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TO INVESTIGATE THE EROSIVE WEAR OF
THERMAL SPRAYED CERAMIC COATING ON
CAST IRON USED IN ASH SLURRY DISPOSAL
PUMP
A THESIS SUBMITTED
IN THE PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE AWARD OF DEGREE OF
MASTER OF TECHNOLOGY
In
Mechanical Engg. Machine Design
SUBMITTED BY
Sunil Kumar
M80801161019
PUNJAB TECHNICAL UNIVERSITY
JALANDHAR, PUNJAB, INDIA
July 2012
TO INVESTIGATE THE EROSIVE WEAR OF
THERMAL SPRAYED CERAMIC COATING ON
CAST IRON USED IN ASH SLURRY DISPOSAL
PUMP
A THESIS SUBMITTED
IN THE PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE AWARD OF DEGREE OF
MASTER OF TECHNOLOGY
In
Mechanical Engg. Machine Design
SUBMITTED BY
Sunil Kumar
M80801161019
PUNJAB TECHNICAL UNIVERSITY REGIONAL CENTER
Baba Banda Singh Bahadur Engineering College
Fatehgarh Sahib (Punjab)
July 2012
ii
ACKNOWLEDGEMENTS
I express my sincere gratitude to the Punjab Technical University, Jalandhar for
giving me the opportunity to work on the thesis during my final year of M.Tech.
I would like to thank Dr. M.S. Grewal, Coordinator, PTU Regional Center, Baba
Banda Singh Bahadur Engineering College, Fatehgarh Sahib for their kind support.
I also owe my sincerest gratitude towards Prof. Jasbir Singh Ratol, Associate
Professor & Workshop Superintendent, Department of Mechanical Engineering, Baba
Banda Singh Bahadur Engineering College, for acting as supervisor and giving valuable
guidance during the course of this investigation, for his ever encouraging and timely
moral support.
I would like to thank Dr. A.P.S. Sethi, Prof. & Head, Mechanical Evgineering
Department, Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib for
providing me the facilities in the department for the completion of my work.
I would like to thank the Dean Academics and members of the Departmental
Research Committee for their valuable suggestions and healthy criticism during my
presentation of the work.
I would also like to thank my parents for their blessings and my wife who helped
me in my thesis.
I would also like to thank everyone who has knowingly & unknowingly helped
me throughout my thesis.
Last but not least, a word of thanks for the authors of all those books and papers
which I have consulted during my thesis work as well as for preparing the report.
At the end thanks to the Almighty for all good deeds.
Sunil Kumar
M80801161019
iii
ABSTRACT
Centrifugal pumps are used to transport the ash slurry from the thermal power
plant to ash pond. Erosive wear occurs on the impeller of the pump due to the
impingement of the ash particles, suspended in water, on the impeller with a high
velocity. The service life of centrifugal pump, handling solid-liquid mixture can be
increased by reducing the erosive wear. Due to erosion, pump life become very short
and need to be replaced periodically. It affects both the initial cost and life of the
component.
In the present work, three parameters were selected to investigate the erosive
wear i. e., rotational speed, ash concentration and ash particle size. Detonation gun
sprayed Al2O3 coating is provided on the surface to reduce the erosive wear and hence
to improve the performance of centrifugal pump. The experimentation on high speed
slurry erosion tester has been carried out to test the erosive wear of uncoated and
coated material. The erosive wear behavior was investigated by Response surface
methodology (RSM). Statistical analysis was performed in the form of the analysis of
variance (ANOVA) to determine the interaction of experimental parameters. At the end,
the graphs were compared for uncoated and coated material to get the final result. The
result shows that the increase in each parameter contributes to erosive wear of pump
impeller. The effect of rotational speed and ash concentration on erosive wear of the
uncoated material is found to be significant as compared to ash particle size. For Al2O3
coated cast iron, the effect of rotational speed on erosive wear is found to be significant
followed by ash concentration and ash particle size. Al2O3 coating on the surface of the
pump impeller improves the performance of centrifugal pump.
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Contents
CANDIDATE’S DECLARATION i
ACKNOWLEDGMENTS ii
ABSTRACT iii
LIST OF TABLES vi
LIST OF FIGURES vii
1 INTRODUCTION 1
1.1 Ash Disposal System 2
1.1.1 Mechanical handling system 4
1.1.2 Hydraulic system 4
1.1.3 Pneumatic system 7
1.1.4 Steam jet system 8
1.2 Principal Requirements of a Good Ash Handling Plant 8
1.3 Ash Disposal Using Centrifugal Pump 8
1.3.1 Working principle of centrifugal pump 8
1.3.2 Main parts of a centrifugal pump 9
1.3.3 Advantages of using centrifugal pumps for transportation of
slurry
14
1.4 Wear 15
1.4.1 Types of wear 15
1.4.2 Parameters affecting erosive wear 17
1.5 Methods for Testing Erosion in Slurry Pumps 19
1.5.1 Jet erosion test 20
1.5.2 Slurry pot test 21
1.5.3 Coriolis erosion test 21
v
1.6 Survey of Thermal Power Plant, Yamunanagar 22
1.7 Thermal Spraying 23
1.8 Motivation of the Work 25
2 LITERATURE REVIEW 27
3 SLURRY EROSION TESTING 37
3.1 Hydraulic Ash Handling System 37
3.2 High Speed Slurry Erosion Tester 39
3.3 Erosion Testing of High Chrome Cast Iron 41
3.3.1 Experimental design (Response Surface Methodology) 45
3.3.2 Analysis of variance (ANOVA) 46
3.3.3 Modeled equation in terms of actual factors 48
3.4 Erosion Testing of Ceramic Coated Cast Iron 49
3.4.1 Experimentation and statistical analysis 49
3.4.2 Modeled equation in terms of actual factors 52
4 RESULTS AND DISCUSSIONS 54
4.1 Results 54
4.1.1 Uncoated cast iron 54
4.1.2 Ceramic coated cast iron 57
4.2 Discussions 59
5 CONCLUSIONS AND SCOPE FOR FUTURE WORK 62
REFERENCES 64
A APPENDIX - Weight of each uncoated and coated specimen before and
after experimentation
68
B APPENDIX - Physical and chemical properties of fly ash 70
C PUBLICATIONS 71
vi
LIST OF TABLES
Table
No.
Caption Page
No.
1.1 Technical data used for slurry disposal pumps 22
3.1 Sieve analysis chart 44
3.2 Factors affecting erosion with levels 45
3.3 Box-Behnken design matrix 46
3.4 ANOVA for response surface linear model 47
3.5 Regresion analysis for response surface linear model 47
3.6 Experimental and modeled values for the response (uncoated material) 48
3.7 ANOVA for response surface reduced quadratic model 51
3.8 Regresion analysis for response surface reduced quadratic model 52
3.9 Experimental and modeled values for the response (Coated Material) 53
vii
LIST OF FIGURES
Figure
No.
Caption Page
No.
1.1 Schematic diagram of a coal-fired thermal power station 1
1.2 General outline of generation of ash 3
1.3 Mechanical handling system 4
1.4 Low pressure handling system 5
1.5 High pressure handling system 6
1.6 Pneumatic system 7
1.7(a) Fluid flow in a centrifugal pump 9
1.7(b) Model of a centrifugal pump 9
1.8(a) Front view of impeller 10
1.8(b) 3D model of impeller 10
1.9 Open impeller 12
1.10 Semi-open impeller 12
1.11 Enclosed impeller 12
1.12 Methods to test erosion 20
1.13 Schematic of thermal spray process 23
1.14 Schematic of the detonation thermal spray process 25
3.1 Clean water for making slurry 37
3.2 Slurry before entering the impeller 38
3.3 Hydraulic ash handling system 38
3.4 Slurry pumps in series 39
3.5(a) Monitoring panel of high speed slurry erosion tester 40
3.5(b) Experimental set-up of high speed slurry erosion tester 40
3.6 Test specimen holder for holding maximum of 12 specimens 42
3.7 Test specimens 42
3.8 Ash percentage versus particle size 44
3.9 Al2O3 coated specimens affected by erosive wear 50
4.1 Weight loss vs. rotational speed 54
4.2 Weight loss vs. ash concentration 55
4.3 Weight loss vs. ash particle size 56
4.4 Contributions of selected parameters for erosive wear of high
chrome cast iron 56
4.5 Weight loss vs. rotational speed 57
4.6 Weight loss vs. ash concentration 57
4.7 Weight loss vs. ash particle size 58
4.8
Contributions of selected parameters for erosive wear of Al2O3
coated cast iron 59
4.9 Comparison of erosion for coated and uncoated material 60
1
Chapter 1
INTRODUCTION
A thermal power station is a power plant in which the prime mover is steam
driven. A huge quantity of ash is produced in central stations, hundreds of tons of ash
may have to be handled every day in large power stations and mechanical devices
become indispensable. Figure 1.1 shows a schematic diagram of a Coal-Fired Thermal
Power Station.
Fig. 1.1 Schematic diagram of a Coal-Fired Thermal Power Station
(Source: en.wikipedia.org)
1. Cooling tower 10. Steam control valve 19. Super heater
2. Cooling water pump 11.High pressure steam turbine 20.Forced draught fan
3. Three-phase transmission line 12. Deaerator 21. Reheater
4. Step-up transformer 13. Feed water heater 22.Combustion air intake
5. Electrical generator 14. Coal conveyor 23. Economizer
6. Low pressure steam turbine 15. Coal hopper 24. Air preheater
7. Boiler feed water pump 16. Coal pulverizer 25. Precipitator
8. Surface condenser 17. Boiler steam drum 26.Induced draught fan
9. Intermediate pressure steam turbine 18. Bottom ash hopper 27. Flue gas stack
2
1.1 ASH DISPOSAL SYSTEM
All the available coals have some percentage of ash. The percentage of ash
content (by weight) in anthracite coal, bituminous coal and lignite Coal ranges from 9.7
to 20.2 %, 3.3 to 11.7% and 4.2 to 5.3 % respectively. In the modern large steam power
plants where huge amounts of coal are used, the amount of ash may be go up to many
thousands tones of ash per year. Theoretically whole of the ash from the furnace should
get deposited in the ash hoppers, but actually from 5 to 40% of it leaves with the flue
gases. Figure 1.2 shows a general outline of generation of ash.
Ash handling comprises the following operations:
1. Removing the ash from the furnace ash hoppers.
2. Conveying this ash to a fill or storage by means of conveyors.
3. Disposal of the stored ash.
To handle huge amounts of ash per day, mechanical means are employed. The ash
handling and disposal system can work continuously or intermittently.
The following are some of the places where the ash can disposed off:
1. Where seaborne coal is used, barges may take the ash to sea for disposal into a
watery grave.
2. Disused queries within reasonable distance of the power station may be used for
dumping the ash into evacuated land.
3. Building contractors may use it to fill the low lying areas.
4. Wasteland sites may be reserved for the disposal of the ash.
5. Deep ponds may be constructed and the ash can be dumped into these ponds
and when they are completely filled, they may be covered with soil and seeded
with grass.
3
Ash handling is major and difficult problem due to the following difficulties
encountered in its handling and disposal:
1. Ash is dusty and so irritating and annoying in handling.
2. It is hot when it comes out of the boiler furnace.
3. It is abrasive and wears out the containers.
4. Poisonous gases are produced.
5. Corrosive acids are produced in water.
6. It forms clinkers by fusing together in lumps.
The following points should be kept in view while handling and disposing ash:
1. Locate the ash plant on the leeward side of the power station to avoid blowing
in and drawing into the buildings of the dry ash.
2. If the ash is cold and cannot be disintegrated, it may be better to crush it before
passing it further or otherwise it might choke the sluiceway.
3. In case of pulverised fuel firing, 60 to 80% of ash is in the form of dust and fly
ash; the plant should be designed accordingly.
Figure 1.2 General Outline of Generation of Ash
(Source: en.wikipedia.org)
4
The modern ash handling systems are mainly classified into four groups:
1.1.1 Mechanical Handling System
This system is generally employed for low capacity power plants using coal as
fuel. The hot ash released from the boiler furnaces is made to fall over the belt
conveyor after cooling it through water seal. This cooled ash is transported to an ash
bunker through the belt conveyor. From ash bunker the ash is removed to the dumping
site through trucks. Figure 1.3 shows a mechanical ash handling system.
Figure 1.3 Mechanical Handling System
(Source: en.wikipedia.org)
1.1.2 Hydraulic System
In this system ash is carried with the flow of water with high velocity through a
channel and finally dumped in the sump. This system is subdivided as follows.
(a) Low pressure system.
(b) High Pressure system.
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(a) Low Pressure System
In this system a trough or drain is provided below the boilers and water is made
to flow through the trough. The ash directly falls into the troughs and is carried by
water to sumps. In the sump the ash and water are made to pass through a screen so
that water is separated from ash; this water is pumped back to the trough for reuse and
ash is removed to the dumping yard. The ash carrying capacity of this system is 50
tons/hour and distance covered is 500 meters. Figure 1.4 shows a low pressure
hydraulic ash handling system.
Figure 1.4 Low Pressure Handling System
(Source: en.wikipedia.org)
(b) High Pressure System.
The hoppers below the boilers are fitted with water nozzles at the top and on
the sides. The top nozzles quench the ash while the side one provides the driving force
for the ash. The cooled ash is carried to the sump through the trough. The water is
again separated from ash and recalculated. The ash carrying capacity of this system is as
large is 120 tons per hour and the distance covered is as large as 5000 meters. Figure
1.5 shows a high pressure hydraulic ash handling system.
6
Figure 1.5 High Pressure Handling System
(Source: en.wikipedia.org)
Advantages of Hydraulic System:
1. The system is clean and healthy.
2. It can also be used to handle stream of molten ash.
3. Working parts do not come into contact with the ash.
4. It is dustless and totally closed.
5. It can discharge the ash at a considerable distance (8000 m) from the power
plant.
6. The unhealthy aspects or ordinary ash basement work is eliminated.
7. Its ash carrying capacity is considerably large, hence suitable for large thermal
power plants.
7
1.1.3 Pneumatic System
This system can handle abrasive ash as well as fine dusty materials such as fly
ash and soot. It is referable for the boiler plants from which ash and soot must be
transported some far off distance for final disposal. The exhauster provided at the
discharge end creates a high velocity stream which picks up ash and dust from all
discharge points and then these are carried in the conveyor pipe to the point of
delivery. Large ash particles are generally crushed to small sizes through mobile
crushing units which are fed from the furnace ash hopper and discharge into the
conveyor pipe which terminates into a separator at the delivery end.
The separator working on the cyclone principle removes dust and ash which pass
out into the ash hopper at the bottom while clean air is discharged from the top. The
exhauster may be mechanical or it may use steam jet or water jet for its operation.
When a mechanical exhauster is used it is usually essential to use a filter or washer to
ensure that the requirements are less. Steam exhauster may be used in small and
medium size stations. Where large quantities of water are easily and cheaply available
water exhauster is preferred. The ash carrying capacity of this system varies from 25 to
15 tons per hour. A Pneumatic ash handling System is shown in figure 1.6.
Figure 1.6 Pneumatic System
(Source: en.wikipedia.org)
8
1.1.4 Steam Jet System
In this case steam at sufficiently high velocity is passed through a pipe and dry
solid materials of considerable size are carried along with it. In a high pressure steam jet
system a jet of high pressure steam is passed in the direction of ash travel through a
conveying pipe in which the ash from the boiler ash hopper is fed. The ash is deposited
in the ash hopper. The system can remove economically the ash through a horizontal
distance of 200 m and through a vertical distance of 30 m.
1.2 PRINCIPAL REQUIREMENTS OF A GOOD ASH HANDLING PLANT
1. The plant should be able to operate with minimum personal attention and
should be able to handle large clinkers as well as dust and soot.
2. Precautions should be taken against abrasiveness of ash.
3. It should be able to handle both wet and dry ash and operate with little noise
and keep the dust menace to minimum.
4. Disposal of ash from the plant site should be speedy.
5. The plant should have adequate capacity to deal with the ultimate station
capacity.
1.3 ASH DISPOSAL USING CENTRIFUGAL PUMP
The selection of a pumping system for any slurry transportation system is
governed more by the practical considerations rather than purely on economical
considerations of maximum efficiency. However, discharge pressure and the abrasivity
are the two key factors for the selection of a pump.
1.3.1 Working Principle of Centrifugal Pump
The centrifugal pump works on the principle of forced vortex flow, which means
that when a certain mass of liquid is rotated by an external flow, the rise in pressure
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head of the rotating liquid takes place. The rise in pressure head at any point of the
rotating liquid is proportional to the square of tangential velocity of the liquid at that
point. Thus at the outlet of the impeller where the radius is more, the rise in pressure
head will be more and the liquid will be discharged at the outlet with high pressure
head. Due to high-pressure head, the liquid can be lifted to a high level. Figure 1.7 (a, b)
shows a flow of fluid and 3-D model of centrifugal pump.
Figure 1.7 (a) Fluid Flow in a Centrifugal Pump; (b) Model of a Centrifugal Pump
(Source: en.wikipedia.org)
1.3.2 Main parts of a centrifugal pump
The following are the main parts of centrifugal pumps
1) Impeller
2) Casing
3) Suction pipes with a foot valve and a strainer
4) Delivery pipe
1) IMPELLER
The rotating part of centrifugal pump is called impeller. It consists of a series of
backward curved vanes. The impeller is mounted on a shaft, which is connected to the
shaft of an electric motor. An impeller is usually made of iron, steel, aluminium or
10
plastic, which transfers energy from the motor that drives the pump to the fluid being
pumped by forcing the fluid outwards from the centre of rotation. Figure 1.8(a) shows
the axial, radial and tangential component of flow and figure 1.8(b) shows a model of
impeller [1].
Figure 1.8(a) Front view of impeller; (b) 3D model of impeller
Components of impeller
a) Blade
Blades are the series of backward or forward curved vanes which transfers the
power from shaft to the fluid.
b) Hub and Shroud
The hub is the surface of the machine closest to the axis of rotation. It defines
the inner fluid flow surface. The shroud is the surface of the machine farthest from the
axis of rotation. It defines the outer fluid flow surface. The hub and shroud can be
11
defined only after the machine data has been defined, although all of these objects can
be defined in one step.
c) Leading and trailing edges
The leading edge curve is the most upstream part of the blade. Any change to
the leading edge changes the blade surfaces, which changes the periodic surfaces as
well as the hub and shroud surfaces. The trailing edge curve is the most downstream
part of the blade.
Impellers are classified as:
Open impeller
Most totally open impellers are found on axial flow pumps. This type of impeller
would be used in a somewhat conventional appearing pump to perform a chopping,
grinding action on the liquid. The totally open axial flow impeller moves a lot of volume
flow, but not a lot of head or pressure. With its open tolerances for moving and grinding
solids, they are generally not high efficiency devices. Open impeller is shown in figure
1.9.
Semi-open impeller
A semi-open impeller has exposed blades, but with a support plate or shroud on
one side. These types of impeller are generally used for liquids with a small percentage
of solid particles from the bottom of a tank or river, or crystals mixed with the liquid.
The efficiency of these impellers is governed by the limited free space or tolerance
between the front leading edge of the blades and the internal pump housing wall. Semi-
open impeller is shown in figure 1.10.
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Figure 1.9 Open Impeller Figure 1.10 Semi-Open Impeller
(Source: Slurry System Handbook by Baha Abulnaga, 2002-McGraw Hill Companies Inc.)
Enclosed impeller
Totally enclosed impellers are designed with the blades between two support
shrouds or plates. These impellers are generally used clean liquids because tolerances
are tight at the eye and the housing, and there is no room for suspended solids, crystals
or sediment. Figure 1.10 shows a type of enclosed impeller.
Figure 1.11 Enclosed Impeller
(Source: Slurry System Handbook by Baha Abulnaga, 2002-McGraw Hill Companies Inc.)
13
2) CASING
Casing of a pump is an airtight passage surrounding the impeller and is designed
in such a way that the kinetic energy of the water discharged at outlet of the impeller is
converted into the pressure energy before the water leaves the casing and enters the
delivery pipe.
Types of casing:
a) Volute casing
Volute casing is of spiral type in which area of flow increase gradually. The
increase in the area of flow decreases the velocity of flow. The decrease in velocity
increases the pressure of the water flowing through the casing. In case of volute casing
the efficiency of the pump increase slightly as a large amount of energy is lost due to
formation of eddies in this type of casing.
b) Vortex casing
If a circular chamber is introduced between the casing and the impeller, the
casing is known as vortex casing. By introducing the circular chamber, the loss of energy
due to formation of eddies is reduced considerably. Thus, the efficiency of the pump is
more than the pump with volute casing.
c) Casing with guide blades
In this type of casing, the impeller is surrounded by series of guide blades
mounted on ring, which is known as diffuser. The guide vanes are designed in such a
way that water from the impeller enters the guide vanes without stock. Also as the area
of guide vanes increase, thus reducing the velocity of flow through guide vanes and
consequently increasing the pressure of water. The water from the guide vanes then
passes through the surrounding casing, which is concentric with the impeller
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3) SUCTION PIPE WITH A FOOT VALVE AND A STRAINER
A pipe whose one end is connected to the inlet of the pump and the other end
dips into water in a sump is known as a suction pipe. A foot valve, which is non-return
valve or one-way type of valve, is fitted at the lower end of suction pipe. The foot valve
opens only in the upward direction. A strainer is also fitted at the lower end of suction
pipe.
4) DELIVERY PIPE
A pipe whose one end is connected to the outlet of the pump and the other
delivers the water at the required height is known as delivery pipe.
1.3.3 Advantages of using Centrifugal Pumps for Transportation of Slurry
1) Simplicity of design
2) Easier installation
3) Low maintenance
4) Lower weight
5) Handles suspensions and slurry easily
Centrifugal pumps are best suited for short distances and for in-plant slurry pipe
line systems. Though the discharge pressure of centrifugal pumps is relatively low, they
can also be used for moderate pressure requirements when used in series. The
centrifugal pumps are used for over 97% of all short distance slurry pipelines. The
design of a centrifugal pump for slurry handling system needs special consideration to
ensure that the flow passage are such as to offer no restriction to the passage of solids.
The abrasivity of solids cause wears in the pumps. A vertical split casing design is
necessary to provide ready access to the wearing parts for replacement. For a given
duty, centrifugal pumps are usually cheaper, occupy less space and have lower
maintenance costs than positive displacement types, and can handle much larger solids.
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1.4 WEAR
Wear is defined as progressive volume loss of material from a solid surface due
to corrosion, abrasion and erosion. Wear is one of the most common problems
encountered in industrial applications. It is defined as the erosion of material from a
solid surface by the action of another solid. In the domain of wear, particularly the wear
encountered in handling abrasive solid particles, much work has been done in the past
half century with regard to dry abrasivity, but only in more recent years has interest
grown in ‘wet’ abrasivity, namely slurries [24].
1.4.1 Types of wear
1) Adhesive wear
Adhesive wear is the only universal form of wear; it arises from the fact that,
during sliding, regions of adhesive bonding, called junctions, form between the sliding
surfaces. If one of these junctions does not break along its original interface, then a
chunk from one of the sliding surfaces will have been transferred to the other surface.
In this way, an adhesive wear particle will have been formed. Initially adhering to the
other surface, adhesive particles soon become loose and can disappear from the sliding
system.
One of the significant things about adhesive wear is that at the interface, or the
point where it touches another metal surface, it must be very hot in order for the micro
welding to take place at all. That is what adhesive wear is — microscopic welding. The
heat produced at the contact interface is very high — near the melting point of the two
metals touching each other. This heat mostly comes from the stress of contact and not
from the temperature of the environment [29].
2) Abrasive wear
Abrasive wear is produced by a hard, sharp surface sliding against a softer one
and digging out a groove. The abrasive agent may be one of the surfaces or it may be a
16
third component (such as sand particles) Abrasive wear coefficients are large compared
to adhesive ones. Thus, the introduction of abrasive particles into a sliding system can
greatly increase the wear rate; automobiles, for example, have air and oil filters to catch
abrasive particles before they can produce damage. On the abrasive specimen, the
surface shows a scratched appearance from hard particles digging into it as they were
moved across the surface [29].
3) Corrosive wear
Corrosive wear arises when a sliding surface is in a corrosive environment, and
the sliding action continuously removes the protective corrosion layer, thus exposing
fresh surface to further corrosive attack. Corrosive wear occurs as a result of chemical
reaction on a wearing surface. The most common type of corrosion is mainly due to
reaction between metal and oxygen. These oxides are wiped away with the flow and
cause pitting of the surfaces. Corrosion is accelerated as impacted surfaces are exposed
to slurry chemistry [35].
4) Erosive wear
Erosive wear is the dominant process and can be defined as the removal of
material from a solid surface due to mechanical interaction between the surface and
the impinging particles in a liquid stream. Erosion involves the transfer of kinetic energy
to the surface. This means that in erosion material removal is a function of particle
velocity squared to higher power. Erosive wear depends on the predominant impact
angle of particle impingement with the material surface. Impact angle will vary from
zero degree to 90 degrees and depend on both fluid particle and particle- particle
interaction [19].
This type of wear can be found on impellers and volute casing in slurry pumps,
angled pipe bends, turbines, pipes and pipe fitting, nozzles, burners etc. The material
loss due to erosion increases with the increase in kinetic energy of the particles
impacting at the target surface.
17
The volume loss due to erosion is a troublesome problem foe slurry
transportation systems e.g. mineral transport systems, ash disposal systems. The
erosion wear due to the air borne particles in some devices such as jet planes and
turbines is also significant due to very high impact velocity. It is thus a challenging task
to control the erosion wear in many engineering applications. The material removal due
to erosion is caused by two dominant mechanisms namely brittle fractures and platelet
deformations.
In brittle type material, the solid particles impacting on the target surface forms
cracks in longitudinal and lateral directions. These cracks propagate due to impact of
succeeding particles and broken materials pieces will be carried out by flowing fluid.The
material removal rate due to brittle fracture increases with increase in normal
component if the particle velocity and thus the brittle type material show maximum
wear near normal impact angles.
In Platelet mechanism, the impact of solid particles deforms the target surface
to forms hills and valleys. The repeated impacts of particles remove the material and
forms crater at the surface. This mechanism along with micro-cutting and chipping
dominates in ductile type materials, which show the maximum wear in the impact angle
range of 20-40 degree. Apart from the target surface characteristics like brittle or
ductile type, many other parameters such as solid particles, carrier fluid, flow conditions
etc. affect the erosion wear. It is, therefore, difficult to estimate wear for a given
operating conditions [19].
1.4.2 Parameters Affecting Erosion Wear
1) Impact angle
Impact angle is defined as the angle between the target surface and the
direction striking velocity of the solid particle. The variation of erosion wear with the
impact angle depends on the characteristics of the target surface material namely
brittle or ductile type.
18
2) Velocity of solid particles
Velocity of solid particle strongly affects the erosion wear. As particle velocity
increases there is significant increase in erosion rate [25]. The erosion rate is generally
related to the particle velocity using power law relationship in which the power index
for velocity varies in the range of 2-4.
3) Hardness
Hardness is the characteristic of a solid material expressing its resistance to
permanent deformation. Surface hardness as well as hardness of solid particles has
profound effect on the erosion wear mechanism. Hardness ratio has been defined as
the ratio of hardness of target material to the hardness of solid particles.
4) Particle size and shape
Particle size and shape is also one of the prominent parameter, which affect
erosion wear. Many investigators have considered solid particle size important to
erosion. The erosive wear increases with increase in particle size according to power
law relationship. The effect of particle shape on the erosion is not very well established
due to difficulties in defining the different shape features. Generally roundness factor is
taken into consideration. If roundness factor is one then the particles are perfectly
spheres and a lower values show the particle angularity [4].
Sizing of the ash particles by sieve analysis: A sieve analysis is a procedure used to
assess the particle size distribution of a granular material. Being such a simple
technique of particle sizing, it is probably the most common. A mechanical shaker is
used for sieve analysis in which sieves are used for gradation test. A gradation test is
performed on a sample of aggregate. A sieve analysis involves a column of sieves with
wire mesh cloth (screen). A weighed sample is poured into the top sieve which has the
largest screen openings. Each lower sieve in the column has smaller openings than the
one above. At the base is a round pan, called the receiver. The column is placed in a
mechanical shaker. The shaker shakes the column for some fixed amount of time. After
19
the shaking is complete the material on each sieve is weighed. The weight of the sample
of each sieve is then divided by the total weight to give a percentage retained on each
sieve [11].
Where WSieve is the weight of aggregate in the sieve and WTotal is the total weight
of the aggregate.
5) Solid concentration
Concentration is amount of solid particles by weight or by volume in the fluid. As
concentration of particle increases more particles strike the surface of impeller which
increase the erosion rate, the concentration of slurries can vary from 2% to 50%
depending upon the type of slurry. However, at very high concentrations particle-
particle interaction increases and this decreases the striking velocity of particle on the
surface.
1.5 METHODS FOR TESTING EROSION IN SLURRY PUMPS
The erosion wear has been estimated by different ways such as field tests, pilot
plant tests, bench scale tests or numerical analysis. Field tests give the actual wear but
required a long time and the results are particular to a specified condition. It is
therefore preferable to conduct pilot plant test where the field conditions are to be
simulated in a scaled test loop to generate data for different operating conditions with
relatively less efforts. However, such tests are laborious, time consuming and require
large materials.
Hence, most of the wear test data have been generated using bench scale test
rigs. These rigs are suitable to small laboratory being small in size and some of the rigs
can generate the data at an accelerated rate. The large data available for different
%Retained = ×100%
20
operating conditions have been generally used to develop empirical correlations for
estimating erosion wear. In some of the numerical analysis, the correlations obtained
through bench scale rigs have been used to estimate the erosion wear based on local
values of the effecting parameters.
Erosion in slurry pumps can be tested by following methods:
1.5.1 Jet erosion test
In jet erosion, testing a high velocity jet strikes a flat specimen at some
adjustable angle. In jet erosion tests, the amount of material removed is determined by
the weight loss. The material which accumulates on the specimen surface interferes
with the incoming particle. The weight loss of the specimen corresponds to the average
erosion over the surface. Figure 1.12 (a, b, c) shows a principle of Jet erosion test, Slurry
pot test and Coriolis erosion test.
Jet erosion tester has been developed to investigate the effect of different
parameters particularly the impact angle under controlled environment. In jet erosion
tester, a circular jet of solid–liquid mixture strikes at the wear specimen fixed in a
fixture, which can be oriented at any angle with respect to the former. Generally a
pump is used to drive water at high pressure and the solid particles are being sucked
through an injector. The mixing of the solid–liquid is formed in the mixing chamber
before the jet comes out through a nozzle.
Figure 1.12 (a) Jet erosion test; (b) Slurry pot test; (c) Coriolis erosion test
(Source: Slurry System Handbook by Baha Abulnaga, 2002-McGraw Hill Companies Inc.)
21
1.5.2 Slurry pot test
In slurry pots, a specimen rod of circular cross-section, fixed to the end of an
arm, is rotated in a circular container filled with the slurry assuming homogeneous
suspension of the solid particles. In slurry pot tests, also the amount of material
removed is determined by the weight loss. The samples are weighted before and after
the tests [13].
Slurry pot tester is simple in design, easy to fabricate and operate. In a slurry pot
tester, generally two cylinder wear specimens have been rotated in solid –liquid
mixture. The rotational movement of the wear specimens and sometimes an impeller
attached at the end of 6th shaft keep the solid particles suspended in the liquid. The
rotating test specimen moves at a velocity relative to the solid-liquid suspension, which
is assumed stationary and homogeneous inside the pot.
In a slurry pot tester the rotational velocity of the specimen has been taken as
the relative velocity between the particles and wear specimens assuming suspension as
stationary. However, due to rotation of the wear specimen and propeller the mixture
inside the pot are highly turbulent conditions. Baffles are used to break the vortex
produced, which also develops some amount of non-homogeneity in the suspension.
These phenomena result in some error in evaluating the independent/isolated effect of
various parameters such as velocity, concentration, and impact angle etc on erosion
wear.
1.5.3 Coriolis erosion test
The Coriolis tester is used to simulate the motion of dense slurries and their
interaction with surfaces such as slurry pumps or pipelines. In this type of tester slurry
passes rapidly through a revolving rotor containing a wear specimen. In the Coriolis
testing slurry is introduced into the centre of gravity of a rotor and exists through two
radial channels. The specimen is mounted into the walls of a radial channel facing the
22
direction of rotation and is eroded by slurry particles, which are pressed against the
specimen face by the Coriolis acceleration. The erosion groove is transversed using a
profilometer and the local erosion depth is determined to within a few microns. In the
Coriolis erosion tester, a relatively small batch of slurry, from an over-head tank passes
through the rotor in one pass. At rotor speeds of 6000 rpm, a measurable groove is
worn into the specimen within a few minutes [14].
1.6 SURVEY OF THERMAL POWER PLANT YAMUNANAGAR
Visit Report
Fly Ash was collected directly for thermal power plant having physical and
chemical properties shown in Annexure 2. There are two stages where twelve slurry
pumps are used to dispose the ash at a distance of 8 Km. away from the power plant.
For each pipeline, three slurry pumps are used in series. Table 1 shows the technical
detail used for slurry disposal pumps.
Table 1.1: Technical data used for slurry disposal pumps
No. of
Stages
No. of
pumps in
series
Speed
(RPM)
Flow
Rate
(m3/hr)
Lift
(kgf/cm2)
Power
Input
(KW)
Pipe
Length
(Km)
Impeller
Dia.
(mm)
1st
3 600-1600 600 6-7 125 8
545
3 600-1600 600 6 125 8 545
2nd
3 600-1600 600 6 125 8 545
3 600-1600 600 6 125 8 545
23
1.7 THERMAL SPRAYING
Thermal spraying techniques are coating processes in which melted (or heated)
materials are sprayed onto a surface. The "feedstock" is heated by electrical (plasma or
arc) or chemical means (combustion flame). Thermal spraying can provide thick coatings
(approx. thickness range is 20 microns to several mm, depending on the process and
feedstock), over a large area at high deposition rate. Figure 1.13 shows a schematic of
thermal spray process [15].
Figure 1.13 Schematic of thermal spray process
(Source: Thermal spray coatings-ASM Handbook, Surface Engineering, Vol. 5)
Coating materials available for thermal spraying include metals, alloys, ceramics,
plastics and composites. They are fed in powder or wire form, heated to a molten or
24
semi-molten state and accelerated towards substrates in the form of micron-size
particles. Combustion or electrical arc discharge is usually used as the source of energy
for thermal spraying. Resulting coatings are made by the accumulation of numerous
sprayed particles. The surface may not heat up significantly, allowing the coating of
flammable substances. Coating quality is usually assessed by measuring its porosity,
oxide content, hardness, bond strength and surface roughness. Generally, the coating
quality increases with increasing particle velocities [16].
Types of thermal spraying processes
1. Plasma spraying
2. Detonation spraying
3. Wire arc spraying
4. Flame spraying
5. High velocity oxy-fuel coating spraying (HVOF)
6. Cold spraying
Detonation Gun Thermal Spraying Process
The Detonation gun basically consists of a long water cooled barrel with inlet
valves for gases and powder. Oxygen and fuel (acetylene most common) is fed into the
barrel along with a charge of powder. A spark is used to ignite the gas mixture and the
resulting detonation heats and accelerates the powder to supersonic velocity down the
barrel. A pulse of nitrogen is used to purge the barrel after each detonation. This
process is repeated many times a second. The high kinetic energy of the hot powder
particles on impact with the substrate result in a build up of a very dense and strong
coating [28]. Figure 1.14 shows a schematic of the Detonation Thermal Spray Process.
25
Figure 1.14 Schematic of the Detonation Thermal Spray Process
(Source: Thermal spray coatings-ASM Handbook, Surface Engineering, Vol. 5)
1.8 MOTIVATION OF THE WORK
Power generation in India is primarily coal based in the present scenario. The
coal used in our thermal power plants produce a large amount of ash, which is around
100 million tons per year, at present. Out of this, around 80 million tons will be fly ash
and the remainder will be bed ash. It is expected that at the best 30% of the total ash
produced will be utilized which will still leave the remaining 70% for safe disposal. The
visit of the ash handling unit of the Deen Bandhu Chhotu Ram Thermal Power Plant
Yamuna Nagar has motivated me to undertake the thesis work currently, both the type
of ash are being mixed together and transported hydraulically from thermal power
plant to ash ponds through pipelines at the solid concentration of around 10-20% by
weight. The operation of ash disposal is highly uneconomical due to requirement of
large quantity of water and increase in power consumption for pumping 80-90% of clear
water. The other problems associated in the slurry pump are excessive wear due to high
velocity of the ash water mixture which reduces their working life.
26
Following are the objectives of the present work:
1. To decrease the wear rate of pump.
2. To study the effect of thermally sprayed ceramic coating on cast iron.
3. To get the increased ash disposal rate.
4. To assess the damage to the impeller and casing of ash slurry disposal pump.
5. To improve the performance of ash slurry disposal pump.
6. To reduce the maintenance cost.
7. To save the amount of water used to dispose large amount of ash into the ash
ponds by increasing the ash concentration in the slurry.
8. To suggest the coating materials for the impellers to protect them from erosive
wear at high ash concentration in the slurry.
9. To reduce the down time.
27
Chapter 2
LITERATURE REVIEW
Gupta et al. [1995] studied the effect of velocity, concentration, and particle size on
erosive wear. The experiment was performed by pot tester for two pipe materials
namely, brass and mild steel. They evaluated that for a given concentration, erosive
wear increases with increase in velocity and for a given velocity, erosion wear also
increases with increase in concentration but this increase is comparatively much
smaller. They also concluded that erosive wear decreases with decrease in particle size.
Dasgupta et al. [1998] evaluated the effect of sand slurry concentration on steel using
DUCOM made TR-41 erosion tester. They also varied the rotational speed and
transverse distance during the test. They concluded that increase in the concentration
of sand reduces the erosion rate. They also concluded that the erosion rate deteriorates
with rotational speed.
Walker et al. [2000] conducted experimental work on different slurry pump impeller
and side-liner geometries to determine the effects of solid particle size, slurry
concentration and pump speed on wear. Explanations are offered for the resultant wear
patterns and empirical wear relationships are outlined for the key variables. The
experimental work outlined has compared the wear performance of side-liners running
with three different impeller designs. The major difference in location and quantity of
wear occurs between the HE ‘‘wear-ring’’ style side-liner impeller and the STD and RE
‘‘expelling vane’’ style impeller and side-liner.
Walker [2001] compares the wear rate of the white cast-iron with rubber material. He
found that both material show excellent similarities in wear rate trend with particle size
28
but rubber show lower wear rate than the metal for equivalent particle size < 700 μm.
This is because the rubber surface can absorb smaller particle impact energy without
significant cutting tearing.
Clark et al. [2001] discussed the nature and significance of the ‘particle size effect’ on
the rate of slurry erosion and the problem of its experimental evaluation. Experiments
involving the change of erodent particle size in typical laboratory test equipment, while
revealing a decrease in erosion rate with decreasing erodent particle size, also produce
significant changes in the slurry flow conditions and particle motion which, unless
evaluated quantitatively, will mask the nature of the particle size effect. Wear results
have been obtained using a slurry pot erosion tester, using 0.94 wt. % suspensions of
SiC (varying in mean particle size from 14 to 780 μm) in diesel oil for aluminium and
Pyrex glass. For aluminium no damage threshold was observed and the experimental
erosion rate varied as dp
2
over the whole particle size range. A contribution to this of dp
was found to be ascribable to fluid flow changes in the experimental apparatus and the
remaining contribution of dp between 100 and 780 μm tentatively assigned to the
decrease in surface energy of debris particles with increasing particle size. Below 100
μm there is a transition from wear by particle impact to wear by wet abrasion by
particles moving across the surface.
Hawthorne [2002] has conducted Coriolis tests for the evaluation of slurry erosion on
different materials. Slurries consisting of glass beads of size 90- 200- micron size with
10% slurry concentration were taken and tests were performed on 1020 steel and
copper at different impingement angles of 90-20 degrees. It was also observed that in
slurry jet testing, most particles impact the specimen above its critical velocity resulting
severe plastic deformation. In contrast, in the Coriolis test most particle impacts result
in only elastic deformation or mild plastic deformation. Hence, elastic as well as plastic
properties of specimen materials affect their performance in a Coriolis slurry erosion
evaluation, thus the results obtained from Coriolis tests were more accurate.
29
Clark [2002] has investigated the effect of Particle velocity and particle size in slurry
erosion. A list of factors affecting slurry erosion such as concentration of particles,
Slurry flow speed (particle impact speed), particle impact angle, particle size, particle
density, hardness, friability, nature of suspending liquid, nature of slurry flow (esp. local
turbulence),nature of target material were explained. It is emphasized that material loss
must be measured by changes in surface profile rather than mass loss, and that the best
specimen form for this analysis is a cylinder.
Tahsin Engin et al. [2003] have evaluated some existing correlations to predict head
degradation of centrifugal slurry pumps. A new correlation has been developed in order
to predict head reductions of centrifugal pumps when handling slurries. The proposed
correlation takes into account the individual effects of particle. The proposed
correlation is therefore recommended for the prediction of performance factors of
‘‘small-sized’’ slurry pumps having impeller diameters lower than 850 mm. size, particle
size distribution, specific gravity and concentration of solids, and impeller exit diameter
on the pump performance.
Goodrich et al. [2003] observed that the main mixing tanks are located under the ash
silos. Fly ash slurry is conveyed into these tanks from the fly ash mix tanks which are
located under the fly ash silos. Bed ash is then added to the fly ash slurry. Once the
combined slurry density reaches target level, a centrifugal slurry pump conveys the
slurry out of the plant.
Clark [2004] has studied the influence of the squeeze film in slurry erosion. A ‘squeeze
film’ may be taken as any liquid layer separating two approaching surfaces. The
presence of a squeeze film generally leads to a significant retardation of any particle
closely approaching a wearing surface at any speed and may even prevent direct impact
altogether. The experiments were performed in slurry pot tester. In addition to this it is
has also found that if the Reynolds number (i.e. velocity or mass) of the approaching
30
particles is low enough, penetration of the squeeze film on rebound or even approach
may not be possible, resulting in particle entrapment at the target surface and a change
in erosion mechanism. He observed that small particles (less then100 micron) and
concentrated slurries were especially liable to behave in this way.
Gandhi et al. [2004] have developed a methodology to determine the nominal particle
size of multi-sized particulate slurry for estimation of mass loss due to the erosion wear.
The effect of presence of finer particles (less than 75 microns) in relatively coarse
particulate slurry has also been studied. They have observed that addition of particles
finer than 75 micron in narrow-size or multi-sized slurries reduce the erosion wear. In
addition, the effective particle size for narrow-size particulate slurries can be taken as
the mean size whereas the weighted mass particle size seems to be a better choice for
multi-sized particulate slurries. The reductions in erosion wear due to addition of fine
particles decreases with increase in the concentration of coarse size particles.
Rajesh et al. [2004] have carried out experiments on the effect of impinging velocity on
the erosive wear behaviour of polyamides. The impact angles were 30 and 60 degrees
at two impact velocities (80 and 140 m/s). Silica sand is used as an erodent. Surface
blackening at the impact zone was observed for all the materials at normal impact and
at both the impact velocities. At normal impact and at lower impinging velocity (i.e. 80
m/s), a mass gain in the initial period was observed for all the materials except
amorphous. The extent of increase in wear, however, depended on the materials and
the angle of impact. The velocity effect was more prominent at the oblique angle of
impact.
Aino Helle et al. [2004] investigated that at impingement angle closer to 90 degree, the
ceramic coating do not offer any advantages over uncoated metallic surface, while at an
impingement angle closer to zero degree, the ceramic coatings seems to offer wear
31
protection. Harder erodent particles cause more wear on ceramic coatings than do
softer particles.
Neville et al. [2005] studied the erosion-corrosion behavior of WC-Metal Matrix
composites (EFM, EFW, EGC, EGG). The materials were eroded by two sizes of silica
sand with stream velocities of 10 and 17 m/s at 65oC. Test was conducted by varying the
concentration. They evaluated that WC grain size fractions has very little effect on wear.
They also concluded that the erosion-corrosion rate is strongly dependent upon
erodent size, impinging velocity and solid loading.
Tian et al. [2005] have observed the erosive wear of some metallic materials such as
high chromium white iron and aluminium alloy using Coriolis wear testing approach. In
the present study, the correlation between wear rate and particle size on the tested
materials is discussed. Factors, which should be considered in wear modeling and
prediction, have also been addressed. It can be seen that larger solids particles resulted
in higher mass loss in all test materials. Although the wear rates at smaller particle sizes
were relatively close within each material group, the wear rate difference was
significantly widened with larger particle sizes. The tested high-Cr white irons showed a
wear resistance some 27–140 times higher than that of the aluminium alloys in the
Coriolis test conditions Both flow rate and solids concentration of slurry affected the
wear results of the test materials. The higher the flow rate, the higher the wear rate of
test materials.
Tian et al. [2005] have experimented on Coriolis wear testing. Wear coefficients (or
specific energy coefficients) have been determined for different slurry conditions over a
large range of particle sizes. Among the test materials, the harder Cr–Mo white iron
alloy demonstrated the best wear resistance under slurry testing conditions. It is also
observed that Coriolis wear testing is an excellent approach to simulate the erosive
wear condition within a slurry pump. Beside particle sizes, other particle properties such
32
as particle shape and size distribution also exhibited significant effect on the values of
wear coefficients. Silica sand and copper ore slurries were used as examples. The
relationship between linear wear rate and solid particles was also shown.
Mann et al. [2006] compared erosive wear of WC-10Co-4Cr, Armcore ‘M’, Stellite 6 and
12 HVOF coatings, Ti-Al-N PVD coatings. Impact angle of 60o and velocity of 20 m/s was
kept constant for all experiments. Mineral sand was used as solid particle of slurry. They
concluded that WC-10Co-4Cr HVOF coatings show best performance against slurry
erosion. They also evaluated the corrosion performance and found that WC-10Co-4Cr
HVOF coating corroded significantly. WC-10Co-4Cr HVOF coatings have very good
erosion resistance but not corrosion resistance.
Desale et al. [2006] shows the effect of erodent properties on slurry erosion.
Experiments have been performed in a pot tester to evaluate the wear of two ductile
materials namely, AA6063 and AISI 304L steel. Solid–liquid mixtures of similar particle
sizes of three different natural erodents namely, quartz, alumina and silicon carbide
have been used to evaluate the mass loss of the two target materials at different
orientation angles. The result shows that the maximum angle for erosion wear is a
function of target material properties and does not depend on the erodent. The erosion
rate of ductile materials varies with the erodent properties other than its size and
hardness. The effect of erodent properties namely, shape and density is more dominant
at shallow impact angles compared to higher impact angles.
Desale et al. [2007] have conducted experiments to visualize the effect of slurry erosion
on ductile materials under normal impact condition. These tests were being done in
slurry pot tester. It is being observed that wear depends upon hardness of target
material and hardness of solid particles. The various ductile materials tested were
copper, mild steel brass etc. The erodent materials used were quartz, alumina, and
silicon carbide. Experiments were performed at 3m/s velocity and 10 percent by weight
33
concentration of 550 micron size particles for combination of different erodent and
target materials at normal impact condition.
Santa et al. [2007] studied the slurry erosion of two coatings applied by oxy fuel
powder (OFP) and wire arc spraying (WAS) processes onto sand-blasted AISI 304 steel
and the results were compared to those obtained with AISI 431 and ASTM A743 grade
CA6NM stainless steels, which are commonly used for hydraulic turbines and
accessories. Slurry erosion tests were carried out in a modified centrifugal pump, in
which the samples were placed conveniently to ensure grazing incidence of the
particles. The slurry was composed of distilled water and quartz sand particles with an
average diameter between 212 and 300 µm and the solids content was 10 wt % in all
the tests. The mean impact velocity of the slurry was 5.5 m/s and the erosion resistance
was determined from the volume loss results. The coated surfaces showed higher
erosion resistance than the uncoated stainless steels, with the lower volume losses
measured for the E-C 29123 deposit.
Vasile Hotea et al. [2008] shows that variety of mechanical and chemical cleaning and
pre-treatment techniques are used prior to coating. The thermal spray techniques to
deposit coatings consist of atomization and deposition of molten or semi-molten
droplets of the coating material on substrates. The feed material is melted by using
energy from fuel combustion, electric arc or plasma.
Desalea et al. [2009] studied the effect of particle size on erosive wear of aluminium
alloy (AA 6063) using slurry pot tester. Quartz particles were used as slurry of eight
different sizes varying between 37.5 and 655 μm. Keeping the concentration of 20% by
weight and velocity as 3 m/s experiment was conducted. They found that the erosive
wear increases with increase in mean particle size.
34
Santa et al. [2009] compared the erosion and corrosion resistance of various thermal
spray coatings on martensitic stainless steel. Nickel, chromium oxide and tungsten
carbide coating were applied by oxy fuel powder whereas chromium and tungsten
carbide coatings were applied by HVOF. A modified centrifugal pump was used to
evaluate the performance. They found that thermal spray coating has more erosion
resistance as compared to bare steel. They also found that coatings do not help in
protecting cavitations, in fact shows poorer performance than bare steel.
Mishra et al. [2009] studied all the parameters affecting the erosive wear using jet
erosion tester on fly ash-quartz coating. By varying different parameters they evaluate
that impact angle is the most significant factor influencing the erosive wear of fly-ash-
quartz coating. They also evaluated that maximum erosion takes place at impact angle
of 90o.
Mishra et al. [2009] studied the erosion wear behavior of fly ash–illmenite coating using
dry silica sand as an erodent. The Taguchi technique was used to acquire the data in a
controlled way. An orthogonal array and signal-to-noise ratio was employed to
investigate the influence of the impact angle, the velocity, the size of the erodent, and
the standoff distance (SOD) on erosion wear. The objective was to investigate which
design parameter significantly affects the erosion wear. It was found that the impact
angle is the most powerful factor influencing the erosion wear rate of the coating.
Further, when erosive wear behavior of fly ash–illmenite coating was investigated at
three impact angles (i.e., at 30◦, 60◦, 90◦), it was revealed that the impact angle is the
prime factor and maximum erosion takes place at α = 90◦.
Nagarajan et al. [2009] studied through experimental investigation the effects of ash
particle physical properties and transport dynamics on the erosive wear of three differ-
ent grades of low alloy steel, using three different power-station ash types. The study
used a Taguchi fractional-factorial L27 DOE. The experimental data were used to derive
35
a model for the prediction of erosion rates. The model incorporates the properties of
the ash particles and the target metal surface, as well as the characteristics of ash
particle motion in the form of the impingement velocity and the impingement angle. It
became possible to predict the relationship between erosion loss and the factors
influencing the rate of room-temperature erosion. Fly-ash particulate size and
concentration, moisture and titania content, impact velocity and angle, duration of
impact, and alloy surface roughness were determined to be first-order effects. The
results show that the rate of erosion increases with increasing impact velocity of fly-ash
on metal and also increases with increasing concentration of fly-ash.
Khuri et al. [2010] provide a survey of the various stages in the development of
response surface methodology (RSM). This includes a review of basic experimental
designs for fitting linear response surface models, in addition to a description of
methods for the determination of optimum operating conditions. Discussion was made
on more recent modeling techniques in RSM, in addition to coverage of Taguchi’s
robust parameter design and its response surface alternative approach. The prediction
capability of any response surface design does not remain constant throughout the
experimental region.
Yassine et al. [2010] experimentally investigated the performance characteristics of a
centrifugal pump using different sand concentrations ranging from 0 to 15 wt % of sand.
The flow behavior of different sand/water mixtures has been inspected using a test rig
with 50 mm diameter PVC pipes with various fittings and valves. It was observed that
the head and the efficiency of a centrifugal pump with slurries are, in general, lower for
traditional designs in comparison to water due to the presence of suspended solids. The
head and efficiency of the pump decrease with increase in sand concentration (by
weight). Power consumption on the other hand increases with increase in sand
concentration at a rate that is higher than the rate of increase of the mixture specific
gravity (S). The study has also shown that the head ratio, efficiency ratio, and power
36
ratio do not vary significantly with flow rate and the variation is within ± 9 % at any sand
concentration tested.
Chawla et al. [2011] concluded that hot corrosion and erosion are recognized as serious
problems in coal based power generation plants in India. The coal used in Indian power
stations has large amounts of ash (about 50%) which contain abrasive mineral species
such as hard quartz (up to 15%) which increase the erosion propensity of coal. An
understanding of these problems and thus to develop suitable protective system is
essential for maximizing the utilization of such components. These problems can be
prevented by either changing the material or altering the environment or by separating
the component surface from the environment. Corrosion prevention by the use of
coatings for separating material from the environment is gaining importance in surface
engineering.
Telfer et al. [2012] investigated the effect of particle and target material on the erosion-
corrosion mechanisms. The performance of Fe as the target material has been modeled
when considering particle concentration and size. A comparison was made between
the erosion-corrosion mechanisms of Fe, Ni, Al and Cu under different conditions of
particle size and concentration. By producing several maps, the regimes and wastage
rates predicted as functions of velocity and applied potential.
Singh et al. [2012] studied that detonation gun spraying is one of the thermal spraying
techniques known for providing hard, wear resistant and dense micro structured
coatings. Process parameters of detonation spraying influence the microstructure,
mechanical and other properties of the coatings. Research is needed in optimization of
the process parameters of detonation spraying process. Detonation gun separation
device designed by researchers resulted in good performance of the detonation gun
spraying in high performance requirement.
37
Chapter 3
SLURRY EROSION TESTING
3.1 HYDRAULIC ASH HANDLING SYSTEM
The handling and disposal of ash is a major problem because due to its dusty,
irritating and abrasive nature. When it comes out of boiler furnace, it is very hot. In the
thermal power plant, 60 % to 80 % of ash is in the form of fly ash and remaining is the
bottom ash. Ash handling systems are mechanical, hydraulic and pneumatic.
In Hydraulic System ash is carried with flow of water with high velocity through a
pipeline and finally dumped in the sump. This system is preferred because it is clean and
healthy. It is dustless and completely closed. It can discharge large amount of ash at
large distance from the power plant, therefore is suitable for large thermal power
plants. Clean water is collected in a tank as shown in figure 3.1. A high pressure pump
(90 KW, 6 to 8 kgf/cm2) is used to draw the clean water from water tank to the collector
tank where the ash is mixed with it. Ash is collected from the ash hoppers and mixed
with clean water.
Figure 3.1 Clean water for making slurry
38
The ash-water mixture before entering the impeller is shown in figure 3.2. Slurry
pumps are used to draw the ash slurry from collector tank and dispose it at a large
distance (1000 meters to 8000 meters) away from power plant. The hydraulic ash
handling and disposal system is shown in figure 3.3. Slurry pumps are used in series to
increase the pressure head as shown in figure 3.4.
Figure 3.2 Slurry before entering the impeller
Figure 3.3 Hydraulic ash handling system
39
Figure 3.4 Slurry pumps in series
3.2 HIGH SPEED SLURRY EROSION TESTER
In slurry erosion tester, a specimen, fixed to the specimen holder, is rotated in a
circular container (slurry pot) filled with the slurry which is made homogeneous. In
slurry erosion test, the amount of material removed from the specimen is determined
by the weight loss. The samples are weighed before and after the tests.
Slurry pot tester is simple in design and easy to operate. In a slurry pot tester,
generally two specimens have been rotated in solid-liquid mixture. The rotational
movement of specimens keeps the solid (ash) particles suspended in the liquid (water).
The rotating test specimens move at a velocity relative to the solid-liquid suspension.
The monitoring penal and experimental set-up of high speed slurry erosion tester is
shown in Figure 3.5a and Figure 3.5b.
40
Figure 3.5a - Monitoring Panel of High Speed Slurry Erosion Tester
Figure 3.5b - Experimental Set-up of High Speed Slurry Erosion Tester
41
3.3 EROSION TESTING OF HIGH CHROME CAST IRON
The fly ash which was collected from directly thermal power plant was used as
the erodent and the particle size varied from 10 to 100 μm. Sizing of the ash particles
was done by sieve analysis and the average particle size was 75 μm. Three levels were
selected for ash particle size in micron as 50, 75 and 90. The test specimens of impeller
material (High Chromium Cast Iron) were prepared and cut into the cylindrical pieces of
12 mm OD × 6 mm ID × 10 mm long and polished using emery paper (200 μm grit size).
The hardness of the specimen was measured to be 500 BHN. The rotation per minute
(r/min) maintained as 1200, 1400 and 1600 whereas the ash concentration taken as
100,000 ppm, 200,000 ppm and 300,000 ppm. Specimens were cleaned with acetone
and weighed before and after the experimentation. The weight of the specimen
material was reduced due to erosion.
Rotations of specimen, ash concentration in slurry and particle size were
considered to be significant factors. Three levels were taken for each factor as shown in
table 3.2. Two test specimens were tightened opposite to each other. The test
specimens were tightened properly to the specimen holder with the help of the screws.
There was a drum of water carrying capacity of 30 liters was properly cleaned and slurry
is prepared and drawn into the drum. The specimens were rotated in the slurry for 30
minutes.
The slurry was made homogeneous in the drum by rotating it continuously
during the experimentation. The test specimen holder is shown in Figure 3.6 having the
capacity to hold maximum of 12 specimens and the prepared test specimens (before
experimentation) are shown in figure 3.7.
42
Figure 3.6 Test specimen holder for holding maximum of 12 specimens
Figure 3.7 Test Specimens
The total number of tests and the values of variables for each test were
determined with the help of Response Surface Methodology (RSM). Table 3.4 shows the
analysis of variance for response surface linear model. The modeled values for the
response were calculated and tabulated in Table 3.5.
43
Apparatus used: High Speed Slurry Erosion Tester
No. of Specimens: 34 (02 for each test)
No. of factors: 03
No. of levels: 03
Material of specimen: Chromium alloy cast iron
Time Taken: 30 minutes for each test
Ash used: Fly ash
Software used: Design-Expert 8.0.7.1
The chemical composition of the material used was checked with the help of
spectrometer. The detail is as under:
Material C Mn Si S P Cr Ni
High Chrome Cast Iron 3.01 1.12 0.39 0.006 0.02 14.64 0.42
A sieve analysis was used to assess the particle size distribution of fly ash (table
3.1). The results were presented in a graph of percent retained in each sieve versus the
sieve size range (figure 3.8) by using the following equation:
%age Retained =
×100%
where
WSieve: weight of fly ash in the sieve
WTotal: total weight of the fly ash
44
Table: 3.1 Sieve Analysis Chart
Particle size range %age retained in sieves
(Up to 35μm) 15/500*100=3 %
(35-45 µm) 30/500*100=6 %
(45-50 µm) 72/500*100=14.4 %
(50-75 µm) 235/500*100=47 %
(75-90 µm) 105/500*100=21 %
(90-105 µm) 23/500*100=4.6 %
(105-125 µm) 12/500*100=2.4 %
(125-150 µm) 8/500*100=1.6 %
Particle Size Distribution
90, 21
75, 47
35, 3
50, 14.4
45, 6105, 4.6
125, 2.4 150, 1.6
0
10
20
30
40
50
0 20 40 60 80 100 120 140 160
Ash Particle Size (micron)
Ash
%ag
e
Figure 3.8: Ash percentage versus particle size
45
Table 3.2: Factors affecting Erosion with levels
Sr.
No. Factors Level 1 Level 2 Level 3
1
Rotation of specimen
(rpm) 1200 1400 1600
2
Ash concentration
(ppm) 100x103 200 x10
3 300 x10
3
3
Particle size
(μm) 50 75 90
3.3.1 Experimental Design (Response Surface Methodology)
Experimental design is the design of any information-gathering exercises where
variation is present, whether under the full control of the experimenter or not. Design-
Expert 8.0.7.1 software was used in current investigation for statistical analysis i.e.
Response Surface Methodology (RSM) and analysis of variance (ANOVA). In statistics,
RSM explores the relationships between several explanatory variables and one or more
response variables. The main idea of RSM is to use a sequence of designed experiments
to obtain an optimal response. It is sufficient to determine which explanatory variables
have an impact on the response variable(s) of interest. An estimated optimum point
need not be optimum in reality, because of the errors of the estimates and of the
inadequacies of the model [17].
In Box-Behnken design for RSM, each numeric factor is varied over 3 levels
hence used in current experimentation. This model is only an approximation, but uses it
because such a model is easy to estimate and apply, even when little is known about
the process. Box-Behnken design model has given 17 numbers of tests for randomized
values of variables as shown in Table 3.3.
46
Table 3.3 Box-Behnken design matrix
Run Rotational
Speed (rpm)
Ash Concenteration
(ppm)
Particle Size
(microns)
1 1400 200000 75
2 1200 300000 75
3 1600 200000 90
4 1400 100000 50
5 1400 200000 75
6 1200 100000 75
7 1200 200000 90
8 1600 200000 50
9 1400 200000 75
10 1600 300000 75
11 1400 100000 90
12 1200 200000 50
13 1400 200000 75
14 1400 300000 50
15 1600 100000 75
16 1400 200000 75
17 1400 300000 90
3.3.2 Analysis Of Variance (ANOVA)
In statistics, analysis of variance (ANOVA) is a collection of statistical models, and
their associated procedures, in which the observed variance in a particular variable is
partitioned into components attributable to different sources of variation. ANOVA
provides a statistical test of whether or not the means of several groups are all equal,
and therefore generalizes t-test to more than two groups. Doing multiple two-sample t-
47
tests would result in an increased chance of committing a type I error. For this reason,
ANOVA is useful in comparing two, three, or more means.
The analysis of variance has been studied from several approaches, the most
common of which use a linear model that relates the response to the treatments and
blocks. Even when the statistical model is nonlinear, it can be approximated by a linear
model for which an analysis of variance may be appropriate.
Table 3.4 ANOVA for Response Surface Linear Model
Source
Sum of
Squares
df Mean
Square
F
Value
p-value
Prob > F
Model 0.083 3 0.028 17.10 < 0.0001 significant
A-rotation 0.040 1 0.040 24.96 0.0002
B-ash concen. 0.041 1 0.041 25.58 0.0002
C-particle size 1.243E-003 1 1.243E-003 0.77 0.3956
Residual 0.021 13 1.610E-003
Lack of Fit 0.017 9 1.923E-003 2.12 0.2434 Non-significant
Pure Error 3.621E-003 4 9.052E-004
Cor Total 0.10 16
The Model F-value of 17.10 implies the model is significant. Values of "Prob > F"
less than 0.0500 indicate model terms are significant. In this case A, B are significant
model terms. The "Lack of Fit F-value" of 2.12 implies the Lack of Fit is not significant
relative to the pure error. Non significant lack of fit is good.
Table 3.5 Regression Analysis for Response Surface Linear Model
Std. Dev. 0.040 R-Squared 0.7979
Mean 0.28 Adj R-Squared 0.7512
C.V. % 14.47 Pred R-Squared 0.6293
PRESS 0.038 Adeq Precision 14.656
48
The "Pred R-Squared" of 0.6293 is in reasonable agreement with the "Adj R-
Squared" of 0.7512. "Adeq Precision" measures the signal to noise ratio. A ratio greater
than 4 is desirable. The ratio of 14.656 indicates an adequate signal. This response
surfece linear model can be used to navigate the design space.
3.3.3 Modeled equation in terms of actual factors
Table 3.6: Experimental and modeled values for the response (Uncoated Material)
Run Rotation Ash conc. Particle size Experi. Values Modeled Values
(rpm) (ppm) (microns) (gms.) (gms.)
1 1600 300000 75 0.497 0.421
2 1400 200000 75 0.281 0.279
3 1200 200000 90 0.248 0.217
4 1400 200000 75 0.282 0.279
5 1600 200000 90 0.365 0.359
6 1200 100000 75 0.175 0.136
7 1200 200000 50 0.186 0.193
8 1200 300000 75 0.288 0.280
9 1400 200000 75 0.208 0.279
10 1600 200000 50 0.308 0.334
11 1400 100000 50 0.239 0.192
12 1400 100000 90 0.163 0.216
13 1600 100000 75 0.294 0.278
14 1400 300000 90 0.353 0.360
15 1400 200000 75 0.262 0.279
16 1400 200000 75 0.258 0.279
17 1400 300000 50 0.307 0.335
Weight reduction due to erosion for each uncoated and coated specimen is shown in
appendix 1.
Weight Loss = (-0.40687) + (3.54375x10-4) x Rotational Speed + (7.17500x10-7) x
Ash Concenteration + (6.13078x10-4) x Particle Size
49
3.4 EROSION TESTING OF CERAMIC COATED CAST IRON
Ceramic coating can be applied to increase the performance of a slurry pump by
reducing the erosive wear. Before applying coating, specimens should be cleaned by
acetone and abrasive shot blast. The detonation gun thermal spray technique may be
used for applying coating. The extreme hardness and high density of Al2O3 is expected
to have a superior erosion and corrosion resistance. The detonation gun thermal spray
technique provides the dense microstructure with less porosity.
Alumina ceramic coated impellers are particularly resistant to the ripping and
tearing damage caused by sharp objects often found in slurries. Alumina ceramic
impellers are chemically inert and suitable for use in hazardous and hostile
environment. The superior wear characteristics of alumina ceramics prevent wear in
critical areas and this enables the impeller to function for far longer at a constant rpm.
3.4.1 Experimentation and Statistical Analysis
The problem was the high wear rate of the pump impeller during its continuous
use. The suggested solution to this problem is applying alumina ceramic coating on the
surface of the impeller so as to reduce the wear rate on the surface by keeping the
mechanical properties of the impeller material unchanged. Rotation of specimen, ash
concentration in slurry, and particle size were considered to be significant factors. Same
levels are taken for each factor as discussed in case of uncoated material. The values of
rotational speed of specimens and ash concentration in slurry were estimated with the
help of technical data obtained from thermal power station. The particle size range was
selected by sieve analysis.
Experiments were performed on the high speed slurry erosion tester (DUCOM
Bangalore make, Model TR 401). Total number of experiments was decided by the
Response surface technique. After performing experiments, ANOVA was used to
calculate the modeled equation of the response. Response was the weight loss of the
50
material due to erosive wear. Weight of the specimens was taken before and after the
experimentation to get the weight reduction of the material. Total number of runs was
given by RSM. Box-Behnken design matrix for ceramic coated material in terms of actual
values is same as shown in Table 3.3. ANOVA for Response Surface Reduced Quadratic
Model is shown in Table 3.7. Experiments were performed on the Al2O3 coated
specimens having the coating thickness of 250 μm. Test Specimens affected by erosive
wear as shown in figure 3.9.
Figure 3.9: Al2O3 coated specimens affected by erosive wear
51
Table 3.7: ANOVA for Response Surface Reduced Quadratic Model
Source
Sum of
Squares
df Mean
Square
F
Value
p-value
Prob > F
Model 1.889E-003 6 3.149E-004 10.84 0.0007 significant
A-rotation 7.313E-004 1 7.313E-004 25.17 0.0005
B-ash concent. 6.301E-004 1 6.301E-004 21.69 0.0009
C-particle size 2.101E-004 1 2.101E-004 7.23 0.0227
AC 5.455E-007 1 5.455E-007 0.019 0.8937
A2 1.540E-004 1 1.540E-004 5.30 0.0441
C2 1.765E-004 1 1.765E-004 6.07 0.0334
Residual 2.906E-004 10 2.906E-005
Lack of Fit 2.694E-004 6 4.490E-005 3.47 0.1289 non-
significant
Pure Error 2.120E-005 4 5.300E-006
Cor Total 2.180E-003 16
The Model F-value of 10.84 implies the model is significant. There is only a 0.07 %
chance that a "Model F-Value" this large could occur due to noise. Values of "Prob > F"
less than 0.05 indicate model terms are significant. In this case A, B, C, A2, C2 are
significant model terms. The "Lack of Fit F-value" of 3.47 implies the Lack of Fit is not
significant. There is only a 12.89% chance that a "Lack of Fit F-value" this large could
occur due to noise. Non-significant lack of fit is considered as good. Regresion analysis
for response surface reduced quadratic model is shown in table 3.8.
52
Table 3.8: Regresion Analysis for Response Surface Reduced Quadratic Model
Std. Dev. 5.39E-03 R-Squared 0.8667
Mean 0.024 Adj R-Squared 0.7267
C.V. % 22.8 Pred R-Squared 0.6665
PRESS 1.38E-03 Adeq Precision 11.319
The "Pred R-Squared" of 0.6665 is close to the "Adj R-Squared" of 0.7267 as one
might normally expect. "Adeq Precision" measures the signal to noise ratio. A ratio
greater than 4 is desirable. Ratio of 11.319 indicates an adequate signal. This model
can be used to navigate the design space. Experimental and modeled values for the
response are shown in Table 3.9.
3.4.2 Modeled Equation in Terms of Actual Factors:
Weight reduction = 0.30368 - 3.80604 x10-4 x (rotational speed) + 8.87500 x 10-8 x
(ash concentration) - 2.32409 x 10-3 x (particle size) + 9.09091 x 10-8 x (rotational
speed) x (particle size) + 1.50987 x 10-7 x (rotational speed2) + 1.75219 x 10-5 x
(particle size2).
53
Table 3.9: Experimental and modeled values for the response (Coated Material)
Run Rotation Ash
concent.
Particle
size
Experimental
Values
(Wt. reduction)
Modeled Values
(Wt. reduction)
(rpm) (ppm) (microns) (gms.) (gms.)
1 1600 300000 75 0.053 0.043
2 1400 200000 75 0.02 0.018
3 1200 200000 90 0.027 0.025
4 1400 200000 75 0.014 0.018
5 1600 200000 90 0.041 0.045
6 1200 100000 75 0.006 0.006
7 1200 200000 50 0.013 0.015
8 1200 300000 75 0.023 0.023
9 1400 200000 75 0.017 0.018
10 1600 200000 50 0.028 0.034
11 1400 100000 50 0.013 0.010
12 1400 100000 90 0.026 0.020
13 1600 100000 75 0.025 0.025
14 1400 300000 90 0.033 0.038
15 1400 200000 75 0.015 0.018
16 1400 200000 75 0.016 0.018
17 1400 300000 50 0.032 0.027
54
Chapter 4
RESULTS AND DISCUSSIONS
4.1 RESULTS
Graphs were plotted between the factors affecting erosive wear and the weight
reduction of specimen for uncoated high chrome cast iron and ceramic coated cast iron.
The effect of each variable on the weight loss due to erosion was undertaken by
considering other variables at lower, middle and higher level.
4.1.1 UNCOATED CAST IRON
It has been found form these graphs that all the three parameters behave
linearly with the weight loss.
Figure 4.1 Weight Loss vs. rotational speed
55
As the rotational speed of the impeller increases, the force with which slurry
strikes the impeller increases which in turn increases the erosion rate (Figure 4.1). It
shows that at 1200 rpm rotational speed, 100000 ppm ash concentration and 50 μm
particle size, the weight loss of the material due to erosive wear is very less. For 1200-
1600 rpm, the erosive wear is increased linearly.
Figure 4.2 Weight Loss vs. Ash Concentration
The weight loss of the material increases linearly with the increase of ash
concentration as shown in figure 4.2. It shows that at 100000 ppm ash concentration,
1200 rpm rotational speed and 50 μm particle size, the weight loss of the material due
to erosive wear is very less. For 100000-300000 ppm, the erosive wear is increased
linearly.
56
Figure 4.3 Weight Loss vs. Ash Particle Size
The effect of particle size on the weight loss due to erosion is shown in figure
4.3. The behavior of particle size with erosive wear is linear. The particle size has low
effect on erosive wear when other values are taken at lower level, but increase in wear
is more by taking other parameter level at middle and higher.
Reduction in weight is primarily depends upon the ash concentration in the
slurry followed by the rotational speed of the pump impeller and ash particle size as
shown in figure 4.4.
Rotational Speed40%
Ash Concent.
41%
Particle Size2%
Error3%
Lack of Fit14%
Figure 4.4 Contributions of selected parameters for erosive wear of high chrome cast iron
57
4.1.2 CERAMIC COATED CAST IRON
Figure 4.5 Weight loss vs. Rotational Speed
For Al2O3 Coated High Chrome Cast Iron, the effect of rotational speed on the
weight loss due to erosion is shown in figure 4.5. As the rotational speed of the impeller
increases, the force with which slurry strikes the impeller increases which in turn
increases the erosive rate. It shows that from 1200 to 1400 rpm the increment in wear
is comparatively less than for 1400 to 1600 rpm. In the range of1400-1600 rpm, the
wear increases exponentially.
Figure 4.6 Weight loss vs. Ash Concentration
58
The effect of ash concentration in slurry on the weight loss due to erosion is
shown in figure 4.6. Increase in ash concentration increases the viscosity of the slurry
which in turn increases the wear rate. The behavior of ash concentration with erosive
wear is linear.
Figure 4.7 Weight loss vs. Ash Particle Size
Erosive wear occurs on the impeller of slurry pump when the impact of ash
particles flows in water is at very high velocity. The kinetic energy of the moving
particles and ash particle size are responsible for the loss of material due to erosion.
The weight loss of the material increases with the increase of particle size in the range
of 75 – 90 µm as shown in Figure 4.7. From 50- 75 µm, there is no effect of particle size
on the erosion.
Reduction in weight is primarily depends upon the rotational speed of the pump
impeller followed by ash concentration and ash particle size as shown in figure 4.8.
59
Rotational Speed41%
Ash Concent.
28%
Particle Size18%
Lack of Fit12%
Error1%
Figure 4.8 Contributions of selected parameters for erosive wear of Al2O3 Coated cast iron
4.2 DISCUSSIONS
Erosion is a complex process in which three co-existing phases, namely
conveying fluid (water), solid particles (ash) and the surface (pump impeller) interact in
many ways. The kinetic energy of the moving particles and amount of ash
concentration in slurry are mainly responsible for the loss of material. The impingement
attack is either by solid or by liquid. A high erosive wear is observed at high rotation
speed. The ash particles at high velocity were associated with more kinetic energy,
causing severe impingement on the specimen surface.
The effect of the rotational speed, ash concentration and particle size on the
erosive wear for Al2O3 coated high chrome cast iron was compared with uncoated high
chrome cast iron and shown in figure 4.9. The effect of these parameters was studied at
the middle level.
60
Figure 4.9 Comparison of erosion for coated and uncoated material
61
By comparing the results of uncoated and Al2O3 coated high chrome cast iron, it
is clear that up to 1400 rpm the effect of erosion is not much but after 1400 rpm there
is an exponential growth in wear rate for coated material. Hence the speed must be
kept near 1400 rpm for the maximum slurry transport at low wear rate. The graphs for
ash concentration vs. wear rate are linear for both metallic and ceramic coated surface
but the wear rate is less in the later case due to the high wear resistance of Al2O3
coating. It is shown that if the particles are finer than 75 microns, there will be no any
significant effect on the wear rate but from 75 microns to 90 microns the increase in
wear rate is significant, hence can not be ignored. When the particle size is very small,
it is not having the sufficient internal energy to penetrate or erode the high wear
resistant material.
62
CHAPTER 5
CONCLUSIONS AND SCOPE FOR FUTURE WORK
If the slurry pumps has to work the range where the erosive wear is less; the
performance of the slurry disposal pump may be improved. Erosive wear of the pump
impeller would decreases with decreasing ash concentration, rotation speed and
particle size. Wear takes place by cutting and removal of material from the specimen
surface. Ceramic coating is very helpful to protect the pump due to slurry erosion.
Instead of applying coating to whole impeller, the coating can be applied only to blades
to cut manufacturing cost.
In the present case, erodent directly hit the cylindrical surface. Specimens can be
fitted to the specimen holder at different angles with respect to the slurry rotation
inside the slurry pot. They can hit by the ash particles at different angles as rotating at a
particular speed. There is no provision to hold the specimen at a particular angle in the
current set-up on which the experimentation is done. This is the limitation of present
experimental work. A special attachment can be provided to avoid this problem.
Ash particles may only tend to slide over the specimen surface at a lower angle
without effectively impinging on it. For Brittle materials such as cast irons and ceramics,
at a lower impingement angle, the tendency of erosive particles towards deflection
from the surface becomes prominent, resulting in a low erosive wear. As the specimen
angle increases up to 90o, the impact of impingement increases and the material is
removed due to the high impact pressure. To investigate the effect of impingement
angle, the test specimen should be flat. On cylindrical specimen, the impact angle can
not be measured accurately.
63
Instead of applying coating on the metallic surface, it is suggested that if whole
impeller is made by alumina ceramic, the pump life and performance increases very
much.
There are advanced materials available such as nano composites and cermets
whose wear resistance is very high. Advanced Wear-Resistant Nano-composites are a
new class of super-hard materials having the potential for obtaining a high-wear-
resistance. A cermet is a composite material composed of ceramic and metallic
materials having the combined properties of both a ceramic and a metal. The
experimentation can be performed with such kind of advanced materials. Various
coating materials such as zirconia, chrome carbide, silicon carbide, tungsten carbide,
titanium carbide etc. are available for experimentation on the different machines.
Experiments are changed as the working condition changes therefore for a new working
condition the same modeling technique can not be applied. Different experimental
design techniques can be used for different set of parameter.
64
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67
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68
APPENDIX-A
Weight Difference Due To Erosion for Each Uncoated Specimen:
Material – High Chrome Cast Iron
Experimentation on Slurry Erosion Tester
Specimen
No.
Initial Weight
(grams)
Final Weight
(grams)
Wt. Difference
(grams)
1 6.921 6.882 0.039
2 6.764 6.731 0.033
3 6.674 6.618 0.056
4 7.156 7.148 0.008
5 6.897 6.876 0.021
6 6.987 6.833 0.154
7 6.729 6.675 0.054
8 6.959 6.935 0.024
9 6.912 6.857 0.055
10 6.906 6.895 0.011
11 6.959 6.728 0.231
12 6.957 6.702 0.255
13 4.936 4.921 0.015
14 6.778 6.613 0.165
15 6.985 6.959 0.026
16 6.654 6.461 0.193
17 6.618 6.339 0.279
18 7.046 6.822 0.224
19 7.547 7.318 0.229
20 7.128 6.899 0.229
21 7.151 6.793 0.358
22 6.915 6.908 0.007
23 7.321 7.112 0.209
24 7.159 6.875 0.284
25 6.281 6.247 0.034
26 6.595 6.471 0.124
27 7.024 6.715 0.309
28 6.559 6.427 0.132
29 6.519 6.351 0.168
30 5.684 5.545 0.139
31 7.119 6.963 0.156
32 7.071 6.828 0.243
33 7.049 6.897 0.152
34 7.079 6.981 0.098
69
Weight Difference Due To Erosion for Each Coated Specimen:
Material - Ceramic Coated Cast Iron
Experimentation on Slurry Erosion Tester
Specimen
No.
Initial Weight
(grams)
Final Weight
(grams)
Wt. Difference
(grams)
1 6.825 6.813 0.012
2 7.055 7.053 0.002
3 6.902 6.899 0.003
4 6.899 6.897 0.002
5 6.724 6.699 0.025
6 6.426 6.423 0.003
7 6.925 6.921 0.004
8 6.847 6.835 0.012
9 7.093 7.069 0.024
10 6.636 6.623 0.013
11 7.209 7.198 0.011
12 7.221 7.199 0.022
13 6.854 6.851 0.003
14 6.268 6.265 0.003
15 7.063 7.042 0.021
16 6.955 6.952 0.003
17 6.998 6.994 0.004
18 6.736 6.733 0.003
19 6.437 6.433 0.004
20 6.889 6.868 0.021
21 6.768 6.756 0.012
22 6.438 6.427 0.011
23 6.815 6.794 0.021
24 7.129 7.118 0.011
25 6.925 6.919 0.006
26 6.827 6.811 0.016
27 6.599 6.593 0.006
28 6.422 6.417 0.005
29 6.899 6.868 0.031
30 7.046 7.044 0.002
31 7.378 7.374 0.004
32 7.212 7.208 0.004
33 7.339 7.337 0.002
34 6.951 6.945 0.006
70
APPENDIX-B
Physical Properties of Fly Ash
Particle Size Distribution (% Finer by Weight)
Size
(μm)
300 200 100 75 64 50 41 36 25 17 13 10 4
%
Finer
100 98 90 85 70 64 61 55 43 28 16 8 3
Specific gravity of fly ash (unsieved) =1.992
Weighted mean diameter = 65 μm
Chemical Properties of Fly Ash
Chemical Composition of Fly Ash
SiO2 Al2O3 Fe2O3 CaO K2O TiO2 Na2O MgO
54.77 33.83 2.79 0.80 1.97 2.76 0.72 2.35
(Source: DBCRTPP, Yamunanagar)
71
APPENDIX-C
PUBLICATIONS
1. Kumar, S. and Ratol, J. S., 2012. “Investigation of Erosive Wear on Al2O3 Coated
Cast Iron Using Response Surface Technique”, International Journal of Advances
in Engineering & Technology, Vol. 3, No. 2, pp. 403-410.
2. Kumar, S. and Ratol, J. S., 2012. “Experimental Investigation of Erosive Wear on
the High Chrome Cast Iron Impeller of Slurry Disposal Pump Using Response
Surface Methodology”, Materials Engineering- Materiálové inžinierstvo, Vol. 19,
No. 3, pp. 110-116.