Department of Mechanical Engineering, Pulchowk Campus

 

COPYRIGHT

COPYRIGHT

 

The author has agreed that the library, Department of Mechanical Engineering, Pulchowk Campus, Institute of Engineering may make this thesis freely available for inspection. Moreover author has agreed that permission for extensive copying of this thesis for scholarly purpose may be granted by the professor(s) who supervised the thesis work recorded herein or, in their absence, by the Head of the Department wherein the thesis was done. It is understood that the recognition will be given to the author of this thesis and to the Department of Mechanical Engineering, Pulchowk Campus, and Institute of Engineering in any use of the material of this thesis. Copying or publication or the use of this thesis for financial gain without approval of the Department of Mechanical Engineering, Pulchowk Campus, Institute of Engineering and author’s written permission is prohibited.

 

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Head

Department of Mechanical Engineering

Institute of Engineering, Pulchowk Campus

Lalitpur, Kathmandu

Nepal

 

 

 

 

 

 

 

 

 

TRIBHUVAN UNIVERSITY

INSTITUTE OF ENGINEERING

PULCHOWK CAMPUS

DEPARTMENT OF MECHANICAL ENGINEERING

 

The undersigned certify that they have read, and recommended to the Tribhuvan University, Institute of Engineering, for acceptance, a thesis entitled “Cavitation and Sediment Erosion Analysis in Francis Turbine with Modelling and Simulation (A Case Study of Middle Marsyangdi Hydropower Plant)” by Durga Bastakoti, Harendra Kishor Karn, Indira Khadka and Khadananda K.C. in partial fulfillment of the requirements for the degree of Bachelor of Mechanical Engineering.

 

   

Supervisor, Dr. Rajendra Shrestha

Head

Department of Mechanical Engineering

Tribhuvan University, Nepal

 

 

                                                            External Examiner, Er. Suresh Raut

                                                            Assistant Manager

                                                            Middle Marsyangdi Hydropower Plant, Lamjung

 



 


                                                            Committee Chairperson, Dr. Rajendra Shrestha

                                                            Head

                                                            Department of Mechanical Engineering

                                                    

Date:                         

ABSTRACT

 

The total available energy in Nepal for the year 2010 was 368.27 GWh and 57% of it was supplied by hydropower plants of Nepal. And the total installed capacity of major and small hydropower stations in Nepal is 472.49 MW. Middle Marsyangdi Hydropower Plant is a major hydropower station with total installed capacity of 70MW with an average annual energy generation of 398 GWh.

 

In MMHP, the critical regions of erosion are sealing region of guide vanes and outlet of runner blades, which was found from modeling and simulation of runner. The main reason is high velocity of sediment laden water, mainly consisting of quartz and feldspar at these regions.

 

The Air Admission System incorporated in the MMHP is effectively working by admitting air into the draft tube region of runner. The air helps to increase the pressure thereby decreasing the cavitation. Furthermore, the vibration meter reading indicates that the vibration is not increased in part load condition which means the effect of cavitation causing vibration is low.

 

 

 

 

 

 

 

 

 

 

 

 

ACKNOWLEDGEMENT

 

First and foremost, we express our deepest gratitude towards our Project Supervisor   Dr. Rajendra Shrestha, Head, Department of Mechanical Engineering, IOE Pulchowk Campus, for his constant supervision and guidance. Our project would not have been possible without his assistance.

 

Our heartfelt appreciation goes to Er. Shiva Kumar Shah of Middle Marsyangdi Hydropower Project for his genuine support throughout the project duration. We are really thankful to Associate Prof. Dr. Tri Ratna Bajracharya and Prof. Dr. Bhakta Bahadur Ale for their supportive guidance regarding our project matters.

 

We are indebted to Prof. Dr. Bhola Thapa, Dean, Kathmandu University and Associate Prof. Dr. Bim Prasad Shrestha, Head, Department of Mechanical Engineering, Kathmandu University, for their valuable suggestions. We are also grateful towards Dr. Hari Prasad Neopane and Er. Biraj Kumar Thapa of Kathmandu University whose guidance played an integral part in the completion of our project.

 

We would like to extend our thanks to Sr. Research Engineer Mr. Padam Prasad Pokharel and Laboratory Manager Mr. Yogesh Khadka, at Hydro Lab Pvt.  Ltd., for their kind support in the testing of sediment samples for Particle Size Distribution (PSD) Analysis and Mineral Content Analysis. Their cooperation was indeed helpful for the success of the project.

 

We are very grateful to Er. Ramesh Tiwari and Mr. Purna Prasad Chauhan, Kulekhani – II Hydropower Plant for providing us Vibration Meter instrument which helped us in the vibration analysis.

 

At last, but not the least, we would like to thank our friends and family for the continuous support and encouragement.

 

 

 

Table of Contents

 

COPYRIGHT.. 2

ABSTRACT.. 4

ACKNOWLEDGEMENT.. 5

LIST OF TABLES. 9

LIST OF FIGURES. 10

LIST OF ABBREVIATIONS. 11

CHAPTER ONE: INTRODUCTION.. 13

1.1 Background. 13

1.1.1 Energy Scenario of Nepal 15

1.1.2 Status of Alternative Energy in Nepal 16

1.1.3 Status of Hydropower in Nepal 17

1.2 Problem Statement 19

1.3 Objectives. 20

1.4 Rationale. 20

1.5 Methodology. 22

1.6 Limitations. 23

CHAPTER TWO: LITERATURE REVIEW... 24

2.1 Development of Hydropower Plant of Nepal 24

2.2 Classification. 25

2.2.1 ROR and Storage Plant 25

2.3 Types of Hydraulic Turbine. 26

2.3.1 Francis Turbine. 27

2.4 Main Problem in Hydropower Plants. 27

2.4.1 Sediment Erosion. 27

2.4.2 Cavitation. 29

2.4.2.1 Types of Cavitation. 29

2.4.2.2 Cavitation Damage Inspection. 31

2.4.2.3 Prevention of Cavitation. 32

2.4.3 Vibration. 33

CHAPTER THREE: CASE STUDY OF MMHP. 34

3.1 Introduction. 34

3.1.1 Salient Features of Middle Marsyangdi Hydropower Plant 35

3.1.2 Special Features of Middle Marsyangdi Hydropower Plant 38

3.2 Mechanical Components of MMHP. 39

3.2.1 Francis Turbine of MMHP. 39

3.2.2 Turbine Runner 41

CHAPTER FOUR: OBSERVATIONS, GRAPHS AND RESULTS. 44

4.1 Hydraulic Turbine (Main Dimensions) 44

4.2 Sediment Sample Testing. 45

4.2.1 Particle Size Distribution (PSD) Analysis. 46

4.2.2 Mineral Content Analysis. 46

4.2.3 Minerals in the Sediment Sample. 47

4.2.4 Result of Mineral Content Analysis. 48

4.3 Suction Velocity Measurement 49

4.3.1 Specifications of Digital Anemometer 49

4.4 Vibration Measurement 52

4.4.1Vibration Level Measurement 53

4.4.2 Portable Vibration Meter VM-62. 53

4.4.3 Measuring Range. 54

4.4.4 Graphs and Tables for Vibration. 54

4.4.4.1 Vibration in terms of Velocity (m/s) 55

4.4.5 Vibration in terms of Displacement (µm) 56

CHAPTER FIVE: MODELLING AND SIMULATION.. 60

5.1 Introduction. 60

5.2 Computational Fluid Dynamics Analysis: 60

5.3 Different terms used in simulation. 61

5.4 Reference geometry. 63

5.5 Simulation Results. 64

CHAPTER SIX: CONCLUSION AND RECOMMENDATIONS. 67

6.1 Conclusion. 67

6.2 Recommendations. 68

REFERENCES. 69

APPENDIX A: SEDIMENT DATA OF MARSYANGDI RIVER.. 70

APPENDIX B: DATA OF VIBRATION OF MMHP. 72

APPENDIX C: LOG SCALE OF VIBRATION CONVERSION.. 75

APPENDIX D: VAPOUR PRESSURE CHART.. 76

APPENDIX E: SKETCH OF GUIDEVANE. 77

APPENDIX F: SKETCH OF TURBINE BLADE. 78

APPENDIX G: THREE VIEWS OF BLADE PROFILE. 79

APPENDIX H: SIMULATION RESULT OF VELOCITY.. 80

APPENDIX I: REPORTS OF SIMULATION.. 81

 

 

 

 

 

 

 

 

 

 

 

 

 

LIST OF TABLES

 

Table 1.1: Status of Alternative Energy in Nepal

Table 1.2: List of existing Hydropower Plants in Nepal

Table 1.3: List of Under Construction Medium and Large Hydropower Plants

Table 4.1: Hydraulic Turbine (Main Dimensions)

Table 4.2: Runner Dimension

Table 4.3: Wicket Gate Dimension

Table 4.4: Intake Sample

Table 4.5: Draft tube Sample

Table 4.6: Tail Race Sample

Table 4.7: Measurement Range

Table 4.8: Suction velocity and pressure at different power output

Table 4.9: Unit 1 at 34 MW (Full Load condition)

Table 4.10: Unit 1 at 23.1 MW (Part Load condition)

Table 4.11: Unit 1 at 23.1 MW (Part Load condition)

Table 4.12: Unit 1 at 23.1 MW (Part Load condition)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LIST OF FIGURES

 

Figure 1.1: Energy Consumption Scenario of Nepal, 2010

Figure 1.2: Electricity Generation and Consumption (MW/HR)

Figure 2.1: Power development map of Nepal

Figure 2.2: Francis Turbine (front view and top view)

Figure 2.3:  Erosion in Guide vanes

Figure 2.4: Erosion in Turbine

Figure 2.5: Main types of cavitation in Francis turbines

Figure 3.1: Middle Marsyangdi Hydropower Scheme

Figure 3.2: Wicket gate blade

Figure 3.3: Guide vane and Stay ring

Figure 3.4: Francis Runner

Figure 3.5: Guide Bearing

Figure 4.1: Results of PSD Analysis

Figure 4.2: Anemometer

Figure 4.3: Suction Velocity Measurement at MMHP                                        

Figure 4.4: Power vs. Suction Velocity

Figure 4.5: Power vs. Draft Tube Pressure

Figure 4.6: Vibration Measurements on Intermediate Shaft Cover using Vibration Meter

Figure 4.7: Position in Turbine vs. Velocity 34MW

Figure 4.8: Position in Turbine vs. Velocity 21.3MW

Figure 4.9: Frequency of Vibration in Unit 1

Figure 4.10: Frequency of Vibration in Unit 2

Figure 4.11: Displacement of Vibration in Unit 1

Figure 4.12: Displacement of Vibration in Unit 2

Figure 5.1: Reference Geometry of Blade and Guide vane

Figure 5.2: Reference Geometry of Runner with Guide vane

Figure 5.3: Velocity Profile at Full Load

Figure 5.4: Velocity Profile at Part Load

 

 

LIST OF ABBREVIATIONS

 

BKPC

Bhotekoshi Power Company

BPC

Butwal Power Company

CD

Cascade Desander

CES

Center for Energy Studies

CFD

Computational Fluid Dynamics

Ft/MIN

Feet per Minute

GWh

Giga Watt hour

HPL

Himal Power Limited

HPS

Hydro Power Station

IOE

Institute of Engineering

ISCR

Intermediate Shaft Cover Radial

ISCA

KE

Intermediate Shaft Cover Axial

Kinetic Energy

KU

Kathmandu University

KW

Kilo Watt

m ASL

meter Above Sea Level

MMHPS

Middle Marsyangdi Hydropower Station

MMHP

Middle Marsyangdi Hydropower Project

mm

millimeter

MoF

Ministry of Finance

MVA

Mega Volt Ampere

MW

Mega Watt

NEA

Nepal Electricity Authority

NHPC

PE

Nepal Hydropower Company

Potential Energy

PROR

Pondage Run-off-River

PSD

Particle Size Distribution

ROR

Run-Off-River

RPM

Revolution Per Minute

SERD

Sand Erosion Rate Density

TBHC

Turbine Bearing Head Cover

TOE

Tonnes Of Equivalent

U/L

Upper/Lower

IWM

Improved Water Mills

 

 




 

 

 

CHAPTER ONE

 INTRODUCTION

1.1 Background

Nepal is blessed with immense amount of hydroelectric potential and is ranked second in terms of water resources after Brazil on global scale. Nepal is rich in natural resources. The most effective sector of all the resources is clean water and the concomitant electricity generated from its adequate water resources. Nepal is surrounded by mountainous hills having about 6000 rivers and small streams and rivulets. As most of the water flows through the densely populated southern region, the importance of hydroelectricity and clean water will be immense in the future. In the coming 10-30 years, the demand for clean water will increase considerably along with the tremendous increase in power demand (both at the domestic front and regional market).[1]

 

Though Nepal has electricity generating potential of 83,000MW, about 43000MW power is seems to be economically and technically feasible. Hydropower is a big resource and leading sector for Nepal because of its adequate water potential (~225 billion cubic meter of water). Total installed capacity of electricity is 697 MW out of which energy from hydro is 644 MW and contribution by NEA is 477.53MW (including off-grid). Currently, power deficit of 400MW is prevalent in the country. Nepal’s electricity needs are mostly met by the hydropower generation. Despite the huge hydropower potential in Nepal, only 40% population has access to electricity; among them 33% get electricity from NEA and rest 7% from other alternative sources of energy like thermal plants, solar, bio-gas. According to 10th plan, in 2010, only 42% of the present energy consumption was met by hydropower.

 

Hydropower is not only about energy production for productive sectors, but also powerful means of bringing in socio-economic transformation and development of villages. Hydropower leads to development activities in villages mostly as hydropower plants need to be constructed in the villages. The poor- the target group can benefit from this because the socio-economic benefit from a hydropower project to the rural populace is extensive.

 

In Nepal, the first hydropower was established in 1911 A.D., named Pharping (Chandrajoyti) Hydropower by Chandra Shamser with capacity of 500kW. Around 40 small-scale hydropower projects (between 100 kW and 10 MW) have been built in the Nepal. Except for a handful of these schemes (built in the 1990s and thereafter), the rest were built by Nepal Electricity Authority, (NEA), or its predecessor with financing from a number of bilateral and multilateral donors. Of the small hydropower plants, some are grid connected and others are isolated.

 

Unlike thermal and nuclear power plant, a hydropower project is very site specific particular site for the hydro power project will produce a specific amount of power and energy. The design and consequently the associated costs of hydropower project are very much dependent on the site.

 

Electricity crisis in Nepal is increasing day by day and many people are forced to live in dark night with low life standard. Micro hydro plant can only facilitate power to small village or a few number of houses, it cannot supply national wise. To overcome this problem development in large hydropower plant should be focused and utilization of water power risen. Various reasons are associated for less development of hydropower: their unfavorable topography, less industrial growth, limited network expansion, lack of adequate infrastructure, lack of skilled experts with the political instability. The development of hydropower plant is good option for using energy because of its minimal contribution to global warming, cleanliness, stability, more secure and environment friendly.

 

Several problems arise in the hydropower plants leading to the reduction in power output hence decreasing the efficiency of plant. In Nepal, hydropower suffers from the cavitation and erosion problem mainly, so our group decided to analyze the cause of this problem by taking Middle Marsyangdi Hydropower Plant as a case in final year project of Bachelor of Mechanical Engineering. MMHP is one of the major hydropower and has a special cavitation tube to reduce the cavitation effect which motivated us to select MMHP as the reference to analyze the cavitation and erosion problem.

 

 

 

1.1.1 Energy Scenario of Nepal

World average energy consumption per year per person is 68 Gigajoule. Per capita energy consumption of Nepal is 15 Gigajoule. Contribution of hydropower is  only 1.82%  while the petroleum products has share of 8% energy consumption in the Fiscal Year 2009/10 arose by 5.5 percent as compared to the Fiscal Year 2008/09 totaling  9,911 tons of oil equivalent (TOE). In the first eight months of current Fiscal Year 2010/11, energy consumption totaled 6,571 TOE. In the Fiscal Year 2009/10 the ratio of traditional, commercial, and renewable energy consumptions was 84.4 percent, 14.9 percent and 0.7 percent respectively. In the first eight months of fiscal year 2010/11 this ratio was recorded as 86.5 percent, 12.8 percent and 0.7 percent respectively. The data reveals that high dependence of Nepalese economy on traditional energy has not changed.

 

Traditional

       87.89%

 

Petroleum

 
From all the existing power projects across the country, the total installed capacity (Including Private and Others) is 697.846 MW. Of the total electricity generated, 477.53MW (grid connected is 472.49MW and off grid is 4.536MW) has been generated from hydropower, NEA. Contribution from the thermal generation is about 53.4MW and that from the solar energy generated is 100KW. Total energy generated from hydropower in Nepal is 644.34MW.

Description: C:\Users\om jai jagdish\Desktop\1.PNG

Figure 1.1: Energy consumption scenario of Nepal, 2010

(Source: Conference on Energy Efficiency and Renewal Energy, October 25-29, 2010, Islamabad, Pakistan)

In FY 2009/10, on a sector-wise basis, industries consumed 37.7 percent of electricity, while the households consumed 41.4 percent, business sector 7.2 percent, non-business sector 2.8 percent and miscellaneous sectors 10.9 percent. In FY 2010/11, industries are estimated to consume 37.44 percent of electricity, households 42.74 percent, business sector 7.7 percent and miscellaneous sectors 12.1 percent.



Description: C:\Users\om jai jagdish\Desktop\Capture1.JPG

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 


             Figure 1.2: Electricity Generation and Consumption (MW/HR)

1.1.2 Status of Alternative Energy in Nepal

 A Rural Energy Fund has been set up under the AEPC to mobilise economic resources received from the Government of Nepal and donor agencies in a well-managed, simple and speedy way so as to invest in micro hydroelectricity projects, solar energy electricity system and improved bio-fuel system and to coordinate with banks to manage necessary credits. The establishment of the Fund has increased the access of rural citizens into new and sustainable energy systems, has taken ahead electrification process where national electricity transmission is not possible, has brought improvements in education and health and has directly contributed in income generation and employment. Likewise, to increase the access of low-income people to bio-gas (gobar gas), AEPC has been disbursing credits under the assistance of KfW (Forderung von Biogasanlagen) establishing a separate Bio-gas Credit Unit. [2]

 

 

 



Table 1.1: Status of Alternative Energy in Nepal

 
 


RETS

Total installation

Unit

District Covered

Number

Capacity

Hydropower(Micro, Pico, IWM)

Micro hydro

804

13.24

MW

59

Pico hydro

1274

2.45

MW

53

Improved Water Mills

6609

 

 

46

Biogas

Household biogas plant

217429

 

 

72

Institutional biogas plant

111

 

 

25

Community biogas Plant

61

 

 

20

Solar

Household Solar PV

192820

5.36

MW

74

Institutional Solar PV

259

 

 

42

Solar Pumping

79

 

 

26

Solar cooker/dryer

1408

 

 

30

Biomass

ICS

474922

 

 

48

Wind

19

 

 

11

 

1.1.3 Status of Hydropower in Nepal

Hydropower is one of the main sources of energy in Nepal. In the past 96 years, only 634MW of energy from the hydropower has been harnessed (NEA, 2009). Besides the total hydropower potential of Nepal stands at around 200000MW against the popularly assumed figure of 83000 MW. From more than 6000 rivers and rivulets of Nepal, around one million GWh of electricity can be generated. This potential is adequate enough to meet the total domestic and part of regional energy demands for many years. More importantly, there exists a robust potential of consuming this generated electricity for the next 3 decades.

 

The hydropower system in Nepal is dominated by run-off-river Projects. There is only one seasonal storage project in the system. There is shortage of power during winter and excess of power during wet season. The load factor is quite low as the majority of the consumption is dominated by household use. This imbalance has clearly shown the need for storage projects, and hence, cooperation between the two neighboring countries is essential for the best use of the hydro resource for mutual benefit. [3]

 



Table 1.2: List of existing Hydropower Plants in Nepal [4]

 
 


S.N.

Name

Installed

Type of

Facility of

Type of

 

 of Plants

Capacity(KW)

 Turbine

 De-silting basin

 Plants

1

Pharping

500

Pelton

Small Water  pond

 ROR

2

Sundrijal

640

Pelton

Collecting Pond

 ROR

3

Panauti

2400

Francis

Collecting Pond

 ROR

4

Phewa

1088

Francis

Taal (as reservoir)

Canal drop

5

Trishuli

24000

Francis

Pond type

 PROR

6

Sun Koshi

10050

Francis

Pond type

ROR

7

Tinau

1024

Francis

NA

 ROR

8

Gandak

15000

Kaplan

Canal type

 CD

9

Kulekhani-1

60000

Pelton

Reservoir

 STORAGE

10

Devi Ghat

14100

Francis

Pond type

 ROR

11

Seti

1500

Francis

Desander

 CASCADE

12

Kulekhani-2

32000

Francis

Desander

 CASCADE

13

Marsyangdi

69000

Francis

Pond type

 PROR

14

Andhikhola

5100

Pelton

NA

 ROR

15

Tatopani and Myagdi-I&II

2000

Francis

NA

 ROR

16

Jhimruk

12500

Francis

Desander-S4

 ROR

17

Puwa Khola

6200

Pelton

Desander

 ROR

18

Khimti

60000

Pelton

 

 ROR

19

Modi Khola

14800

Francis

Pond type

 ROR

20

Chatara

3200

Bulb type

 

 CD

21

Bhote Koshi

36000

Francis

 

 ROR

22

Indrawati

7500

Francis

 

 ROR

23

Kaligandaki

144000

Francis

 

 PROR

24

Chilime

20000

Pelton

 

 PROR

25

Small Sun –Koshi

2500

Turgo

 

 ROR

26

Middle -Marsyangdi

70000

Francis

 

 PROR

 

 Table 1.3: List of Under Construction Medium and Large Hydropower Plants.

S.N.

Name of Plants

Installed Capacity(KW)

Type of Turbine

Type of Plants

1

Upper Tamakoshi

456000

Pelton

PROR

2

Chamelia

30000

Francis

PROR

3

Kulekhani-III

14000

Francis

CASCADE

4

Rahughat

30000

Francis

NA

5

Upper Seti

127000

Francis

STORAGE

 

1.2 Problem Statement

Air Admission system in MMHP is placed for the reduction of cavitation. It is in the testing phase and is of only kind in Nepal. Its effectiveness needs to be known and for that, the suction velocity can be measured against the power output. For these reasons the job of analysis of the role of the Air Admission System in preventing the cavitation.

 

Likewise, only the hand counted skilled persons are here in Nepal who works in CFD for the flow analysis and erosion in the mechanical components. So, till date almost no Hydropower Plants were found to opt for the simulation in prediction erosion in the mechanical components of turbine. Thus, the study of erosion in MMHP through modeling and simulation was undertaken.

 

 

 

 

 

 

 

1.3 Objectives

Main objective

·       Analysis of Cavitation and Sediment Erosion in Francis Turbine of MMHP with Modelling and Simulation.

 Specific objectives

·       To observe the nature of the sediment particles present in water of MMHP.

·       To study the effect of sediment particles in guide vane and runner of Francis turbine.

·       To study the performance of Air Admission Tube.

·       To perform software modelling of turbine parts in SOLIDWORKS.

·       To perform simulation in ANSYS CFX.

1.4 Rationale

There are huge technical challenges to develop new hydropower projects involving risks of sediment erosion, especially the run-off-river ones. The declining performance of hydro turbines has become one of the major technical issues in the development of hydropower plants. Sediment transportation from the rivers is a natural phenomenon, it neither can be completely controlled, nor can be completely avoided; it should however be managed. Withdrawal of the clean water from the river for power production is expensive due to design, construction and operation of sediment settling basins. Even with the settling basins, 100% removal of fine sediments is impossible and uneconomical.

 

Different factors are needed to be considered for predicting the erosion. Therefore, dealing with sediment erosion problem requires a multidisciplinary approach. More research and development is needed to investigate the relationship between the particle movement and erosion inside a turbine and to establish the operating strategy for the turbine operating in sediment-laden water.

 

Extensive research has been done to develop a wear model in terms of the material properties involved but little attention has been given to clarify the influence of fluid motion, especially in the turbulent flow regime. In the context of Nepal there are rare cases of opting for the modeling and simulation for the clear understanding and visualization of the phenomenon, problems or conditions with prior acknowledgement of end results.

 

The cavitation phenomenon consists in formation and collapse of vapor bubbles in a fluid due to decreasing of local pressure under the equilibrium vapor pressure of water. The effects of cavitation are noise, vibrations and cavitation erosion of runner blades and adjacent areas. Cavitation erosion affects the components of mechanical systems which work in liquid environment. The biggest economic impact is found in hydraulic turbines, were runner blades and adjacent areas of the runner are submitted to cavitation.[2]

 

Up to now the best results regarding repair techniques applied to turbine runner blades were obtained by overlay welding of cold hardening austenitic stainless steels. The main problems in case of repair welding in situ are connected to the residual stresses and to the important structural modifications of the base and filler materials, which appear during the welding process. These effects, especially when extensive welding is applied, can lead to the damage of the repaired component during following operation.

 

In case of Middle Marsyangdi Hydropower Plant, there is an Air Admission Tube which facilitates the suction of air from open air tube to the draft tube region of the turbine sets whenever the significant suction pressure prevails. This thereby results in the decrease in cavitation formation. As Middle Marsyangdi hydropower plant is the only plant in Nepal that incorporates Air Admission System, it is challenge as well as opportunity to study the cavitation reduction mechanism.

 

With enthusiasm of doing some kind of project in the field of hydropower, the case of MMHP was chosen to study cavitation and erosion phenomenon also to perform modeling and simulation to predict the critical regions of erosion in the turbine.

 

 

 

1.5 Methodology

The methodologies of this project are:

1.     Literature review

·       A detail study of the Middle Marsyangdi Hydropower Plant was made and the relevant data observation works were performed.

·       The literature review of the relevant subject matters had been collected from websites, internet, course books and similar thesis reports.

 

2.     Site visits

·        Middle Marsyangdi Hydropower Plant was visited frequently and consulted with the mechanical engineers to get the relevant help regarding our project wok and to get the details about the mechanical components.

·       Kathmandu University was visited for grasping the experts’ opinion regarding the project matters.

 

 

3.     Data collection

·       The sand samples were collected from the intake, draft tube and the tail race and was given to the Hydro Lab Pvt. Ltd. to observe the nature of particle reaching the turbine from the penstock.

·       The air suction velocity at the Air Admission Tube and draft tube pressure were recorded with respect to variable power output. Instrument used for the velocity measurement was Digital Anemometer.

·       The vibration of the mechanical components such as turbine casing and the draft tube were measured using an instrument named Vibration Meter.

 

4.     Analysis

·       Particle Size Distribution Analysis and Mineral Content Analysis were done in the Hydro Lab Pvt. Ltd.

·       Suction velocity and draft tube pressure were correlated with the power output.

·       Vibration measurement of different parts of the turbine unit was correlated with cavitation.

5.     Modelling and Simulation

·       Modelling of the Francis Turbine, Guide Vanes and the other turbine parts were done in SOLIDWORKS.

·       Flow simulation was done using ANSYS CFX.

 

1.6 Limitations

 

  • Unavailability of the complete drawings of the runner and turbine parts.
  • The data at varying power generation was recorded only for two hours during day time.
  •  The results of the software simulation may not solely be trustworthy.
  • The parameters such as inlet velocity and reference pressure were assumed for CFD analysis of the turbine.
  • Reverse-engineering process was adopted for measuring the dimension of runner, guide vane and blade profile.
  • Calibration of the Vibration Meter and Anemometer were not performed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER TWO

 LITERATURE REVIEW

 

2.1 Development of Hydropower Plant of Nepal

Hydropower is environment friendly, clean source of renewable energy. World’s one- fifth electricity supply is fulfilled by the hydropower. The Pharping Hydropower Plant having capacity 500kW commissioned in 1911 is the first hydropower plant installed in Nepal and second in Asia. Then Sundarijal Hydropower Plant was commissioned in 1965 having capacity of 640kW.The 2.4MW Panauti Hydropower Plants was then installed. In 1982, Kulekhani I and II were commissioned having capacity of 60MW and 32 MW respectively. Kulekhani Hydropower Plant is the only plant offering seasonal water storage in Nepal. In 2003, the biggest hydropower in Nepal, Kaligandaki “A” Hydropower Plant with 144 MW was commissioned.

 

NEA (Nepal Electricity Authority) owned most of the hydropower while some of the hydropower projects in Nepal is designed, constructed and financed by the international consultant supported by the international assistant. Private hydropower developer also got involved in this sector after 1992. The companies are BPC (Butwal Power Company), HPL (Himal Power Limited), NHPC (Nepal Hydropower Company), BKPC (Bhotekoshi Power Company), Sanima Hydropower. Nepalese skilled expert are also getting involved in planning, designing and constructing hydropower like Chalice Hydropower Plants of 20 MW capacity. Upper Tamakoshi, Chamelia, Kulekhani III, Upper Seti Hydropower Project are under construction. [5]

Figure 2.1: Power development map of Nepal

2.2 Classification

Hydro power plants can be classified as based on head as low, medium and high head plants. Present categorization of hydropower according to power generated is:

Less than 1MW          : Mini and micro hydro

1MW to 10 MW         : Small hydro

10MW to 300MW      : Medium hydro

Plants above 300MW : Big hydro

2.2.1 ROR and Storage Plant

Run-off-river plants depend on the discharge on the river, it has no more storage capacity. Diversion dam is constructed on the river to divert water and the water which exceeds the turbine capacity is spilled away. For ROR hydropower, head is up to 1000m. ROR plants with diversion type are best option to minimize the ecological impact. Most of the developed hydropower plants of Nepal are of ROR type. Because of the seasonal variation in the flow of river, there is excess power supply during the monsoon and load shedding in dry season.

 

In storage based hydropower project, the dam is built such that it creates large water storage capacity behind the dam to the river bed in the downstream portion, which is used to generate power. Kulekhani Hydropower Plant is a storage type and its only limitation is that it cannot fulfill the supply during dry season. There is now increasing trend of building ROR project than the storage type because in hilly region big gradient is available in short distance and no more area is needed like storage plants. MMHP is the run of river type with daily pondage of five hour peaking.

 

2.3 Types of Hydraulic Turbine

Turbine can be classified in various aspects. Based upon whether the pressure head available is fully or partially converted into the kinetic energy in the nozzle they are categorized into impulse and reaction turbines.

 

a)     Impulse turbine

In this type, the available hydraulic energy is first converted into kinetic energy by means of an efficient nozzle. The velocity of jet is very high when coming from the nozzle. Then it strikes the suitably arranged bucket around the rim of wheel. The bucket changes the direction of the jet without changing its pressure. The motion of wheel and bucket change into rotary motion. It is then converted into mechanical energy which is available at the turbine shaft. When the fluid jet leaves the runner, then energy is reduced. Some impulse turbines are: Pelton Turbine, Girad Turbine, Banki Turbine and Jonval Turbine etc.

 

b)    Reaction turbine

In this turbine only the portion of the fluid energy is transformed into kinetic energy before the fluid enters the turbine runner. A substantial part remains in the form of pressure energy. Subsequently both the velocity and pressure energy changes simultaneously as water glide along the turbine runner. The flow from inlet to outlet of the turbine is under pressure, and therefore, blades of a reaction turbine are closed, and passage is sealed from atmospheric conditions. Some important reaction turbines are: Fourneyron, Thomson, Francis, Kaplan and Propeller Turbine. Francis and Kaplan turbines are widely used at present.

2.3.1 Francis Turbine

In 1849 A.D. James B.  Francis, an American Engineer, first developed an inward radial flow type of Reaction turbine. Later it was modified to modern Francis turbine, mixed flow type turbine. In Francis turbine water enters radially and leaves axially. It runs by the reaction force of the exiting fluid [6]. PE and KE of the fluid come to stationary part of Turbine and partly changes PE into KE. Moving part (runner) utilize both PE and KE. It works above atmospheric pressure and will be fully immersed in water. It has draft tube to use suction head thus increases the efficiency.

Figure 2.2: Francis Turbine (front view and top view)

 
 


Working

Water under pressure (from reservoir and penstock) enters the runner through guide vanes towards the center in radial direction and discharges out of the runner axially. Energy of water is transfer into the rotational energy of runner that rotates of shaft. There is a difference of pressure in between the guide vanes and the runner which is called reaction pressure and is responsible for the motion of the runner.

2.4 Main Problem in Hydropower Plants

2.4.1 Sediment Erosion

In water there is sand, silt, calcium oxide, quartz and various hard abrasive minerals which can erode the portion of turbine. Sediment erosion in the hydropower plant is the biggest problem and no one can ignore it. It is impossible to eliminate the sediment but we can somehow minimize it. So the extent of erosion of turbine and hydro machinery is also dependent on the size of particles. Sources of sediment are glacial deposits, land slide and intensively cultivated hill slopes. Desander or settling basin is used for sedimentation but complete removal of sediment particle do not occur and hence it passes through the penstock to the turbine and erode the blade of turbine , bush, draft tube cone, block the filter and damage the pipeline. Even small problems due to sediment may affect the efficiency of the power production and also the life of turbine and power efficiency.

 The erosion of turbine component depends upon the

 (i) Eroding particles - size, shape, hardness,

(ii) Substrates–chemistry, elastic properties, surface hardness, surface morphology, and (iii) Operating conditions – velocity, impingement angle, and concentration.







 

 

 

 

 


 

 

 

                                                         

 

 

Figure 2.3:  Erosion in Guide vanes







 

 

 


 

 

 

 

 

 

 

 

(Source: Mack R., P. Drtina, and E. Lang, 1999. Numerical prediction of erosion on guide vanes and in labyrinth seals in hydraulic turbines. Wear. 233 235: 685 691.)

 

 
Figure 2.4: Erosion in Francis Turbine

 

 

Depending on the gradient of the river and distance traversed by the sand particles, the shape and size of sediment particles vary at different locations of the same river system, whereas mineral content is dependent on the geological formation of the river course and its catchment area. [7]

2.4.2 Cavitation

Cavitation, which occurs in reaction water turbines, presents unwanted consequences such as flow instabilities, excessive vibrations, damage to material surfaces and degradation of machine performance. Cavitation is the condition when a liquid reaches a state at which vapour cavities are formed and grow due to dynamic-pressure reductions to the vapour pressure of the liquid at constant temperature. In a flowing liquid, cavities are subjected to a pressure increase that stops and reverses their growth, collapsing implosively and eventually disappearing. The violent process of cavity collapse takes place in a very short time of about several nano seconds and resulting the emission of  shock-waves, If the amplitude of the resulting pressure pulse is larger than the limit of the material mechanical strength, a hollow or indentation of several micrometers called ‘‘pit’’ will be formed on the surface. If an accumulation of pits take place in a narrow area, the material is finally eroded and mass loss occurs due to the repetitive action of the cavity collapsing. In a flowing liquid, these cavities can take different forms that can be described as travelling bubbles, attached cavities or cavitating vortices.

Cavitation is the process that occurs in the draft tube, lower region of the turbine blades because pressure is decreased  here. Thus the effect of cavitation is seen in these regions. The guide vanes and other upper parts of the turbine are unaffected by this damage.

 

2.4.2.1 Types of Cavitation

The main types of cavitation that can be induced in the Francis turbine are:

1.     Leading edge cavitation

It takes the form of the attached cavity on the suction side of the runner blade due to operation at the high head than the machine design head when the incidence angle of the inlet flow is positive and largely deviated from the design value, it can also occur at the pressure side during operation at the lower head than the machine design head when the angle of incidence is zero. If unstable, this is a very aggressive type of cavitation that is likely to deeply erode the blades and to provoke pressure fluctuations.

 

 

       2.  Travelling bubble cavitation

It takes the form of separated bubbles attached to the blade suction side near the mid-chord next to the trailing edge. These travelling bubbles appear due to low plant cavitation number and they grow with load reaching their maximum when the machine operates in overload condition with the highest flow rate. This is a severe and noisy type of cavitation that reduces significantly the machine efficiency and that can provoke erosion if the bubbles collapse on the blade.

 

  3.   Draft tube swirl

It is a cavitation vortex-core flow that is formed just below the runner cone in the center of the draft tube. Its volume depends on the cavitation number and it appears at the partial load and over load. Due to the residual circumferential velocity component of the flow discharge from the runner, this vortex rotates in the same direction as the runner at part load and in opposite direction at the overload. From 50% up to 80% of the best efficiency flow rate, the vortex core takes a helical shape and presents a precession rotation at 0.25–0.35 times the runner rotating speed. In this case, circumferential pressure pulsations are generated at this low frequency. Strong fluctuations may occur if the precession frequency matches one of the free natural oscillation frequencies of the draft tube or the penstock. This provokes the large bursts of the pressure pulses in the draft tube causing strong vibrations on the turbine and even on the power house. Beyond the best efficiency point the vortex is axially centered in the draft tube cone.

 

        4. Inter blade vortex cavitation

This is formed by secondary vortices located in the channels between blades that arises due to the flow separation provoked by the incidence variation from the hub to the band. They can attach to the intersection of the blade inlet-edge with the crown or mid-way of the crown between the blades close to the suction side. Only if their tip is in touch with the runner surface they can result in erosion. These vortices appear in the partial load and resulting the high noise level. They can also appear and cavitate in extremely high head operation range because the cavitation number is relatively low and in this case they become unstable and cause strong vibration.

 

Description: C:\Users\om jai jagdish\Desktop\c.PNG

Figure 2.5: Main types of cavitation in Francis turbines: (1) leading edge cavitation,

(2) Travelling bubble cavitation, (3) draft tube swirl and (4) inter-blade vortex cavitation

Source: www.elsevier.com/locate/jnlabr/ymssp

 

2.4.2.2 Cavitation Damage Inspection

Periodic inspection of a turbine for cavitation damage is an important part of turbine maintenance. Frequent inspections are particularly important during the initial period for operation of a new turbine or new runner, as they will enable cavitation damage to be detected at an early stage and remedial measures to be effected before pitting becomes extensive. When a new turbine is placed into operation, the unit should be inspected after about 1500, 4000, and 8000 hours of operation. For pump-turbines, this frequency should be 750, 1500, and 3000 hours because of the potential for more severe cavitation pitting during pump operation. For this inspection, a hydraulic engineer from the turbine manufacturer should be present to inspect any areas of cavitation pitting and to report on the cause of the damage and possible remedial work to mitigate the damage. Subsequent to the initial inspection program, the frequency of future inspection should be dictated by the extent of damage which is occurring. "Cavitation damage inspection should be made from both the draft tube area below the runner and from the stay ring/wicket gate area in the spiral (or semi-spiral) case. Inspection from the draft tube area should normally be done from a temporary maintenance platform installed below the runner. Most areas of the runner which are susceptible to cavitation damage can be seen from the draft tube side. The leading edge of the blades, however, can best be inspected from the wicket gate area. On small units where access to the runner from the wicket gate area is poor, a polished metal mirror can be used for observing the leading edge area from the draft tube side.

 

 For units which are experiencing minimum damage, the period between inspections can be increased. However, the frequency of inspection should not be less frequent than:

 For Turbines . . . . . . . . . . . . . . . . . . Every 24,000 operating hours or every 4 years

 For Pump-turbines . . . . . . . . . . . . . Every 12,000 operating hours or every 2 years

 

"Operating and maintenance experience, other than cavitation damage, may dictate a more frequent inspection (e.g., blade cracking, seal wear). Also, a more frequent inspection is warranted when making runner profile modifications so as to adequately monitor the effectiveness of the changes.[8]

2.4.2.3 Prevention of Cavitation

To prevent cavitation from occurring altogether, a redesign of the turbine system would have to be undertaken where special consideration is given to the fluid pressure near the turbine runner. For this purpose the turbine runner should be designed such that the designed velocity at runner outlet will prevent pressure drop at this region. This is supported by the well-known Bernoulli’s equation which implies the inverse relationship between velocity and pressure. This prevents cavitation as there will be lower negative pressure. A significant decrease in fluid velocity will also decrease the efficiency of the turbine, and a compromise must be met. Ideally, the design should incorporate a tolerable amount of cavitation damage while minimizing the decrease to turbine efficiency.

2.4.3 Vibration

Vibration problems occur where there are rotating or moving parts in machinery. Apart from the machinery itself, the surrounding structure and human also face the vibration hazards because of the vibrating machinery. The effects of vibration are excessive stresses, undesirable noise, looseness of part and partial or complete failure of parts.

 

The draft tube is used to recuperate the kinetic energy downstream of the turbine runner. A well performing draft tube is therefore essential to achieve high efficiencies in hydroelectric power plant. However, at off-design operating conditions the fluid flow in the draft tube contains not only a through flow component equivalent to the discharge but also a swirl component. This type of flow can roll up to a vortex that rotates with a fraction of the rotational speed of the runner. Normally the rotational speed of the vortex lies between 30% and 50% of the runner speed. The corresponding pressure field rotates with the same speed leading to noise and vibration for the adjacent walls of the building.

 

Vibration on the external draft tube wall can be measured at several power levels to identify the power where the conditions are critical for the structure. Also, measurements can conducted at partial load with air injection from the middle of the turbine thru hollow shaft.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER THREE

CASE STUDY OF MMHP

3.1 Introduction

The Middle Marsyangdi Hydro Electric Project (MMHEP) proposed for development by Nepal Electricity Authority (NEA) has a capacity of 70 MW and it generates annual average energy of 380 GWH. It is designed to meet the energy demand by utilizing the available Marsyangdi River flow and head without serious implications to the environment.

 

The MMHEP is located on the Marsyangdi River in Lamjung District of the Western Development Region of Nepal. The project site is accessed by Dumre–Besisahar single lane black-topped Feeder Road which connects district headquarters of Lamjung "Besisahar". The project site can be approached from Dumre situated at 130 km west from Kathmandu and 70 km east from Pokhara on Kathmandu-Pokhara National Highway (Prithibi Rajmarga). The Project site extends more than 10 km and is situated by the side of Dumre-Besisahar road at about 33 km north-east from Dumre Bazar.

 

Description: C:\Users\om jai jagdish\Desktop\Capture.JPG

Figure 3.1: Middle Marshyangdi Hydropower Scheme

Source:(DYWIDAG International GmbH: A case study of MMHP)

3.1.1 Salient Features of Middle Marsyangdi Hydropower Plant

General                     

Type of project                       : Run of river with daily pondage for five hour peaking

Location                                  : Phaila Sangu/Siudibar (Head works/Powerhouse)

Maximum gross head/net head (80m3/s)            :           120/98m

Total length of the waterways                        :           5940m

 

Power & Energy                                                                                                

Installed capacity                                  :                                                               70 MW

Annual average energy                         :                                                        398GWh

 

Hydrology                                                                                                          

Catchments area                                    :                                                       2,729km2

Average annual flow                             :                                                         99.5m3/s

 

Reservoir                                              :                                                              

Minimum/Maximum operating levels  :                                  621.0mASL/626.0masl

Live storage volume                              :                                                               1.65 million m3

Surface area at maximum

operating level                                       :                                             427,000m2(42.7 ha)

 

River diversion                                                                                                  

Diversion flood                                     :                                                          560m3/s

Upstream cofferdam                              :                                                              Mass concrete dam, crest el.605.00masl

Downstream cofferdam                         :                                                             Earth- rock fill dam, crest el. 593.00 masl

Diversion Tunnel                                  : Length 375.00m cross section          

 

                                                               63.6m2, concrete/shotcrete lined

Dam                                                                                                                                                                                   

Type of dam                                          :     Combined concrete gravity and rockfill dam

Crest el. Of dam                                    :                                        629.00/630.00masl

Crest length of dam (in total)                :                                                               95m

Height above foundation                       :                                                           34.50m (54min gorge)

Spillway capacity                                  : 4,270m3s (Design flood, 10,000 years      return period) at headwater level of 626.00 m ASL

Spillway gates3                                     :                                                            Radial gates, W x H =12.00x19.54m

Energy dissipation                                 :                                                             Roller bucket

 

Intake Structures                                                                                              

Type                                                      : Submerged tunnel intakes with trash racks and gates.

Sill/Platform levels                               :                                                            616.00 m ASL/629.00 m ASL

Length                                                   : 25m (intake structure) 70-75m Connecting tunnels to desanding basins)

 

Desanding basins                                                                                               

Type                                                      :                                                  Underground desander in caverns

Length/width/height per cavern            :                                                    130m/15m-27m

Number of basins                                  : 6 accommodated in 3caverms, i. e. 2 in                     each

Length/width/height per basins             :                                   100.75/7.50m/25.10m

Efficiency                                              : 95%of particle size 0.2 mm max. sediment concentration 20,000 ppm

Flushing operation                                :                                                          Vertical flushing; BIERI system.

Operating levels (80m3/s)                     :                                       Max.el.626.0mASL

                                                                                                                                                                                            

Power Tunnel and Surge Tank                                                                        

No. of adits                                            :                                                               5

Diameter                                                :                                                               5.4m

 

Power Tunnel (low pressure line)                                                                    

Length                                                   : 5,230m (between desander outlet and surge tank)

Excavated tunnel diameter                    :                                                             6.20m (branch tunnels=4.90m)

Lined tunnel diameter                           :                                                             5.40m (branch tunnels=4.30m)

Lining                                                    :                                                       Concrete, partly reinforced

Surge tank                                                                                                          

Type                                                      : Vertical, circular surge tank, underground, concrete lined with throttle

Net diameter                                          :                                                               20m, orifice Ø 2.90m

Height                                                    :                                                               45m

Max. up- /down surge                           :                                                      El.640.93/ el.606.71 m ASL

 

Power tunnel (high pressure line)                                                                    

Length                                                   : 225m (between surge tank and discharge measurement chamber)

Excavated tunnel diameter                    :                                                             5.80m

Lined tunnel diameter                           :                                                             4.60m

Lining                                                    :                                                               Steel

 

Penstock                                                                                                             

Number, Type                                       :                                                               1, concrete cased steel pipe (cut and cover)

Diameter penstock/

bifurcation-manifold                             :                                                               Ø 4.60m / Ø3.00m to Ø2.40m

Length                                                   : 212-218m (between discharge measurement chamber and turbine inlet valve)

 

Powerhouse and Service Building                                                                   

Type                                                      : Concrete structure below the ground constructed in an open excavation pit, 27m deep. Service building on top, above ground with bridge crane

Bridge Crane capacity                           :                                                            130/10 tons

Tailrace length                                      :                                                             42.2m

 

 

Switchyard at MMHEP                                                                                    

Type                                                      :                                                              Open - air switchyard at el. 540.00 masl

Dimensions (L×W)                               :                                                             43×50

 

 

Turbines                                                                                                             

Number and Type                                 :                                                               2 Francis, vertical shaft

Rated discharge                                     :                                                               40 m3/s

Gross head                                             :                                                              110m

Rated output                                          :                                                               35.9 MW

Rated speed                                           :                                                            333.33  r.p.m.

Axis of scroll case                                 :                                                               515 m ASL

                                                                                                                                                                                            

Generators                                                                                                         

Number and Type                                 :                                                              Two, 3-phase synchronous

Rated output                                          :                                                               39 MVA

Rated voltage                                        :                                                    11kv±7.5%

Rated frequency                                    :                                                              50Hz

Power factor                                          :                                                               0.85-0.90

Rated speed                                           :                                                            333.33 r.p.m(18 poles)

Fly wheel effect (GD2)                         :                                                               600 tm2

 

Transmission line                                                                                              

Route                                                     : Middle Marsyangdi HPS-Lower   Marsyangdi HPS

Nominal voltage/Length                       :                                                               132 kV, 38.2 km [5]

 

3.1.2 Special Features of Middle Marsyangdi Hydropower Plant

1. Air Admission Tube (Cavitation tube)

2. Underground desander (Bieri type)

3. Soft coating on turbine blades

 

 

 

 

 

1. Air admission system: When water hits the turbine and it leaves then suddenly pressure changes from very high pressure to low pressure and causes the formation of bubble and it called as cavitation. So to balance the pressure, an Air Admission Tube is passed through the turbine to the draft tube portion and this tube is located in front of power house. When the pressure is below the atmospheric pressure then tube sucks the required air from the atmosphere and increases the pressure of the draft tube portion. Cavitation effect on the lower side of the runner, draft tube cone and draft tube portion. So this cavitation tube somewhat help to minimize this effect.

 

2. Underground desander: There are three underground desander and we can reach there from 500m long tunnel. Water enters from the trashrack and reach to the desander, each desander contains 8 parts, total is 24 parts and only one part operates at a time and deposited sand is remove from there from the flushing channel.

 

3. Soft coating in turbine blade: The base metal of turbine is stainless steel .Actually 2 coating is done in turbine that is primary and secondary coating. The special coating (soft coating) material is named as metalline 785 elastometic compound. It has rebounding properties and water does less effect in soft coating turbine than in hard coating turbine. It is applied to blade through the spray gun or brush.

3.2 Mechanical Components of MMHP

3.2.1 Francis Turbine of MMHP

The turbines of Middle Marsyangdi Hydropower Plant are of vertical shaft orientation fitted with the clockwise runner rotation, when viewed from the generator. On 333.33 rpm maximum power output of 39.9 MW can be achieved if the head water and tail water level are between 626 m ASL and 530 m ASL. The spiral case as well as the draft tube elbow is embedded in concrete. All static and dynamic forces resulting from weight, pressure, torque and thermal expansions are included into the foundation via upper and lower stay ring flanges.

 

The spiral inlet pipe has a thrust ring which transfers the axial forces directly into the concrete when the turbine inlet valve closed. The Turbine shaft line consists of Turbine main shaft and the intermediate shaft. The runner is coupled to the turbine shaft flange. The generator is coupled to the upper intermediate shaft flange. The guide bearing supported by the head cover and bearing cone, guide the lower shaft line. The thrust bearing is arranged below Generator.

 

The distributor is actuated by two servomotor anchored in the foundation through the operating ring links and levers, the position and angle of wicket gates are adjustable. The turbines and power house design allows disassembly of the turbine without dismantling the generator. The opening in concrete below spiral case allows dismantling of the bottom ring and draft tube cone from down.

 

 Discharge ring  

Stainless steel has a length of 1000mm and thickness of 30mm

 

 Spiral casing

 It is designed and constructed for a maximum operating pressure of 14 bars.  The water from the penstock is conducted through the scroll casing and distributed completely around the circumference of the guide vane cascade.

The shape of casing helps to maintain the velocity of water high enough all around the runner Spiral casing shell is made from welded steel plates (material S355N) .it has manhole diameter 600m for inner inspection and maintenance.

 

Inlet pipe with thrust ring

Thrust ring will transmit the hydraulic force of the closed turbine inlet valve to the concrete structure and will ensure the exact position of the spiral casing, jointly with the pit liner under all operating condition.

 

Stay ring

It is a welded steel plate material of S355NZ35. It has three different shape with different stay vane angle.

 

Wicket gates

These vanes direct the water into the runner at an appropriate angle to the design. All the guide vanes are connected to a regulating ring. The governor through a rod rotates the ring to adjust the opening of the guide vane according to the load. The guide vanes have aerofoil shape which is shown in the picture. There are 24 wicket gates. They are machined cast material and supported by three bearings. They are designed to withhold the maximum dynamic pressure and vibration free operation. The gates are 60-65% opened. The soft coating on the gates protects from river abrasives substance.

 

Description: SANY5085

Figure 3.2: Wicket gate blade                       

 

    Figure 3.3: Guide vane and stay ring

 

 
Description: Picture4

           

Source: Middle Marsyangdi Hydropower Plant

 
 

 


3.2.2 Turbine Runner

It is the main unit of turbine in which high velocity water gives impulse and reactive effort to the blade due to which runner rotates with high rpm. In MMHP there are two Francis runners each of capacity 35MW. The runner is converts the pressure energy into mechanical energy. The power is thus developed in the shaft on which runner is mounted.

 

Runners are welded from casted parts: Crown, the runners installed in middle Marsyangdi are free from internal stress and casting imperfections. There are no hollows, depressions, waviness and projections. The runner blade inlet edges are hard coated while the surface are soft coated with metaline 785 elastomeric chemical. The chemical material is G-X5 Cr Ni 13 4.

 

The wearing ring at the crown side has the hardness of 300HB, and the chemical compound is X 3 Cr Ni 13 4. The hard coating thickness is 0.3mm.the total weight of runner and wearing ring is 5580kg.

 

Figure 3.4: Francis Runner

 

Shaft seal: It is located in the head cover and designed for easy inspection. It comprises the split seal, the rotating seal ring, stationary seal plate, remote wear indicator and a stainless steel shaft protecting sleeve. They are rust proof and wear/ tear resistant.

 

Guide bearing: The guide bearing is double segmented type with integrated oil tank mounted at the turbine shaft. The inner core has Babbit–metal lining of 3.5mm thick anchored firmly in two bearing segments. It is self-lubricating because of rotation of the lubricating oil tank with turbine shaft around the sliding segments. It is located at the head cover. It allows vertical movement of the runner and shaft for required adjustment and dismantling of thrust bearing and sufficient clearance between turbine and generator shaft. The guide bearing assures that no water can enter in lubrication system through leakage or condensation.

 

Description: SANY5132

Figure 3.5: Guide Bearing

Shaft: The shaft of MMHP is hollow type. It has a bore of 150mm in the centre for natural air admission system. The shaft material is forged metal of CK35 N. The shaft is divided as turbine shaft and intermediate shaft. The turbine shaft is the lowest part of main shaft and has the tolerances of 0.02mm. The intermediate shaft is between turbine shaft and generator shaft.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER FOUR

 OBSERVATIONS, GRAPHS AND RESULTS

 

4.1 Hydraulic Turbine (Main Dimensions)

Table 4.1: Hydraulic Turbine (Main Dimensions)

Spiral case inlet diameter

 φ2600mm

Distributor apparatus pitch circular diameter

 φ2462mm

No. of wicket gates

 33.45degree φ145mm between gates

Wicket vane height

 622.4mm

Runner inlet diameter

 φ1786.9mm

Runner discharge diameter

 φ2103.0mm

Runner height

 1210.5mm

No. of blades

 13

Turbine shaft with hollow bore

 φ499.6mm h6/φ150mm (+2)

Intermediate shaft with hollow bore

 φ500.0mm h6/ φ150mm (+2)

Turbine shaft range, upper/lower

 φ900mm h9/φ920mm h9

Intermediate shaft flange U/L

 φ900mm h9/ φ900mm h9

Turbine shaft length

 1598mm

Intermediate shaft length

 2158mm

Turbine guide bearing inner diameter

 φ500mm H7

 

 

 

 

Runner Dimensions

Table 4.2 : Runner Dimensions

 

Type

 f710/4(f599)

Material

 G-X5CrNi134

(134=13% Cr, 4%Ni remaining stainless steel)

Diameter (mm)

2200

Z2(Number of Blade of runner)

13

 

Wicket Gates

Table 4.3 : Wicket Gates Dimensions

 

γ max(degree)

33.45

Zo(Number of wicket gates)

24

bo(height of wicket gate)

623 mm

Dz(Diameter of wicket gate)

246.2mm

 

4.2 Sediment Sample Testing

The sediment samples were collected from Intake Site, Draft Tube, and Tail Race of the Middle Marsyangdi Hydropower Plant as on 29 Ashad, 2068 and delivered to the Hydro Lab Pvt. Ltd.  for the Particle Size Distribution (PSD) and Mineral Content Analysis.

 

 

 

 

4.2.1 Particle Size Distribution (PSD) Analysis.

The results of PSD analysis are given in the figure below:

 

Description: C:\Users\om jai jagdish\Desktop\Capture.JPG

 

Source:  Hydro Lab Pvt. Ltd.

 
Figure 4.1: Results of PSD Analysis

 

 

Above particle size distribution curve shows that the particle size at draft tube is decreased than that of intake. It signifies the effectiveness of desander. From particle size distribution analysis, we found that mainly the particles between 0.01mm and 1mm (large particles) are settled in desander. The diameter of particle at tailrace is slightly greater than that of draft tube. It may be due to mixing the sand from river.

 

4.2.2 Mineral Content Analysis

In order to carry out mineral content analysis, the sample was divided into three sets and analysed separately. Finally, the results of the three sets were averaged to have good representation of minerals.

 

 

 

4.2.3 Minerals in the Sediment Sample

Following are the minerals found in the sediment sample.

Quartz

Quartz is made up of a continuous framework of SiO4 silicon–oxygen tetrahedral, with each oxygen being shared between two tetrahedral, giving an overall formula SiO2.

Density: 2.65-2.66 gm/cm3

 

Feldspar

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming minerals which make up as much as 60% of the Earth's crust.

Density: 2.55 to 2.76 gm/cm3

 

Mica

Mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage.

Density: 1.60  gm/cm3

 

Tourmaline

 Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of color

Density: 2.82 to 3.32 gm/cm3

 

Beryl

The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. Pure beryl is colorless.

Density: 3.1 to 4.3 gm/cm3

 

Carbonate

 Density: 2 to 2.9 gm/cm3

 

 

 

 

4.2.4 Result of Mineral Content Analysis

 

Table 4.4: Intake Sample

 

Table 4.5: Draft tube Sample

 

Table 4.6: Tail Race Sample

 

Notes:

1. Others A: Tourmaline, Garnet and Beryl.

              2. Others B: Carbonate (9%) and Clay.

 

 

Above tables show the concentration and hardness of sediment particles. Quartz is available in highest concentration, which has highest hardness. From this we conclude that the erosion is one of the major turbine damaging factors in MMHPP. So sediment is one of important parameter to be studied. At draft tube the concentration of quartz is decreased than that of intake but the concentration of mica is increased. It means that quartz is settled more than mica in desander. It may be due to the high density of quartz and low density of mica. Though mica is not settled effectively in desander, its effect is not significant because of low hardness of mica, the hardness value of feldspar is significant for erosion but its concentration is small. So its effect on erosion is not significant. Other particles (Tourmaline, Garnet, Beryl, Carbonate and clay) are in small concentration so their effect is also minimized. Mainly the quartz is responsible for erosion.

 

4.3 Suction Velocity Measurement

The suction velocity at the inlet of the Air Admission Tube was measured using an instrument named Digital Anemometer which was made available from Center Of Energy Studies (CES), Institute of Engineering, Pulchowk Campus.

An anemometer is a device for measuring wind speed. The term is derived from the Greek word anemos, meaning wind, and is used to describe any air speed measurement instrument used in meteorology or aerodynamics.

 

Description: G:\Memories\gorkha tour\ashis\DSC05860.JPG

 
4.3.1 Specifications of Digital Anemometer

 (MODEL No. : AR826 by SMART SENSOR)                                                  

1) Features:

  • Wind speed measurement                         
  • Measuring unit options
  • Data hold
  • Max/Min/Avg measurement
  • LCD backlight
  • Auto power off

Figure 4.2: Anemometer

 
 


2) Operation Conditions:

Humidity: 40% to 85%

Temperature: 10oC to 50oC (14oF to 122oF)

 

3) Storage Conditions:

Humidity: 10% to 90%

Temperature: 20oC to 60oC (-4oF to 140oF)

 

4) Measurement Range

Table 4.7: Measurement Range

Unit

Range

Resolution

Threshold

Accuracy

m/s

0-45

0.1

0.3

±3%±0.1dgts

Ft/min

0-8800

19

60

±3%±10dgts

Knots

0-88

0.2

0.6

±3%±0.1dgts

Km/hr

0-140

0.3

1

±3%±0.1dgts

Mph

0-100

0.2

0.7

±3%±0.1dgts

5) Power Supply : 9V DC

Figure 4.3: Suction Velocity Measurement at MMHP

 
Description: G:\Memories\gorkha tour\ashis\DSC05859.JPG

Table 4.8: Suction velocity and pressure at different power output

Power(MW)

Suction Velocity(ft/min)

Spiral Casing Pressure(bar)

Draft tube pressure(bar)

20.08

3405

9.68

0.64

21

3661

9.65

0.64

22

3090

9.48

0.65

23.5

2913

9.69

0.65

25

2834

9.3

0.67

26.3

2248

9.42

0.69

26.9

1870

9.44

0.66

28

1791

9.56

0.69

29.2

1520

9.36

0.7

29.4

874

9.36

0.7

31.4

668

9.3

0.7

33

374

9.24

0.7

34

59

9.21

0.7

34.4

3

9.12

0.7

 

Graphs for Suction Velocity

The suction velocity of the Air Admission Tube of MMHP was recorded using Digital Anemometer and the following graphs and conclusions were observed.

 

Figure 4.4: Power vs. Suction Velocity

 
 

 


Above graph shows the relationship between suction velocity of Air Admission System and power output. The velocity and the power are inversely proportional. When the turbine runs in the full load i.e. producing 35MW then the suction velocity from the cavitation tube is near about zero.  As the power generation decreased, the air suction velocity increased. At the varying power generation the draft tube pressure was recorded in similar manner, which is given below:

 

Figure 4.5: Power vs. Draft Tube Pressure

 

 
 

 


The above graph shows that at the full load condition pressure in the draft tube is more i.e. 0.7 bars. When the power is lowered, the pressure is not changed and draft tube parts and cause pitting. So to maintain the pressure on the draft tube portion suction velocity is more when the power is decreased. For example, when the power is 21 MW then suction velocities is about 3661 ft/min maintaining the pressure of 0.64 bars by flow pressure from atmospheric pressure to draft tube pressure.

4.4 Vibration Measurement

Currently, there is a trend to operate turbines in conditions far from their best efficiency point imposed by the variable demand on the energy market; therefore, it is important to determine the range of load where the turbine presents instability and seek ways of reducing this instability or adjusting the design so the structural elements can withstand the conditions at partial load. Air admission was an effective way to decrease pressure pulsation and wall stress. An air flow of certain % of the water discharge proved sufficient to improve the hydraulic stability.

 

Any increase in vibration level in machine or machine component in accordance with international/national standard gives an immediate indication of degradation of machine and occupational health exposure to workers. Vibration of the different mechanical components such as Regulating Ring, Turbine Bearing Head Cover, Intermediate Shaft Cover and Draft Tube were measured using Vibration Meter and the result was correlated to the cavitation.

 

4.4.1Vibration Level Measurement

Vibration level was recorded in both turbine units of MMHP at full load (power output 34MW) and partial load (power output 21.3MW) condition using vibration meter. Vibration Meter was held at a level for approximately 10 seconds to get a stable reading of RMS value of vibration data.

 

The reading was taken in four dimensional positions, 90 degree to each other in horizontal plane of each part, viz. regulating cover, turbine bearing head cover, intermediate shaft cover at radial position, intermediate shaft cover at axial position and draft tube.

4.4.2 Portable Vibration Meter VM-62

Portable Vibration Meter Model VM-62 is equipped with basic functions required to measure and evaluate wide range of machinery vibration. The used vibration pick up is a shear structure piezoelectric type with built-in preamplifier. This structure reduces cable noise and Pyro noise which cause error in small vibration measurements.

 

The vibration Pickup Model PV-57 assembled with the VM-62 converts the detected vibration into electric signal. As with this model vibration acceleration, velocity and displacement can be measured, we selected this vibration meter for the purpose of vibration measurement in the turbine casing and outer part of draft tube.

 

 

                                    

 

Figure 4.6 Vibration Measurement on Intermediate Shaft Cover using Vibration Meter

4.4.3 Measuring Range

The instrument had the following measuring range switch and frequency range:

Acceleration: 0.003~100 m/s2, at 3Hz~5 KHz

Velocity: 0.03~100 cm/s, at 3Hz~5 KHz

Displacement: 0.03~10mm, at 10Hz~500Hz

 

The vibration measurement level in Acceleration unit was recorded by keeping the measurement range on m/s2. Similarly, the vibration measurement levels in velocity units were recorded by keeping the measurement range on cm/s. The displacement measurement was out of range for the device. The acceleration and velocities both were calculated for RMS value.

4.4.4 Graphs and Tables for Vibration

With the help of Vibration Meter, the vibration of the different parts of the turbine unit (such as Regulating Ring, Turbine Bearing Head Cover, Intermediate Shaft (ISC) Covering Radial and ISC Axial) was measured after 10 seconds interval.

 

 

 

4.4.4.1 Vibration in terms of Velocity (m/s)

 

 

Figure 4.7 Positions in Turbine vs. Velocity 34MW

 

 

Figure 4.8 Positions in Turbine vs. Velocity 21.3MW

The velocity of vibration at different position of turbine at full load and part load is shown in above graphs. These graphs are plotted by using the data of vibration that was measured in terms of velocity (m/s) (see appendix F).

 Excluding at intermediate shaft cover (ISC axial), overall pattern of vibration in unit one is more than that of unit two for the same power output. Being both turbines identical water borne vibration is identical. From this it is concluded that unit one has more structural vibration, which may be due to some fault during commissioning or other unknown reasons.

4.4.5 Vibration in terms of Displacement (µm)

The collected data of velocity and acceleration are used to find the frequency (Hz) and displacement (µm) from the log scale vibration chart (see appendix G). The frequency range of the vibration at any parts of the turbine of MMHP is within the safe range.

 

Table 4.9: Unit 1 at 34 MW (Full Load condition)

Table 4.10: Unit 1 at 23.1 MW (Part Load condition)

Table 4.11: Unit 2 at 34 MW (Part Load condition)

Table 4.12: Unit 2 at 23.1 MW (Part Load condition)

 

 

 

Figure 4.9: Frequency of Vibration in Unit 1

 

Figure 4.10: Frequency of Vibration in Unit 2

 

Figure 4.11: Displacement of Vibration in Unit 1

 

Figure 4.12: Displacement of Vibration in Unit 2

We know the violent process of cavity collapse takes place in a very short time of about several nano seconds. Partial load condition provides a low pressure condition in runner outlets. The guide vanes allow less water to flow into the runner. This creates obstruction to the flow of water, which causes water hammering thus leading to the vibration.

The above graph clearly shows that the displacement is decreased in every parts of the turbine, except in intermediate shaft cover, when the power output is lowered to 23.1 MW from 34MW. On the other hand, the frequency of the vibration has increased in the part load condition in both the turbine units, except for the frequencies at turbine head cover and intermediate shaft cover in axial position.

This decrease of displacement and increase of frequency at the part load indicates that the nature of vibration changes from low frequency-high amplitude to high frequency-low amplitude.

As there is change in nature of the vibration in partial load condition, with the lapse of time it further changes due to the increase in intensity of erosion added with the pitting effect of the cavitation. The increase in the frequency resembles more vigorous vibration. Whenever the frequency exceeds the allowable limit of the machine, maintenance of the turbine should be started. In the case of MMHP, the maximum frequency measured is 110Hz and it is well below the optimum range.

Thus it can be concluded that there is a clear effect of cavitation at part load. But there is no drastic increment in the frequency of vibration (not more than 60 Hz) at part load condition which is due to the Air Admission System that does not allow the pressure to drop below 0.26bar. It signifies the satisfactory performance of the Air Admission System in reducing the cavitation.

 

 

 

 

 

CHAPTER FIVE

 MODELLING AND SIMULATION

5.1 Introduction

For the analysis of erosion, cavitation and vibration in turbine modelling and simulation become important tools and for this purpose we selected SOLIDWORKS for modelling and CFX for simulation. At first the turbine Model was created with the available data and with the input parameters of pressure 101.325 kpa, temperature 293.2k and turbine inlet velocity of 10m/s. with these references we obtained the simulated result for the pressure and velocity distribution in the turbine.

 

A model is a simplified representation of a system at some particular point in time or space intended to promote understanding of the real system and simulation is the manipulation of a model in such a way that it operates on time or space to compress it, thus enabling one to perceive the interactions that would not otherwise be apparent because of their separation in time or space.

 

Modeling and Simulation is a discipline for developing a level of understanding of the interaction of the parts of a system, and of the system as a whole. The level of understanding which may be developed via this discipline is seldom achievable via any other discipline.

 

A simulation generally refers to a computerized version of the model which is run over time to study the implications of the defined interactions. Simulations are generally iterative in there development. One develops a model, simulates it, learns from the simulation, revises the model, and continues the iterations until an adequate level of understanding is developed.

5.2 Computational Fluid Dynamics Analysis:

CFD is one of the tools for the simulation of fluid flow problems. It has various advantages like:

  • Low cost in designing phase
  • Less time consuming
  • More accurate  than manual calculation for complex geometry like turbines

The main objectives of CFD analysis are as follows:

·        To find out the critical region of erosion.

·        To find out the best operating point in erosion point of view.

 

Computational Fluid Dynamics (CFD) Analysis is the analysis of systems involving fluid flow, heat transfer and related physical processes by means of computer based simulation. ANSYS CFX is generalpurpose CFD tool. It is a finite volume technique. In this technique, the region of interest is divided into small subregions, called control volumes. The equations are discredited, and solved iteratively for each control volume. As a result, an approximation of the value of each variable at specific points throughout the domain can be obtained. From the result of individual control volume, the full picture of flow is obtained. It is a powerful CFD tool. It is capable of modeling: steady state and transient flows, laminar and turbulent flows, subsonic, transonic and supersonic flows, heat transfer and thermal radiation, buoyancy, nonNewtonian flows, transport of nonreacting scalar components, multiphase flows, combustion, flows in multiple frames of reference, particle tracking, etc. CFD analysis is a five step process, modeling, meshing, preprocessing, solving and post processing.  Three softwares have been used to perform these tasks. SOLIDWORKS, for modeling, ICEMCFD for meshing and ANSYS CFX for analysis.[9]

 

5.3 Different terms used in simulation:

A) Domain

The domain is the bounding volume within which CFD analysis is performed. CFX-Pre uses the concept of domains to define the type, properties and region of the fluid, porous or solid. Domains are regions of space in which the equations of fluid flow or heat transfer are solved. In our analysis we set up an environment that has water and sand defined one way. The sand properties are defined in material named sand.

 

B) General Erosion Model:

Erosion models are useful for design of turbine components, sediment settling basins and optimization of hydropower plant operation in sand laden-rivers .The wear of a wall due to the erosive effect of particle impacts is a complex function of particle impact, particle and wall properties. For nearly all metals, erosion is found to vary with impact angle and velocity according to the relationship.

 

The fundamental and simplest equation of erosion is:

Erosion = f (operating condition, properties of particles, properties of base material)

 

Generally this equation is given as a function velocity, material hardness, particle size, and concentration. Truscott on his literature survey of publications of 20 years on abrasive wear of hydraulic machinery has found that the most often quoted expression for Erosion ∞ (velocity).

Bardal describes the most general formula for pure erosion as:

W = Kmat .  Kenv .c. Vn . f (α )                   (mm/year)………………..eq.1

Here , W is erosion rate in mm/year, Kmat is material constant and Kenv is constant depending on environment, c is concentration of particles and f(α) is function of impingement angle α. V is the velocity of particle and n is the exponent of velocity.[10]  

 

C) Boundary condition:

It is the condition of variables on the external boundaries. Boundaries are the points from where the solving process starts. Boundary conditions are applied in boundary region of domain and in domain interface. Boundary conditions can be inlets, outlets, openings, walls, and symmetry planes. In CFX, any unspecified boundary is named as default boundary and is a wall type boundary condition.

 

D) Inlet

The inlet boundary is the surface from where the fluid enters to the domain.  We have used the normal velocity inlet boundary.

 

E) Outlet

The outlet boundary is the surface from where the fluid leaves the domain. We have used pressure outlet boundary.

 

 

F) Wall

The wall boundary is the water tight wall. It isolates the inner condition from the outer environment. All the restitution coefficients are defined in wall boundary.

 

5.4 Reference geometry

The reference geometry of turbine was made in solidworks. Image sketch feature of solidworks was used for modelling. Reverse engineering technique was used to sketch blade profile and guide vane(see appendix B). Further the model was meshed in ICEMCFD. This reference geometry is composed of 1,346,573 elements.

 

Figure 5.1: Reference Geometry of Blade and Guide Vane

 

Figure 5.2: Reference Geometry of Runner with Guide Vane

 

5.5 Simulation Results

 

As the manual iterative process for the static analysis of turbine is time consuming and less accurate, computer simulation is chosen. The simulation in ANSYS CFX is an iterative process that iterates the input data 100 times so as to give the best result.

The inputs for this process are velocity at inlet boundary and pressure at outlet boundary. The output of simulation is in the terms of velocity. It displays the velocity distribution in the domain.

 

The following are the velocity distribution profiles for full load and part load. These velocity distribution can be used to predict the erosion regions.

Figure 5.3: Velocity Profile at Full Load

 

Figure 5.4: Velocity Profile at Part Load

 

The above pictures are the top view of velocity profile of simulation result. The first profile belongs to velocity profile at full load and the second one is of part load condition. The velocity at the outlet of turbine blade is almost same for full load as well as part load. From the general model of erosion (equation no.1) the velocity gives significant effect on erosion rate. It shows that the erosion rate is almost same for full load and part load.

 

In case of guide vane, the velocity near the sealing region is increased at part load. It indicates that the erosion of guide vane increases, at part load.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER SIX

 CONCLUSION AND RECOMMENDATIONS

 

6.1 Conclusion

Erosion and cavitation are inevitable in Hydropower stations. Erosion rate and parts most affected can be predicated by sand sampling and simulation. Cavitation, on the other hand, can be controlled by increasing the pressure in low pressure region of reaction turbines by air admission system. If erosion and cavitation is controlled then the life of mechanical components increases as wear and tear is reduced. And under these conditions, efficiency of turbine increases. From our analysis, we come to the following conclusions:

1)     The sediment has Quartz as the major constituent, 60% at intake and 51% at both the draft tube and tailrace, which is the hardest among the other mineral contents. Though 15% of the Quartz is settled in the desander its abrasive action in runner and guide vanes is very much significant.

2)     The critical regions of erosion as indicated by simulation are sealing regions of guide vanes; and leading and trailing ends of runner blades.

3)     The guide vanes are eroded with greater intensity at part load condition and the rate of erosion in runner blades is almost equal in full and partial load. Thus the best operating condition is full load condition.

4)     Air suction maintains the draft tube pressure not lower than 0.64 bars at 21 MW, a part load condition the minimal range of increase of  vibration frequency (60 Hz) at part load condition of turbine power output concludes the satisfactory effectiveness of the Air Admission System.

 

 

 

 

 

6.2 Recommendations

1.     Erosion analysis by using appropriate erosion model is recommended.

2.     The pressure variation at draft tube, compared with other hydropower station of similar capacity with no Air Admission System (Lower Marsyangdi Hydropower Plant) , would give better result.

3.     Further research in evaluating the performance of desander is suggested.

4.     It would be better to perform dynamic analysis of turbine model.

5.     Noise measurement to relate it with the occurrence of cavitation is also recommended.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REFERENCES

·       Dr. Jagadish Lal, Fluid Mechanics and Hydraulics, 10th edition, published by Metropolitan Book Co. Pvt. Ltd.

  • Law, A. M., and W. D. Kelton. 1991. Simulation, Modeling and Analysis, second edition, Mc Graw-Hill
  • M. K. Natarajan, Principles of Fluid Mechanics Second Edition, OXFORD & IBH PUBLISHING CO. PVT.LTD.
  • R. K. RAJPUT, Textbook of Fluid Mechanics and Hydraulic Machines, S.CHAND AND COMPANY LTD., RAM NAGAR NEW DELHI-110055.
  • Thapa, Bhola, 2004.July,”Sand Erosion in Hydraulic Machinery”, MSc Thesis, Department of Mechanical Engineering
  • The license of ANSYS CFX software was used under the prior permission of Bim Prasad Shrestha, Head, Department of Mechanical Engineering, Kathmandu University.
  • Nepal Electricity Authority, A Year in Review – Fiscal Year 2009/10

[1] www.voithhydro.de/media/hypow_11_6.pdf

[2] www1.eere.energy.gov/industry/forest/pdfs/pulppaper_profile.pdf

[3] www.sari-energy.org/Publications/bbmalla.pdf

[4] Er. Suresh Raut, Final Year MSc Thesis, Department of Mechanical Engineering   Tribhuvan University, Nepal.

[5] http://hydropower.inl.gov/turbines/pdfs/doeid-13741.pdf

[6] www.voithhydro.de/media/t3339e_Francis_72dpi.pdf

[7] www.tn.gov/environment/wpc/sed_ero_controlhandbook

[8] http://www.absoluteastronomy.com/topics/Cavitation

[9] http://www.featflow.de

[10]http://www.frenchriverland.com/cavitation_&_vibration_of_a_draft_tube [11]http://www.gtz.de/de/dokumente/en-divestment-of-nea-plants.pdf [12]http://www.hydroworld.com/index/display/article-display

[13]http//en.structurae.de/structure/data

[14]http://www.begellhouse.com/journals

[15]http://www.powervision-eng.ch/Profile/Publications/pdf/IAHR_2004.pdf

[16]http://acad-tim.tm.edu.ro/iSMART-flow/pdf/Muntean2_CIEM2009.pdf.

[17]http://jaibana.udea.edu.co/grupos/revista/revistas/.../Articulo%209.pdf

APPENDIX A: SEDIMENT DATA OF MARSYANGDI RIVER

 

 

Date

Reservoir water level

Intake   (PPM)    

De-sander   (PPM)

 Draft tube   (PPM)

Gate opening( cm)

Remarks

1

2

3

7-Jul-10

622.35

3417

 

 

150

 

 

 

8-Jul-10

622.19

3279

 

 

160

 

 

 

9-Jul-10

622.28

3421

 

 

170

 

 

 

10-Jul-10

622.22

4889

 

 

235

 

 

 

11-Jul-10

622.19

5000

 

 

175

 

 

 

13-Jul-10

621.94

1122

 

 

125

50

 

 

14-Jul-10

622.03

1531

 

 

 

 

127

 

15-Jul-10

622.03

1692

1639

1475

 

 

133

 

17-Jul-10

622.98

5440

4615

2240

 

155

 

 Highest ppm in draft tube

19-Jul-10

622.47

3469

2800

1818.2

 

145

 

 

21-Jul-10

622.14

1920

1718.8

1562.5

 

 

170

 

23-Jul-10

622.14

3556

960

937.5

 

171

 

 

25-Jul-10

622.03

3368

1111

1083.3

120

90

 

 

27-Jul-10

622.26

2558

2321

2667

 

200

 

 

29-Jul-10

621.98

2558

952

152

 

185

 

 

31-Jul-10

622.13

2609

1087

2459

90

140

 

 

2-Aug-10

622.03

2459

1522

667

 

150

 

 

4-Aug-10

621.77

1000

1087

682

 

180

 

 

6-Aug-10

622.23

2000

1304

1488

 

185

 

 

8-Aug-10

622.19

1538

1333

1000

 

 

140

 

10-Aug-10

622.04

971

591

434

 

125

 

 

12-Aug-10

622.16

581

434

321

 

145

 

 

14-Aug-10

622.13

824

696

786

 

130

 

 

16-Aug-10

621.97

2435

983

2210

 

1175

 

sand 57%silt 43%(intake)

18-Aug-10

622.13

2342

943

1787

100

100

 

sand 41%silt 59%(intake)

20-Aug-10

622.00

1002

754

1883

210

 

 

sand 48%silt 52%(intake)

22-Aug-10

622.08

1843

1676

735

 

252

 

sand 52.4%silt

47.6%(intake)

24-Aug-10

622.33

1647

734

719

155

73

 

sand 49.5%silt 50.5%(intake)

26-Aug-10

622.02

948

715

981

190

 

 

sand 47%silt

53%(intake)

28-Aug-10

622.13

2155

940

1609

 

 

220

sand 55%silt

45%(intake)

30-Aug-10

622.35

1827

961

1175

200

105

 

sand 49%silt

51%(intake)

1-Sep-10

622.1

2225

1372

1371

200

65

 

sand 65%silt

35%(intake)

3-Sep-10