Coal Mill

Full text

(1)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 and Manufacturing Engineering) (specialisation Maintenance Engineering). UNIVERSITY OF JOHANNESBURG September 2008 Registration number: 200838373 Study leader: Dr Jasper L Coetzee Co-leader: Dr Johann Pretorius.

(2) 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 Holtshauzen – My wife; “For all her support and motivation”.. •. Mr. Keith Williams – ex. Babcock Design Engineer; “Assistance and advice when needed”.. •. Mr. Kevin Dugdale – ex. Mitsui Babcock Design Engineer; “Opening a new world for me”.. •. Mr. Brett King – Steinmüller Engineering Principal Engineer; “Assistance with the Financial Model”.. •. Mr. Chris Loots – Steinmüller Engineering Services Continuous Improvement Manager, “Assistance with the English language”.. •. Dr Jasper Coetzee – Study Leader.. ii.

(3) TABLE OF CONTENTS ACKNOWLEDGEMENTS...............................................................................................ii LIST OF TABLES ..........................................................................................................iv LIST OF FIGURES........................................................................................................ v LIST OF SYMBOLS ......................................................................................................vi ABBREVIATIONS ........................................................................................................ vii SUMMARY ....................................................................................................................... 1 OPSOMMING ................................................................................................................... 3 CHAPTER 1 – PROBLEM DEFINITION........................................................................... 5 1.1 PROBLEM DEFINITION ..................................................................................... 5 1.2 ROAD MAP FOR THE DISSERTATION ............................................................. 6 1.3 RESEARCH METHODOLOGY........................................................................... 7 1.4 KEY RESEARCH QUESTIONS.......................................................................... 8 1.5 SUMMARY ......................................................................................................... 8 CHAPTER 2 – BACKGROUND...................................................................................... 10 2.1 SA COAL AND ELECTRICITY ......................................................................... 10 2.2 COAL PULVERIZING FOR ELECTRICITY GENERATION .............................. 11 2.3 DEFINITION OF A MILL ................................................................................... 12 2.4 COAL PULVERIZATION .................................................................................. 13 2.5 BASIC MILL OPERATION ................................................................................ 16 2.6 SUMMARY ....................................................................................................... 17 CHAPTER 3 – LITERATURE SURVEY.......................................................................... 18 3.1 INTRODUCTION .............................................................................................. 18 3.2 PROFITABILITY OF STRATEGIC MAINTENANCE ......................................... 18 3.3 THE EVOLVEMENT OF MAINTENANCE STRATEGIES ................................. 19 3.4 PROFITABILITY BY INNOVATIVE MAINTENANCE ........................................ 22 3.5 TECHNICAL AND FINANCIAL FEASIBILITY ................................................... 23 3.6 SUMMARY ....................................................................................................... 25 CHAPTER 4 – SOLUTION DEVELOPMENT ................................................................. 26 4.1 TESTING PROCEDURES ................................................................................ 26 4.2 ROTATING THROAT ASSEMBLIES ................................................................ 26 4.2.1 PROCEDURE FOR MILL THROUGHPUT / PERFORMANCE TESTING .. 30 4.2.2 MAINTENANCE INTERVAL TESTING ON RTA’S..................................... 31 4.3 MILL SPIDER AND GUIDE WEAR PLATES..................................................... 32 4.3.1 TEST PROCEDURE FOR MILL SPIDER WEAR PLATES ........................ 33 4.3.2 TRITON WEAR MATERIAL ....................................................................... 34 4.4 MILL BALL LOADING CYLINDERS.................................................................. 35 4.4.1 AIRBAGS .................................................................................................. 35 4.5 CLASSIFIER CONE ......................................................................................... 36 4.6 HIGH CHROME MILL GRINDING MEDIA ........................................................ 38 4.7 SUMMARY OF TESTING PROCEDURES/METHODS .................................... 38 CHAPTER 5 – SOLUTION TESTING ............................................................................. 39 5.2 TECHNICAL RESULTS .................................................................................... 39 5.2.1 Rotating Throat Assembly.......................................................................... 39 5.2.2 Spider and spider guide wear plates.......................................................... 40 5.2.3 Airbag installation ...................................................................................... 40 5.2.4 Classifier cone modification ....................................................................... 40 5.2.5 High chrome mill grinding media................................................................ 40 5.3 FINANCIAL MODEL ......................................................................................... 41 5.4 MODIFICATIONS ............................................................................................. 42 5.4.1 RTA installations........................................................................................ 42 5.4.1.1 Efficiency ............................................................................................ 42 5.4.1.2. UCLF / PCLF Improvement: ............................................................... 42.

(4) 5.4.1.3. Project Implementation Cost:.............................................................. 42. 5.4.1.4. Project Operating Cost: ...................................................................... 42. 5.4.1.5. Project Maintenance Cost:.................................................................. 42. 5.4.1.6. Status Quo Replacement Cost: .......................................................... 43. 5.4.1.7. Status Quo Operating Cost:................................................................ 43. 5.4.1.8. Status Quo Maintenance Cost: ........................................................... 43. 5.4.1.9. Financial Results / Parameters from the Model:.................................. 43. 5.4.2 Spider and spider guide wear plates.......................................................... 44 5.4.3 Airbag installation ...................................................................................... 44 5.4.3.1 Financial Results (appendix H for full detail): ...................................... 44 5.4.4 Classifier cone ........................................................................................... 44 5.4.4.1 Financial Results (appendix G for full detail): ...................................... 44 5.4.5 High chrome mill grinding media................................................................ 44 5.4.5.1 Financial Results (appendix I for full detail):........................................ 44 5.5 FINANCIAL INDICATORS FOR THE INTRODUCED TECHNOLOGIES .......... 45 5.6 SUMMARY OF TECHNICAL RESULTS AND FINANCIAL INDICATORS ........ 46 CHAPTER 6 – CONCLUSION........................................................................................ 47 6.1 INTRODUCTION .............................................................................................. 47 6.2 CONCLUSION.................................................................................................. 47 APPENDIX A – KRIEL PERFORMANCE TESTS .......................................................... 53 A1 Coal analysis: ................................................................................................... 53 A.2 Mill differential pressure:................................................................................... 53 A.3 Mill motor power consumption: ......................................................................... 54 A.4 PA fan motor power consumption: .................................................................... 54 A.5 Mill reject rate: .................................................................................................. 54 A.6 Mill rejects density: ........................................................................................... 55 A.7 PF fineness (75 µm sieve): ............................................................................... 55 A.8 PF fineness (150 µm sieve): ............................................................................. 55 A.9 PF fineness (300 µm sieve): ............................................................................. 56 A.10 Recirculation Load: ....................................................................................... 56 APPENDIX B – PERFORMANCE TESTS AT ADDITIONAL POWER STATIONS ........ 57 B.1 Performance test at Arnot Power Station .......................................................... 57 B.2 Performance test at Duvha power station ......................................................... 57 B.3 Performance test at Hendrina power station ..................................................... 58 B.4 Performance test at Matla power station........................................................... 58 APPENDIX C – FINANCIAL MODEL ............................................................................. 59 APPENDIX D – FINANCIAL MODEL GUIDE ................................................................. 60 APPENDIX E – RTA FINANCIAL MODEL ..................................................................... 63 APPENDIX F – WEAR PLATES..................................................................................... 64 APPENDIX G – CLASSIFIER CONE ............................................................................. 66 APPENDIX H – BALL LOADING SYSTEM.................................................................... 68 APPENDIX I – MILL GRINDING MEDIA ........................................................................ 70 I.1 Steel ball and high chrome ring cycle regime (current philosophy)................... 70 I.1.1 Background on the Current Ball Cycle Regime: ......................................... 70 I.1.2 Calculation:................................................................................................ 70 I.2 High chrome ball and ring cycle regime (proposed ball cycle philosophy)......... 71 I.2.1 Background on the High Chrome Ball Cycle Regime: ................................ 71 I.2.2 Calculation: ...................................................................................................... 71 I.3 Ring Life ........................................................................................................... 71 I.4 Mill Grinding Media Prices ................................................................................ 72 I.5 Financial Model Inputs (Theoretical) – Current Steel Ball Cycle Regime........... 72. ii.

(5) I.6 I.7. Financial Model Inputs (Theoretical) – High Chrome Ball Cycle Regime........... 73 Added Benefits: ................................................................................................ 73. iii.

(6) 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. iv.

(7) 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. v.

(8) 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. vi.

(9) 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 Kg/m. Internal Rate of Return 3. 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. Maintenance management system. MTTR. Mean time to repair. MW. Mega Watt. MWh. Mega Watt hour. No. Number. NPV. Net Present Value. O&M. Operating and Maintenance. OEM. Original Equipment Manufacturer. PA. Primary Air. PCD. Pitch Centre Diameter. PCLF. Planned capability loss factor. PF. Pulverised Fuel. PSM. Power Station Manager. R&M. Renovate and Modernisation. R/t. Rands per ton. vii.

(10) 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. viii.

(11) 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. The new technologies tested were thus evaluated from a technical and financial point of view. Each technology was implemented at different time periods and nowhere was any tests performed in parallel on a single mill. To get approval from an investment committee for release of money for tests/modifications, technical and financial assumptions need to be made regarding the performance parameters of the modification/change. Once a modification is being tested, actual plant data can be used as inputs into the execution phase of the modification as assumptions can be replaced with test data. A financial model was developed to “test” the financial feasibility of the proposed changes/modifications. With new technology successful implemented in the plant the current maintenance strategies for maintenance interventions can be re-evaluated as the proposed modifications removed historic barriers that determined the current used based maintenance intervals. An example is where a mill is taken from service to replace or repair the stationary air throat (typically every 5 000 operating hours). With the newly applied technology (rotating throat. 1.

(12) 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.. 2.

(13) 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. Die nuwe komponente getoets sal dus geïvalueer word uit ‘n tegniese en finansiële oogpunt. Om goedkeuring te kry van ‘n beleggingskommitee, om gelde beskikbaar te stel vir die toetse/modifikasies, moet talle tegniese en finansiële aannames bemaak word rakende die beoogde verandering/modifikasie. As die fisiese toetse afgehandel is, kan werklike toetsdata gebruik word om die finale implementering van die verandering/modifikasie te motiveer. ‘n Finansiële model is ontwikkel waarmee die finansiële lewensvatbaarheid van veranderinge getoets kan word. Met nuwe tegnologie suksesvol geïmplimenteer in ‘n aanleg kan die huidige instandhoudings aksies herevalueer word. Hierdie omdat historiese bottelnekke in die instandhouding en bedryfs omgewings die oorspronklike instandhoudingsfrekwensies bepaal het. ‘n Voorbeeld is waar statiese lugblokke vervang is met ‘n roterende lugblok konstruksie. Die oorspronklike komponent kon slegs 5 000 ure bereik voordat dit vervang moes word in vergeleke met ‘n moontlike 60 000 ure vir die nuwe komponent.. 3.

(14) 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.. 4.

(15) 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 thus more maintenance is required on the equipment. For any technical problem there are technical solutions, the problem being that investors expect a certain return of investment. This implies that the plant can be modified, but it has to be done in such a manner that the modifications done must guarantee a certain return on investment. Most modifications and changes on a pulveriser need to be tested empirically as data are not readily available, and the coal being pulverised may differ substantially between different plants. For this reason the implementation of modifications need to be tested financially and technically before the full implementation of such a project or modification get the green light for full implementation.. 5.

(16) 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 between actual performance parameters and the assumptions made in the concept phase of the modification can now be weighed against each other. This comparison in the end determines if the modification will be fully implemented.. 1.2. ROAD MAP FOR THE DISSERTATION. How plant maintenance strategies have evolved and how important finances are in industry will be the focus points of chapter 3. The conceptual undertone is that any anticipated change or modification has to be tested for technical and financial success before it can be fully implemented. Only a positive outcome in terms of both parameters will ensure the successful implementation of the proposed change if human safety is not a factor to be considered. For the dissertation to be successful, a roadmap will be developed to dictate the manner in which the research will be undertaken. Section 2.3 will pose research questions that have to be answered to ensure good feedback on the research undertaken. The word success was. 6.

(17) 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 “key”- / research – questions posed in this thesis will be used as guideline for the research undertaken. Once the research questions have been answered, a merged approach (technical and financial) of the technology introduced will be taken to assess the impact of the introduction of the new technology on the test mill. The new technology (more modern components) identified that will be tested for technical and financial successes are: •. Rotating throat assemblies compared to the traditional stationary mill’ throat slots.. •. Replacement of the OEM Bennox mill wear plates with a new material type called Triton.. •. The introduction of a new generation classifier cone without typical gladiator skirt fingers.. •. Replacement of the hydro-pneumatic loading cylinders with the patented “Airbags” from Lockhyd Industries.. •. Replacement of the steel mill balls with high chrome mill balls.. 7.

(18) 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.. •. A more reliable classifier cone arrangement.. •. The hydro-pneumatic mill loading cylinders has a very poor life expectancy and needs to be replaced with new cylinders or an alternative technology that is more reliable and cost-effective.. •. The introduction of mill grinding element material that is completely different to the currently technology being applied in the RSA.. 1.5. SUMMARY. From the key questions posed, it is evident that there is a lot of testing involved to determine the success of the introduced technology. This chapter acts as guide to the research undertaken. With the key questions in mind and the technical and financial testing of each technology being considered, a structured approach is given to the complex tests and results.. 8.

(19) 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.. 9.

(20) 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).. •. Hendrina (10 x 200 MW = 2 000 MW total).. •. Kendal (6 x 686 MW = 4 116 MW total).. •. Kriel (6 x 500 MW = 3 000 MW total).. •. Letabo (6 x 618 MW = 3 708 MW total).. •. Majuba (3 x 657 + 3 x 713 = 4 110 MW total).. •. Matimba (6 x 657 MW = 3 942 MW total).. •. Matla (6 x 600 MW = 3 600 MW total).. •. Tutuka (6 x 609 MW = 3 654 MW total).. Note 1: This power station is in the process of a capacity increase project where the units are upgraded from 350 to 400 MW per unit. In May 2008 unit 3 was complete with this process with the other 5 units following suite in the next two years. Note 2: Unit 1 at Camden was not on commercial load in May 2008 as turbine spares delayed the return to service of the plant. Commercial load was achieved on 31 July 2008.. 10.

(21) 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.. If maintenance costs are not kept within acceptable levels, the profitability of a utility can change from positive to negative. An example of this is when half of Arnot Power Station (near Middelburg, Mpumalanga) was partially mothballed in the 1990’s. This decision was based on the fact that the characteristics of the coal properties caused the mills to wear so fast that it was not economically feasible to keep the units in production. The pulveriser plant at Arnot is installed with redundancy where 5 out of 6 mills need to be in operation to deliver full boiler load. The wear on the mill components were so high that the mill maintenance interventions started to overlap and the Maintenance Department could not finish maintenance activities quick enough before the next mill was due for a service. Due to the unavailability of the redundant plant, the generating capacity of the unit was negatively affected. Arnot Power Station could thus not produce at full capacity when needed. Note 3: In May 2008 Grootvlei Unit 1 was on commercial load with unit 2 being in the commissioning phase. Note 4: Unit 9 at Komati was being commissioned and tested in May 2008 with commercial load for the unit planned for July 2008.. 11.

(22) 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 Engineering.. Figure 1 - Alstom Deep Bowl Mill [20]. 12.

(23) 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 figure 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. Figure 2 - Basic layout of a Babcock and Wilcox 10.8E mill [15]. 2.4. COAL PULVERIZATION. Figure 3 schematically shows how coal is processed in a pulverising plant. A coal feeder feeds the mill with raw coal from the coalbunkers above the mill. The feeder regulates the quantity of raw coal being supplied to the mill. A typical feeder used to deliver coal to a 10.8E B&W E-type mill is a Stock volumetric or gravimetric feeder. For a volumetric feeder coal feeder density (ρ) is assumed to be of constant nature. A gravimetric feeder weighs the. 13.

(24) 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 mill. The mill housing is cylindrical in shape and is bolted into the support top plate. Inside the mill housing are the drive yoke, bottom and top rings, grinding balls and mill spider.. (iii). Classifier - The PF is accelerated through the primary classification process and then the flow path passes into the classifier section of the mill. The PF classifier section is cylindrical in shape and houses the classifier blades on the top termination point of the cone.. (iv). PF Outlet turret – PF is graded in the classifier and exits the mill through this outlet turret. The raw coal pipe passes through the middle of this turret, through the top part of the cylindrical section of the mill top where the raw coal is fed onto the mill table.. 14.

(25) Coal Bunker. Coal Gate. Feeder. Mill. Primary Air Seal Air. Pulverised Fuel. Secondary Air. Burner. Core Air. Boiler. Figure 3 - Schematic flow of coal in a pulverizing plant. 15.

(26) 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 are fine enough to the classifier section of the mill.. •. The particles that are not fine enough are pulled back by gravity and are reintroduced into the grinding zone. This is the primary classification zone of the mill.. •. The particles that are fine enough to reach the mill classifier section are subjected to a sudden direction change by the classifier blades.. •. This causes coal particles that are not of the correct fineness to be re-introduced through the classifier cone into the grinding zone. This is referenced as secondary PF classification.. •. The coal particles of the correct size grading then exit the mill through the pulverised coal outlet or also called the mill turret.. •. These particles or PF are transported through PF pipe work to the PF burners of the boiler where they combust.. 16.

(27) Figure 4 - General arrangement drawing of an 8.5E B&W mill. Any boiler is designed to accommodate PF within pre-determined parameters. The pulveriser set-up is of paramount importance for correct and efficient combustion. The milling plant of a power utility can be regarded as the carburettor of the complete boiler process. If this piece of plant is not fully optimised, the “vehicle” will not reach design performance indicators.. 2.6. SUMMARY. The crux of this dissertation is based on the study of the change in maintenance strategy to improve the life of the above mechanical components. By not changing the process performance parameters, the components and its maintenance are changed to achieve optimum performance from the mill. By a change of maintenance, it refers to the type of maintenance being done, i.e. the maintenance strategy of the components that are reevaluated. Optimum performance equates to expected process parameters at an acceptable cost.. 17.

(28) 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.. •. Hardware installed.. •. Type of coal supply being pulverised.. •. Maintenance staff complements and experience.. •. Resources and tools available.. In the above the assumption is made that all the utilities operates at the same load factors. If the Management of a Business Unit sees the maintenance expenditure as a “necessary evil” as quoted by Sherwin [13] and not a “contributor to profits”, the net profit margins expected from investors will never be realised. To operate a power utility at acceptable reliability and availability levels, the correct amount of money spent on maintenance will be determined by the annual profit margins realised while keeping the hardware in the same and/or better condition. Coetzee [10] uses the Failure Rate to illustrate how reliability decreases over time with the plant aging. The more failures experienced, the more proactive money will have to be spent to achieve the desired output requirements from the plant.. 18.

(29) 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 developed superior systems for its application.. •. In the 1960’s preventative maintenance became well-known and was utilised extensively.. •. During the 1980’s the concept of predictive or condition-based maintenance [21] was introduced.. •. From then to now the process and strategy applied to maintenance activities were refined to the diagram of Coetzee [10] (figure 5).. 19.

(30) Maintenance Strategies. Design-out Maintenance. Preventative Maintenance. Corrective Maintenance. Used based Maintenance. Predictive Maintenance. Scheduled Overhaul. Scheduled Replacement. Component Replacement. Block Replacement. 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 availability levels. As can be seen there are 3 (three) basic maintenance philosophies: •. Design-out Maintenance: The design is altered to rid the system of the items limiting expected levels of output. Another option could also be to modify the operating procedure. The need for maintenance is decreased or eliminated. This philosophy is chosen. when. you. cannot. increase. reliability. with. additional. preventative. maintenance. MTTR (mean time to repair) cannot be decreased (planned as well as unplanned activities). When product rate cannot be increased or maintained without decreasing reliability or availability. •. Preventative Maintenance: These maintenance activities are implemented to maintain the system before unplanned failures occur.. •. Corrective Maintenance: Traditional breakdown maintenance or wait to failure strategy.. 20.

(31) 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 system’ inputs. WC Worsham [30] quotes in this regard: “Like any tool, CMMS must be maintained and used properly to achieve the greatest possible potential”. The old saying of garbage in, garbage out, applies. D. Mather [31] in an article Fundamentals of Maintenance planning suggests a review process to ensure data with integrity is gathered, captured and stored. WC Worsham [30] in the above mentioned article, Criteria for CMMS to satisfy facility reliability needs, states that although data is not captured in a company’s balance sheet, it is a big asset and must be treated as such. A CMMS could change an existing maintenance management system (MMS) from a manual human driven system (hard copy system) to a computerised driven system. If the CMMS is properly maintained the impact on the maintenance organisation will be less if one or two key individuals within the maintenance organisation should terminate his / her service with the relevant company. (Knowledge and plant information will not leave the company along with those individuals).. 21.

(32) 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: “Management research confirms that innovative firms – those that are able to use innovation to improve their processes or to differentiate their products and services – outperform their competitors…..” Ferguson [32] re-affirms this thought: “Enhanced reliability of assets is a critical element for the survival of today’s organizations.” This proves that a strategic plan needs to be mapped out in an R & M initiative for a company to be able to stay competitive. Tigges [8] makes it very clear that during the last few years, further development of the firing technology for hard coal has been governed by keywords “more efficient’, “simpler” and “more cost effective”. This forces a strategic thinking process for milling plants, some thirty years after its introduction into power plants, as the milling plants needs to be considered high-tech units. A milling plant must not only meet actual requirements, but also has to have a high availability and reliability at a reasonable cost. It must be realised that theoretical calculations methods have limitations and the practical testing of mills in power plants will continue to be an essential developmental step / tool. The reason for this testing method is because mills traditionally were designed, based on empirically developed curves [17] &. 22.

(33) [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 will be to motivate a technical change. New technology has to be well proven before such a committee will approve its implementation. The successful implementation of a project is thus determined by the technical and financial parameters.. Thuesen and Fabrycky [2] addresses the above mentioned very well by quoting as follows: “Engineers are confronted with two important environments, the physical and the economic.” They clearly spell out that the success of a utility is measured in economic terms. “The usual function of engineering is to manipulate the elements of one environment, the physical, to create value in the second environment, the economical [2].” This concept drives the initiatives of an engineer to always assess the implications of a project/product from both posed environments, otherwise the success of the project/product can be doomed, because the economic parameters were not favourable. In the beginning of every design life cycle economic feasibility has to be tested before valuable resources are used only to realize before implementation that the economic parameters will not give positive returns. Hughes [27] mentions the new concept of designing for life cycle where the entire life cycle of a. 23.

(34) 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 costs.”. Before the list above is scrutinised an organization goes into a recognition phase that there is a problem or there exists an opportunity. “Once aware of the problem or opportunity, actions can be taken to solve or take advantage of it [22].” Financial mathematical calculations are necessary to be able to evaluate and accommodate all the factors listed by Harrison & White. Financial assessment introduces an important factor in financial calculations, which is interest. The time value of money has to be taken into account to be able to make sound decisions in a technical environment.. 24.

(35) 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.. 25.

(36) 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 maintenance (welding inside the mill) also poses a possible health risk to the welding personnel performing the work.. 26.

(37) 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.. Figure 5 - Schematic drawing of a Rotating Throat Assembly (RTA). When rotating throats were first introduced into South Africa, the initial objective was: “The use of Rotating Throats to enhance Milling Plant Performance”. This implied that with the. 27.

(38) 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). Bottom ring. Throat vanes (rotating). Figure 6 - Rotating Throat Assembly in a Babcock and Wilcox E-type mill. The current maintenance philosophy being applied on the B&W mills at Kriel Power Station is time based (used based maintenance with scheduled interventions), with a mill service interval of 5 000 hrs. This interval is derived from the level of wear on the throat slots as well as for the replacement requirement of the mill wear plates. This is however not the only maintenance intervention needed on the mills, as the mill ball wear rate also impacts on the time intervals at which the mills are serviced. The mill ball cycle regime is schematically presented in figure 9.. 28.

(39) 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 manpower and the time it takes for an activity like the opening of a mill for a ball change, the maintenance activity is “drawn forward” to be performed as part of the ball change/add. Mill cycles 768-mm (11 off balls) 0. 1. 2. 3. Ball add 690-mm (12 off balls). 4. 5. 6. 640-mm. Figure 7 - Schematic representation of the mill cycles. 29.

(40) 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.. 30.

(41) 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 analysed or were not conclusive). (ii) To be able to compare “apples with apples”, it is important to look at the complete mill system. This comment reflects to the system resistance of PF pipe work from the mill outlet to the PF burner inlet. Two identical mills with the same PF outlet system needs to be considered for analysis, otherwise false results will be obtained. The ideal situation would be to test all the performance parameters on a mill with the stationary throats, shut it down, install a RTA and then re-test. Due to installation pressures the Plant Manager did not have the luxury of having a mill readily available for testing before and after a RTA installation.. 4.2.2 MAINTENANCE INTERVAL TESTING ON RTA’S One of the most important aspects of an RTA installation is the life of the new technology compared to the original stationary throats. As was shown in section 4.2.1 the stationary throats needed a preventative maintenance intervention every 5 000 hrs, otherwise the mill’s performance becomes unacceptable. With a RTA installed the trend on the performance of. 31.

(42) 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 onto the ring spider. The spider heads and spider guide are both fitted with wear plates on either side. The wear plates keep the spider in position with the mill grinding media going through its various mill cycles. Because the spider guides are longer wear plates, it accommodates the up and down movement of the spider heads as the mill grinding media is running through the various mill cycles. The original mill spider wear plates are manufactured to material specification BS 4659. These plates are hardened and tempered to 56 RC (Rockwell C).. 32.

(43) Figure 8 - General arrangement drawing of mill spider guide wear plates. 4.3.1 TEST PROCEDURE FOR MILL SPIDER WEAR PLATES A test was performed at Kriel Power Station where a set of the original material (BS 4659) wear plates and guides were run to 7 450 hours (Figure 11). At 7 450 hrs the wear plates were completely worn down. In this condition the wear plate clearances between spider head and spider guide were very big (40 mm in total). Running a mill in this condition is very bad practice as the risk of breaking spider heads is high due to the large amplitude of the resultant oscillating motion of the spider. The thickness of a new wear plate (head and guide) is 38 mm.. 33.

(44) 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. Figure 9 - Worn spider and guide wear plates. 4.3.2 TRITON WEAR MATERIAL A weld on wear material called Triton was tested on a 10.8E mill. The process of manufacture is to weld wear resistant material onto a base material on a cylindrical cone. Typically a 300 WA material is used for the base material and a hard-facing weld material for the wear resistant material. When the automated welding process is complete, the cylinder is cut and flattened to a plate that can then be formed and machined into required shapes. The test procedure of the Triton material was to install the wear plates and then trend the wear on these over time to see if acceptable results could be obtained. The maximum allowable wear of the wear plates will be determined by the combined wear on the spider. 34.

(45) 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 ageing system. A new concept used in Europe for the suspension of trains and busses is by using an airbag. This design was adjusted and patented for a mill airbag loading system. This system is only operated with nitrogen and no oil, compared to the original loading cylinders’ oil and nitrogen systems. The test procedure would be to install a set of airbag cylinders and run it for an extended period of time and then compare the reliability of the system with the original installed hydropneumatic loading cylinders. The neoprene bladder in the airbag has a rolling action and no moving parts and therefore the OEM claims no maintenance is required for extended periods of time.. 35.

(46) Figure 10 - Airbag loading cylinder. 4.5. CLASSIFIER CONE. Chapter 2 explained the pulverization process within a mill and the function of the mill classifier cone was also briefly explained. The reader might ask why the classifier cone is being evaluated as it is a stationary component with no effect on mill availability. The reliability of a mill is however affected by the classifier cone as the bottom section of the cone (outlet section that discharges PF back into the grinding zone of the mill) sometimes 36.

No results found