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.

iv

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.

5

(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

9

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.

12

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

14

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.

15

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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Some Metallic Materials using Coriolis Wear Testing Approach”, Wear, Vol. 258,

pp. 458–469.

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.