Cavitation in pumps is known as the formation of vapor bubbles due to pressure drop and collapsing these bubbles. In some conditions, it has been observed that the formation of bubbles occurs at the pressure side of centrifugal pump blades. In this study, the formation of bubbles at the pressure side of blades has been investigated. Water is used in this study as the fluid and performance curves were depicted for different flow rates in an approximately constant speed. The results show that when a centrifugal pump works in low flow rates, a secondary flow namely recirculation starts to begin. In this condition, separation of flow increases which causes vortex formation and local pressure drop and eventually the formation of vapor bubbles starts.

Suction pressure falling below vapor pressure causes bubble formation [3].
The schematic of the experimental setups.
Flow pattern among the rotating vanes of a centrifugal pump; a-Q= 4.5 lit/sec, Suction Pressure= -0.6 mH 2 0, Discharge Pressure= 6.3 mH 2 0, b-Q= 4.2 lit/sec, Suction Pressure= -0.4 mH 2 0, Discharge Pressure= 10.5 mH 2 0, c-Q= 4.0 lit/sec, Suction Pressure= 0.5 mH 2 0, Discharge Pressure= 12.5 mH 2 0, d-Q= 3.4 lit/sec, Suction Pressure= -0.8 mH 2 0, Discharge Pressure= 16.0 mH 2 0 and e-Q= 2.8 lit/sec, Suction Pressure= 0.2 mH 2 0, Discharge Pressure= 19.0 mH 2 0.

Flow pattern among the rotating vanes of a centrifugal pump; a-Q= 4.5 lit/sec, Suction Pressure= -0.6 mH 2 0, Discharge Pressure= 6.3 mH 2 0, b-Q= 4.2 lit/sec, Suction Pressure= -0.4 mH 2 0, Discharge Pressure= 10.5 mH 2 0, c-Q= 4.0 lit/sec, Suction Pressure= 0.5 mH 2 0, Discharge Pressure= 12.5 mH 2 0, d-Q= 3.4 lit/sec, Suction Pressure= -0.8 mH 2 0, Discharge Pressure= 16.0 mH 2 0 and e-Q= 2.8 lit/sec, Suction Pressure= 0.2 mH 2 0, Discharge Pressure= 19.0 mH 2 0.

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Abstract—Cavitation in pumps is known as the formation of

vapor bubbles due to pressure drop and collapsing these bubbles. In

some conditions, it has been observed that the formation of bubbles

occurs at the pressure side of centrifugal pump blades. In this study,

the formation of bubbles at the pressure side of blades has been

investigated. Water is used in this study as the fluid and performance

curves were depicted for different flow rates in an approximately

constant speed. The results show that when a centrifugal pump works

in low flow rates, a secondary flow namely recirculation starts to

begin. In this condition, separation of flow increases which causes

vortex formation and local pressure drop and eventually the

formation of vapor bubbles starts.

Keywords—Cavitation, Centrifugal pump, Recirculation, Vapor

bubble.

I. INTRODUCTION

T recent decades cavitation and the effects of it on

hydraulic design of pumps has been one of the technical–

scientific goals of managers, engineers and scientists. For

investigating this phenomenon, there have been various

studies on centrifugal pumps. As an important result of these

studies, nowadays pumps can operate safely against cavitation

even at high velocities. Several investigators have stressed the

fact that pump failures associated to cavitation depends on a

large number of design and operational parameters such as

type of pump, design type, and the use of suction specific

speed [1].

The term 'cavitation' comes from the Latin word cavus,

which means a hollow space or a cavity. In the context of

centrifugal pumps, the term cavitation implies a dynamic

process of formation of bubbles inside the liquid, their growth

and subsequent collapse as the liquid flows through the pump.

Vaporization of the liquid can occur in centrifugal pumps.

When the local static pressure reduces below that of the vapor

pressure of the liquid at the pumping temperature, the

reduction of static pressure in the internal suction system

Mohammad Taghi Shervani Tabar is with the Department of Mechanical

Engineering, Tabriz University, Tabriz, Iran( email :msherv@tabrizu.ac.ir).

Seyyed Hojjat Majidi is with the Department of Mechanical Engineering,

Iran University of Science and Technology, Tehran, Iran( email :

h_majidi@Mecheng.iust.ac.ir).

Zahra Poursharifi is with the Department of Mechanical Engineering,

Tabriz University, Tabriz, Iran( email: z.poursharifi@gmail.com).

occurs mainly due to the rise in the velocity at the impeller eye

(Fig. 1). Unless there is no change in the operating conditions,

new bubbles continue to form and old bubbles grow in size.

The bubbles then get carried in the liquid as it flows from the

impeller eye to the impeller exit tip along the vane trailing

edge. Due to impeller rotating action, the bubbles attain very

high velocity and eventually reach the regions of high

pressure within the impeller where they start collapsing. As

the vapor bubbles move along the impeller vanes, the pressure

around the bubbles begins to increase until a point is reached

where the pressure on the outside of the bubble is greater than

the pressure inside the bubble and then the bubble collapses

[2]. The process is not an explosion but rather an implosion.

Hundreds of bubbles collapse at approximately the same point

on each impeller vane. Bubbles collapse non-symmetrically

such that a hammering action occurs. The highly localized

hammering effect can pit the pump impeller. After the bubble

collapses, a shock wave emanates outward from the point of

collapse. This kind of cavitation is known as NPSHA

insufficiency.

Fig. 1 Suction pressure falling below vapor pressure causes bubble

formation [3].

Cavitation damage to a centrifugal pump may range from

minor pitting to catastrophic failure and depends on the

pumped fluid characteristics, energy levels, and duration of

cavitation. Most of the damage usually occurs within the

impeller; specifically, to the leading face of the non-pressure

Investigation of Recirculation Effects on the

Formation of Vapor Bubbles in Centrifugal

Pump Blades

Mohammad Taghi Shervani Tabar, Seyyed Hojjat Majidi, Zahra Poursharifi

A

World Academy of Science, Engineering and Technology

International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:5, No:1, 2011

80International Scholarly and Scientific Research & Innovation 5(1) 2011 scholar.waset.org/1999.8/9868

International Science Index, Mechanical and Mechatronics Engineering Vol:5, No:1, 2011 waset.org/Publication/9868

side of the vanes. The net effect observed on the impeller vane

will be a pockmarked, rough surface and severe thinning of

the vanes from metal erosion [3]. It should be mentioned that

cavitation might happen even at stationary components of a

hydraulic machine or at rotating parts (such as the impeller of

a centrifugal pump). Additives in the liquid which increase

vapor pressure, as well as corrosive properties of the liquid,

can increase cavitation damage. Different materials show

different resistance to the cavitation. In general, there is no

material that can be completely resistant to this phenomenon.

Rigid plastics and composites are normally the least cavitation

resistant materials. Cast iron and brass will experience the

most damage of commonly used metals, while stainless steel,

titanium, nickel, aluminum, and bronze will have much less

damage, under the same cavitation conditions [4].

Another type of cavitation seen in pumps is due to a

phenomenon called as recirculation. One of the complex

problems associated with operation of pumps is that of

recirculation. Recirculation is defined as flow reversal either

at the inlet or at the outlet tips of the impeller vanes. This

reversal causes a vortex that attaches itself to the pressure side

of the vane. If there is enough energy available and the

velocities are high enough, damage will occur. There are two

types which may occur together or separately: suction

recirculation and discharge recirculation.

Suction recirculation is a condition created by operating the

pump at low flows, and it frequently dictates the low-flow

limit of stable operation of the pump in relation to the

percentage of BEP. In the petroleum industry, it is usually

referred to as the 'minimum flow for stable operating

condition', and is frequently required to be identified by the

pump supplier for consideration in the pump evaluation

process. This kind of recirculation is the reversal of flow at the

impeller eye. A portion of the flow is directed out of the eye at

the eye diameter, as shown in Fig. 2 and travels upstream with

a rotational velocity approaching the peripheral velocity of the

diameter. A rotating annulus of liquid is produced upstream

from the impeller inlet and through the core of this annulus

passes an axial flow corresponding to the output capacity of

the pump. The high shear rate between the rotating annulus

and the axial flow through the core produces vortices that

form and collapse, producing noise and cavitation in the

suction of the pump.

Fig. 2 Suction recirculation [5].

In the other side, discharge recirculation is another

condition precipitated by low-flow operation that takes effect

at a lower flow than suction recirculation, and also displays

similar symptoms. Discharge recirculation occurs when high

pressure flow streams re-enter the impeller on the low

pressure side of the impeller vane. This is caused by the pump

operating back on its curve or with an inlet restriction [6]. The

reverse flow within the impeller passage shears across the

outgoing flow, sets up vortices along the pressure wall of the

impeller, and causes cavitation along the pressure wall and

shrouds adjacent to the impeller outlet (Fig. 3):

Fig. 3 Discharge recirculation [5].

The flow rates at which the suction and discharge

recirculation occurs are dependent on the design of the

impeller at the inlet and outlet respectively and may occur at

different flow rates as shown in Fig. 4:

Fig. 4 Points on curve where recirculation can be expected [7].

Like suction recirculation, discharge recirculation causes

hydraulic surges and local cavitation at impeller tips. Suction

recirculation will produce a loud crackling noise in and

around the suction of the pump while discharge recirculation

will produce the same characteristic sound as suction

recirculation except that the highest intensity is in the

discharge volute or diffuser. Suction and discharge

recirculation produce cavitation damage to the pressure side of

the impeller vanes. Viewed from the suction of the impeller,

the pressure side would be the invisible, or underside, of the

vane. Guide vanes in the suction may show cavitation damage

World Academy of Science, Engineering and Technology

International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:5, No:1, 2011

81International Scholarly and Scientific Research & Innovation 5(1) 2011 scholar.waset.org/1999.8/9868

International Science Index, Mechanical and Mechatronics Engineering Vol:5, No:1, 2011 waset.org/Publication/9868

from impingement of the back-flow from the impeller eye

during suction recirculation. Similarly, the tongue or diffuser

vanes may show cavitation damage on the impeller side from

operation in discharge recirculation.

II. EXPERIMENTAL S ETUP

Experiments are done on a multipurpose radial flow

machine namely GILKES-GH90. This machine is a UK

production for research and experimental work which consists

of two centrifugal pumps with geometric similarity. One of

these pumps can be used as a reaction turbine using the outlet

flow of the other pump. These pumps are derived by DC

motors with variable speed. The coupled motor to the smaller

pump can work as a generator at the turbine test. Speed can be

controlled from the front panel which also includes an

ammeter and voltmeter for determining motor input power.

The power consumed by pumps is measured by a

dynamometer balancing the electro motors. Fig. 5 shows the

schematic of the experimental setup which consists of a digital

flow meter, speed measuring device, pressure gauges for

measuring the suction and discharge pressures and a

dynamometer for measuring the required torque for rotating

the pumps.

Fig. 5 The schematic of the experimental setups.

The operating characteristics of the studied centrifugal

pump are:

The diameter of impeller: 140 mm

Maximum flow rate: 6.5 lit/sec at 3000 rpm

Maximum output head: 28 m at 3000 rpm

Specific Speed:

4/3

)( P

Sh

Q

NN =

(1)

The NS from the above data is calculated: NS = 19.87 rpm

A stroboscope should be used to monitor the flow patterns

among the rotating vanes. A stroboscope, also known as a

strobe, is an instrument used to make a cyclically moving

object appear to be slow-moving, or stationary and the

principle is used for the study of rotating, reciprocating,

oscillating or vibrating objects. In its simplest form, a rotating

disc with evenly-spaced holes is placed in the line of sight

between the observer and the moving object. In electronic

versions, the perforated disc is replaced by a lamp capable of

emitting brief and rapid flashes of light. The frequency of the

flash is adjusted so that it is an equal to, or a unit fraction

below or above the object's cyclic speed, at which point the

object is seen to be either stationary or moving backward or

forward, depending on the flash frequency. By using this

setup we are able to measure the operating characteristics

such as speed, flow rate, suction and discharge pressures. Our

experimental procedure is based on the appearance of the

vapor bubbles formation by using a stroboscope and a camera.

Experiment was done for five different flow rates in an

approximately constant speed (N = 2500 rpm). At each step,

the valve was closed gradually and the data's such as flow

rate, suction and discharge pressures were recorded. Based on

these data's and also the following equations, the head, input

and output power, efficiency and also the factors related to

cavitation such as NPSHR and Toma Coefficients were

determined:

Head:

12

P

h (2)

Input Power:

60

2N

RFP in

××=

(3)

Output Power:

Pout hQgP

(4)

(g=9.81 m/sec2 )

Efficiency:

100% ×=

in

out

PP

P

η

(5)

Required Net Positive Suction Head:

Suction Specific Speed:

4/3

)(NPSHR

Q

NN SS =

(6)

As a usual standard for single suction pumps in HI institute:

NSS =174.6

World Academy of Science, Engineering and Technology

International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:5, No:1, 2011

82International Scholarly and Scientific Research & Innovation 5(1) 2011 scholar.waset.org/1999.8/9868

International Science Index, Mechanical and Mechatronics Engineering Vol:5, No:1, 2011 waset.org/Publication/9868

Finally:

3/4

)

)(

(

SS

N

QN

NPSHR =

(7)

Toma Coefficient (Cavitation Coefficient):

P

h

NPSHR

=

σ

(8)

III. RESULTS AND DISCUSSION

As mentioned above at each step, the operating

characteristics of pump such as flow rate, suction and

discharge pressures were noted. By using a stroboscope and

a camera, the flow patterns among the rotating vanes were

photographed. The images have been shown in Fig. 6 with

details:

(a)

(b)

(c)

(d)

(e)

Fig. 6 Flow pattern among the rotating vanes of a centrifugal

pump;

a- Q= 4.5 lit/sec, Suction Pressure= -0.6 mH2 0, Discharge

Pressure= 6.3 mH2 0,

b- Q= 4.2 lit/sec, Suction Pressure= -0.4 mH2 0, Discharge

Pressure= 10.5 mH2 0,

c- Q= 4.0 lit/sec, Suction Pressure= 0.5 mH2 0, Discharge

Pressure= 12.5 mH2 0,

d- Q= 3.4 lit/sec, Suction Pressure= -0.8 mH2 0, Discharge

Pressure= 16.0 mH2 0 and

e- Q= 2.8 lit/sec, Suction Pressure= 0.2 mH2 0, Discharge

Pressure= 19.0 mH2 0.

Experiment was done for five different flow rates. As it

can be seen at higher flow rates vapor bubbles form at

suction side of the vanes and the amount of vapor bubbles

decreases by decreasing the flow rate and at a critical flow

rate (Q = 3.4 lit/sec), bubbles vaporize due to recirculation

and at the lowest flow rate (Q = 2.8 lit/sec) there can't be

seen any bubbles. Fig. 6-d shows that at Q = 3.4 lit/sec, the

bubbles form at both side of the vanes; Thus, both suction

and discharge recirculation occur. Fig. 7 shows the result of

the operation of the regarded centrifugal pump at different

flow rates:

World Academy of Science, Engineering and Technology

International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:5, No:1, 2011

83International Scholarly and Scientific Research & Innovation 5(1) 2011 scholar.waset.org/1999.8/9868

International Science Index, Mechanical and Mechatronics Engineering Vol:5, No:1, 2011 waset.org/Publication/9868

(a)

(b)

(c)

(d)

Fig. 7 Operation of the regarded centrifugal pump;

a-Head versus flow rate, b- Input power versus flow rate,

c- Output power versus flow rate and d- Efficiency

versus flow rate.

As we expected, the head decreases as the flow rate

increases. Because increasing the flow rate in an

approximately constant speed, decreases the energy

delivered to fluid. Also it is obvious that the input power

(also known as brake horsepower) increases as the flow rate

increases; This is derived from the fact that the force exerted

on the vanes increases with the increase in flow rate (for low

NS ). From (4) it is obvious that the output power (or

hydraulic power) depends on the head and flow rate. As it

can be seen after the flow rate that bubble vaporization due

to recirculation occurs (Q = 3.4 lit/sec), this power

decreases. This can be met that increasing the flow rate,

decreases the head intensively. Also efficiency decreases

with a great slope after the critical flow rate and there is a

small change in efficiency between Q = 2.8 lit/sec to Q = 3.4

lit/sec. This is because of the fact that in this interval both

input and output powers increase.

Fig. 8 shows the variations of the parameters related to

cavitation such as NPSHR (Net Possitive Suction Head) and

Toma coefficient (or cavitation coefficient):

(a)

(b)

Fig. 8 Diagram of parameters related to cavitation; a- NPSHR

versus flow rate and b- Toma coefficient versus flow rate.

As an important fact, NPSHR depends on velocity

components. Thus, as a certain result, the variation of

NPSHR is directly proportional to the variation of flow rate

and it can be seen obviously in Fig. 8. From (8), Toma

coefficient is a function of NPSHR and head; Then as we

expected this parameter also increases as the flow rate

World Academy of Science, Engineering and Technology

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increases. Fig. 8 shows that this coefficient increases

intensively at higher flow rates and as it can be seen this

coefficient is very high when cavitation occurs due to low

NPSHA.

IV. CONCLUSION

Internal recirculation is a less frequent cause of field

troubles but on occasion, it can be quiet severe. In this paper,

an experiment was done to observe cavitation due to

recirculation in a centrifugal pump. A stroboscope was used to

monitor this phenomenon. From the figures it was obvious

that this kind of cavitation occurs at low flow rates. The

results showed that the efficiency and output power decrease

as recirculation happens. Both Suction and discharge

recirculation can be very damaging to pump operation and

should be avoided for continuous operation. The damage due

to recirculation occurs on the opposite side of the vane where

classical cavitation occurs. Substituting a harder material for

the impeller or even bleeding air into the suction of pump can

be useful for reducing the rate of cavitation damage. Finally,

every impeller design has specific critical flow rate which

recirculation occurs. This flow rate is inherent in the design

and cannot be changed without modifying the design.

Increasing the output capacity of the pump and installing a

bypass between the discharge and the suction of the pump can

be possible corrective procedures for preventing recirculation

to occur.

REFERENCES

[1] J.F., Gülich, "Selection criteria for suction impellers of

centrifugal pumps", J. World Pumps , January 2001; 28-

34.

[2] A.J., Stepanoff, "Centrifugal and Axial Flow Pumps",

John Wiley & Sons, 1948.

[3] P., Girdhar, and O.Moniz , "Practical Centrifugal Pumps:

Design, Operation and Maintenance", Newnes- Elsevier,

2005.

[4] A.Nourbakhsh, A.Jaumotte, Ch.Hirsch, and Parizi H. B.,

"Turbopumps and Pumping Systems", Springer, 2008.

[5] R.Mackay , "Pump Clinic 3: What is Recirculation and

Seperation", Kelair Pumps , May 2006.

[6] H.Frazer, "Flow Recirculation in Centrifugal Pumps",

presented at the ASME Meeting, 1981.

[7] R.Mackay, "Low flow does not have to mean low

reliability", J. World Pumps , July 2005; 22-25.

World Academy of Science, Engineering and Technology

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85International Scholarly and Scientific Research & Innovation 5(1) 2011 scholar.waset.org/1999.8/9868

International Science Index, Mechanical and Mechatronics Engineering Vol:5, No:1, 2011 waset.org/Publication/9868

... When such obstruction to flow is created, the flow rate drops and a secondary flow called the recirculation flow develops. As the separation of flow increases, vortex is formed, resulting in local pressure drop and thus formation of vapour bubbles [3]. The after effect of this is unwanted vibrations due to the resulting flow instabilities. ...

... It may be inferred from the experimental observations that the recirculation effects in the CP without impeller defects/pitted cover plate faults are due to inlet flow restriction [3,4]. While in a pump with impeller defects the cracks govern a pseudo recirculation [4]. ...

It has been observed that centrifugal pumps (CPs) usually fail due to flow instabilities and other factors related to the hydraulic pump design. What is often ignored is the role of one fault in the pump system on the commencement/enhancement of another fault in it. Also, in a CP it is not only important to identify a fault, but it is also important to find the severity of it. In this research, causes of flow instabilities like, the blockage faults, impeller defects, pitted cover plate faults and dry runs are considered with varying severity. These faults are considered both independently and also in combinations with blockage faults. The CP vibration data and the motor line current data in time domain are used for the purpose of fault classification. The multi-fault diagnosis is attempted with the help of support vector machine (SVM) classifier. A fivefold cross-validation technique is used for the selection of optimum SVM hyper parameters. Wrapper model is used to select the best statistical feature(s). Novel features which give the best fault prediction performance of CP faults have been brought out in this work. The fault prediction from the experiments and the established approaches, aid the segregation of all the individual faults, fault combinations and their severities with promising execution, not only at the same training and testing speeds but also at the intermediate and overlying test speeds. The fault prediction of developed methodology has been inspected at various operating conditions of the CP and is found to be remarkably robust.

... Recirculation is the reversal of flow at the inlet or discharge tips of the impeller and contrary to traditional cavitation, recirculation is due to a drop in flowrate and not pressure [22,23]. It happens when the flowrate is outside its reliability zone, which is centered around the Best Efficiency Point (BEP), and one of the reasons that could have caused that drop is the sodium fluorosilicate deposition found on the impeller. ...

The aim of this study is to identify the failure mechanism behind the damage found on the impeller of a slurry pump operating in a phosphoric acid production plant. The failure analysis was carried out first by identifying the solid particles found in the slurry, then by analysing the damaged impeller itself. The observed damage was non-uniform throughout the different regions of the impeller, especially in the back shroud. This is why a series of samples was extracted from different regions to perform hardness tests, microhardness profiles along its cross-section as well as cross-sectional microstructure examinations. Slurry erosion and cavitation erosion were the main damage sources and were particularly effective due to the uncharacteristically low surface hardness value. This was due to decarburization and the continuous network of carbides that facilitated crack propagation in the grain-carbide interface in near surface areas.

... There are two types that may occur together or separately: suction recirculation and discharge recirculation. 36 The variability of our results in repeated tests could be related not only to the unintended changes in the gas content of pumping fluid but also to the appearance of recirculation with alteration of inlet and outlet pressures. ...

Successes of extracorporeal life support increased the use of centrifugal pumps. However, reports of hemolysis call for caution in using these pumps, especially in neonatology and in pediatric intensive care. Cavitation can be a cause of blood damage. The aim of our study was to obtain information about the cavitation conditions and to provide the safest operating range of centrifugal pumps. A series of tests were undertaken to determine the points at which pump performance decreases 3% and gas bubbles start to appear downstream of the pump. Two pumps were tested; pump R with a closed impeller and pump S with a semiopen impeller. The performance tests demonstrated that pump S has an optimal region narrower than pump R and it is shifted to the higher flows. When the pump performance started to decrease, the inlet pressure varies but close to −150 mmHg in the test with low gas content and higher than −100 mmHg in the tests with increased gas content. The same trend was observed at the points of development of massive gas emboli. Importantly, small packages of bubbles downstream of the pump were registered at relatively high inlet pressures. The gaseous cavitation in centrifugal pumps is a phenomenon that appears with decreasing inlet pump pressures. There are a few ways to increase inlet pump pressures: (1) positioning the pump as low as possible in relation to the patient; (2) selecting appropriate sized venous cannulas and their careful positioning; and (3) controlling patient's volume status.

... This re-entering flow results in the formation of vortex on the pressure side of the impeller blade. In case the flow possesses enough energy, the fluid re-circulation results in the material damage [6]. ...

Reliable detection and isolation of centrifugal pump (CP) faults is a challenging and important task in the modern industries. Hence, this paper proposes an artificial intelligence-based multi-fault detection of CPs driven by induction motor. The intelligent fault detection methodology is developed based on the multi-class support vector machine (MSVM). The mechanical and hydraulic faults in CPs are mutually dependent and therefore may exist concurrently. Hence, in the present research, an assortment of various flow instabilities like the suction re-circulation, discharge re-circulation, pseudo-re-circulation and dry runs are considered coexisting with mechanical faults, like the impeller cracks and pitted cover plate faults. The power spectrum of the CP vibration and the induction motor line-current data is used for monitoring the CP condition. The best statistical feature combination is selected based on a wrapper model. Gaussian radial basis function (RBF) is used for the kernel mapping. In addition, the RBF kernel parameter (width) and MSVM parameters are optimally selected using a fivefold cross-validation technique. Also, variation of operating speed of the CP drastically changes the system vibration level owing to the change in fault manifestations; hence, in the present work a methodology that is independent of CP operation is proposed and tested. Thereafter, it is observed that the proposed methodology is remarkably robust and successfully classifies multiple individual as well as coexisting CP faults at all the tested CP speeds.

... Restriction to flow causes, the flow rate to drop and a secondary flow develops, called the recirculation flow. The increase in the separation of flow causes vortex formation, resulting in local pressure drop and thus formation of bubbles [4]. ...

Fault severity detection, assessment and classification in centrifugal pumps is attempted in this paper. Experiments are carried out on two geometrically similar axial-flow centrifugal pumps to understand the effects of change in area on the suction side (blockage) in inducing suction recirculation and bubble formation. One of the two pumps has artificially created cuts on the impeller. The effect of this damage on the flow instability and also the pump vibrations is studied. The combined effect of the blockage and the impeller defect is studied and attempted to be classified using Support Vector Machine (SVM) algorithm, in frequency domain. The pump vibration data is measured using two tri-axial accelerometers. A multi-class classification is presented with parametrically chosen SVM parameters. Gaussian Radial Basis Function (RBF) kernel is used for mapping the classification data input space to feature space.

... When such obstruction to flow is created, the flow rate drops and a secondary flow develops, called the recirculation flow. As the separation of flow increases, vortex is formed which results in local pressure drop and thus formation of vapour bubbles (4). The after effect of this is unwanted vibrations due to resulting flow instabilities. ...

The present work proposes an automation methodology of the vibration based condition monitoring, fault detection and severity assessment in centrifugal pumps using the SVM classifier based on frequency domain data. The inlet pipe blockage, is artificially induced on the pump and is considered in steps of increasing severity. As the flow-rate decreases with increasing blockage, recirculation and associated vapour bubble formation starts. The pump is run at different flow conditions and speeds. Spectral entropy is extracted from frequency domain data. Binary and multi-class classifications are proposed using the Gaussian RBF kernel function of the SVM algorithm. The kernel input parameter (γ) is optimally chosen. A fault prediction performance is presented.

... Two through-through cracks per impeller vane are artificially created on one of the CPs, as shown in Fig. 2. Obstruction to flow on the suction side of the CP, drops the flow rate and results in the formation of secondary flow, called the recirculation flow. As the separation of flow increases, vortex is formed which results in local pressure drop and thus formation of vapor bubbles [15]. Presence of bubbles, (i) reduces the head performance of the CP and (ii) results in microjet formation and material erosion due to pitting. ...

When the hydraulic flow path is incompatible with the physical contours of the centrifugal pump (CP), flow instabilities occur. A prolonged operation in the flow-instability region may result in severe damages of the CP system. Hence, two of the major causes of flow instabilities such as the suction blockage (with five levels of increasing severity) and impeller defects are studied in the present work. Thereafter, an attempt is made to classify these faults and differentiate the physics behind the flow instabilities caused due to them. The tri-axial CP vibration data in time domain is employed for the fault classification. Multi-distinct and multi-coexisting fault classifications have been performed with different combinations of these faults using support vector machine (SVM) algorithm with radial basis function (RBF) kernel. Prediction results from the experiments and the developed methodology help to, segregate the faults into appropriate class, identify the severity of the suction blockage, and substantiate the practical applicability of this study.

Centrifugal pumps play an important role in many industrial applications even in harsh environment for prolong duration. High efficiency with very low power consumption makes them very popular in industry. However, during their operation, they may fail due to some operationally developed faults, which may subsequently lead to the interruption in the continuous operation of pumps. Therefore, monitoring the health status of the centrifugal pumps is essential to prevent unwanted stoppage, which may further lead to the breakdown of the whole system. The main focus of this study is to propose a methodology to identify the presence and severity of blockages, and cavitation in the centrifugal pump using fluid pressure, which is very vital for fluid related faults. To simulate the blockage in the pump, the flow area of the suction pipe is restricted by dividing into six equal intervals (i.e., 0%, 16.7%, 33.3%, 50%, 66.6% and 83.33%) using a mechanical modulating valve. Due to blockage and cavitation, the main parameter which directly gets affected is the fluid dynamic pressure. Hence, in the present study, pressure signatures were captured at different blockage levels and at different running speeds with the help of a pressure transducer, which was mounted on the circumference of the centrifugal pump casing. Deep learning based binary data classification methodology is used to classify the data acquired from the pressure transducer. To get better performance of the data classifier, statistical features are extracted from time domain pressure signals. In order to identify the severity of the faults, binary classification of the data is performed at different blockage levels and running speeds. Finally, based on the results obtained from the classifier, existence of the faults (i.e., blockage and the cavitation), their severity levels are presented.

Aim: The aim of this study was to examine the hydrodynamic performance and gaseous microemboli (GME) activity of two centrifugal pumps for possible use in low-flow extracorporeal CO2 removal. Materials & methods: The performance of a Rotassist 2.8 and a Rotaflow 32 centrifugal pump (Maquet Cardiopulmonary AG, Hirrlingen, Germany) was evaluated in a water-glycerine mixture-filled in vitro circuit that enabled measurement of pressures and GME at the pump inlet and pump outlet. Pressure-flow curves were acquired in a 1,000 to 5,000 rpm range while increasing drainage resistance in one series and outlet resistance in another. Results: Respective minimum pump inlet and maximum pump outlet pressures were -539 mmHg and 754 mmHg for the Rotassist 2.8 and -606 mmHg and 806 mmHg for the Rotaflow 32. Maximum standard deviations on pump pressures and flow amounted to 3.0 mmHg and 0.03 L/min, respectively, regardless of pump type and drainage or outlet resistance. The GME at the pump outlet were detectable at pump inlet pressures below -156 mmHg at 0.2 L/min and 2,500 rpm for the Rotassist 2.8 and below -224 mmHg at 0.9 L/min and 3,000 rpm for the Rotaflow 32. Conclusion: Both the Rotassist 2.8 and Rotaflow 32 centrifugal pumps show a comparably high hydrodynamic stability, but potential GME formation with decreasing pump inlet pressures should be taken into account to ensure safe centrifugal pump-based low-flow extracorporeal CO2 removal.

  • S. Ahmad Nourbakhsh
  • Baron Andre Jaumotte
  • Charles Hirsch Charles Hirsch
  • Hamideh B. Parizi

This book covers most of the subjects that are requested by engineers, those who are directly involved in design and manufacturing the pumps to those who operate the pumps in industrial units. The authors have worked for many years teaching the subject, conducting research, and implementing pump and pump station designs in different academic institutions, industrial sectors, and consulting firms. Based on this extensive background, the material of the book is arranged to cover the most important topics, from basic theories to practical applications. This book can also serve as a useful textbook for students who are taking the courses in the area of turbopumps and hydraulic machineries. The book is divided into two major Parts. In Part I, Turbopumps, the basic information about pumps classification, definitions, principal of operation, and construction elements are presented. In Part II, Pumping Systems, the important parameters in pump operation, selection, pumping systems, and pump stations are discussed in details.

  • P. Girdhar
  • Octo Moniz
  • Steve Mackay

Practical Centrifugal Pumps is a comprehensive guide to pump construction, application, operation, maintenance and management issues. Coverage includes pump classifications, types and criteria for selection, as well as practical information on the use of pumps, such as how to read pump curves and cross reference. Throughout the book the focus is on best practice and developing the skills and knowledge required to recognise and solve pump problems in a structured and confident manner. Case studies provide real-world scenarios covering the design, set up, troubleshooting and maintenance of pumps. A comprehensive guide to pump construction, design, installation, operation, troubleshooting and maintenance. Develop real-world knowhow and practical skills through seven real-world case studies Coverage includes pump classifications, types and criteria for selection, as well as practical information on the use of pumps.

  • Warren H. Fraser

The pressure field produced in a centrifugal pump impeller at a flow corresponding to peak efficiency is more uniform and more symmetrical than at any other flow. At flows less than that of a peak efficiency, the pressure field becomes increasingly distorted until at some point the pressure gradient reverses and a localized reversal of the flow takes place. This is the point of recirculation, which can occur at the discharge or the suction of the impeller, or at both the suction and the discharge. Recirculation characteristics are dependent on the design of the impeller. It is inherent in the dynamics of the pressure field that every impeller design must recirculate at some point - it cannot be avoided. It is important that both the designer and the operator of centrifugal pumps realize the capacity at which discharge recirculation occurs can be reduced through design procedures, but only at a reduction in the rated efficiency of the pump.

  • Ross Mackay

Operating a pump under low-flow conditions can give rise to a range of problems, all of which result in premature failure. In this article, Canadian pump reliability consultant Ross Mackay considers first the definition and causes of low flow in pumping systems before examining the design and operation of the different types of pumps that can provide satisfactory performance in low-flow conditions.

  • Johann F Guelich Johann F Guelich

The occurrence of flow separation and subsequent recirculation at partload in rotodynamic pumps is described. The assessment of the onset and intensity of recirculation is discussed. The factors affecting the occurrence of recirculation in impeller inlets and the effects of recirculation on cavitation damage and vibration are considered. Case histories illustrating the impact of suction specific speed on reliability in an extraction pump, a pipeline pump and two cooling water pumps are presented.

Pump Clinic 3: What is Recirculation and Seperation

  • R Mackay

R.Mackay, "Pump Clinic 3: What is Recirculation and Seperation", Kelair Pumps, May 2006.