International Journal of Civil Engineering and Technology (IJCIET)
Volume 10, Issue 04, April 2019, pp. 28-35, Article ID: IJCIET_10_04_004
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJCIET&VType=10&IType=4
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
Scopus Indexed
CAVITATION IN MATERIALS USED IN THE
MANUFACTURE OF HYDRAULIC TURBINES:
REVIEW
J.D. Betancur, A. Ruiz, M. J. Valdés
Facultad de Ingeniería
Instituto Tecnológico Metropolitano, Medellín, Colombia.
ABSTRACT
Cavitation affects the hydraulic turbines of action and reaction, causing them to
lose efficiency and useful life. It is important to consider this phenomenon to prevent or
mitigate it. In this review we describe the phenomenon of cavitation and its different
forms. Later, studies are presented that discuss the effects of cavitation on the
microstructure of materials and on the surface, how roughness and surface defects
increase the amount of material removed by cavitation. Then we present some
alternatives that are proposed to reduce cavitation by means of coatings or from the
design of the turbines. Finally, the effects of cavitation on the reaction turbines are
presented.
Key words: Cavitation, Erosion, Hydraulic turbine, Materials
Cite this Article: J.D. Betancur, A. Ruiz, M.J. Valdés, Cavitation in Materials Used in
the Manufacture of Hydraulic Turbines: Review, International Journal of Civil
Engineering and Technology 10(4), 2019, pp. 28-35.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=4
1. INTRODUCTION
Worldwide there is an installed capacity in hydroelectric plants between 926GW and 956GW
up to the year 2010 [1]. According to the Colombian Association of Electric Power Generators,
the installed capacity in the country of hydraulic energy is 10.785 MW, which is equivalent to
80.7% of the total generation [2]. The hydraulic turbine is one of the fundamental components
which transforms the potential energy of water into rotational mechanical energy [3]. The
efficiency of the turbines depends to a large extent on their hydraulic profile, for this reason
any change in their geometry affects significantly the efficiency of the system [4]. The main
reasons for loss of mass in the turbine are given by the factors of erosion, cavitation and
corrosion [5]. The present article will focus on cavitation as a factor of wear in hydraulic
turbines.
2. CAVITATION
Cavitation is a physical phenomenon, through which a liquid, under certain conditions, passes
into a gaseous state producing bubbles that a few moments later pass back into a liquid state.
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Cavitation in Materials Used in the Manufacture of Hydraulic Turbines: Review
The vapor bubbles or cavitation bubbles disintegrate as they are swept out of the low pressure
regions, thereby generating extremely destructive high pressure waves [6]. If the bubbles
collapse near a solid body, they can detach small parts of material or cause erosion on the
surface and long-term component failure.
In a timeline, cavitation begins with the formation of microscopic bubbles, expansion of said
bubbles, increase of the vapor temperature inside the bubbles that finally collapse releasing a
large amount of energy. In Figure 1 the growth of the cavitation bubbles in time deltas of 3.7
microseconds can be observed, the compression of the bubble after reaching its maximum point
and later the implosion is also evident.
Cavitation can be generated in four different ways according to Lauterborn. The
hydrodynamic and acoustic cavitation are caused by the tension of the liquid; and optical and
particle cavitation are generated by a local energy reservoir [7]. Hydrodynamic cavitation is
generated by pressure changes due to the shape of the system. Acoustic cavitation is produced
by sound waves in a fluid by the variation of pressure. Optical cavitation is induced by highintensity light photons that traverse the liquid.
The cavitation can generate erosion in a material because of the impulsive pressure generated
by the repetitive collapse of the bubbles, this phenomenon affects hydraulic turbines, pumps,
boat propellers and hydroprofiles [8]. Considering that the turbines are located in rivers where
the water contains additional components, it is necessary to consider, for example, silt particles
in the flow since these can increase the cavitation bubbles through the increase of the nuclei in
the Water. Jyoshiro, et al. reported in a study that initial cavitation in water laden with silt can
increase by 10-15% compared to tap water [9].
Figure 1. Growth of cavitation bubbles in sequence, using Phantom v2511 speed camera. Source:
[10].
3. EFFECTS OF CAVITATION ON MATERIALS
Borkent et al. Conducted a study of the influence of the type of particles on the cavitation
phenomenon for different materials such as polyamide, polystyrene, glass beads, CaSO4,
CaPO4, dynoQ735, dynoQ745, hydroxylapatite and polibead. The results obtained are shown
in Figure 2 where the cavitation activity in the polyamide and polystyrene materials are higher
than others. The main reason to occur is the hydrophobic surface and rough structure of the
particles of the two materials mentioned above. In Figure 3, images obtained by means of an
electron scanning microscope are shown. Although the surface of polystyrene is more
hydrophobic than polyamide, it can be confirmed that the surface of the polyamide is rougher
than that of polystyrene, this makes the gas bubbles that cause cavitation can enter and stabilize
on the surface of the polyamide more easily [11].
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J.D. Betancur, A. Ruiz, M.J. Valdés
Figure 2. Relative cavitation for different particles suspended in water of high purity. Source: [11].
Figure 3. SEM photograph a) Polyamide, b) Polystyrene. c) Glass beads. Source: [11].
Arora, et al. found in a study conducted in 2004, that the surface structure is one of the main
factors for the initiation of cavitation. In the study an acrylic polymer and polystyrene was used,
in Figure 4 the photographs of the surface of both materials taken with an electron scanning
microscope are observed. Figure 4.a belongs to the polystyrene and Figure 4.b is of the acrylic
polymer. In the study it was shown that with the wave increase that starts cavitation, the material
with a smooth surface did not present cavitation even at high voltage levels, while for
polystyrene, initiation of cavitation was observed [12].
Figure 4. Scanning electron microscope image of the particles of the surfaces a) Polystyrene
(copolymer: divinylbenzole, distribution diameter 30 to 150 μm) b) Acrylic polymer (monodisperses
30 μm dynospheres EXP -SS-42.3-RSH). Source: [12].
The studies of cavitation in surfaces for different materials used in the manufacture of
components of hydraulic turbines, pumps, propellers of boats and hydro profiles are of great
importance to estimate the behavior of these materials. For this reason, Gottardi et al.
Developed a cavitation study on different types of aluminum alloys by means of ultrasonic
vibration tests [13]. The study begins with the characterization of the surface microstructure of
each of the alloys by means of an electron scanning microscope as shown in Figure 5, where it
is shown that the surface of A356 and AlSi3Cr-AC present significant particle separations.
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Cavitation in Materials Used in the Manufacture of Hydraulic Turbines: Review
Figure 5. Microstructure of a) A356, b) 6061, c) AlSi3Cr AC, d) AlSi3Cr T6. Source: [13].
In Figure 6 the loss of mass is shown against the time of exposure to cavitation. The alloys
of AlSi3Cr present a better resistance to cavitation than the alloy A356. The AlSi3Cr-T6 alloy
was the one that lost less mass during the entire test period. It is evident that the materials that
initially had more defective spaces on the surface present a greater loss of mass due to
cavitation.
Figure 6. Loss of mass versus time of exposure to cavitation. Source: [13].
On the other hand, the roughness of each material was measured before being exposed to
cavitation and then the average roughness of the eroded surface of the materials was measured
for different test times, the average values are shown in Figure 7. evidence a roughness
variation for all alloys compared to the initial condition after one minute of testing. The samples
of A356 and AlSi3Cr-AC exhibit greater roughness than the other alloys due to the low
hardness in relation to 6061 and AlSiCr-T6. Although authors like Ospina and others [14], they
affirm that there is no evident relationship between the hardness of the material and the
resistance to cavitation. Some authors agree that cavitation produces progressive roughness on
the surface, formation of holes and corrugations that can amplify the surface defect until it leads
to eventual detachment of material due to edge fracture or desquamation [15]–[17]. In fact,
when the pressures exceed the elastic limit of the material, it undergoes a plastic deformation
until a microscopic failure occurs, promoting the removal of the material by ductile cutting of
the superficial asperities [18]–[20].
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J.D. Betancur, A. Ruiz, M.J. Valdés
Figure 7. Average roughness on the surface at the start, 1 minute and 5 minutes after exposure to
cavitation. Source: [13].
4. MITIGATE CAVITATION
Lavigne, et al [21], used an alloy known as CaviTec® to improve the resistance to cavitation
of certain materials by means of coatings, postponing the moment when the material begins to
lose mass. In Figure 8a we can see the surface of a material with Fe, Cr, Mn, Co, Si content in
which CaviTec® powders were used without grinding to make a coating, this has large regions
between particles which are rich in oxides, pores and structural defects. In Figure 8.b the same
surface is shown after 2 hours of exposure to cavitation where it is evident that the areas most
affected by cavitation were initially defective areas.
In the same study a material proportions of Fe, Cr, Mn, Co, Si similar to the previous one
were used, they used CaviTec® powders in a process of grinding with balls to make a coating,
in Figure 9.a a surface with regions is evidenced between smaller particles with respect to the
material of Figure 8.a. In Figure 9.b it is evident that after two hours of cavitation the surface
is less affected than that of the previous material. This material with the coating obtained the
best results of the study in terms of resistance to cavitation, showing that the coating improves
the surface and reduces the loss of material because it decreases the pores and defective places
where cavitation begins.
Other authors used nickel, chromiuoxide and tungsten carbide coatings, performing erosion
tests by cavitation. The results showed that fragile fractures and microcracks on the surface
appear in this coating. However, the coatings studied had lower cavitation strengths than
uncoated stainless steel [22].
Figure 8. Material coating with CaviTec® powders without grinding (Image obtained with electronic
scanning microscope). a) Before the cavitation test. b) Two hours later. Source: [21].
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Figure 9. Coating material with CaviTec® powder ground with balls (Image obtained with an electron
scanning microscope). a) Before the cavitation test. b) Two hours later. Source: [21].
On the other hand, Silva [23], use design techniques to avoid cavitation in hydraulic
turbines, where they calculate the location of minimum pressure points by means of
mathematical models, then simulate these geometries to corroborate the pressures generated in
the turbine and once these points are identified, they make changes in the geometry. Wimshurst
[24], in addition to the study of pressure profiles, shows the effect of the speed of the end of the
blade, concluding that the higher this is, the greater the chances that the turbine will present
cavitation. Chernin, et al. [25], develop a statistical model in which they take into account
variables of pressure, fluid velocity, depth at which the turbine is located and its interaction
with the fluid; finding the values of the variables with which, statistically, a lower probability
of cavitation is obtained.
5. EFFECTS OF CAVITATION IN HYDRAULIC TURBINES
As evidenced in the previous sections, the cavitation bubbles can cause damage to the material
when they collapse near the surface, therefore, detachments of the material that give rise to
cavitation erosion in hydraulic turbines can be generated. In the action turbines the cavitation
is low, in the reaction cavitation has been an important aspect that is considered from the design
and operation of the turbine.
The cavitation in a Francis turbine can be started at the entrance edge near the base of the
blades, it can also occur in the throat of the rotor flow passage [26], in this last point the
cavitation is sensitive since it is can extend to other parts of the turbine [27].
Celebioglu, et al. in a study conducted in 2017 [28], show the increase in cavitation for a
Francis turbine that operates to conditions outside of design, by means of simulation, it predicts
the areas where it will generate the greatest amount of bubbles and performs a redesign where
it increases efficiency to 97% and minimizes cavitation to increase turbine life.
The strong vibrations and noises caused by cavitation can produce mechanical faults in
turbine elements as evidenced by Frunzǎverde [29] in a metallographic study conducted to
determine the reason for the fracture of a blade in a rotor of a Francis turbine. In Figure 10,
several fatigue cracks can be observed, initiated by grooves of weld seams.
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J.D. Betancur, A. Ruiz, M.J. Valdés
Figure 10. Details of the rotor fault of the Francis turbine. Source: [29].
6. CONCLUSIONS
After a search made in the literature about cavitation and erosion by cavitation on hydraulic
turbines, it can be concluded that:
• Roughness and imperfections on the surface of a material influence the generation of
cavitation.
• A defective material increases the likelihood of cavitation initiation and also mass
loss due to long-term bubble collapse.
• To avoid cavitation in hydraulic turbines, the operating parameters must be taken
into account in accordance with the design parameters.
• To increase the time in which the turbine does not lose mass due to cavitation, the
choice of material is of the utmost importance.
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