Introduction to Cavitation in Pumps and Their Types

7/30/2019 Introduction to Cavitation in Pumps and Their Types

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Department of Polymer and Process Engineering,University of Engineering and Technology, Lahore

Assignment # 2Fluid FlowSubject Code:

PolyE-203Submitted to:

Sir SajidNaseemSubmitted by:Mujadidul Hassan Khawaja

2010-PE 51

Dated: 18th October 2011


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Assignment #2

Topic:

A brief introduction to

Cavitation

Table of Contents:

1. What is Cavitation?

2. Why is Cavitation an Unacceptable Phenomena?

3. Why and how Cavitation does occur??

4. Types of Cavitation

5. How can we reduce Cavitation??


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1. What is Cavitation??

Cavitation is defined as the occurrence of vapor-filled cavities in a liquid.

Cavitation is a general fluid mechanics phenomenon which can occur whenever a

Liquid used in a machine inducing pressure and velocity fluctuations in the fluid (e.g.

Pumps, turbines, propellers, bearings, even the heart and knee joints).

Luca D Agostino in his book, Fluid Dynamics of Cavitation and Cavitating Turbo pumps ,

defines cavitation as;

Cavitation is the development of vapor structures in an originally liquid flow. Contrary to

boiling, the phase change takes place at almost constant temperature and is due to a local drop

in pressure generated by the flow itself

The occurrence of this low pressure region in flows is a well known phenomenon. For example,

in the case of a venture ie a converging duct followed by a diverging one , the velocity is

maximum at the throat where the cross section is minimum. Then , according to Bernoullis

equation, the pressure is minimum there and the risk of cavitation is maximum.

Another example is the flow around the foil at a given angle of attack which is representative of

that around the blades of a hydraulic machine. From classical hydrodynamics, it is well known

that a foil is subjected to a lift because of a lower pressure on the suction side in comparison to

the pressure side. Hence the suction side of expected to be the place where cavitation will first

develop.

Figure: cavitation on a hydrofoil.


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A fnal example is that of vortices which are very common structures in many flows. Because of

the rotation and the associated centrifugal forces, the pressure in r=the core of such structures

is lower than outside. Hence vortices are likely to cavitate in their core. There are actually many

situations in which cavitating vortices can be observed as tip vortices or coherent vertical

structures in turbulent flows like wakes or shear layers.

Figure: cavitation in centrifugal pump

2. Why is cavitation an unacceptable phenomenon??

A cavitating Centrifugal pump can be found unacceptable in a number of ways.

Those of industrial significance can be categorized as follows:1. Hydraulic performance loss:

The presence of cavitation in a centrifugal pump impeller can give rise to a loss of generated

head. Whilst a modest amount of cavitation may have no measurable effect, a large volume of

cavities can result in the generating head being reduced considerably, which results in the

failure of required causing huge economic loss.

A catastrophic reduction in head caused by large volumes of cavitation, which gives rise to

vapour locking of the pump and lead to an immediate cessation of pumping. This is sometimes

called Vapor binding. Mechanical damage to the rotating element of the pump nearly always

occurs as a consequence.


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Figure4: an impellor destroyed due to cavitation

Other drawbacks include:

2. Cavitation surging (hydrodynamically induced)

3. Cavitation surging (Thermodynamically induced)

4. Cavitation erosion:

Cavitation erosion is caused as the vapor filled cavities enter the region of higher pressure

within a pump and collapse. When this violently implosive cavitation process takes place in

close proximity to a metal surface, a damage mechanism that produces local pitting can

result. These pittings decrease the age of pump and it wears down causing great financial

loss.

Figure: Cavitation erosion in pump


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Some Other potentially serious consequences thart can arise when a pump cavitates are:

1. Loss of economic performance

2. Damage to the expensive equipment

3. Noise pollution; the pump sounds like its pumping rocks

4. High vacuum reading on suction line5. Low discharge flow/ High Flow

So for all these reasons, Cavitation is an unacceptable phenomenon.

3.Why and how Cavitation does occur??Much about the working of cavitation has been discussed in the above portion. But for convenience,

detailed process of cavitation is discussed as under:

One of the reasons for cavitation is a condition in which a pump is either operating with no water thepump or is filled with water and operating but the system does not allow water to flow. In either case, thepump is not cooled properly by the appropriate amount of water flow. Water within the pump may thenvaporize from the heat build-up and therefore cause cavities to form in the water that is within the pump.In both cases, heat will build up due to a lack of cooling and damage to the pump will then occur.

Discussing the phenomenon in a more scientific way; There may be, on the low-pressure side of the

runner, regions in which the pressure falls to values considerably below atmospheric. In a liquid,

however, the pressure cannot fall below the vapour pressure at the temperature concerned. If at any

point the vapour pressure is reached, the liquid boils and small bubbles of vapour form in large

numbers. These bubbles are carried along by the flow, and on reaching a point where the pressure is

higher they suddenly collapse as the vapour condenses to liquid again. A cavity results and the

surrounding liquid rushes in to fill it. The liquid moving from all directions collides at the centre of the

cavity, thus giving rise to very high local pressures (up to 1 GPa). Any solid surface in the vicinity is also

subjected to these intense pressures, because, even if the cavities are not actually at the solid surface,

the pressures are propagated from the cavities by pressure waves similar to those encountered in water

hammer. This alternate formation and collapse of vapour bubbles may be repeated with a frequency of

many thousand times a second. The intense pressures, even though acting for only a very brief time

over a tiny area, can cause severe damage to the surface. The material ultimately fails by fatigue, aided

perhaps by corrosion, and so the surface becomes badly scored and pitted. Parts of the surface may

even be torn completely away. Associated with cavitating flow there may be considerable vibration and

noise; when cavitation occurs in a turbine or pump it may sound as though gravel were passing through

the machine.


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Figure: A cavitating pump

4.Types of Cavitation:the types of cavitation occurring in propellers and pumps depend generally upon

two factors;i) Location on the blade of a propeller

ii) Physical appearance

i) Cavitations Depending upon location on a blade:The angle and the assembly of the impeller blade gives rise to different types of

cavitations. Ill simply give the names of these types of cavitations, since the

details and description of these types is beyond our knowledge.

a) Back cavitation

b) Face cavitation

ii) Depending upon physical appearance of cavitationa) Tip and Hub vortex cavitation

b) Sheet cavitation

c) Bubble cavitation

d) Root cavitation

e) Propeller-Hull vortex cavitation

f) Unsteady sheet cavitation (cloud cavitation)


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Figure: Different types of cavitations

a) Tip and Hub vortex cavitation:

The tip vortex cavitation is normally first observed some distance behind the tips ofthe propeller or impellerblades which is said to be unattached but as the vortex

becomes stronger, either through higher blade loading, it moves towards the blade tip

and ultimately becomes attached.The hub vortex is formed by the combustion of individual vortices shed from each

blade root and although individually these vortices are unlikely to cavitate, under the

influence of a converging propeller cone the combination of the blade root vorticeshas a high susceptibility to cavitate.

This type of cavitation may also harm the rudders behind the propeller causing

erosion on them. It can be avoided using Propeller Boss Cap Fins (PBCF) or other

type vanes.


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Figure: Tip and Hub vortex Cavitation

b) Sheet cavitationSheet cavitation occurs when the pressure distribution has a strong adverse pressuregradient and the flow separates from the blade surface. Sheet cavitation initially

becomes apparent at the leading edges of the propeller blades on the back when theblade sections are working at positive angle of attack. Conversely if the sections are

operating at negative angle of attack, the sheet cavitation may initially appear on the

face of the blade. The sheet cavitation occurs when a leading edge suction peak islower than the vapour pressure.

Figure: Sheet Cavitation

c) Bubble Cavitation:Bubble cavitation is primarily affected by the pressure distribution which causes high

suction pressure in the mid- chord region of the blade section. Thus the combination

of camber line and section thickness plays an important role on the susceptibility of apropeller towards bubble cavitation. When the blade sections are relatively thick and

operate at a small angle of attack the bubble cavitation occurs.

For example near the root of controllable pitch propeller where the chord length is

restricted and strength requires thick blade sections.


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Bubble cavities collapse very violently so that this cavitation is noisy, erosive and

bad.

Figure: Bubble cavitation

d) Root Cavitation:This type of cavitation may occur at the blade root and has the shape of a wedge. The topof the wedge can be at the leading edge, but it can also start on the blade itself. Root

cavitation is related to the horse shoe vortex developed at root as well as inclined shaft

and wake shadow effect created by the shaft brackets, boss rings, etc. It is commonlyobserved on controllable pitch propellers (CPP).

Figure: Root Cavitation

e) Propeller-Hull Vortex (PHV) Cavitation:A special form of cavitation reported in early 1970s is the PHV cavitation. This type of


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cavitation can be described as the arching of a cavitating vortex between propeller tip

and ships hull and it is pronounced for small tip clearance of the propeller and hull.

Figure: PHV cavitation

5.How can we reduce Cavitation?Cavitation can be avoided by:.

1. Removing debris from suction line

2. Moving pump closer to source tank/sump

3. Increasing suction line diameter

4. Decreasing suction lift requirement

5. Installing larger pump running slower which will decrease the Net Positive

Suction Head Required by the pump(NPSHR)

6. Increasing discharge pressure

7. Fully open Suction line valve


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References and Bibliography:

BOOKS:

1. Fluid Dynamics of Cavitation and turbo pumps

http://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=e

n&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwB

Q#v=onepage&q&f=false

2. Cavitation and Centrifugal pumps

http://books.google.com.pk/books?id=F6dJ36W-

nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=bo

ok_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=false

Web References:

3. http://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdf

4. http://www.pumpworld.com/troubleS2.htm

5. http://www.engineeringtoolbox.com/cavitation-d_407.html

6. http://cavity.ce.utexas.edu/kinnas/cavphotos.html

7. http://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-

Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdf

8. http://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdf

9. http://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitation

10.http://www.irrigationcraft.com/cavitation_types.htm

11.http://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitation

12.http://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitation

13.http://www.sugartech.co.za/piping/npsh/index.php
http://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdfhttp://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdfhttp://www.pumpworld.com/troubleS2.htmhttp://www.pumpworld.com/troubleS2.htmhttp://www.engineeringtoolbox.com/cavitation-d_407.htmlhttp://www.engineeringtoolbox.com/cavitation-d_407.htmlhttp://cavity.ce.utexas.edu/kinnas/cavphotos.htmlhttp://cavity.ce.utexas.edu/kinnas/cavphotos.htmlhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdfhttp://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdfhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.irrigationcraft.com/cavitation_types.htmhttp://www.irrigationcraft.com/cavitation_types.htmhttp://www.irrigationcraft.com/cavitation_types.htmhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.sugartech.co.za/piping/npsh/index.phphttp://www.sugartech.co.za/piping/npsh/index.phphttp://www.sugartech.co.za/piping/npsh/index.phphttp://www.sugartech.co.za/piping/npsh/index.phphttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.irrigationcraft.com/cavitation_types.htmhttp://www.slideshare.net/DelftOpenEr/types-of-cavitation-bubble-cavitationhttp://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdfhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://www.nstcenter.com/docs/PDFs/TechResourcesPresentations-Whats%20New%20in%20the%20Fight%20Against%20Cavitation.pdfhttp://cavity.ce.utexas.edu/kinnas/cavphotos.htmlhttp://www.engineeringtoolbox.com/cavitation-d_407.htmlhttp://www.pumpworld.com/troubleS2.htmhttp://www.gidb.itu.edu.tr/staff/emin/Lectures/Ship_Hydro/cavitation.pdfhttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=F6dJ36W-nIEC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=falsehttp://books.google.com.pk/books?id=B5rXBLCzI8QC&printsec=frontcover&dq=cavitation&hl=en&ei=FT6gTsbIEcfrrQf4r_X9Ag&sa=X&oi=book_result&ct=result&resnum=6&ved=0CEcQ6AEwBQ#v=onepage&q&f=false

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Introduction to Cavitation in Pumps and Their Types