Coal Mill

COAL PULVERISER MAINTENANCE PERFORMANCE ENHANCEMENT THROUGH THE APPLICATION OF A COMBINATION OF NEW TECHNOLOGIES

GERHARD HOLTSHAUZEN A dissertation submitted in partial fulfilment of the requirement for the degree Magister Ingeneriae in the FACULTY OF ENGINEERING (Mechanical (Mechanical and Manufacturing Engineering) (specialisation (specialisation Maintenance Engineering)

UNIVERSITY OF JOHANNESBURG September 2008

ACKNOWLEDGEMENTS Firstly I want to thank my Saviour and God who gave me the strength, courage and perseverance to complete this dissertation. I also wish to thank:

•

Me. Hanlie Hanlie Holtshauzen Holtshauzen – My wife; “For all her her support support and and motivation”.

•

Mr. Keith Williams – ex. Babcock Design Engineer; “Assistance and and advice when needed”.

•

Mr. Kevin Dugdale – ex. Mitsui Babcock Babcock Design Engineer; “Opening a new new world for me”.

•

Mr. Brett Brett King – Steinmüller Steinmüller Engineering Engineering Principal Engineer; “Assistance with the Financial Model”.

•

Mr. Chris Loots – Steinmüller Steinmüller Engineering Services Continuous Continuous Improvement Improvement Manager, “Assistance with the English E nglish language”.

•

Dr Jasper Coetzee – Study Leader.

TABLE OF CONTENTS ACKNOWLEDGE ACKNOWLEDGEMENTS MENTS................ ................................ ................................ ................................ ................................ ...............................ii ...............ii LIST OF TABLES TABLES.............. .............................. ................................ ................................ .............................. .............................. ..............................iv ..............iv LIST OF FIGURES................. FIGURES................................. ................................ ................................ ............................... ............................... ........................ ........ v LIST OF SYMBOLS SYMBOLS ............................... ............................................... ................................ ............................... ............................... ........................vi ........vi ABBREVIATIO ABBREVIATIONS NS ............................... ............................................... ................................ ................................ ................................ .........................vii .........vii SUMMARY SUMMARY ................................ ................................................ ................................ ................................ ................................. ................................. ...................... ...... 1 OPSOMMING....................... OPSOMMING....................................... ................................ ................................ ................................ .............................. ............................ .............. 3 CHAPTER CHAPTER 1 – PROBLEM PROBLEM DEFINITION................... DEFINITION................................... ................................ ................................ ........................ ........ 5 1.1 PROBLEM PROBLEM DEFINITION....................... DEFINITION....................................... ................................ ................................ .............................. .............. 5 1.2 ROAD MAP FOR THE DISSERTAT DISSERTATION.......................... ION........................................ .............................. ..................... ..... 6 1.3 RESEARCH RESEARCH METHODOLO METHODOLOGY................... GY................................... ................................ ................................ ........................ ........ 7 1.4 KEY RESEARCH RESEARCH QUESTIONS................ QUESTIONS.............................. .............................. ................................ ............................ ............ 8 1.5 SUMMARY SUMMARY ................................. ............................................... .............................. ................................ ................................ .......................... .......... 8 CHAPTER CHAPTER 2 – BACKGROUND. BACKGROUND................ ............................... ................................ ................................ ................................ ...................... ...... 10 2.1 SA COAL AND ELECTRICITY ELECTRICITY ............................. ............................................. ................................ ............................ ............ 10 2.2 COAL PULVERIZING PULVERIZING FOR ELECTRICITY ELECTRICITY GENERATION GENERATION ............................. ............................... 11 2.3 DEFINITION DEFINITION OF A MILL................ MILL ................................ ................................ .............................. ................................ ..................... ... 12 2.4 COAL PULVERIZAT PULVERIZATION ION ............................... ............................................... ................................ ............................... ................... .... 13 2.5 BASIC MILL OPERATION........................ OPERATION........................................ .............................. .............................. .......................... .......... 16 2.6 SUMMARY SUMMARY ................................. ............................................... .............................. ................................ ................................ ........................ ........ 17 CHAPTER CHAPTER 3 – LITERATURE LITERATURE SURVEY................ SURVEY................................ ................................ ................................ .......................... .......... 18 3.1 INTRODUCTIO INTRODUCTION N ................................ ................................................. ................................. ................................ ............................. ............. 18 3.2 PROFITABILIT PROFITABILITY Y OF STRATEGIC MAINTENANCE MAINTENANCE ............................... ......................................... .......... 18 3.3 THE EVOLVEMENT EVOLVEMENT OF MAINTENANC MAINTENANCE E STRATEGIES STRATEGIES .............................. ................................. ... 19 3.4 PROFITABILIT PROFITABILITY Y BY INNOVATIVE INNOVATIVE MAINTENANCE MAINTENANCE ................................ ........................................ ........ 22 3.5 TECHNICAL TECHNICAL AND FINANCIAL FINANCIAL FEASIBILITY FEASIBILITY .............................. ............................................... ..................... .... 23 3.6 SUMMARY SUMMARY ................................. ............................................... .............................. ................................ ................................ ........................ ........ 25 CHAPTER CHAPTER 4 – SOLUTION SOLUTION DEVELOPMENT DEVELOPMENT ................................ .............................................. .............................. ................... ... 26 4.1 TESTING TESTING PROCEDURES PROCEDURES............. ............................. ................................ ................................ ............................... ................... .... 26 4.2 ROTATING ROTATING THROAT THROAT ASSEMBLIES ASSEMBLIES 26

5.4.1.3

Project Project Implementa Implementation tion Cost:.................................. Cost:.................................................. ............................ ............ 42

5.4.1.4

Project Project Operatin Operating g Cost: .............................. .............................................. ................................ ........................ ........ 42

5.4.1.5

Project Project Maintenan Maintenance ce Cost:............................ Cost:............................................ ................................ ...................... ...... 42

5.4.1.6

Status Status Quo Replacemen Replacementt Cost: .............................. .............................................. ............................ ............ 43

5.4.1.7

Status Status Quo Operatin Operating g Cost:............................. Cost:........................................... ................................ ..................... ... 43

5.4.1.8

Status Status Quo Maintenance Maintenance Cost: ............................... ............................................... ............................ ............ 43

5.4.1.9

Financial Results / Parameters from the Model:.................... Model:.......... ................... .............. ..... 43

5.4.2 Spider and spider guide wear plates.......... plates ................... .................. .................... ..................... ................... ......... 44 5.4.3 Airbag Airbag installati installation on .............................. .............................................. ................................ .............................. ........................ .......... 44 5.4.3.1 Financial Financial Results Results (append (appendix ix H for full detail): detail): .............................. ...................................... ........ 44 5.4.4 Classifier Classifier cone............................ cone............................................ .............................. .............................. ................................ .................. 44 5.4.4.1 Financial Results (appendix (appendix G for full detail): detail): .................... .......... ................... .................. ......... 44 5.4.5 High chrome mill grinding media................ media..... ...................... ..................... ................... .................. .................. ......... 44 5.4.5.1 Financial Results (appendix (appendix I for full detail):...... detail):...... ...................... ........... ..................... ............ .. 44 5.5 FINANCIAL INDICATORS INDICATORS FOR FOR THE THE INTRODUCED INTRODUCED TECHNOLOGIES TECHNOLOGIES .......... ........ .. 45 5.6 SUMMARY SUMMARY OF TECHNICAL RESULTS RESULTS AND FINANCIAL FINANCIAL INDICATORS INDICATORS ........ 46 CHAPTER CHAPTER 6 – CONCLUSION......... CONCLUSION....................... .............................. ................................ ................................ ................................ .................. 47 6.1 INTRODUCTIO INTRODUCTION N ................................ ................................................. ................................. ................................ ............................. ............. 47 6.2 CONCLUSION.... CONCLUSION.................... ................................ ................................. ................................. ................................ ............................. ............. 47 APPENDIX APPENDIX A – KRIEL PERFORMA PERFORMANCE NCE TESTS ................................ ................................................ .......................... .......... 53 A1 Coal analysis: analysis: .............................. .............................................. ................................ ................................. ............................... .................... ...... 53 A.2 Mill differenti differential al pressure pressure:: .............................. .............................................. ................................ ............................... ..................... ...... 53 A.3 Mill motor powe powerr consumptio consumption: n: ............................. ............................................. ................................ ............................ ............ 54 A.4 PA fan motor powe powerr consumptio consumption: n: .............................. .............................................. ................................ ...................... ...... 54 A.5 Mill reject reject rate: rate: ............................. ............................................. ................................ ................................. ............................... .................... ...... 54 A.6 Mill rejects rejects density: density: ............................. .............................................. ................................. ................................ ............................. ............. 55 A.7 PF fineness (75 µm sieve): sieve): .............................. .............................................. ................................ ................................ .................. 55 A.8 PF fineness (150 µm sieve): sieve): ............................... ................................................ ................................. ............................. ............. 55

I.6 I.7

Financial Model Inputs (Theoretical) – High Chrome Ball Cycle Regime........... 73 Added Benefits: ................................................................................................ 73

LIST OF TABLES Table 1 - Summary of financial indicator for technologies tested Table 2 - Mill diff pressure Table 3 - Mill kW’s consumed Table 4 - PA fan kW’s consumed Table 5 - Reject rate Table 6 - Reject density Table 7 - 75 µm sieve fineness Table 8 - 150 µm sieve fineness Table 9 - 300 µm sieve fineness Table 10 - Recirculation load Table 11 - Financial model Table 12 - RTA financial model Table 13 - Wear plates financial model Table 14 - Classifier cone modification financial model Table 15 - Airbag financial model Table 16 - Current grinding media financial inputs Table 17 - Proposed grinding media financial inputs Table 18 - High chrome mill grinding media financial model

45 53 54 54 54 55 55 55 56 56 59 63 65 67 69 72 73 74

LIST OF FIGURES Figure 1 - Alstom Deep Bowl Mill [20] 12 Figure 2 - Basic layout of a Babcock and Wilcox 10.8E mill [15] 13 Figure 3 - Schematic flow of coal in a pulverizing plant 15 Figure 4 - Typical stationary mill throat (throat plate ring) in a Babcock & Wilcox E-Type Mill 27 Figure 5 - Schematic drawing of a Rotating Throat Assembly (RTA) 27 Figure 6 - Rotating Throat Assembly in a Babcock and Wilcox E-type mill 28 Figure 7 - Schematic representation of the mill cycles 29 Figure 8 - General arrangement drawing of mill spider guide wear plates 33 Figure 9 - Worn spider and guide wear plates 34 Figure 10 - Airbag loading cylinder 36 Figure 11 - General arrangement drawing of new classifier cone design 37

LIST OF SYMBOLS Symbol

Description

Unit of measure

ρ

Density

kg/m3

φ

Diameter

Mm

µ

micron

1 x 106

%

percentage

Fraction of 100

°C

Temperature

Degrees Celsius

ABBREVIATIONS B&W

Babcock and Wilcox

BPE

Boiler Plant Engineering at Kriel Power Station (unless otherwise stated)

BS

British Standards

CMMS

Computerised Maintenance Management System

DIN

Deutches Institut für Normung (German National Standards Organisation)

Dr

Doctor

ERV

Equipment replacement value

ESKOM

Electricity Supply Commission of South Africa

ex.

Previous

GDP

Gross Domestic Product

hrs

hours

IRR

Internal Rate of Return

Kg/m3

kilogram per cubic meter

kg/s

Kilogram per second

kPa

Kilo Pascal

kW

Kilowatt

M. Eng

Masters Degree in Engineering

M. Ing

Meesters Graad in Ingenieurswese

m/s

meter per second

mm

millimetre

MMS

RSA

Republic of South Africa

RTA

Rotating Throat Assembly

TLC

Tender Loving Care

Tn

Test number n

UCLF

Unplanned capability loss factor

UK

United Kingdom

vs.

Versus (opposed to)

WW II

World War II

WA

Weld Ability

SUMMARY The dissertation is an investigation on the implementation of new technologies (five off) in a coal pulverising with main aim to optimise mill maintenance interventions. The technologies in question are: •

Stationary air throat replaced with a rotating throat assembly.

•

Hydro-pneumatic mill loading cylinders replaced with airbags.

•

Classifier cone modification.

•

Introduction of triton material for the mill spider guide plates.

•

High chrome mill grinding balls.

Every maintenance intervention, even if planned, negatively affects a plant’s availability and reliability. A Babcock and Wilcox (B&W) at Kriel power station (ESKOM) was used for the testing of the mentioned technologies. The mill model/size is a B&W 10.8E mill. The aim of the introduction of new technology on a mill is to optimise the period between required maintenance activities. A higher availability will assist in achieving good plant maintenance performance indicators. It needs to be noted that the dissertation focussed on the financial and technical parameters of a specific modification. This in an effort to increase uptime and reduce costs as part of a business drive for bigger profit margins.

assemblies), there is no need for maintenance interventions every 5 000hrs as a rotating throat assembly can run without major interventions for 60 000hrs.

The scientific proof that the introduced technology was successfully applied in the case study plant from a technical and financial point of view yields positive results for full implementation of the proposed modifications. In conclusion the current indication is the mill maintenance interventions can be stretched to 8 000hrs without negatively affecting the performance of the pulverising plant, if all 5 technologies are implemented together. This 60% stretch between mill maintenance interventions however needs to be tested to determine if there are not other restrictions not thought of that prohibits this action.

Possible future work in this field will be to determine if the plant cannot be run on a condition based maintenance strategy. Tests in this regard will have to be performed to determine the impact on plant and personnel with the change from used base to condition based maintenance. The author is confident that the perfect balance between optimum plant performance and optimum maintenance cost is within this approach.

OPSOMMING Die verhandeling handel oor die implementering van 5 nuwe komponente in ‘n kool vergruisings/meule aanleg en die dienooreenkomstige toetse en afleidings om te bepaal of die huidige instandhoudings frekwensie die optimum is. Die komponente ter sprake is: •

Statiese lugblokke vervang met ‘n roterende lugblok konstruksie.

•

Hidroliese lug meul ball belastings silnders vervang met lugsak silinders.

•

Klassifikasie kegel verandering.

•

Bekendstelling van Triton material vir die “spider” slytasieplate.

•

Hoë chroom meulballe.

Elke instandhoudings aksie, selfs al is dit beplan, beïnvloed die beskikbaarheid en betroubaarheid van ‘n aanleg. ‘n Babcock en Wilcox (B&W) meule by Kriel kragstasie (ESKOM) was gebruik as toetsmeule vir al die genoemde tegnologie. Die meule grootte is ‘n B&W 10.8E. The doel vir die nuwe tegnologië is die soeke na die optimum periode tussen instandhoudingsaksies. ‘n Hoër beskikbaarheid op die aanleg sal hulp verleen tot beter aanleg betroubaarheid en uitsette. Dit is belangrik om daarop te let dat dat die verhandeling sal fokus op die tegniese en finansiële implikasies op die komponente getoets.

Wetenskaplike bewyse dat die tegnologië geïmplementeer suksesvol is uit ‘n tegniese en finansiële oogpunt, is genoeg motivering vir die volle implementering van die veranderinge. Die huidige aanduiding is dat die instandhoudingsintervalle gerek kan word na 8 000 ure sonder om die aanleg in ‘n risiko situasie te plaas. Die 60% verlenging in diensinterval sal egter eers getoets moet word om te bepaal of daar nie ander dele van die aanleg is waar nie die beoogde 8 000 ure sal bereik nie.

Voorgestelde toekomstige werk in hierdie vakgebied sal wees om te bepaal of die aanleg nie op ‘n toestandsgebasseerde instandhoudingsstrategie bedryf sal kan word nie. Toetse in hierdie verband sal die impak bepaal op die aanleg en personeel. Die outeur is egter vol vertoue dat ‘n perfekte balans tussen aanleg uitsette en aanleg instandhouding sal lei tot die laagste moontlike instandhoudingskoste.

CHAPTER 1 – PROBLEM DEFINITION 1.1

PROBLEM DEFINITION

Coal resources are available commercially in 70 countries of the world. Current supply shortages are being dictated by transportation costs and logistical problems. Due to the high demand for coal in China and India, the South African market is benefiting from the international boom of coal prices. With the high volumes of coal being exported from South Africa, the pressure on the domestic market is forcing coal prices higher.

ESKOM (Electricity Supply Commission of South Africa), being the biggest user of coal in South Africa, has been forced to look at lower grade coals to fire their power stations due to the cost of the typical “design coal”. The coal the power stations were designed for has become uneconomical to purchase as the South African Energy regulator is trying to keep the annual electricity price increases close to the inflation rate. The bulk of the higher grade coal is being shipped off to the international buyers, paying higher fees for the coal than the South African economy can afford.

The lower grade coal being utilised at the ESKOM power stations has a big impact on the pulverizing plants installed, as the pulverization process poses a harsh environment even with the higher grade coal previously processed, with a high amount of associated component wear. Lower grade coal equates to higher load factors on the pulverisers and

When changes are implemented on a pulveriser plant, the maintenance thereof will also be influenced. A very careful approach and analysis need to be performed during the concept phase of any modification or change. Parameters of the modification to consider are: •  Efficiency. •

Project implementation cost.

•

Plant availability.

•

Project operating cost.

•

Project maintenance cost.

Once the modification is installed the plant can be run and actual performance data can be compared to: •

Status quo replacement cost.

•

Status quo operating cost.

•

Status quo maintenance cost.

With actual test data available the modification can be compared to before the modification was performed. From the parameters that need monitoring it can be gathered that the modification is being evaluated from a technical and financial perspective. The comparison

purposefully not used, as negative results do not necessarily indicate the research was in vain. Negative results should move the strategic direction of the research into the perusal of alternative routes for positive, less negative or the least cost option/s.

1.3

RESEARCH METHODOLOGY

The research undertaken will be technically evaluated industry wide and then tested for financial success on a single case study. The case study milling plant will be from Kriel power station.

For mill maintenance to be optimised, the conventional way of practical (empirical) testing was used as very little, or no performance data was available. Due to the vast differences of plant hardware, operating philosophies and coal being pulverised, no known plant could be referenced before the tests were undertaken. The reference mill for the research is a B&W 10.8E vertical spindle mill (Figure 2). The first technological advance that was tested on the “test” mill was that of a rotating throat assembly in place of the traditional stationary mill throat. With this technology being tested, other modifications were also being evaluated in parallel to “move the envelope” to be able to determine optimum maintenance intervals to achieve optimum maintenance costs for the applicable plant. It can thus also be reasoned that this approach forced the research into a testing phase of additional new technology to determine the impact on the system and its maintenance interventions.

The new technologies being tested were chosen primarily to increase component life expectancy and thus to stretch maintenance intervals without subjecting the plant to unnecessary risks in terms of planned or unplanned load losses. The end result of the research will be a proposed mill maintenance flow chart that indicates how maintenance interventions should be planned for a specific milling plant for optimum mill availability and reliability at the least possible cost.

1.4

KEY RESEARCH QUESTIONS

The introduction of the complete complement of technical modifications on the specific mill will be evaluated technically and financially in this dissertation, as it has a direct impact on the pulverizing process that has been described. The financial feasibility of each change / modification will be tested according to the criteria that will be defined in Chapter 4.

As was stated in the previous section, the introduction of a range of new technologies will be needed to support a change in the mill maintenance frequency and duration. Other components that are gate-keepers in terms of the time they can last before failure has to be considered with the RTA’s. The areas of plant that will be evaluated for success are mentioned in section 1.3. A case gap analysis has revealed that the following components must be upgraded before condition based maintenance principles can be applied with financial success: •

Mill spider wear plates.

The end results will give an indication whether or not the current maintenance strategies should still be applied. A review of the current maintenance strategy will be done to determine the optimum levels of maintenance activities and / or interventions. With an optimum level of maintenance, costs will be at the lowest possible level for the applicable operating philosophy and this is where any competitive business unit wants to be.

CHAPTER 2 – BACKGROUND 2.1

SA COAL AND ELECTRICITY

South Africa (RSA) is a country where coal resources are readily available at a relative low cost compared to international coal prices. For example, the average cost of coal in SA is R120/ton, whereas prices in Europe is up to €85/ton, which equates to roughly R1 020/t. The availability of coal biased the decision of the South African Government to build power stations in the 1960’s to 1990’s that are of fossil fuel type. ESKOM, a para-statal company, currently has 11 (eleven) operational fossil fuel power plants, all situated within the borders of South Africa. Two additional fossil power plants are in a partial mothballed state. These are currently being rehabilitated for production. The last unit of these two mothballed power stations has to be available for commercial load by mid 2010. This date is not fixed as South Africa’s electricity shortage escalates, and thus the return to service of the mothballed power plants could happen sooner than indicated if spares and resources could be sourced in time.

The eleven fossil fuel power stations in SA that are in operation are: •

Arnot (6 x 350 MW = 2 100 MW total) 1.

•

Camden (8 x 200MW = 1 600 MW total)2.

•

Duvha (6 x 600 MW = 3 600 MW total).

The mothballed power stations in order of being returned to production are: •

Grootvlei (6 x 200 MW = 1 200 MW total)3.

•

Komati (5 x 100 MW + 4 x 125 MW = 1 000 MW total)4.

2.2

COAL PULVERIZING FOR ELECTRICITY GENERATION

For a steam boiler to be able to combust fossil fuel, the coal first has to be pulverised to a certain fineness before flow and burning characteristics are within acceptable parameters. To achieve the desired fineness the raw coal is processed within a milling plant (also referred to as crushing or pulverization plant) to get the desired characteristics to ensure stable and effective combustion of these particles in the furnace of a boiler. The milling plant will be the focus of this dissertation. The usage of mills is however not unique to the electricity generating utilities and a large amount of mills are also utilised in the cement and mineral processing industry. The mills of an electricity generating utility are a big contributor to the profit or not of the company because: •

The work on mills is labour intensive.

•

The spare parts of a mill are usually expensive as only selected companies can manufacture the spares to the correct requirements.

•

A mill is a high wear component and is designed to sacrifice its grinding media to be able to process the coal.

•

A milling plant uses an amount of auxiliary power when in operation that has to be factored into the cost of production for a pulveriser.

2.3

DEFINITION OF A MILL

The definition of a roller mill as per DIN 24100 [14] : “The roller mill is a machine, in which the grinding path is of a ring form. On this, grinding elements (rollers or bowls) are rolling. Their own weight presses the grinding elements on the grinding path by centrifugal action, by springs, by hydraulic or pneumatic system. The driving power can be expected on the grinding elements as well as the grinding path. The machine is used for grinding, which means to produce materials in mostly fine grain form. The size of the grains depends on industrial utilisation.” The roller mill is one type of vertical spindle mill. ‘Vertical-spindle’ because the mill grinding media rotates around a virtual vertical axis. Typical OEM’s (Original Equipment Manufacturers) of roller mills are: Loesche, MPS, PHI, Lapulco, Alstom, Steinmüller Engineering Services, and Combustion E ngineering.

The mill type that will be used as basis of discussion in this document is also of the vertical spindle mill type, but with different grinding media than that indicated in DIN 24100. The grinding elements are not rollers or bowls (figure 1), but hollow cast steel balls running in a top and bottom ring. The OEM’s for this type of mill are Babcock and Wilcox (B&W), Mitsui Babcock and Claudius Peters. Refer to f igure 2 for a general arrangement drawing of a B&W 10.8E mill. A Babcock and Wilcox 10.8E coal mill is described as a vertical spindle, low speed, type “E” pressurised, ball mill. The 10.8E refers to the PCD (Pitch Centre Diameter) of the mill being 108 inches.

Spider guide wear plate

coal delivered to the mill (kg/s) and no assumptions are made in terms of the coal tonnage delivered to the mill.

After the feeder, raw coal enters the mill through the raw coal pipe from where it falls, with the assistance of gravity onto the mill table (yoke cover plate). With the mill table that is rotated by the mill electric motor and mill gearbox, the raw coal migrates, by centrifugal force to the outside of the mill table. It then enters the grinding zone as a coal bed between the mill bottom ring and mill balls. The Primary Air (PA) fan blows hot air into the mill through the mill throat slots. The mill throat slots, which act as a venturi for the air, accelerate the PA to have enough kinetic energy to transport small coal particles. The primary air aerates the pulverized coal particles in a process described as primary classification. Pulverised fuel (PF) with the correct properties then enters the mill classifier and if of correct weight and quality, is delivered via pipe work mounted on top of the mill, as aerated PF to the boiler for combustion. Coal particles not fine enough or still too heavy are re-introduced into the mill’s grinding zone.

A B&W mill can, for descriptive purposes, be divided into four sections or levels: (i)

The Mill Housing Support – This part of the mill is cylindrical in shape and within it is the mill gearbox. The housing support is mounted on its own steel foundation frame and the gearbox is mounted on its own foundation frame. On top of the mill housing is the wear plate section.

(ii)

The Mill Housing – The support plate provides the support for the second level of the

Coal Bunker

Coal Gate

Feeder

Mill

Primary Air Seal Air

Pulverised Fuel

2.5

BASIC MILL OPERATION

Before the research questions, an overview will be given into the insight of the operating of a vertical spindle mill. The start-up and pulverizing process in a B&W 8.5E vertical spindle mill can be summarised to be as follow: •

The coal bunker isolating valve (slide gate) above the mill’s feeder is opened whilst the mill is being warmed by Primary Air (PA) – Figure 3.

•

Once the mill is at the correct temperature (mill outlet temperature ≈ 100oC) the mill feeder and motor is started.

•

Raw coal is fed through the raw coal feed pipe to the mill via the mill’s feeder.

•

At the bottom of the raw coal chute (pipe), coal is thrown onto the mill’s table (yoke cover plate).

•

The centrifugal force of the rotating ring, throws the coal in the mill grinding zone (between rings and balls).

•  The mill balls, as they rotate with the mill bottom rings, crush the coal into finer

particles (Refer to figure 5). •

A grinding force is exerted between the mill grinding elements (rings and balls) as well as by the weight of the components plus an external loading system.

•

PA enters the mill through stationary throat slots, which act as a venturi and cause primary pulverised fuel (PF) classification.

•

The PA blowing through the venturi picks up coal particles and transports those that

Figure 4 - General arrangement drawing of an 8.5E B&W mill

CHAPTER 3 – LITERATURE SURVEY 3.1

INTRODUCTION

Chapter 2 gave a basic insight to the application of pulverisers and where these units are utilised within Industry. The operation of a coal mill was also briefly explained. This chapter serves as introduction into the anticipated research work that will be undertaken. A structured approach will be used to test the new technology technically (Chapter 4), followed by the development of a financial model (Chapter 5) to test the financial feasibility of the change/s undertaken. The various maintenance philosophies that can be considered in a maintenance environment will be introduced and discussed in this chapter from a return on investment point of view. The chosen strategy must typically give a power utility a favourable return on investment; otherwise the plant will not be competitive and be outperformed by its competitors.

3.2

PROFITABILITY OF STRATEGIC MAINTENANCE

The maintenance budget of the electricity power generating utilities in the Republic of South Africa contributes a large portion to the cost of operation of a Business Unit, and is thus largely decisive regarding its profitability. The maintenance budget in some utilities contributes up to 25% of a Business Unit’s annual budget [12]. The maintenance budget for different fossil-fired utilities differs, mainly based on the following considerations: •

Age of the plant.

Idhammar   [26] also encourages a pro-active approach to maintenance where “everyone in the organization is jointly focussed on reliability performance”   and not on the typical cost cutting exercises. Idhammar   [26] also quotes that “increased throughput to sales generate 20% more revenue” compared to cost cutting initiatives.

3.3

THE EVOLVEMENT OF MAINTENANCE STRATEGIES

As this dissertation is based on a vertical spindle milling plant, the focus will be on the type of maintenance strategies applied for such units to achieve optimum performance at the least possible cost. In any maintenance environment there are various strategies to consider. Coetzee   [10] summarises the different strategies in the figure shown in figure 5. The indicated strategies evolved over time and were developed and streamlined to achieve optimum returns from a profitability point of view.

The history of when the different maintenance strategies developed is summarised as follow: •

During the latter stages on WWII, the Americans realised the importance of reliability.

•

Prior to this period in maintenance history, the only maintenance strategy applied was that of breakdown (corrective maintenance).

•

In the 1950’s the concept of preventative maintenance was applied for the first time [21].

•

The Japanese copied the American concepts on preventative maintenance and

Maintenance Strategies

Design-out Maintenance

Preventative Maintenance

Corrective Maintenance

Used based Maintenance

Scheduled Overhaul

Scheduled Replacement

Component Replacement

Block Replacement

Predictive Maintenance

Routine Service

Opportunistic Maintenance

Condition Monitoring

Inspections

Figure 5 - Maintenance Strategies (Coetzee[10])

Within figure 4 a process or road map is captured to assist in the effective management of maintenance. With the optimum maintenance philosophy applied, an organisation will contain maintenance costs and still be able to operate plant at desirable reliability and

For preventative maintenance 2 (two) sub activities are devised that are: •

Used base (Time Based) Maintenance: Statistical analysis of historic failures to schedule appropriate maintenance activities at the correct intervals. Maintenance activities could include visual inspections, wear measurements, adjustment of settings, replacement of parts etc.)

•

Predictive (Condition Based Maintenance): Condition Base Maintenance to apply maintenance only when a component’s condition indicates performance levels cannot be maintained. There is a risk component coupled to this type of maintenance.

To ensure the effectiveness of maintenance activities applied and executed, regular audits on technical performance should be done to measure the condition of the plant vs. designed outputs. The operating philosophy will always dictate the maintenance philosophy which in turn will dictate the maintenance strategies used. This implies that maintenance strategy for a plant could change over time due to the age of the plant, change in plant utilisation and rate of production output. A “Finger on the Pulse” approach is applied to immediately determine deviations from set or desired standards. The maintenance plan should in essence also be maintained in order to support the various maintenance strategies used.

CMMS (Computerised Maintenance Management Systems) will assist with this function. It should be noted that the CMMS is only as good as the inputs the people operating the

3.4

PROFITABILITY BY INNOVATIVE MAINTENANCE

Most of the Power Generating Utilities in RSA and worldwide have been designed for a life expectancy of 20 - 25 years. Since most of the design codes were more conservative in the designing years (1960’s & 1970’s for SA) compared to modern day codes, the concept of renovate and modernisation (R&M) presents major opportunities for Industry. The cost of R & M is a fraction of that of an investment in a new Plant. Varley  [1] quotes R & M costs to be 15 % – 25 % of the cost of new capacity. R & M costs also avoid the lengthy and uncertain approval process for new plant. As coal quality is constantly deteriorating in RSA, the possible shortfall on performance in Milling Plants can cost an economy millions of Rands. In India, Varley   quoted this “loss of income ” as high as 3% of the Gross Domestic Product (GDP); costing India’s economy tens of billions of dollars due to of un-optimised milling plants. Note that the R&M cost as quoted by Varley  is for renovating and modernizing a plant that is operational. For the mothballed power stations in South Africa the R & M investment percentage compared to new plant can be higher than the quoted 15 to 25 % due to the fact that the plants have not been in operation for quite some time (10 to 15 yrs for some plants). The electricity demand in South Africa has however grown at such a rate the last three years that Government is prepared to spend the additional money on the return to service of the mothballed plants to be able to bridge the gap between capacity installed and available and system electricity demands.

R & M initiatives must be driven with a strategic innovative approach, as Tidd   [11] quotes:

[18]. Idhammar   [26] in an article “Current best practices”   introduces the concept of maintenance costs compared to estimated replacement cost. In the paper and pulp industry acceptable cost for maintenance is 4.2% per annum of ERV (equipment replacement value). Peterson  [25] also touches on this concept. International acceptable standards are available per industry type where the concept of benchmarking can be applied.

3.5

TECHNICAL AND FINANCIAL FEASIBILITY

The technical success of a modification / change to plant does not necessary guarantee the implementation of the change / modification in question. This statement as the technical success does not always imply financial success. Financial success is determined by the type of returns the modification can guarantee. An Investment Committee of a power utility will set guidelines for the approval of projects to be able to ensure scientifically and mathematically that they approve projects for the correct reasons. For all projects that are voluntary and do not involve safety or environmental contraventions and or corrections, the Committee can set standards such as: •

Internal Rate of Return ≥ 15%.

•

Payback period < 5 years.

•

Cost / Benefit Ratio >2:1.

These parameters are determined by the investors of the company as they expect certain returns on their money. The more conservative a company’s approach, the more difficult it

product needs to be considered. Performance and maintenance must be considered as part of the original design.

Harrison & White   [22] quotes a listing of main financial factors that may be relevant for projects under consideration as follows: •

“First cost, installed and ready to run (or net realizable value).

•

Insurance and property tax.

•

The life period of the machine until displaced from the proposed job.

•

The salvage value at the date of disposal.

•

The degree and pattern of utilization; the percent of capacity at which the machine will operate on the intended job with allowances for possible future changes in utilization.

•

Routine and Maintenance repair costs.

•

Major repair items or periodic overhauls.

•

Direct operating costs, including operating labour, fuel or power, scrap material and rework.

•

Indirect costs: indirect labour, tooling, supplies, floor space, inventory.

•

Fringe benefits.

•

Hazards and losses related to equipment, material and labour time.

•

Changes in sales volumes or price resulting from choice.

•

Changes in cost of labour, power, supplies, etc. resulting in changes in operating

In the milling plant specific maintenance environment basically only used based maintenance is the accepted norm for maintenance to guarantee acceptable levels of reliability. The aim of the research undertaken would be to investigate the option of a preventative maintenance strategy to reach high levels of availability and reliability at a cost that justifies the money spent on maintenance activities.

3.6

SUMMARY

This chapter set the scene for the dissertation to be developed from a technical and financial point of view. The maintenance impact by the introduction of new technology, assemblies or parts will be evaluated. It is of paramount importance in the competitive market for a company to be able to adapt effectively and efficiently to an ever-changing technical and economic environment. State of the art cost effective equipment as well as up to date maintenance philosophies will give a utility a competitive advantage in the market.

CHAPTER 4 – SOLUTION DEVELOPMENT 4.1

TESTING PROCEDURES

The technology introduced in this chapter must be explained and understood before technical and financial conclusions are made. The aim of this chapter is to give insight into the technology and testing methods to determine the technical success of the proposed modifications/changes. The maintenance philosophy currently being applied to the components will also be mentioned for the reader to gain insight into the maintenance of a typical coal pulveriser. Each technology introduced was tested and evaluated individually and independently.

4.2

ROTATING THROAT ASSEMBLIES

The throat slot (throat plate ring as seen in figure 6) openings in a mill act as a venturi and produce very high PA velocities (up to 80 m/s), which creates primary pulverised fuel (PF) classification. This action causes extreme erosion on the throat slots, which in turn leads to intense maintenance. The maintenance is intensive and very time-consuming, as the throat slots need to be welded up, cut out and/or replaced. Where throat plates are welded up, the

Figure 4 - Typical stationary mill throat (throat plate ring) in a Babcock & Wilcox E-Type Mill

A Rotating Throat Assembly (RTA) in turn is where the rotating throat is fixed to the mill table by a clamped or bolted arrangement. This throat rotates with the mill’s table when in operation and has an air seal on the ledge cover. Refer to figure 7 for a general arrangement drawing of a RTA.

introduction of the RTA’s there would be a positive performance improvement associated with the upgrading to a rotating throat assembly. Rotating Throats were tested on the following power stations for possible performance improvements: •  Hendrina. •  Arnot. •  Duvha. •  Kriel. •  Matla.

The initial key questions of the research however addressed the final conclusion and focus of the “modified research objectives” very well. This is where the research project moved from a performance (PF fineness) and throughput point of view to a maintenance point of view. The reason for this was that the performance improvement and throughput of the RTA were not conclusive on all installations tested.

Mill Ball

Ledge cover (stationary)

At the start of cycle 0 (zero), the mill is equipped with new grinding rings and 11 off steel balls. The mill balls are worn down from φ 768 mm to an intermediate size of 690 mm. This set of balls is then removed, stored and replaced with a new set of φ 768 mm balls which will then be referenced as cycle 1. This is repeated two more times till the end of cycle 3 is reached at a ball size of φ  690 mm. Now the φ  690 mm balls are not removed, but an additional 690 mm ball added. There are now 12 (twelve) 690 mm balls for cycle 4 (four) that are worn down to 640 mm which is the ball scrap size. The 640 mm scrap balls are removed and then replaced with the 10 off balls from cycle 0 plus 2 balls from cycle 1. This process continues till cycle 6 is reached on the mill. At the end of cycle 6 the ring is usually also at the end of its life and the mill grinding media (rings and balls) are then replaced with new rings and balls where the mill cycle starts at cycle zero again. A typical ring life for a Kriel 10.8E B&W mill is 60 000 operating hours. This equates to a wear rate of 980 hrs/mm.

A typical wear rate on a steel cast mill ball on a Kriel 10.8E B&W mill is 105 hrs/mm. This equates to operating hours for each cycle from 0 to 3 to [(768-690) x 105] 8 190 hrs. A factor that will override a mill service before the typical 5 000 hrs services will be the ball size. A mill in cycle 2 for example was serviced at 5 000 hrs, now only (8 190 – 5 000) 3 190 hrs later the mill has to be opened for the mill balls to be changed as part of the ball cycle regime. Usually the maintenance department then does a complete mill service as is prescribed for every 5 000 hrs. Logically this does not make any sense and the aim should be to only service the mill the next service interval of 5 000 hrs is reached. Due to the cost of

The utopia will be if mill maintenance intervals and ball change/add activities could be coordinated. This implies a condition-based approach as the wear rate on the mill balls vary because of changing coal qualities, different mill cycles, various mill conditions, etc.

4.2.1 PROCEDURE FOR MILL THROUGHPUT / PERFORMANCE TESTING Before the test procedure is discussed, it must be reiterated that there is a definite split between mill performance and throughput of a mill. A basic definition of the two terms as follows: •

Mill performance is the fineness achieved in the pulverising process.

•

Mill throughput is the tons of coal pulverised per hour by the mill in question.

By carrying out performance tests a mill would be tested before a rotating throat installation and the same tests repeated after the RTA installation. This methodology to indicate performance parameters/figures before and after the modifications. With both sets of results a proper evaluation can be performed on the modification’s impact.

Typical tests that were performed as part of the test program are: •

Clean Air Curve.

•

Load Line.

•

PF Fineness.

•

Coal Analysis -

Abrasiveness Index.

-

Ash Content.

-

Total Moisture.

-

Surface Moisture.

-

Bulk density.

-

Calorific Value – Coal.

-

Calorific Value – Rejects.

-

Hardgrove Index.

•

Mill reject rate.

•

Mill Power Consumption.

•

PA Fan Power Consumption (only where dedicated PA Fans per mill).

For the analysis of performance and throughput of a mill, two Kriel Power Station mills were identified for testing after the testing on all the vertical spindle mill power stations were complete. The reason for this was twofold: (i) No coal samples were taken during previous tests (if they were taken, they were not

the mill as well as the wear on the hardware needs to be tracked. Consistent performance results over time will indicate a superior product compared to the stationary throats.

The key questions in section 5.4 introduced 4 (four) other technologies to be able to extend the service life of a mill to be able to go beyond the indicated 5 000 operating hours. These technologies mentioned are: •

Mill spider the wear plates

•

Mill loading cylinders

•

Classifier cone

•

High chrome mill grinding media

These technologies do not impact on the performance or throughput of the mill directly, but will support longer maintenance times for maintenance activities or interventions. Each of the new components will be tested and compared to the old/original installed component’s lifetime. This to test the technical success of the modification.

4.3

MILL SPIDER AND GUIDE WEAR PLATES

In a vertical spindle Babcock and Wilcox E-type mill, the top ring is held in position by the mill spider. The spider keeps the top ring stationary by keeping it from rotating by means of 4 spider heads that fit into spider guide plates in the mill’s casing and spider guides (Figures 7 and 2). The spider guides are situated in the mill casing and the spider heads are bolted

The test procedure for an alternative material for the spider wear plates will be to install a test set, made of the new material and then trending the wear over time. The 7 480 hrs achieved with the original material should not be regarded as 100% life as at this stage the clearances between the spider head and guide plates were far greater than OEM maximum allowable tolerances of 15 mm. 5 000 hrs should be regarded as the 100% life of the wear plates for a point of reference.

Spider head wear plate

Spider guide wear plate

wear plate and the spider guide plate. The maximum allowable gap on any one of the 4 spider wear plate/guide combinations is 15 mm. The installed gap is 6 mm .

4.4

MILL BALL LOADING CYLINDERS

Every E-type B&W mill is fitted with a loading system to apply a force on the top ring to have a certain grinding force in the grinding media. This external force on the top ring ensures grinding efficiency and a constant force on the coal bed as the loading system’s pressure can be adjusted to have different forces. A 10.8E mill is fitted with 8 off hydro-pneumatic loading cylinders. Hydro-pneumatic refers to oil providing the pressure (force) and nitrogen providing the damping effect required for an effective pulverization process.

For such hydraulic cylinder to be overhauled, the barrel section of the cylinder needs to be re-chromed and machined to ensure achieving a good sealing surface on the hydraulic seals. Once the chrome thickness exceeds a certain thickness (after multiple re-chroming events), the parent material of the cylinder loses its structural integrity and the chrome and machining cannot ensure a barrel with good tolerances for hydraulic sealing. Refer to figure 2 for the position and geometry of the mill loading cylinders.

4.4.1 AIRBAGS Due to the age of the hydro-pneumatic system of the mills, alternative solutions needed to be investigated as the mill reliability and availability was at unacceptable levels due to this

get blocked. The root cause of the blocking is blasting wires, rags and foreign material that get stuck in the flat plate skirt (figure 2). This section is also referred to as the gladiator skirt. If the gladiator skirt is kept open, a percentage of the PA will take the path of the least resistance and bypass the classifier blades. This causes excessive amounts of coarse PF to be fed to the boiler that in turn leads to numerous combustion problems.

“Labyrinth sealing in bottom of

Refer to figure 13 for a general arrangement drawing of the new classifier cone design. There are no gladiator skirts in this design and the PF/PA seal is established by PF in the inside of the cone that that form a labyrinth seal in the bottom section of the cone. The sleeve at the bottom can be adjusted to optimise the level of PF inside the cone.

4.6

HIGH CHROME MILL GRINDING MEDIA

The current mill grinding media installed in a 10.8E B&W mill m ill is as follows: •

Cast steel balls.

•

High chrome rings (top and bottom).

As mentioned in section 4.2, the wear on the mill grinding media is of such a nature that the 5 000 hour service interval and ball change/ball add interventions are not aligned. As was mentioned the ideal would be to align the ball add or ball change activities with the mill service activities or to extend the grinding media life so that there are less frequent chances of service intervals being shortened due to a ball add or ball change activity.

The test performed for this modification will be to introduce grinding media of different material compositions that will deliver longer service hours between ball change/add activities. The ball wear rate (hrs/mm) thus needs to be improved. An important consideration is not to jeopardise the ring life as the norm in tribology wear maps is that two hard materials in a wear situation will wear quicker than a hard and soft material combination. The optimum levels of wear on the ball/ ring combination is the main driving motive for this test.

CHAPTER 5 – SOLUTION TESTING 5.1

TECHNICAL

AND

FINANCIAL

EVALUATION

OF

TECHNOLOGIES INTRODUCED Applicable test procedures for the technologies in question have been described in Chapter VI. The financial viability of the technology has however not been evaluated or proven. Chapter VII will test t he introduction of the new technology against a financial model that was developed for the plant in question. The purpose of the financial model is to supply an investment committee with financial parameters to be able to determine if the expenditure can be undertaken. The technical performance results of the 5 different technologies are also be reviewed and discussed.

The financial impact of the 5 introduced technologies is tested against set financial parameters. The purpose of this approach is aimed at enabling the business to establish if the money invested in the technology is a financial success or not. Note that all financial calculations are for a single mill only. The benefits (positive or negative) can thus be extrapolated to determine the impact on the full power plant. All calculations done in the financial model are in 2003 Rands. A detailed explanation of the financial calculations performed for a rotating throat installation is shown in section 5.4.1. Detail on the financial results for the other technologies tested, is captured in the t he appendixes.

Life expectancy of a RTA however proved to be far superior to that of the original stationary throats. The RTA can operate without replacement of any part for at least the ring life in a B&W 10.8E mill which is in the region of 60 000 hours, compared to the stationary throat slots that needs major repairs and/or replacement every 5 000 hrs.

5.2.2 Spider and spider guide wear plates The test result achieved was that the Triton wear plates could last at least 5 (five) times longer than the OEM Bennox wear plates. A figure of 25 000 running hours is achievable with these wear plates. The Triton wear plates are however not maintenance free as the wear plates must be shimmed after 10 000 hours of operation to keep the maximum allowable gap between the wear plates less than the OEM specified 15 mm.

5.2.3 Airbag installation The airbags installed performed excellently and at the time that the test was declared technically successful, the first set of airbags installed in mill 3A had just over 43 000 operating hours without any maintenance intervention. This set was however opened at about 25 000 hours to inspect the neoprene bladders. These were all in excellent condition.

It is thus safe to state that the airbags can last for at least 50 000 operating hours without maintenance being required. These running hours equates to replacement every 6 years. There is a big environmental spin- off from the implementation of this project as the airbags

running a non ductile casting with a relative thin wall thickness. The high chrome OEM’s claims in terms if the absorption of shock loads by their high chrome cast balls were true and no incidents were recorded.

5.3

FINANCIAL MODEL

The format of the financial model can be found in Appendix A with the manual / guideline of its usage in Appendix B. When studying the financial model the logic behind the model can be explained as follows: The financial indicators of a technology introduced are compared to the option of keeping the status quo. An imaginary line can be drawn between columns “G” & “H”. Columns “C” to “G” are where all financial implications of the new technology (project as indicated in the model) are inputted. To maintain status quo (keep all as is) are entered in columns “H” to “J”. The model then automatically calculates the financial indicators for technology introduced. These indicators are found in columns “J” to “M” in rows 6 to 9 and columns “N” to “Q” in rows 6 to 8.

The financial return as indicated in the financial model can be explained as follows (the position of the parameter is indicated [A1]; column A, row 1): •

Benefits to improve performance [M6]: This is the additional financial returns because of either improved efficiency in the Rankine cycle or the additional capacity delivered due to less load losses or a combination of both returns if both (efficiency and UCLF) are applicable to a model.

5.4

MODIFICATIONS

Each modification will be discussed separately as a merged approach will not indicate which modification is contributing what to the financial indicators. The rotating throat assemblies will be handled in full detail whilst only summaries of the other technologies will be presented. This in an attempt not to add too much detail, but to focus on the results obtained.

5.4.1 RTA installations For the financial analysis conventional stationary mill slots will be compared to the proposed rotating throat installation. The financial impact on each area will be evaluated separately. The financial model is captured in appendix “E”.

5.4.1.1

Efficiency: Because this is a maintenance case study, NO  efficiency improvement is assumed as mentioned in section 5.2.1.

5.4.1.2

UCLF / PCLF Improvement: The UCLF on a mill will reduce as the six-mill availability factor will increase where RTA’s are installed due to less time needed to do maintenance. The load losses for 2002 on the 10.8E mills at Kriel P/S equated to 2 217 MWh’s lost. This

5.4.1.6

Status Quo Replacement Cost: The cost of a new set of air blocks for a stationary throat will amount to R 120 000 and assume the installation costs are the same as for the RTA installation (R25 000). Total R 145 000 incurred in 2004.

5.4.1.7

Status Quo Operating Cost: None.

5.4.1.8

Status Quo Maintenance Cost: The maintenance cost will be broken down into labour and consumables. LABOUR COSTS: The work done by two welders and two artisan aids in 5 days is mandatory to maintain the stationary throats to an acceptable condition. This equates to R 4 941.56 (50.01A×1.7B×8C×5D  + 22.66E×1.7F×8G×5H) per annum. This cost will be carried till the end of the life of the power station. A = Cost for a qualified welder (R/hr) B = Factor to multiply R/hr to get cost to company C = 8 hr per day D = 5 days E = Cost for a artisan aid F=B G=C

5.4.2 Spider and spider guide wear plates 7.4.2.1 Financial Results (appendix F for full detail): •

IRR (Internal Rate of Return)

= 702.519%

•

Payback period

= 1 year

•

Benefit / Cost Ratio

= 3.461

•

NPV (Net Present Value)

= R107 000

5.4.3 Airbag installation 5.4.3.1

Financial Results (appendix H for full detail): •

IRR (Internal Rate of Return)

= 125.056%

•

Payback period

= 1 year

•

Benefit / Cost Ratio

= 7.491

•

NPV (Net Present Value)

= R459 000

5.4.4 Classifier cone 5.4.4.1

Financial Results (appendix G for full detail): •

IRR (Internal Rate of Return)

= 96.164%

•

Payback period

= 1 year

•

Benefit / Cost Ratio

= 3.487

•

NPV (Net Present Value)

= R147 000

5.5

FINANCIAL INDICATORS FOR THE INTRODUCED

TECHNOLOGIES Before it can be determined if a modification is successful in financial terms, an investment committee has to set standard benchmark parameters against which all projects will be measured. Typical figures used for the 5 modifications tested were: •

IRR > 20%.

•

Payback Period < 4 yrs.

•

Benefit / Cost > a 1.9 factor.

In table I it can clearly be seen that all modifications evaluated gives a better return of investment than the minimum requirements of the investment committee, except for the benefit/cost ration for the high chrome mill grinding media. With the high amount of maintenance labour cost not factored into the case study, it is felt that the return of investment is worth spending the money on this technology. This then implies that all modifications tested is a “financial go” and can be implemented in a structured program as and when required. Cash flow as well as plant condition will determine the program and phasing of implementation.

Financial indicator IRR (%)

RTA 27.508

Wear plates 702.519

Airbags 125.056

Classifier cone 96.164

High chrome 151.737

correctly, be implemented during a normal 5 000 hour mill service. The mill outage plan can then be used as program for installation of this technology.

By comparing the results with the tested modifications, it is observed that all five fits the criteria to be successful in f inancial terms.

5.6

SUMMARY OF TECHNICAL RESULTS AND FINANCIAL INDICATORS

In chapter 4 the technical performance results of the 5 different technologies introduced were discussed. As all the tests were done empirically, the results can be deemed to be extremely accurate. Every technology was first reviewed from a technical point of view to determine its performance. The costs, maintenance requirements, etc. were recorded for accurate data.

A financial model was developed and introduced. With this model it is possible to evaluate the financial success of each technology. Table 1 summarises these results and it is clear that all indicators have a positive outcome if compared to the requirements set out by the investment committee (refer to section 5.5).

The 5 000 hrs service intervals can now be challenged as all bottle necks were removed that warranted the mill to be serviced every 5 000 hrs. How this possible decision to extend

CHAPTER 6 – CONCLUSION 6.1

INTRODUCTION

The previous chapter clearly indicated that the 5 technologies tested on the test 10.8E B&W mill was a technical and financial success. The topic of this dissertation however introduces the concept of enhanced maintenance performance after successful implementation of tested and evaluated technologies.

This chapter will conclude the test results of the technologies introduced. An optimised maintenance strategy, to have the plant available, reliable and operable at acceptable performance levels with a cost saving will conclude the work for this dissertation. The conclusion will also pose the question: “Is there is still room for improvement from a performance (reliability & availability) or cost (maintenance complement) point of view”. If applicable, this will be suggested as future work in this field.

6.2

CONCLUSION

The original equipment manufacturer (Babcock and Wilcox) determined a scheduled maintenance service interval of 5 000 hours. The main components in the mill that influenced and determined this interval were the stationary throat slots and mill wear plates. These components were now proven to be improved with the introduction of new technologies:

advantage of this approach is that there is a cost saving added with maintenance and plant performance enhancement.

The airbags and classifier cone modification were introduced to improve the mill’s reliability and availability. The added benefit is life cycle cost saving with the new technologies. The high chrome mill grinding media was introduced for the purpose of extending mill grinding media life. This material change proved to be successful and with longer material life, the service intervals for ball changes and/or ads are extended. This has the benefit of being able to coordinate service intervals and ball change/add intervals. Refer back to section 4.2.1 where a 5 000 hour mill service will be done after 3 910 hrs due to the wear ion the mill balls. This modification poses a big opportunity for future maintenance cost savings due to a smaller required mill maintenance crew as the amount of required services will be less.

The author would like to suggest future work in this field where a condition based maintenance strategy needs to be investigated. The plant reliability, plant availability and then the cost of maintenance must be compared between a used base and predictive maintenance strategies. The most interesting thing will be to determine the optimum crew size to support a condition based mill approach as the inspection pillar in this approach will need to be extremely reliable for predictions, planning and scheduling of maintenance activities.

The technologies implemented was purposefully first tested for hardware performance and

be trended to be able to compare the maintenance results achievable by comparing a time based to a condition based maintenance strategy.

REFERENCES [1]

Varley, J. (April 1999). Looking for India’s lost generation. International Journal – Modern Power Systems.

[2]

Thuesen G.J. & Fabrycky W.J. (2001, ninth edition). Engineering Economy.

[3]

Simon, E., Bischoff, W. & Schuster, H. (August 2000). Recent Findings about the Use of an Extended Coal Range in a Hard Coal-Fired Steam Generator . International Journal – VGB Power Tech.

[4]

Jacobs, B. (1990). E-Type Mill Ball Reduction in Wall thickness. Report done by

Engineering

Investigations

Division

of

Eskom,

Project

Number

E89C4405/I. [5]

Williams,

K.N.

(1984)

Babcock

‘E’

Mills:

Achievements

&

Future

Developments. Marketing report for Babcock Engineering Contractors (Pty) Ltd. [6]

Angleys, M. & Gehrke, B. (September 1998). Development of the Modern EVT Bowl Mill. International Journal – VGB Power Tech.

[7]

Chercea, G., Radulescu, M., Mangu-Totolo, C., Ispas, C. & Belausov, G. (October 1999). Experiments for Choosing the Optimal Mill Type for Grinding the Middling Product of Romania. International Journal – VGB Power Tech.

[8]

Tigges, K.D., Bischoff, W. & Steinhage, T. (November 1998).  Ring-and-Roller Mills as Components of Modern Firing Technology . International Journal –

[17]

Williams,K. (Various discussions). Ex. Babcock Africa Commissioning Engineer.

[18]

Dugdale, K. (Various discussions) Ex. Mitsui Babcock Design Engineer.

[19]

International Energy Agency. (1995). Coal Pulverisers – Performance and Safety. Report number IEACR / 79.

[20]

EPRI Technical Report. (Volume 1). Pulveriser Maintenance Guide . Raymond Mills.

[21]

Nakajima, S. (1998). Introduction to Total Productive Maintenance .

[22]

Harrison & White. (n.d.). Capital Replacement.

[23]

Investec Client securities article for investors. (December 2007). The global market for coal .

[24]

Scott, D.H. (1995). Coal pulverisers – performance and safety . International Energy Association coal research report.

[25]

Peterson,

B.S.

Creating

a

successful

maintenance

council,

www.samicorp.com [26]

Idhammar, C. Current best practices. Available from www.idcon.com.

[27]

Hughes, R. Designing for the life cycle. Available from www.reliability.com.

[28]

Liptrot, D. Achieving maximum equipment reliability. Available from www.mtonlie.com.

[29]

Idhammar, C. What constitutes world-class reliability and maintenance ? Available from www.idcon.com.

[30]

Worsham, W.C. Criteria for CMMS to satisfy facility reliability needs. Available

APPENDIXES

APPENDIX A – KRIEL PERFORMANCE TESTS A1

Coal analysis:

For comparison purposes the following conclusions were made: •

Total moisture on Test 4 (T4) for mill 1C cannot be correct; as the mill outlet temp. during the test was ± 90°C, showing that a grab coal sample is not very accurate.

•

Hardgrove Index for all the tests is more or less the same and can be assumed to be constant at 59 for all tests.

•

The Calorific Value was only taken as a matter of interest.

•

The ash content of tests 2, 4 & 5 differ by less than a percentage point. The differences on the other tests are as follow: T1 = 28%, T3 = 10.6% & T6 = 11.1%. The ash content will influence wear rates more than mill performance and can be assumed to constant for all tests.

•

In conclusion the coal can be assumed as a constant for comparison purposes. No major deviations were noted.

A.2 Mill

Mill differential pressure: Test 1

Test 2

Test 3

Test 4

Test 5

Test 6

A.3

Mill motor power consumption:

Mill

T1

T2

T3

T4

T5

T6

2C (kW)

125

104

98

95

110

101

1C (kW)

131

110

113

104

110

119

Variance (%)

+ 4.8

+ 5.8

+ 15.3

+ 9.5

same

+ 17.8

Table 3 - Mill kW’s consumed

The mill motor kW’s is on average 8.9% higher with the rotating throat fitted.

A.4

PA fan motor power consumption:

Mill

T1

T2

T3

T4

T5

T6

2C (kW)

149

137

134

128

151

128

1C (kW)

146

137

131

125

149

125

Variance (%)

-2

Same

-2

-2

- 1.3

-2

Table 4 - PA fan kW’s consumed

The PA Fan kW’s is 1.6% lower for the RTA installation compared to the stationary throat.

A.5 Mill

Mill reject rate: T1

T2

T3

T4

T5

T6

A.6

Mill rejects density:

Mill

T1

T2

T3

T4

T5

T6

2C (kg/m3)

952

1092

1007

1001

1050

986

1C (kg/m3)

2102

2176

2121

2143

1996

1990

Variance (%)

220

200

211

214

190

201

Table 6 - Reject density

The RTA fitted mill (2C) rejects only pyrites  at an average of 206% more dense  product than mill 1C.

A.7

PF fineness (75 µm sieve):

Mill

T1

T2

T3

T4

T5

T6

2C (%)

66.3

70.2

73.1

77

67.4

73.3

1C (%)

62

68.3

70.6

75.4

67.6

69.2

Variance (%)

- 6.9

- 2.7

- 3.4

- 2.1

- 2.9

-5.6

Table 7 - 75 µm sieve fineness

The Stationary Throat (2C) pulverises the coal 3% finer  than the RTA installed mill (75 micron sieve).

A.9

PF fineness (300 µm sieve):

Mill

T1

T2

T3

T4

T5

T6

2C (%)

93.2

99.6

99.8

?

99.5

99.6

1C (%)

92.3

99.7

99.6

99.9

99.7

99.8

Variance (%)

+ 1.7

- 0.1

+ 0.1

+ 0.2

+ 0.2

+ 0.2

Table 9 - 300 µm sieve fineness

On the 300 micron sieve the RTA pulverises 0.4% finer than for the stationary throat.

A.10 Recirculation Load: This calculation is the difference between PA fan outlet pressure & mill differential pressure.

Mill

T1

T2

T3

T4

T5

T6

2C (kPa)

2

1.5

1.2

1.1

1.5

1.5

1C (kPa)

3.5

2.3

2.5

0.9

2.7

2

Variance (%)

+ 75

+ 53

+ 208

+ 81

+ 44

+ 33

Table 10 - Recirculation load

From the above it can be seen that the RTA mill has a much higher recirculation load than the normal stationary throat. This can be linked to the higher mill amps drawn for the RTA.

APPENDIX B – PERFORMANCE TESTS AT ADDITIONAL POWER STATIONS B.1

Performance test at Arnot Power Station

South Western Corporation rotating throat (5E) & Standard Loesche stationary throat (5F) The RTA shows the following: •

PF Fineness is marginally better

•

Mill Differential Pressure is lower

•

Higher throughput of ≈ 10% achieved

•

Reject Rate is abnormal high

•

Rejects bulk density is > 1 500 kg/m3

•

Very little difference in Power Consumption

Loesche “test” static throat (3D) compared to a standard Loesche stationary throat (3E): •

This test stationary throat was not part of the scope of the research of this project, but it was included as Loesche offered a static throat free of charge for testing purposes. This test proved no real performance improvement. Due to a lower mill differential pressure, the mill will have a higher throughput. All test comparisons were difficult as the controls on Unit 1 - 3 are very old and to get repeatable conditions is difficult at this power plant.

B.3

Performance test at Hendrina power station

Because of the test methodology at Hendrina power station, the tests cannot be used with a great deal of confidence. This as coal qualities were not linked to the results obtained. This implies that the results obtained can be used for basic trending, but detail conclusions cannot be made as coal quality would have varied.

Tests were done on Unit 5, mills A to F. At the time of the start of the tests, mill 5F was already fitted with a RTA. On the other mills on the Unit, tests were done before a RTA and after a RTA installation. Summary of the Hendrina power station results: •

The mill differential pressure is 33% lower on the RTA fitted mills

•

PA Fan amps are only 5% lower on the RTA fitted mills

•

The throughput on the mills increased by at least 5%, depending on the coal quality

•

PF fineness on the 75-µm sieve is on average 10.9% finer with the RTA fitted

•

Reject density is higher on the RTA mills and only stones (pyrites) are rejected

•

Reject rate is down by as much as 75% on the RTA mills

•

Mill amps are slightly higher due to the mill’s higher recirculation load

B.4

Performance test at Matla power station

At Matla power station 3 (three) throat types were tested and compared. It was comparing

APPENDIX D – FINANCIAL MODEL GUIDE PROJECT ECONOMIC EVALUATION MODEL MANUAL ALWAYS SAVE MASTER DOCUMENT WITH ZERO INPUTS AS REFERENCE SHEET Notes: •

If it is necessary to retain a copy of any case study, please use the ‘Save As’ function to save a copy to another folder using some variations of the project title!

•

All costs in the model are in Rands x 106 and in current Rands.

•

All money values must be inputted as current Rand values, irrespective of whether these are historic (actual) or future costs or benefits. Historic costs must be recalculated to the current year values, using CPI escalation rates from the “Inflation Data”  spreadsheet, before being inputted on the “Main” spreadsheet! 

•

No manual changes should under any circumstances be made in any of the cells other than those shaded blue in this Workbook!

OVERVIEW: Enter all the relevant data into the blue shaded cells [‘MAIN’ worksheet] FOR THE ANTICIPATED PROJECT Project title [Cell D4].

•

Project Implementation Cost [Cell E15:E52] - To include all costs (Project Management fees for example. The capital needed to realise the project plus all future costs for replacement/refurbishment.

•

Project Operating Cost [Cell F15:F52] - Direct Operating Costs relating to this project. Total operating costs associated with the project.

•

Project Maintenance Cost [Cell G15:G52] - Capture all maintenance costs for the lifecycle of the project. Maintenance costs projected for this project, total cash flow to be considered.

•

Project Revision Number [Cell C55].

TO MAINTAIN STATUS QUO • Status Quo Replacement Cost [Cell H15:H52] - Cost to replace the current system on a 1 to 1 basis at the time it cannot be deferred any more (current operating regime). •

Status Quo Operating Cost [Cell I15:I52] - Operating Cost relating to the current system.

•

Status Quo Maintenance Cost [Cell K15:K52] - Maintenance Cost relating to the current system.

EXPLANATION ON METHODOLOGY USED FOR THE FINANCIAL MODEL'S CALCULATIONS Column K L M

N O

P Q R S T U

Output ERA Project Cost amounts in nominal Rands ("Rands of the year") - The Total Project approval amount is reflected in Cell K55. Depreciation on Project expenditure - Capital Projects are depreciated over 5 years – Non capital tax benefit in the year of the expenditure Tax savings due to implementation of the project - calculated on total cash flow difference between the project (including performance benefits) and maintaining the status quo. Operating projected cost savings stemming from the project which will result in a possible reduction in future budgets. Calculated value – Column ‘I’ – Column ‘F’. Maintenance projected cost savings stemming from the project which will result in a possible reduction in future budgets ( Calculated value, = Column'J' Column'G') Additional energy produced as a result of improvements in efficiency and/or UCLF/PCLF stemming from implementation of the project. Additional cash inflow due to the increased energy output reflected in Column P. Additional costs incurred to generate the increased output reflected in Column P Net cash inflow/outflow per annum stemming from the implementation of the project. Present Value per annum of the amounts reflected in Column S. Cumulative NPV of the project based on the amounts reflected in Column T.

APPENDIX F – WEAR PLATES For the financial analysis original OEM wear plates (spider and guides) are compared to new Triton material. The financial impact on each area will be evaluated separately. The financial model is captured in appendix “DD”.

F.1

Efficiency: Because this is a one to one replacement of a component, no efficiency

gains can be claimed. F.2

UCLF / PCLF Improvement: As the OEM specified wear plates are changed during

every 5 000 hr service intervals; there is no UCLF or PCLF impact. F.3

Project Implementation Cost: The cost of a set of Triton wear plates is R 18 500.00

and will only be replaced every 25 000 operating hours. A set of shims will be installed every 3 years at a cost of R2 900.00 per set. F.4

Project Operating Cost: None as the operating of the mill will not change before and

after the installation. F.5

Project Maintenance Cost: None

F.6

Status Quo Replacement Cost: R16 400.00 for a set of original Bennox wear plates

very 5 000 operating hrs. F.7

Status Quo Operating Cost: None.

F.8

Status Quo Maintenance Cost: None

APPENDIX G – CLASSIFIER CONE For the financial analysis the newly modified classifier cone will be compared to the original OEM classifier cone. The financial model follows all the inputs.

G.1

Efficiency Improvement: None

G.2

UCLF/PCLF Improvement:

•

Assume 2 incidents of 18 hrs each will be prevented per year. This is to shut down a mill and clean the gladiator skirt

•

UCLF improvement

= (2A x 100G x 18B) / (100 x 24C x 475D x 365E x 0.9F) = 0.00096%

A – 2 Incidents per year on the specific mill B – 18 hrs per incident to clean the gladiator skirt C – 24 hrs / day D – 475MW load loss if a mill is out of service E – 365 days / year F – 90% availability of the generating unit G to A mill contributes 100MW’s to the production process

G.3 •

G.4

Project Implementation Cost: The cost of a classifier cone is R47 164.00 Project Operating Cost:

•  None

APPENDIX H – BALL LOADING SYSTEM For the financial analysis original hydro pneumatic loading cylinders are compared with airbags. The financial model follows all the inputs.

The present system is expensive to maintain and labour intensive. The life expectancy of 2 years is relatively poor in today's technologically advanced environment. 43 000 operating hours has been achieved and this will be used as 100% life for a set of airbags.

H.1

Efficiency Improvement: None

H.2

UCLF / PCLF Improvement: None

H.3

Project Implementation Cost:

•

The cost of an airbag will be R12 722.00 per loading cylinder

•

The commissioning cost will be R2 106 per mill

•

Total cost

= 8 x 12 722 + 2 106 = R102 982.00

H.4 •

H.5

Project Operating Cost: Nitrogen costs per year = R6 25.00 Project Maintenance Cost: The airbags will need to be replaced every 6 years.

APPENDIX I – MILL GRINDING MEDIA For the financial analysis high chrome rings and cast steel balls are compared with a set of high chrome rings and high chrome balls. •

Note: For the motivation of this modification, only look at the ball cycle material costs. All Maintenance service interventions will be left out of the financial equation at this point in time. This approach to be able to determine the impact of the new grinding media.

I.1

Steel ball and high chrome ring cycle regime (current philosophy)

I.1.1

Background on the Current Ball Cycle Regime:

With the current steel cast ball cycle regime, an average wear rate of between 105 – 115 hrs per millimetre is realized. This wear rate is empirically determined from measurements on mills balls in service. For calculation purposes, a conservative wear rate of

105 hrs/mm  on

the cast steel balls will be used.

I.1.2

Calculation:

In the steel ball and high chrome ring cycle regime, the critical ball dimensions are 768 (new size), 690 (filler ball size) & 640 (ball scrap size for the steel balls). The times between the sizes are as follow: •

768 - 690

= (768mm-690mm)×105hrs/mm = 8 190 hrs   (continuous

running hours)

I.2

High chrome ball and ring cycle regime (proposed ball cycle philosophy)

I.2.1

Background on the High Chrome Ball Cycle Regime:

With the high chrome mill grinding media test, empirical results on the ball and ring wear rate were obtained during mill inspections and/or maintenance interventions. Wear rates in the range of 301 hrs/mm down to 254 hrs/mm were realised on the mill balls. For calculation purposes on the 10.8E mill, a conservative wear rate of

250 hrs/mm  on

the high chrome mill

balls will be used.

I.2.2 Calculation: The ball sizes are the same as for the steel ball cycle regime. The times between the sizes are as follow: •

768 to 690

= (768 mm-690 mm)×250 hrs/mm = 19 500 hrs   (continuous

•

690 to 640

running hours)

= (690 mm-640 mm)×250 hrs/mm = 12 500 hrs   (continuous

Cycle 0 to 1

= 2 x 19 500 hrs

Cycle 2 to 3

= 2 x 12 500 hrs

running hours)

= 64 000 hrs For a complete ring cycle, 2 sets of 768 mm & 2 off 690 mm filler balls are expected to be

I.4

Mill Grinding Media Prices •

10.8E High Chrome Ring

= R207 023.00 per ring (2 x for set)

•

768 mm cast steel ball

= R21 222.24

•

690 mm cast steel ball

= R17 553.00

•

768 mm high chrome ball

= R31 398.00

•

690 mm high chrome ball

= R29 495.00

I.5

Financial Model Inputs (Theoretical) – Current Steel Ball Cycle Regime

Year

Reason for expenditure

Costs

for

Service @ yr

grinding media 0

1 sets of 10.8E rings & 11 off (207 023 × 2) +

Mill Cycle

0

0

768 mm balls (steel)

(21 222.24 × 11)

1

11 off 768 mm balls

11 × 21 222.24

1.3

1

2

11 off 768 mm balls

11 × 21 222.24

2.6

2

3

11 off 768 mm balls

11 x 21 222.24

3.9

3

5

12 off 690 mm balls (11 off balls Zero

5.2

4

from cycle 3 + 1 from cycle 0)

I.6

Financial Model Inputs (Theoretical) – High Chrome Ball Cycle Regime

Year

Reason for expenditure

Costs

Service @ yr

Mill Cycle

0

1 sets of 10.8E rings & 11 off (207 023 × 2) + 768 mm balls (high chrome)

0

3.1

1

6.2

2

8.2

3

(31 398 × 11)

1

Zero

2

Zero

3

0

1 sets of 10.8E rings & 11 off 31 389 x 11 768 mm balls (high chrome)

4

Zero

5

Zero

6

1 off 690 mm filler ball

7 8

29 495 x 1 Zero

1 off 690 mm filler ball

29 495 x 1

End of ring life at year 9.4

Table 17 - Proposed grinding media financial inputs

I.7

Added Benefits: