noise levels of the discrete
frequency tone at 100-200 Hz before the incipient of
cavitation and after cavitation fully developed is
between 10 and 25 dB .This is enough to use the signal
of the noise, to detect the onset of the cavitation process
and to prevent further development of the cavitation
process even in an industrial environment, where a
background noise is present. The cavitation
phenomenon causes the onset of bubble formation and
their implosion with bombardment of the pumps inner
surfaces with consequences of pressure fluctuation in
liquid, vibrations in structure and noise in the
surrounding air. In spectra of the emitted noise, there are
discrete frequency tones, which are in close correlation
with the development of the cavitation process. The
discrete frequency remains unchanged, whereas its
magnitude changes according to the development of the
cavitation so that it can even be used to determine the
NPSH available or critical value.
The emitted noise of a pumping set depends on the
speed and load of the pump and on the instability in the
pump. Instability in the pump can also appear due to
cavitation in the pump. Cavitation as a source of
instability causes vibration, cavitation noise, pitting and
Material erosion and deterioration of pump
performance.
Average sound pressure over a specific central
frequency of octave band analysis can be used as
cavitation detection parameters in centrifugal pumps.
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ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
[13]
Nakayama.Y, Aoki.K and Ohta.H, ―Visualization
of pressure distribution due to impact
accompanying collapse of cavity on the vanes of
mixed flow pump-turbine,‖ International
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Visualization ASME, Vol. 22, pp. 101–108,
1985.
[14] H.Ohki,
Y.Yoshinaga
and
Y.Tsutsumi,
―Visualization of relative flow patterns in
centrifugal impellers,‖ The 3rd International
Symposium on Flow Visualization,‖ Ann Arbor,
Michigan, U.S.A., September 1983.
[15]
P.J.McNulty and I.S.Pearsall, ―Cavitation
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[16]
P.J.McNulty, National Engineering Laboratory
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techniques and analysis of fluid-borne noise in
pumps. 1981.
[17]
The stability of pumping systems Freeman
scholar lecture. Texas A&M University, College
Station, Modern cavitation criteria for centrifugal
Pumps, pp. 3–11, 1980.
[18]
ISO 3555 1977(E) Centrifugal, mixed flow and
axial pumps} code for acceptance tests} Class B.
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