This document discusses cavitation in pumps. It defines cavitation as the formation of vapor bubbles in a liquid when low pressure causes the liquid to transition to a vapor state. There are two types of cavitation described - inertial (transient) cavitation where bubbles quickly form and collapse, releasing energy as shockwaves, and non-inertial cavitation where bubbles undergo controlled collapse. Cavitation in pumps can be suction cavitation where low pressure at the pump inlet causes bubble formation, or discharge cavitation where high pressure makes it difficult for fluid to exit and bubbles form. Causes of cavitation include poor pump design, low net positive suction head, high fluid temperature, high velocities, debris buildup, and

1. NEW YORK CITY COLLEGE
State of Art in Cavitation
Control in Pumps
ME I 3100: Steam and Gas Turbines
Johnaton McAdam
Fall 2016

2. 1
Contents
1. Abstract..........................................................................................................................................2
2. Background Information..................................................................................................................2
2.1 What is Cavitation? ...................................................................................................................2
3. Cavitation in Pumps ........................................................................................................................3
3.1 What is it? ................................................................................................................................3
3.2 Cavitation Behaviors .................................................................................................................3
3.2.1 Inertial Cavitation ...............................................................................................................3
3.2.2 Non-Inertial Cavitation........................................................................................................3
3.3 Cavitation Type.........................................................................................................................4
3.3.1 Suction Cavitation...............................................................................................................4
3.3.2 Discharge Cavitation...............................................................................................................4
3.4 Causes of Cavitation in Pumps ...................................................................................................5
3.5 Identifying Cavitation Formation................................................................................................6
4. Cavitation Control...........................................................................................................................6
4.1 Parameters................................................................................................................................6
4.2 Limiting Cavitation ...................................................................................................................7
4.2.1 Pump Design ......................................................................................................................7
4.2.2 External Contributions.........................................................................................................7
5. Reflection.......................................................................................................................................8
6. Future Scope of Research................................................................................................................9
7. Conclusion .....................................................................................................................................9
8. Nomenclature ...............................................................................................................................10
9. Table of Figures............................................................................................................................10
10. References..................................................................................................................................11

3. 2
1. Abstract
Discuss the problems arising due to cavitation in pumps along with the recent
technological development to tackle these problems. The different type of cavitation and their
formations will be address in this report along with the possible prevent and solutions. An
analytical approach will be used to develop an optimize cavitation system in the pump, the
advantages and disadvantages of using that specific system will be comprehensively examined.
2. Background Information
2.1 What is Cavitation?
Cavitation is the formation of the vapor phase of a liquid when it is subjected to low
pressure at a constant temperature. According to Bernoulli's equation
𝑝𝑣1
2
2
+ 𝑝1 =
𝑝𝑣2
2
2
+ 𝑝2 =
𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (1) , the pressure can only decrease if the velocity increases as long as the
temperature remains constant. An example of this situation can be shown in propellers of
submarines where in areas of low pressure steam bubbles are form due to the high velocities and
in areas of decreased area in pipes or pumps inlet.
Figure 1: Pressure vs
Temperature for Water

4. 3
3. Cavitation in Pumps
3.1 What is it?
In a pump voids, air bubbles are developed around the impeller and due to the unstable
nature of these bubbles intense shockwaves are created from their collapsing bodies and that
release of energy damages the impeller or the pump housing. If left untreated significant
problems such as failure of the pump housing, destruction of the impeller, decrease in flow rate
or pressure, undamped vibrations and higher power consumption. The behavior of cavitation in
pumps can be divided in to two types of cavitation inertial or transient and non-inertial. One of
the most common pumps that is likely for cavitation to occur is the centrifugal pump which
operates on the principle of creating low pressure at the eye and atmospheric pressure that forces
the liquid to the impellor. The two types of cavitation that will occur are suction and discharge.
3.2 Cavitation Behaviors
3.2.1 Inertial Cavitation
This the process when the bubbles are unable to hold its shape then quickly collapses thus
producing a shockwave from the released energy. The reason why the bubbles implode is due to
the flowing liquid subjected to pressure above the vapor pressure. This phenomenon can be
observed near areas of low pressure or high velocities since they are inverse proportional.
3.2.2 Non-Inertial Cavitation
This spectacle occurs when there is a forced vibration on the bubble thus creating a
controlled collapse of the air bubbles. The forced oscillations are normally created by the
energy input of an acoustic field or vibrations created from the impellors of the pump or the
propellers of a submarine. Not all uses of cavitation are destructive for example non-inertial
Figure 2: Shows the imploding air
bubble collapsing as time increase
Figure 3: shows the bubbles collapsing due
to increase pressure

5. 4
cavitation can be used as cleaning baths or as a form of nondestructive testing, by measuring the
wavelength the material to find different engineering properties.
3.3 Cavitation Type
3.3.1 Suction Cavitation
Suction Cavitation occurs when there is low pressure or high vacuums conditions in the
pump, was a result of this circumstance there is reduction in flow to the pump. The pump will
“starve” and cavities will begin to form at the eye of the impellor due to the water becoming
vapor pressure. As the bubbles are moving to the discharge area of the
pump the change in fluid condition will compress the bubble into a liquid
thus resulting the bubbles collapsing. The imploding cavities will then
transfer its energies in the form of strong local shock waves which may
be audible, thus resulting in major damages to the impellor. An impellor
exposed to prolong suction cavitation may have chunks of its material
removed thus leading to the premature failure of the pump.
3.3.2 Discharge Cavitation
Discharge cavitation occurs when the discharge pressure is exceedingly high which
generally occurs when the pump runs below 10% of its best efficiency point (BEP). Due to the
high pressure at the discharge it will be very difficult for fluid flow out of the pump, resulting in
the fluid circulating inside the pump. As the fluid passes between the impellor and housing
through a small clearance, the velocity will increase resulting in a vacuum that will turn the
liquid into vapor at the housing wall. Due to the intensity of the fluid activity at the housing wall
the bombardment of cavities will create powerful shockwaves that will damage the tip of the
Figure 4: A control collapse of
air bubble
Figure 5: Illustration of Suction
Cavitation

6. 5
impellor, housing walls and in extreme cases break the shaft of the impellor. Due to the extreme
conditions created by discharged cavitation any pump exposed under a prolong period of time
will also experience mechanical failure of the seals and bearings.
3.4 Causes of Cavitation in Pumps
There are a lot of reason why cavitation bubbles could form in a pump such as poor pipe
design, drop in pressure at the suction nozzle due to low Net Positive suction head (NPSHa)
because if the pump pressure is not greatly higher than vapor pressure then cavities will form. If
there is an increase of temperature of the liquid at the pumps suction point there will be an
increase in vapor pressure limit and this will lead to the formation of bubbles. Increasing the
inlet velocity will also create low pressure as seen from Bernoulli’s equation and frictional
pressure drop from sharp elbow, valves and other fittings will create areas of low pressure. If the
pump is not properly cleaned there will be a buildup of junk that will later reduce the net flow
that will create vapor in the pump due to the high velocities, this will cause the pump to starve.
Lastly the most common mistake that could lead to cavitation is the selection of the wrong pump
and if the pump is selected incorrectly then the net positive suction head will fall below its limit
and due to the pump ability to be unable to perform its task cavitation will occur.
Figure 6: Discharge Cavitation
Figure 7: Areas were cavitation will
be likely form

7. 6
3.5 Identifying Cavitation Formation
One of the most common signs to tell if a pump undergoing cavitation is if the pump is
vibrating and the outflow is less than the desired amount. The vibration field the pump may emit
may start off as a low intensity and increase over time signifying the material is being chipped
away over time. The surfaces of the part that is being contacted by the chipped material pieces
will become rough thus its frictional force will be increased and the pump will require more
energy to push the fluid and this will lead to more power consumption. Many time consumers
may confuse Air entrainment to cavitation because both process involves the process of vapors,
the difference between the two is air entrainment is the process when air enters the pump through
the suction section and expands reducing fluid flow and damages the material. Air entrainment
can be easily fixed by identifying the leaks and patching them, this process is not completely
destructive meaning very skilled technicians can use the air to cushion the imploding bubbles
thus reducing the cavitation. This process has its risks, if too much air is introduced to the
system the pump will overflow and will be unable to handle the cavitation therefore reducing the
overall efficiency of the pump.
4. Cavitation Control
4.1 Parameters
Because cavitation is likely to occur at the inlet of the pump since there is a lower
pressure Bernoulli’s equation can be applied between the surface of the liquid and the impellor.
𝑣𝑖
2
2𝑔
+
𝑃𝑖
2𝑔
+ 𝑧 =
𝑃𝐴
𝜌𝑔
− ℎ𝑓 (2)
We can also describe the net positive suction head, NPSH for pumps in the following equation.
𝑁𝑃𝑆𝐻 =
𝑃𝑖
𝜌𝑔
+
𝑣𝑖
2
2𝑔
−
𝑃𝑣
𝜌𝑔
(3)
With the help of equation (2) we can write equation (3) as
𝑁𝑃𝑆𝐻 =
𝑃𝐴
𝜌𝑔
−
𝑃𝑣
𝜌𝑔
− 𝑧 − ℎ𝑓 (4)

8. 7
The Thomas cavitation parameter and critical cavitation parameter can be written respectively as.
𝜎 =
𝑁𝑃𝑆𝐻
𝐻
(5)
𝜎𝑐 =
(
𝑃 𝐴
𝜌𝑔
)−(
𝑃 𝑖
𝜌𝑔
)−𝑧−ℎ𝑓
𝐻
(6)
𝜎 > 𝜎𝑐 (7)
Another Parameter to determine if cavitation will occur by using the following equation
𝑁𝑠 =
𝑁 ∗ √ 𝑄
(𝑔 ∗ 𝐻𝑠)
3
4
(7)
Where, 𝐻𝑠 =
𝑃 𝑜−𝑃𝑣
𝜌𝑔
(8)
If Ns > 0.47 cavitation will occur
If N is in radians then Ns ≥ 4
4.2 Limiting Cavitation
4.2.1 Pump Design
One of the serval ways to limit cavitation in a pump is to design a pump with NPSHA
being higher than the NPSHR, and one method to do is to increase the suction pressure of the
pump which can be done by reducing the vertical displacement of the liquid and pump. Also
reducing the temperature of the liquid that is being pumped will decrease the saturation pressure
thus making the NPSHA higher. Another method to increase NPSHA is to increase the pipe
diameter, number of elbows, valves and length of pipe this is simultaneously decrease the head
loss as well. The reason why the NPSHR is difficult to manipulate is due to its properties being
a function of the pump, if the pump speed decreases the NPSHR also decreases.
4.2.2 External Contributions
Although redesigning a pump may not be ideal alternative methods to limit cavitation
formation may implemented such making sure the pump strainers and filters are properly
maintained and clean. Another way to reduce the cavity bubbles is to reduce the suction head
height due to the fluid and the suction pipe length. Reducing the temperature of the temperature
of Newtonian fluids such as water may also help to lower the risk of creating cavitation bubbles
because the vapor pressure is also simultaneously being lowered as well this making it harder for

9. 8
the fluid to vaporize at low pressure. Monitoring cavitation is always a good option; this could be
from checking the filters for reduced flow, installing pressure gages to check the pump curves and
determine if the problem is hydraulic or mechanical. .
5. Reflection
If I were to design a pump impellor, I would start off by studying a submarine’s propeller
because both fundamentally serve the same purpose which is to “push fluid”. Many government
submarines developed very silent propellers meaning they create very little cavitation and can
travel at optimal speeds. The difference between the propeller and impellor is the location they
are used, propellers are used in an open environment and impellors are used in a closed
environment such as a pump. By studying a propeller I would research various angle of twists,
gaps and aerodynamics and why the reasons why this type of propeller is effective. One of the
most fundamental ways to decrease cavitation in a pump is to design with a slower rpm, however
with a slower rpm this means there will be an increase in the pump size due to the vibrational
displacement being bigger. After I have completed my design study of propellers I can optimize
my impellor now by calculating the Meridional velocity which is a function and the flow rate (9).
I will also need the rotational velocity (10) and the inlet flow angles (11). This will give my
velocity triangle and because my impellor size is hidden in the Meridional velocity I can
optimize it to find my optimal area. By control the area I can control the gap size also and type of
impellor I want can be calculated from the velocity triangles.
𝑉 𝑚(ℎ,𝑠) =
𝑄
𝐴
(9)
𝑈 𝑛(ℎ,𝑠) = 𝜔𝑟(ℎ,𝑠) (10)
𝛼 𝑚(ℎ,𝑠) = 𝑡𝑎𝑛−1 𝑉 𝑚(ℎ,𝑠)
𝑈 𝑛(ℎ,𝑠)
(11)
Figure 8: Velocity Triangle
Method

10. 9
6. Future Scopeof Research
Cavitation control is a very powerful research topic because of its potential to greater
improves the efficiency along with the lifespan of turbo machinery and other aerodynamic
systems. If the cavitation problem that arises in much fluid system can be reduced there will be
an influx of wealth a company can save yearly from damages created from cavitation. I would
like to thank my Professor Rishi Raj for assigning this research topic to me. I did learn a lot
about pump designs, I hope to continue to research areas similar to this topic and couple that
knowledge with my creativity to design wonderful and efficient systems.
7. Conclusion
In conclusion, the cavitation effects created from turbomachines can substantially
decrease the overall efficacy and destroy the materials properties within a short period of time,
however not all cavitation process is destructive. The nondestructive cavitation, non-inertial can
be used to clean pumps or propellers and is created by an acoustic force to control the decay of
the bubbles. In centrifugal pumps the category of cavitation can be describe in two ways suction
which is created from low pressure conditions and discharge cavitation resulted from high
discharge pressure. Although cavitation can be created due to low pressure build up around the
impellor it can be prevented with proper pipe design, lowering the pump speed and with the help
of valve control technologies. Many may confuse cavitation with air entailment because both
process are similar, however the difference is air entailment is when air molecules enters directly
in to the pump whereas cavitation is created. Many skill technicians can use air entailment to
cushion the cavitation bubbles as the implode to the surrounding materials thus reducing its
destructive properties, however this technique comes with its risk and if they are not careful they
can essential “starve” the pump making matters worse. After the engineers are finish developing
their pumps there are a few criteria that must be checked to make sure there is little to no
cavitation in pump at optimal conditions and these parameters can be calculated with equations
(2) to (8). Although there are many advances in this field of turbomachinery there is always
room for improvement because science and engineering always venturing into the unknown.

11. 10
8. Nomenclature
𝑃 − 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝑝- Density
ℎ𝑓 − ℎ𝑒𝑎𝑑 𝑙𝑜𝑠𝑠
𝜔 − 𝑎𝑛𝑔𝑢𝑙𝑎𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝜎 − 𝑇ℎ𝑜𝑚𝑎𝑠 𝐶𝑎𝑣𝑖𝑎𝑡𝑖𝑜𝑛 𝑃𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟
𝜎𝑐 − 𝐶𝑟𝑡𝑖𝑐𝑎𝑙 𝐶𝑎𝑣𝑖𝑎𝑡𝑖𝑜𝑛 𝑃𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟
𝑉 − 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝑄 − 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒
𝛼 𝑚(ℎ,𝑠) − 𝐼𝑛𝑙𝑒𝑡 𝑓𝑙𝑜𝑤 𝑎𝑛𝑔𝑙𝑒
𝑈 𝑛(ℎ,𝑠) − 𝑅𝑜𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝑉 𝑚(ℎ,𝑠)− Meridional velocity
r- Radius
9. Table of Figures
Figure 1: Pressure vs Temperature for Water........................................................................................2
Figure 2: Shows the imploding air bubble collapsing as time increase....................................................3
Figure 3: shows the bubbles collapsing due to increase pressure ............................................................3
Figure 4: A control collapse of air bubble ............................................................................................4
Figure 5: Illustration of Suction Cavitation...........................................................................................4
Figure 6: Discharge Cavitation ............................................................................................................5
Figure 7: Areas were cavitation will be likely form ...............................................................................5
Figure 8: Velocity Triangle Method .....................................................................................................8

12. 11
10. References
[1] (Parkhurst, 2014) http://www.engineeringtoolbox.com/cavitation-d_407.html
[2] Christopher Earls Brennen (1995), CAVITATION AND BUBBLE DYNAMICS, Oxford
University Press
[3](Dale Conway, 2010) http://www.flowcontrolnetwork.com/qa-pump-cavitation-diagnosis-
control
[4] Prakash Shinde, Ajinkya Satam (2014) , Cavitation Effect in Centrifugal Pump, International
Journal of Researchers, Scientists and Developers, Vol. 2 No. 2
[5] Thermo-fluid system analysis and design- Prof. R.S Raj