Practical low-cost method to sustain mine compressed air savings

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Practical low-cost method to sustain

mine compressed air savings

J Taljaard

orcid.org 0000-0002-6665-4475

Dissertation submitted in fulfilment of the requirements for the

degree

Master of Engineering in

Mechanical Engineering

at the

North-West University

Supervisor:

Dr JC Vosloo

Graduation ceremony: May 2019

Student number: 24165964

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ABSTRACT 2

ABSTRACT

Title: Practical low-cost method to sustain mine compressed air savings

Author: J. Taljaard

Supervisor: Dr J. Vosloo

School: North-West University, School for Mechanical Engineering

Degree: Master of Engineering (Mechanical)

Keywords: Compressed air systems, compressed air inefficiencies, compressor control, pressure set-points, deep-level gold mining

The future of the South African mining industry is uncertain as a result of declining ore grades, reduced gold production and increasing operational costs. However, the industry remains a significant contributor to the country’s economy. Efforts to ensure a sustained growth path for the mining industry are therefore vital for its survival, as well as the communities that depend on it. Mining companies in South Africa need to focus on reducing their operational costs to remain competitive.

One such area offering potential is the production of compressed air, usually the largest consumer of electricity on a deep-level mine. Mines can reduce their operational costs by optimising compressed air production and curbing oversupply. Infrastructure, instrumentation and capital expenditure is limited. As a result, complex and expensive solutions are impractical, and the need for a simple, low-cost solution to match compressed air supply with the required demand is evident.

A step-by-step methodology was developed that is easy and inexpensive to implement. The methodology focused on identifying cost-saving initiatives to reduce compressed air network inefficiencies. These inefficiencies were evaluated, and a suitable solution strategy was developed. Appropriate baselines were developed to determine the cost-saving impact after implementation.

The methodology was implemented on the compressed air network of Mine A, located in the Free State province of South Africa. The method utilises setpoint control, effective compressor selection and compressed air prediction. The solution strategy was incorporated into an existing Energy Management System (EnMS), with no additional infrastructure costs. The EnMS platform was used to communicate the compressor running schedule to the control

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ABSTRACT 3

room operators, based on the demand requirements. This was shown visually by a message on a computer screen to either start or stop a specific compressor.

The implemented solution strategy proved to have no negative effects on the production of the mine, while annual energy cost savings of R1,1-million was shown to be viable. This proves that the solution strategy can be sustainably implemented, and that compressed air savings can be achieved and sustained through practical and low-cost methods.

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ACKNOWLEDGEMENTS 4

ACKNOWLEDGEMENTS

I would like to thank the following parties for their support and guidance for the duration of this study.

• Firstly, I would like to give thanks to Jesus Christ my Lord and Saviour. “I can do all things through Christ who strengthens me” – Philippians 4:13. Thank you for the opportunity You gave me and Your continued love and mercy.

• I would like to thank my study mentor, Dr Handré Groenewald, for his support, guidance and open-door policy throughout this study.

• I would like to thank my study leader, Dr Jan Vosloo, for his guidance throughout this study.

• I would like to thank my co-workers for their support and valuable inputs, especially Mr Pieter Peach and Dr Charl Cilliers, for their personal time to assist me during this study. I am truly grateful.

• I would like to thank my parents, Francois and Christelle Taljaard, for their love, support and motivation. Thank you for making me the person that I am today.

• I would like to say thanks to ETA Operations (Pty) Ltd, Enermanage and its sister companies for their financial support and resources to complete this study.

• Lastly, I would like to thank my fiancé, Therien Roodt, for her support, motivation and love. I am grateful to have you in my life. Thank you for the late-night coffees and treats.

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TABLE OF CONTENTS 5

TABLE OF FIGURES

Figure 1-1: Mine worker underground at a typical gold mine1_{... 14}

Figure 1-1: Shares of the nominal GDP (adapted from [1]) ... 15

Figure 1-2: World gold production vs SA gold production (adapted from [7], [6], [9]) ... 16

Figure 1-3: Gold grade in ores mined in South Africa (adapted from [6], [9]) ... 17

Figure 1-4: Electricity price increase and inflation (%) of South Africa (adapted from , [11], [14]) ... 18

Figure 1-5: Productivity and wages (adapted from [17]) ... 19

Figure 1-6: Electricity consumption breakdown for each type of mining system ... 20

Figure 1-7: Typical compressed air requirements of a normal production day [21], [26] ... 22

Figure 1-8: Underground haulage with train tracks ... 26

Figure 2-1: Compressor types based on main categories [34] ... 28

Figure 2-2: Typical compressor operating conditions ... 29

Figure 2-3: Multi-stage centrifugal compressor at a mining complex ... 30

Figure 2-4: Operational schedule of a typical gold mine [20], [43] ... 31

Figure 2-5: Illustration of a typical compressed air network with end users ... 32

Figure 2-6: Typical refuge chamber layout (adapted from [46]) ... 35

Figure 2-7: A typical characteristics curve of a compressor (adapted from [50]) ... 37

Figure 2-8: Compressed air control valve ... 40

Figure 2-9: Auto compression at different discharge pressures ... 45

Figure 2-10: Annual cost of a leak ... 51

Figure 2-11: Typical KYpipe simulation ... 54

Figure 2-12: Typical SolidWorks flow simulation ... 54

Figure 2-13: Graphical User Interface of PTB [29] ... 55

Figure 2-14: Underground image at a typical deep-level mine ... 64

Figure 3-1: Diagram of the simplified methodology ... 66

Figure 3-2: Compressed air pressure measuring points of a typical gold mine ... 69

Figure 3-3: Typical electrical power baseline ... 70

Figure 3-4: Example of peak scaling on the power usage of a compressed air network ... 71

Figure 3-5: Compressed air supply side ... 72

Figure 3-6: Typical control of underground control valves ... 74

Figure 3-7: Parameters to calculate the desired surface pressure ... 75

Figure 3-8: Compressor intensity at different guide vane angles ... 77

Figure 3-9: Future surface pressure trend ... 81

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Table of Figures 8

Figure 3-11: Trimming compressor start process ... 84

Figure 3-12: Mining shaft in the Free State Province ... 90

Figure 4-1: Google Earth image of Mine A ... 92

Figure 4-2: Gold plant pressure control and compressor layout ... 94

Figure 4-3: Mine A compressed air network ... 95

Figure 4-4: Electrical power baseline ... 96

Figure 4-5: Typical pressure and power profile of a compressor at Mine A ... 98

Figure 4-6: Guide van angle of running compressor at Mine A ... 98

Figure 4-7: Downstream pressure control on 23L ... 99

Figure 4-8: Measurement locations of critical mining levels at Mine A ... 100

Figure 4-9: Surface pressure requirement for all critical mining levels at Mine A ... 101

Figure 4-10: Surface pressure vs the required pressure of 23L ... 102

Figure 4-11: Proposed surface pressure set-point at Mine A ... 103

Figure 4-12: Mine A compressor intensity ... 104

Figure 4-13: Surface pressure trend for Mine A... 105

Figure 4-14: Simulation power data compared to actual power data ... 108

Figure 4-15: Optimal required compressors throughout a typical production day ... 109

Figure 4-16: Simulated power usage compared to the scaled baseline ... 109

Figure 4-17: REMS recommendation platform for Mine A ... 110

Figure 4-18: Simulated power usage compared to the actual power usage after project implementation ... 111

Figure 4-19: Pressure measurements of the critical mining level after implementation ... 112

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List of Tables 9

LIST OF TABLES

Table 1-1: Required process parameters [15] ... 23

Table 2-1: Pressure requirements of pneumatic equipment [28], [31], [32], [45] ... 33

Table 2-2: Initiatives to reduce supply-side inefficiencies [22], [25], [32], [33], [43], [54], [55] ... 39

Table 2-3: Flow regime based on the Reynolds number [57] ... 47

Table 2-4: Parameters used to calculate the financial cost of an air leak... 50

Table 2-5: Process parameters to accurately monitor the performance of a compressed air network [15] ... 52

Table 2-6: Portable measurement instrumentation data ... 52

Table 2-7: Simulation Package evaluation [8], [61] ... 56

Table 2-8: Critical analysis of previous studies ... 63

Table 4-1: Mine A compressor information ... 93

Table 4-2: Mine A – KPI summary... 97

Table 4-3: Mine A compressor intensity at different guide vane angles ... 104

Table 4-4: Simulation input and output data for Mine A ... 107

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List of Equations 10

LIST OF EQUATIONS

Equation 2-1: Compressor motor power calculation [15], [52], [57]... 41

Equation 2-2: Compressor electrical power calculation [31], [57] ... 42

Equation 2-3: Ideal gas equation to calculate the density air [57] ... 42

Equation 2-4: Mechanical energy of a compressor [15], [31] ... 43

Equation 2-5: Pressure increase due to auto compression [59] ... 44

Equation 2-6: Darcy-Weisbach equation – Head loss [57], [60] ... 46

Equation 2-7: Reynolds number calculation [57] ... 46

Equation 2-8:Darcy friction factor – Laminar flow regime [57] ... 47

Equation 2-9: Colebrook-White equation - Turbulent flow regime [57] ... 47

Equation 2-10: Darcy-Weisbach equation – Pressure loss [57] ... 48

Equation 2-11: Power wastage due to air leaks [31], [27] ... 49

Equation 2-12: Mass flow rate of compressed air through a leak [31] ... 49

Equation 3-1: Energy reduction ratio calculation [28]... 67

Equation 3-2: Baseline adjustment calculation (adapted from [63]) ... 70

Equation 3-3: Calculating the required surface pressure ... 76

Equation 3-4: Calculating pressure drop from surface to point of use ... 76

Equation 3-5: Equation for a straight line... 79

Equation 3-6: Future surface pressure value ... 80

Equation 3-7: Relative error percentage calculation – MRD (adapted from [29]) ... 86

Equation 3-8: Relative error percentage calculation – MAE (adapted from [29])... 86

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List of Abbreviations 11

LIST OF ABBREVIATIONS

Abbreviation Description

CPI Consumer Price Index

DCS Dynamic Compressor Selector ERR Energy Reduction Ratio ESCos Energy Services Companies GDP Gross Domestic Product GUI Graphical user interface

IDM Integrated Demand Management KPI Key Performance Indicator PLC Programmable Logic Controller PTB Process Flow Toolbox

RCA Root Cause Analysis

REMS Real-time Energy Management System SA South Africa

SCADA Supervisory Control and Data Acquisition VSD Variable-Speed Drive

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Symbols and Units 12

SYMBOLS AND UNITS

Symbol Description Unit

𝑚̇𝑙𝑒𝑎𝑘 Mass flow through leak 𝑘𝑔/𝑠

ℎ𝑓 Head loss due to friction 𝑚

𝐴𝑙𝑒𝑎𝑘 Cross-sectional area of leak 𝑚2

𝐵𝑎𝑑𝑗𝑢𝑠𝑡𝑒𝑑 Baseline adjustment factor −

𝐶𝑑 Discharge coefficient of a leak −

𝐶𝑝 Specific heat capacity of compressed air 𝐾𝐽/𝑘𝑔. 𝐾

𝑃𝑎𝑏𝑠 Absolute air pressure 𝑘𝑃𝑎

𝑃𝑎𝑣𝑔(𝐵) Average peak drilling power consumption (Original baseline) −

𝑃𝑎𝑣𝑔(𝑖) Average peak drilling power consumption (Current) −

𝑃𝑐𝑜𝑚𝑝 Electrical power of the compressor 𝑘𝑊

𝑃𝑖𝑛 Atmospheric pressure 𝑘𝑃𝑎

𝑃𝑙𝑒𝑎𝑘 Atmospheric air pressure at leak 𝑘𝑃𝑎

𝑃𝑚𝑜𝑡𝑜𝑟 Electrical power of the motor 𝑘𝑊

𝑃𝑡(0) Present surface pressure value 𝑘𝑃𝑎

𝑃𝑡(30) Future surface pressure value after 30 minutes 𝑘𝑃𝑎

𝑃𝑡(−30) Surface pressure value 30 minutes ago 𝑘𝑃𝑎

𝑇𝑖𝑛 Inlet air temperature 𝐾

𝑇𝑙𝑒𝑎𝑘 Air temperature at leak 𝐾

𝑈𝑚 Fluid velocity over pipe cross section 𝑘𝑔/𝑚3

𝑊𝑐𝑜𝑚𝑝 Mechanical energy of the compressor 𝑘𝐽/𝑘𝑔

𝑓𝐷 Darcy friction factor −

𝑚̇ Mass flow rate of air 𝑘𝑔/𝑠

𝑛𝑐𝑜𝑚𝑝 Compressor efficiency −

𝑛𝑚𝑜𝑡𝑜𝑟 Efficiency of the compressor motor −

𝑝0 Final pressure 𝑘𝑃𝑎

𝑝𝑖 Initial pressure 𝑘𝑃𝑎

𝑝𝑜𝑢𝑡 Discharge pressure 𝑘𝑃𝑎

𝑧0 Final altitude 𝑚

𝑧𝑖 Initial altitude 𝑚

∆𝑃 Pressure loss due to friction 𝑃𝑎

𝐷 Pipe diameter 𝑚

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Symbols and Units 13

𝑅 Gas constant 𝐾𝐽/𝑘𝑔. 𝐾

𝑅𝑒 Reynolds number −

𝑇 Air temperature 𝐾

𝑉 Average air velocity 𝑚/𝑠

𝑒 Surface roughness 𝑚

𝑔 Gravitational acceleration 𝑚/𝑠2

𝑘 Specific heat ratio of compressed air 𝐾𝐽/𝑘𝑔. 𝐾

𝑛 Polytrophic constant for isentropic compression 𝑘𝑔/𝑠

𝑢 Dynamic viscosity 𝑁𝑠/𝑚2

𝑣 Kinematic viscosity 𝑚2/𝑠

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Chapter 1 14

CHAPTER 1

1

1_{H. Swart, “Class action for silicosis payout tarnishes gold industry,” Mail & Guardian, 2013. [Online]. Available:}

https://mg.co.za/article/2013-01-25-00-class-action-for-silicosis-payout-tarnishes-gold-industry. [Accessed: 06-Nov-2018].

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Chapter 1 15

1 Introduction

1.1 Preamble

This chapter explores the current South African mining environment, as well as some of the challenges faced by the industry. An energy breakdown of a typical deep-level gold mine and the critical role of compressed air within this environment are presented. Relevant background information is used in the formulation of a problem statement. The objectives of the study are presented in such a way as to address the concerns highlighted in the problem statement. The chapter concludes with an overview of the remainder of the document.

1.2 Importance of mining in South Africa

The mining sector remains an important contributor to the South African economy. As shown in Figure 1-1, the mining industry contributes approximately R8 for every R100 produced by the national economy [1]. This industry also employs around 2.5% of the entire workforce in South Africa [1]. Although these figures may not seem to be impressive, it must be considered that mining is more important in some regions than others. In the North West province, mining contributes 33% of the total production and employs 16% of the total provincial workforce [2]. Mining is therefore a major contributor to the economy of the North West province.

Figure 1-1: Shares of the nominal GDP (adapted from [1])

The future of the South African mining industry is uncertain, therefore efforts to ensure a sustained growth path are vital for its survival, as well as the survival of the communities that depend on it [1], [2]. Challenges influencing the mining industry have become more

Agriculture 3% Finance 19% Government 18% Trade 15% Manufacturing 13% Transport & Communication 10% Mining 8% Personal services 6% Construction 4% Electricity 4%

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Chapter 1 16

pronounced in recent years, leading to the planned retrenchment of more than 10 000 mining employees in 2017 [3], [4].

1.3 Gold production in South Africa

Although global gold production increased by 18% since 2005, South Africa’s gold production has seen output decline by over 50% for the same period [5], [6]. When examining production trends in greater detail, it is found that the declining production trend for South African gold mining has been ongoing since the 1970s [7], [8]. Figure 1-2 shows the trends for the global gold mining sector against South Africa’s declining production [7], [6], [9].

Figure 1-2: World gold production vs SA gold production (adapted from [7], [6], [9])

This decline resulted in South Africa losing its position as the top gold producer in the world, a position held for almost a century [10]. South Africa is currently the 7th _{largest gold producer} globally, contributing only 4.4% to global production in 2016 [5], [6]. The country once responsible for producing two-thirds of the world’s annual gold, could only muster 135 tonnes in 2017 [6]. The challenges facing the industry causing lower production output will be elaborated within the following section.

1.4 Challenges influencing the South African gold mining industry

The South African gold mining industry faces challenges that are unique to the country. These challenges have a major impact on productivity which consequently influences the sustainability and profitability of the industry. The impact of the relevant challenges is discussed in terms of product and processing streams in the next sections.

0 50 100 150 200 250 300 350 400 0 500 1000 1500 2000 2500 3000 3500 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 S ou th A fri ca n go ld prod uc ti on [t on ne s] Worl d go ld prod uc ti on [ ton ne s] Year

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Chapter 1 17 1.4.1 Product stream by means of ore grades

An exponential trend can be expected for the grade of ore mined and the corresponding energy consumption [9]. This means that for lower grade ore, the amount of energy and resources used are increasing per unit of gold produced. At present, most of the higher-grade gold deposits are exhausted and mines exploit lower grades [11]. The decreasing trend in gold ore grades mined in South Africa can be seen in Figure 1-3.

Figure 1-3: Gold grade in ores mined in South Africa (adapted from [6], [9])

Several studies indicate that according to the current trend, an average ore grade of 0.9 g/t can be expected by 2050 [9], [12]. This is an indication that most of the current mining operations may become marginal in the near future [11], [13]. The increase in energy consumption, coupled with general input costs, greatly increases the production costs of gold [11]. 0 1 2 3 4 5 6 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 G old grad e [g/t t on ] Year

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Chapter 1 18 1.4.2 Process stream

1.4.2.1 Electricity costs

Eskom’s electricity costs for South Africa have increased 356% in the last decade2_{, whilst} inflation over the same period was 74%3_{. This means that the electricity price increased 4.5} times that of inflation over the same period. The comparison of electricity price increase versus inflation over the same period can be seen in Figure 1-4.

Figure 1-4: Electricity price increase and inflation (%) of South Africa (adapted from 4_{, [11], [14])} As seen from Figure 1-4, the electricity price increased with a compounded rate higher than that of inflation for the past decade. The combination of declining ore grades and increasing electricity costs is the key driver that may severely impact the South African mining industry [11].

2_{S. Moolman, “350% increase in a decade: how expensive is electricity in South Africa compared to other countries? -}

PowerOptimal,” 2017. [Online]. Available: http://www.poweroptimal.com/350-increase-decade-expensive-electricity-south-africa-compared-countries/. [Accessed: 13-Mar-2018].

3_{A. Slabbert, “Eskom tariff increase could cost us R1bn – Sibanye - Moneyweb,” 2017. [Online]. Available:}

https://www.moneyweb.co.za/news/companies-and-deals/eskom-tariff-increase-could-cost-us-r1bn-sibanye/#to-comments. [Accessed: 13-Mar-2018].

4_{ESKOM, “Tariff history,” 2017. [Online]. Available:}

http://www.eskom.co.za/CustomerCare/TariffsAndCharges/Pages/Tariff_History.aspx. [Accessed: 24-Mar-2018].

0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 0.0% 5.0% 10.0% 15.0% 20.0% 25.0% 30.0% 35.0% 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 A v erag e Infl ati on S ou th A fri ca (C P I) E sk om pri ce i nc rease [ % ] Year

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Chapter 1 19 1.4.3 Productivity and wages

In South Africa, 95% of the total gold production comes from underground mines. South Africa has some of the worlds’ deepest mines, reaching depths of up to 4 km [15]. The increase in mining depth increases the traveling times of employees and consequently decreases productivity [16]. Throughout a typical year, South African gold mines are productive for approximately 274 days [8], [17]. During this time, workers are only active for two thirds of the day [17]. This severely influences the production rate and consequently increases operational costs [8], [16].

Coupled with a decrease in productivity, gold mines are adopting essential socio-economic solutions. To maintain strong employee labour relations, gold mines have increased employee remuneration during the last decade. Figure 1-5 illustrates the decline in production and increase in labour costs per employee from 1990 to 2017 [17].

Figure 1-5: Productivity and wages (adapted from [17])

The extrapolated data in Figure 1-5 reveals a major concern, as labour costs have increased, and employee productivity has decreased. Research conducted by the Minerals Council of South Africa (previously known as the Chamber of Mines of South Africa) revealed average labour productivity declined by 35% since 2007 [11], [6]. Even with employee numbers declining in the sector, earnings, however, increased over the same period from R 14.7 billion in 2007 to R 28.5 billion in 2016 [6]. Reducing expenditure in this section of the mining sector, although possible, seems highly unlikely. Mining companies in South Africa therefore need to focus on reducing operational costs.

0 50 100 150 200 250 300 0 20 40 60 80 100 120 140 160 180 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 R ea l lab ou r co sts pe r kg go ld produced kg go ld pe r employ ee Year

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Chapter 1 20

1.5 Electricity consumption of a typical gold mine

1.5.1 Breakdown of major consumers

The mining sector consumes approximately 15% of the total electricity demand in South Africa, while the gold mining industry accounts for 7.5% of the total [18]. As seen in Figure 1-6 the largest electricity consuming system on a South African gold mine is compressed air, responsible for roughly 21% of the mining industry’s total electricity demand. This is due to increasing mining depths and new developing areas [17]. Compressed air networks can reach extreme lengths, to supply the developing ends with sufficient compressed air.

Figure 1-6: Electricity consumption breakdown for each type of mining system

In order to reduce operational costs, energy-saving initiatives should be implemented to reduce the electricity demand of compressed air systems.

1.5.2 Energy management on gold mines

Gold mines previously relied heavily on Eskom’s Integrated Demand Management (IDM) model to fund electricity cost-saving initiatives. This was important due to the large capital expenditure associated with these projects5_{. Energy service companies (ESCos) would} usually mediate between Eskom and the mines by means of project implementation and management [8]. Recently, Eskom applied changes to their IDM model, cutting all

5_{“EscomodelFAQ.” [Online]. Available: http://www.eskom.co.za/sites/idm/Business/Pages/EscomodelFAQ.aspx. [Accessed:}

09-May-2018]. 4% 6% 8% 10% 14% 18% 19% 21% Other Hoisting Processing plant Buildings and hostels Ventilation and cooling Pumping

Mining process Compressed air

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Chapter 1 21

implementation funding for projects [19]. This becomes problematic as marginal mines depended largely on the IDM model for capital funding of electricity cost-saving initiatives6_. Therefore, it is suggested that mines should consider alternative cost-saving initiatives that do not require capital funding.

1.6 Importance of compressed air within the gold mining industry

Compressed air plays an important role in the gold mining industry, since it is relatively safe to handle and easy to use. Mines use pneumatic equipment underground, due to the risk of electrical equipment igniting the methane gas that is sometimes found in underground mines [18], [20].

Compressed air is essential for the production process, as rock drills are one of the largest compressed air consumers [21]. During typical drilling shifts, compressed air consumption is approximately 70% more than during off-peak periods [15].

Unfortunately, the production process of compressed air is a highly inefficient process [22]. This is due to high energy losses during the electrical energy conversion process. Approximately 95% of the energy used during the conversion process of transforming electrical energy to pneumatic energy is wasted [18], [23], [24].

1.6.1 Demand requirements

Operational efficiency of pneumatic equipment is based on pressure and flow demands from the compressed air network [25]. It is important to note that pressure and flow requirements for each compressed air consumer are different. The compressed air supply must be sufficient to meet the pressure and flow requirements of all the consumers on the network [15]. The pressure requirements of deep-level mines change according to the changes in operational

schedules. The operational schedule consists of drilling, blasting and cleaning shifts [21], [26].

1.7 Problem statement

1.7.1 Compressed air requirements

The challenges within the mining industry emphasise the importance of cost-saving initiatives on the compressed air networks of mines. As mentioned in Section 1.6.1, a mine’s compressed air flow and pressure requirements change throughout a typical production day. It is important to identify compressed air demand requirements of different consumers, during

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Chapter 1 22

different operational schedules. Mines tend to oversupply the compressed air network during low demand periods [15], [27], [28]. Oversupply wastes compressed air and should be avoided at all costs. Figure 1-7 illustrates this by comparing the demand requirements of a typical production day with an oversupplying compressed air network [21], [26].

Figure 1-7: Typical compressed air requirements of a normal production day [21], [26]

Demand and supply requirements must be met to ensure optimal use of pneumatic equipment [25]. Pneumatic equipment is one of the key drivers within the production process. If compressed air requirements for drilling are not met, the processing plant will receive no ore to process, consequently stalling production of the entire mining company [15]. However, if the compressed air network is oversupplied, energy wastages occur [9].

1.7.2 Integrating demand and supply requirements

Mines can reduce their operational costs by accurately matching the demand and supply requirements of compressed air networks [29]. A suitable way to improve and sustain operational costs is with the use of a Dynamic Compressor Selector (DCS) controller. The DCS controller simulates the required network pressure and consequently determines the most efficient compressor combinations for optimal compressor utilisation [15], [30].

For the DCS controller to operate optimally, various process parameters are required. These process parameters ensure that the DCS controller simulates the correct control commands [15]. Table 1-1 summarises the required process parameters for optimal utilisation of the DCS controller [15]. 0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 P ressu re (kP a) Time (h) Oversupply

Demand Requirements Supply

Clean in g Ch an ge Ov er Dril lin g Bla sting Clean in g

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Chapter 1 23

Table 1-1: Required process parameters [15]

Location Process Variables Unit

Compressor

Discharge flow rate [m3_/s] Discharge pressure [kPa] Inlet guide vane position [%]

Blow-off valve position [%] Running status [0/1]

Loaded status [0/1] Electrical power consumption [kW]

Consumer

Pressure requirement [kPa] Valve downstream pressure [kPa] Compressed air consumption [m3_/s]

From Table 1-1, it is evident that the accuracy of the DCS controller is limited to the availability of certain process parameters. On marginal mines with limited infrastructure and instrumentation, such data is often unavailable, making it impossible to utilise the DCS controller.

Due to financial constraints discussed in Section 1.4, mines have limited funds available for instrumentation upgrades. This emphasises the need for a practical low-cost method to sustain mine compressed air savings. By reducing the financial burden through optimised compressed air control, the South African gold mining industry may sustain their competitiveness.

1.8 Objectives

The aim of this study is to develop a practical low-cost method to sustain compressed air savings of mines with limited instrumentation and infrastructure. The objectives of this study are summarised as follows:

• Identify compressed air demand and supply requirements • Identify demand and supply cost-savings initiatives

• Implement an improved compressed air control strategy, with no capital funding • Develop a compressor scheduling strategy for sustainable energy savings

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Chapter 1 24

An improved compressed air control strategy will be developed, without influencing the pressure supplied to the mining levels. The main objective of the study is to optimise the compressed air supply based on the demand requirements. The effect of maintenance costs on the compressors due to control strategy alterations will not form part of the scope of this study.

1.9 Overview of study

This study consists of five chapters. A short overview of the chapters will be discussed within this section.

Chapter 1

This chapter discusses the importance of mining for the South African economy, along with the challenges faced by the gold mining industry. The electricity breakdown of a typical gold mining industry will identify the importance of cost-saving initiatives on mine compressed air systems. The importance of compressed air within the gold mining industry will provide a better understanding on how the system operates. The challenges within the gold mining industry can be used to formulate a problem statement. The chapter concludes with a set of objectives to reduce inefficiencies on a mine compressed air network.

Chapter 2

This chapter provides a general overview of a compressed air network on a deep-level gold mine. The compressed air network can be divided into two sections: compressed air supply and demand. The supply and demand sides are characterised and evaluated to identify inefficiencies on the compressed air network. This section focuses on initiatives to reduce identified compressed air inefficiencies. The chapter concludes by evaluating previous cost-saving studies on compressed air networks. The need for the study is formulated within this chapter.

Chapter 3

This chapter focuses on developing a methodology to reduce the operational costs of mines through optimised compressed air control. The methodology consists of evaluating and identifying inefficiencies on a mine compressed air network. The inefficiencies are then rectified with an optimised compressed air solution strategy.

Chapter 4

The methodology formulated during Chapter 3 is implemented on an actual case study. The solution strategy will be verified, implemented and validated to ensure that the objectives

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Chapter 1 25

discussed in Chapter 1 are met. The results of the implemented strategy will also be interpreted throughout this chapter.

Chapter 5

This chapter concludes the study by comparing the results of the solution strategy to the objectives as formulated within Chapter 1. The problem statement will also be addressed. The chapter concludes by making recommendations for further studies.

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Chapter 2 26

CHAPTER 2

Figure 1-8: Underground haulage with train tracks7

8

7_{S. Solomons, “Murray & Roberts awarded R3.8 billion in new underground mining projects,” 2018. [Online]. Available:}

https://www.miningreview.com/murray-roberts-awarded-r3-8-billion-new-underground-mining-projects/. [Accessed: 08-Nov-2018].

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Chapter 2 27

2 Mine compressed air systems

2.1 Introduction

Before the problems defined in the previous chapter can be addressed, compressed air systems on deep-level mines should first be investigated and understood. This first step is known as characterisation. This includes gathering knowledge on basic components within the system, as well as how these components interact with each other.

After characterisation is complete, evaluation of the system can commence. Evaluation consists of understanding how changes to individual components can affect the entire system. Different control methods are investigated to determine how changes can be enacted upon this system. The mathematical modelling and simulation of compressed air systems and their components are presented to better understand the exact effect these changes will have on the system. Different methods of data gathering are presented, as the evaluation phase is heavily dependent on the quality of information being used.

Lastly, a comprehensive literature survey on previous studies is conducted to determine the scope of work presented in this field. These studies will be evaluated based on their objectives and main results. A summary based on the findings, methods used, shortcomings and recommendations of previous research are presented to highlight the need for this study, after which the chapter is concluded.

2.2 Characterise mine compressed air networks

2.2.1 Preamble

As discussed in Chapter 1, compressed air systems are one of the most expensive utilities within the mining sector. The challenges within the mining industry emphasise the importance of cost-saving initiatives on mine compressed air networks. A critical understanding of the network and its operation is required before a suitable solution strategy can be developed. Compressed air networks of deep-level mines can be divided into two main sections: compressed air supply and demand sides [15], [31], [27]. The supply side of the compressed air network constitutes the machinery that is used for the generation of compressed air [28]. The demand side of the compressed air network constitutes the different compressed air consumers [32], [33].

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Chapter 2 28 2.2.2 Characterise supply side

Compressors on surface are used to generate compressed air to the entire network. There is a large range of compressors available on the market which can be divided into two main groups; namely, continuous flow and positive displacement compressors [34]. Continuous flow compressors increase the velocity of the air to increase the pressure, while positive displacement compressors change the volume of the air to increase the pressure. Figure 2-1 shows a diagram of different compressor types that fall under the two main categories.

Figure 2-1: Compressor types based on main categories [34]

Selecting the optimal compressor type depends on the application at hand. Figure 2-2 shows the different operating conditions for various compressor types [34]. The comparison is based on the pressure ratio and the corresponding flow in cubic feet per minute (cfm) of the different compressor types. The pressure ratio is calculated by comparing the outlet air pressure after compression with the inlet air pressure to the compressor.

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Chapter 2 29

Figure 2-2: Typical compressor operating conditions

Multi-stage centrifugal compressors are most commonly found in the South African mining industry, due to their supply capacity and compact size [32], [35], [36]. The large operating range of this compressor suits the dynamic nature of compressed air networks within the mining industry

The compressor consists of intricate components and operating systems to produce compressed air at higher efficiencies. A wide range of literature is available to optimally use these systems [15], [37], [38]. For this study, the mechanics around these compressors will not be investigated. However, the factors that influence the power usage of these compressors will be discussed further throughout this section. A typical multi-stage centrifugal compressor used at one of the mining complexes in South Africa can be seen in Figure 2-3.

0 10 20 30 40 P ressu re rat io Flow [cfm]

Axial flow compressor Positive

displacement

Centrifugal compressor

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Chapter 2 30

Figure 2-3: Multi-stage centrifugal compressor at a mining complex 9

Several multi-stage centrifugal compressors are usually connected in parallel to meet the compressed air demand requirements of deep-level mines [15].

2.2.3 Characterise demand side

Compressed air pipe networks are fed from energy-intensive compressors to sustain demand requirements. The diameters of these pipe networks are usually in the range of 150 mm – 700 mm, reaching lengths of up to 40 km [32].

As discussed in Section 1.6.1, compressed air flow and pressure requirements must be met for optimal use of pneumatic equipment. Various mining operations (for instance workshops, processing plants and the mining shaft) use different pneumatic equipment with discrete compressed air requirements [25]. It is therefore important that the compressors supply the compressed air network with sufficient compressed air, attaining all requirements of the various consumers [39].

A typical gold production day consists of three main shifts: drilling, blasting and cleaning shifts [40]. The compressed air requirements differ for each of these shifts as illustrated in Figure 2-4.

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Chapter 2 31

Figure 2-4: Operational schedule of a typical gold mine [20], [43]

A typical production day starts at 04:00 when workers on surface are transported underground by means of a service cage [15]. A service cage is an enclosed cage that vertically elevates mine workers underground and back to surface. In some cases, the working places for some workers can exceed traveling distances of 4 km [41]. As soon as the workers reach their working places, the drilling shift begins.

Drilling shift

During the drilling shift, mine workers use rock drills to drill holes in rock faces [42]. Pneumatic drills are most commonly used to drill the holes. Air from the compressed air network is mainly used to supply the pneumatic drills. As indicated in Figure 2-4, the compressed air pressure requirement during this shift is the highest throughout the production day. This is due to the large amount of active pneumatic drills [33]. When the workers are finished drilling, they need to travel back to the station where the service cage will elevate them back to surface.

Blasting shift

Explosives are placed within the drilled holes and wired to the centralised blasting panel. The explosives need to be charged manually by responsible mining personnel. The mining levels should be evacuated, and all mining personnel should be on surface before the explosives can be detonated. During this time, the demand for compressed air is reduced and the pressure requirement during this shift is the lowest throughout the typical production day [15], [32]. Once detonated and the dust has cleared, mine personnel travel back to their working areas to start with the cleaning shift [15].

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Chapter 2 32 Cleaning shift

The blasted ore is gathered by face and gully winches and collected by pneumatic loaders. The loaders transport the ore to hoppers (train carts), where they are further transported by other material-handling equipment to the processing plant on surface [27]. Pneumatic loaders are large consumers of compressed air; therefore, during the cleaning shift, high pressures of compressed air are required [28]. The importance and requirements of compressed air during the different mining procedures will be discussed within the following section.

2.2.4 Compressed air requirements

Throughout a typical production day, compressed air is also being consumed on surface. The main surface compressed air consumer is processing plants, consuming approximately 2500 𝑚3/ℎ at a pressure range of 420kPa – 500kPa. Figure 2-5 shows the different compressed air consumers at a typical mining complex.

C o m p re ss o rs Surface consumers Shaft Rock drill Loader Refuge bay Hopper

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Chapter 2 33

Pressure is the most important factor when using pneumatic equipment [43]. When the pressure of the compressed air is higher than recommended, pneumatic equipment may fail, or inefficiencies may occur. Conversely, if the pressure is too low, the pneumatic equipment may not operate at all [44].

It is therefore important to identify the requirements of the different pneumatic equipment on the compressed air network. Vermeulen, Bredenkamp, Marais and Cilliers summarised pressure and flow requirements during a typical production day [28], [31], [32], [45]. The pressure requirements of different pneumatic equipment at a typical mining shaft are shown in Table 2-1.

Table 2-1: Pressure requirements of pneumatic equipment [28], [31], [32], [45]

Pneumatic equipment Pressure requirements [kPa] Image (Where applicable)

Rock drills 500 – 600

Rock loaders 400 – 500

Pneumatic actuators 400

Pneumatic cylinders 400

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Chapter 2 34 Rock drills

Pneumatic rock drills are used underground to drill holes in rock faces of the gold reef. The holes are filled with explosives, and during the blasting shift, the explosives are detonated and the rock is blasted into workable fragments.

Loaders

After the rock has been blasted into workable fragments, the rocks are moved by pneumatic loaders. The loader lifts the rocks from the ground into its hull. The rocks are then transported to surface by means of additional rock-handling equipment.

Pneumatic actuators

Surface and underground compressed air or water supplies are regulated by control valves. These control valves can be controlled remotely by means of pneumatic actuators. The pressure downstream of the control valve should be adequate for optimal operation. The pneumatic actuator can be manually opened or closed during an unplanned event.

Pneumatic cylinders

Pneumatic cylinders are used to mechanically open and close chute doors and loading boxes during the ore transportation phase10_.

Refuge chambers

Refuge chambers are located underground and are used as assembling points in case of an emergency. During such an event, mine workers travel to the nearest refuge chamber. The refuge chamber is equipped with a fire extinguisher, phone, emergency supplies, portable toilet, benches and a compressed air line [46]. The compressed air line ensures that the pressure in the chamber is higher than the atmospheric pressure.

The positive pressure of the chamber ensures that smoke and toxic gases are kept outside the chamber [46]. The supplied pressure of the compressed air line should be maintained at 200 – 300 kPa [47]. This will prevent toxic gases from entering the refuge chamber. Figure 2-6 shows a typical layout of an underground refuge chamber.

10_{“FLSmidth - Measuring and Loading Stations.” [Online]. Available:}

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Chapter 2 35

Figure 2-6: Typical refuge chamber layout (adapted from [46])

The South African mining regulations state that the pressure inside the refuge chamber should be maintained at a positive pressure [46]. This means that the refuge chamber should be supplied with compressed air even if no mine workers are underground.

This section provided a basic overview of the mining process, along with characterising the supply and demand sides of a compressed air network. Capacity control methods on the supply side can ensure that the demand requirements of the compressed air network can be met.

After characterising the compressed air network, the network should be evaluated. The aim of the evaluation step is to identify inefficiencies on the supply- and demand-sides of the compressed air network. During the evaluation step, the purpose of the study should be kept in mind. The purpose of this study is to reduce the operational costs of mines through optimised compressed air control. Therefore, fundamental calculations on the compressed air network are required to determine the effect of operational changes on the power consumption of the compressors.

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Chapter 2 36

2.3 Evaluating mine compressed air networks

2.3.1 Preamble

In this section, inefficiencies are evaluated based on the supply and demand sides of a compressed air network. The evaluation process focuses on various techniques to analyse the performance of a compressed air network.

2.3.2 Improving the supply side of compressed air networks

Capacity control methods can be implemented to accurately match the supply with the required demand. It is important to note that when these methods are implemented, the supply will match the required demand without influencing the production of the mining complex [15]. Capacity control enables mines to optimise their compressed air networks. Capacity control methods are used to control the supply flow of the compressors to accurately match the demand flow requirements. If the demand requirements are not met, production may be affected. However, if the compressed air network is oversupplied, energy wastages occur. The following methods can be used to alter the discharge flow of the compressors:

• Compressor combinations [32], [48], [49] • Loading / off-loading compressors [15], [32] • Compressor guide vane control [50]

• Blow-off control [51] • Speed control [52]

The identified methods will be discussed in detail throughout the rest of this section.

Compressor combinations [32], [48], [49]

Compressors are started and stopped to match the demand requirements. It is important to note that when a compressor is stopped, there is a certain time delay until it can be started again. The time delay on various compressors differs from one to another. The compressor should only be stopped in cases when it can be justified that the compressor is used unnecessarily.

Loading / off-loading compressors [15], [32]

Loading and off-loading compressors involves isolating compressors from the compressed air network. A blow-off valve located at the compressor is opened and the compressed air is released to the atmosphere. When the blow-off valve is opened, the compressor is powered without the need for air compression. During compressor off-loading, the electrical power

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Chapter 2 37

consumed by the compressor is reduced and the compressor need only overcome basic friction.

Compressor inlet guide vane control [50]

The swirl pattern of the inlet air can be changed by controlling the inlet guide vane angles of the compressors. The delivery capacity of the intake air is dependent on the swirl pattern. In other words, the inlet guide vane angles regulate the angle of the intake air, consequently adjusting the supply flow [25]. For example, if the guide vane angle of a compressor is at a maximum position (100%), maximum flow and pressure will be delivered. Subsequently, if the guide vane angle of a compressor is at a minimum position, minimum flow and pressure will be delivered.

The performance of a fixed speed compressor can be increased with the use of guide vane control. For example, guide vane control can reduce the inlet air flow of the compressor within acceptable pressure ranges. The delivery flow of the compressor at different pressure ranges can be defined by the characteristic curve of that specific compressor. A typical characteristic curve of a compressor at a mining complex is shown in Figure 2-7.

Figure 2-7: A typical characteristics curve of a compressor (adapted from [50])

Indicated in Figure 2-7, the pressure ratio of the compressor is plotted against the inlet volume ratio. The operating range of the compressor is between the surge and choke limits. The design point of the compressor indicates the pressure range and flow capacity of the compressor at a maximum guide vane angle.

0 20 40 60 80 100 120 140 40 60 80 100 120 140 D isc ha rge pres su re, he ad or pres su re rat io [% ]

Inlet volume flow ratio [%]

Surge limit Design point 105% 100% 95% 90% 85% Choke (Stonewall) Optimum efficiency Constant efficiency curves

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Chapter 2 38

The choke limit is reached when the air velocity at one of the impellers is at sonic conditions (air flow reaches Mach 1) [53]. When the choke limit is reached, the compressor is unable to increase the pressure at an increased air flow. During this period, the efficiency of the compressor is reduced.

Compressor surging occurs when the compressor cannot handle the outlet system pressure (backpressure). The generated flow of the compressor reverses and a sudden change in axial thrust occurs. When the surge limit is reached, various components of the compressor can be damaged. Most of the compressors at mining complexes have blow-off control to avoid the occurrence of surging.

Blow-off valve control [51]

When a compressor is near its surge limit, a fast-acting blow-off valve opens. The opening of the valve increases the air flow through the compressor and consequently prevents compressor surging. Blow-off control can also be considered as a capacity-control method. However, blow-off control releases compressed air into the atmosphere and is considered as an energy-intensive practise.

Speed control [52]

At any point on the optimum efficiency line, indicated in Figure 2-7, the compressor will consume the least amount of electrical power to deliver the specific amount of compressed air at a constant pressure.

Various studies mentioned that speed control on the electrical motor of a compressor is an effective capacity-control method. This entails installing variable-speed drives (VSDs) on the motors to control the speed of the motors. However, this is an expensive practise and will not be considered throughout this study [32].

Capacity control methods eliminate a wide range of supply-side inefficiencies within the mining industry. These inefficiencies are summarised in Table 2-2 [22], [25], [32], [33], [43], [54], [55]. All the identified inefficiencies increase the power consumption of the compressors during the generation process. Strategies to eliminate the identified inefficiencies are briefly discussed in Table 2-2.

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Chapter 2 39

Table 2-2: Initiatives to reduce supply-side inefficiencies [22], [25], [32], [33], [43], [54], [55]

Inefficiencies on the supply side

Inefficiency Strategy Strategy description

Compressors run unnecessarily throughout a typical production day. The blow-off valve opens and

excess air is released into the atmosphere.

Stop/Start Compressors

One of the most basic control strategies is to stop and start a

compressor. A compressor should be stopped during low

demand periods.

Compressors tend to blow off excess air into the atmosphere.

Load/Unload compressors

The delivery air valve of the compressor should be closed, and the blow-off valve opened.

The compressor will only consume electric power to

overcome basic friction. Compressors often experience

large changes in airflow rates. If the demand flow drops too low,

flow through the compressor decreases. This increases the possibility of compressor surging.

Compressor inlet guide vane

control

Automatically controlling the inlet guide vanes of the compressor, ensures that the

compressor can effectively adjusts to the changing parameters. This will also maintain compressor efficiency. Inefficient compressor

combinations.

Compressor selection

Run the minimum number of compressors at the most

effective combination. 1. Intake filters can block up

over time, causing pressure drops.

2. The moisture traps located on the air network is

blocked, causing corrosion. 3. Ineffective motor cooling

can result in an increased motor temperature.

Regular maintenance

1. Replace/clean air filters regularly.

2. Inspect moisture traps. 3. Regularly clean and

lubricate compressor motor. Inspect water pumps and fans. Maintain the coolers on the compressor.

The identified strategies should be implemented to eliminate the identified problem. The following section will focus on demand-side technologies for effective compressed air distribution.

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Chapter 2 40 2.3.3 Improving the demand side of compressed air networks

Typical mine compressed air networks comprise several interconnected shafts and processing plants [31]. As discussed in Section 2.2.3 compressed air requirements of the consumers on the compressed air network differ throughout a typical production day. Processing plants require a constant air pressure, while the pressure requirements of the shafts vary according to their operational schedules [32]. The lowest network pressure is determined by the highest-pressure requirement of the different consumers on the compressed air network. Due to the constant pressure supply to the processing plant, ineffective compressed air distribution takes place during low demand periods.

Surface control valves can be installed on the compressed air pipelines to separate the processing plants from the shafts [32], [33]. The valves can then control the pressure supplied to the end-users based on their requirements. The pressure requirements of the different consumers are critical for effective valve control [33], [48]. Figure 2-8 illustrates a typical surface compressed air control valve used at one of the mining complexes in South Africa.

Figure 2-8: Compressed air control valve11

During some cases, certain compressed air lines are completely isolated from the rest of the network [15], [33]. This is usually isolated with manually operated butterfly valves [48]. High-performance butterfly control valves and globe valves are used for highly accurate

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Chapter 2 41

compressed air control. These valves are used to accurately control the pressure supplied to the different compressed air consumers, therefore, maintaining the pressure supplied to the processing plant at the desired pressure set-point.

Globe valves are approximately five times more expensive than butterfly valves [31]. Due to financial limitations, mines tend to use standard or high-performance butterfly valves to control the pressure requirements of the different consumers. It is important that the control valves should be installed at strategic places to minimise the implementation costs.

The first step is to identify the locations of the high- and low- pressure consumers on the compressed air network [32], [33], [48]. After the locations have been located, the following criteria should be identified and evaluated before the control valves can be installed:

• The control valves should be sized correctly to prevent valve seat damage.

• The valve should comply with strict noise regulation limits when installed near human activity.

• Available infrastructure (network and electrical cables), so as to reduce installation costs.

• Identified installation location (is it easily accessible and out of harm’s way?).

It can be concluded that demand- and supply-side technologies play an important role in improving the efficiency of the entire compressed air network. The next step is to evaluate the compressed air network in terms of fundamental calculations to accurately define the problem at hand.

2.4 Mine compressed air network fundamentals

2.4.1 Electrical power consumption

The electrical power consumed by the motor of a centrifugal compressor can be calculated when the compressed air requirements and conditions are known. The compression process of a centrifugal compressor is considered as a polytrophic process [56]. Equation 2-1 can be used to calculate the power required by a compressor’s motor during the compression process [15], [52], [57]

Equation 2-1: Compressor motor power calculation [15], [52], [57]

𝑷𝒎𝒐𝒕𝒐𝒓=

𝑷𝒄𝒐𝒎𝒑

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Chapter 2 42

𝑃𝑚𝑜𝑡𝑜𝑟 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑜𝑡𝑜𝑟 [kW]

𝑃𝑐𝑜𝑚𝑝 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 [kW]

𝑛𝑚𝑜𝑡𝑜𝑟 = 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑚𝑜𝑡𝑜𝑟 [-]

The efficiency of the motor is usually obtained from the compressor specifications. If the specifications are unavailable, an efficiency of 0.9 may be assumed [32], [31]. The electrical power of a compressor (𝑷𝒄𝒐𝒎𝒑) can be calculated using Equation 2-2.

Equation 2-2: Compressor electrical power calculation [31], [57]

𝑷𝒄𝒐𝒎𝒑= ṁ𝒂𝒊𝒓𝑾𝒄𝒐𝒎𝒑

𝑃𝑐𝑜𝑚𝑝 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 [kW]

ṁ = 𝑀𝑎𝑠𝑠 𝑓𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑎𝑖𝑟 [kg/s] 𝑊𝑐𝑜𝑚𝑝 = 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 [KJ/kg]

The mass flow rate of the compressed air (ṁ) can be calculated by dividing the volume flow rate (Q) with the density (𝜌) of the air [58]. The volume flow rate is usually measured at the outlet of the compressor, whereas the density of the air can be calculated using the ideal gas law. The density of the air is calculated using Equation 2-3.

Equation 2-3: Ideal gas equation to calculate the density air [57]

𝝆 =𝑷𝒂𝒃𝒔 𝑹𝑻 𝜌 = 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟 [kg/m3_] 𝑃𝑎𝑏𝑠 = 𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑎𝑖𝑟 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 [kPa] 𝑅 = 𝐺𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (0.278) [KJ/kg.K] 𝑇 = 𝐴𝑖𝑟 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 [K]

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Chapter 2 43

As seen from Equation 2-3 the density of the air is dependent on the pressure and temperature of the air. The density of the air will decrease with an increasing temperature and constant pressure value. The physical properties of air at different air temperatures can be found at Appendix A [57].

To calculate the electrical power consumption of a compressor as formulated in Equation 2-2, the mechanical energy consumption of a compressor should be calculated, using Equation 2-4 [15], [31].

Equation 2-4: Mechanical energy of a compressor [15], [31]

𝑾𝒄𝒐𝒎𝒑= 𝒏𝑹𝑻𝒊𝒏((𝒑_𝑷𝒐𝒖𝒕 𝒊𝒏) 𝒏−𝟏 𝒏 − 𝟏) 𝒏𝒄𝒐𝒎𝒑(𝒏 − 𝟏) 𝑊𝑐𝑜𝑚𝑝 = 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑛 = 𝑃𝑜𝑙𝑦𝑡𝑟𝑜𝑝ℎ𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑓𝑜𝑟 𝑖𝑠𝑒𝑛𝑡𝑟𝑜𝑝𝑖𝑐 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 (1.4) 𝑅 = 𝐺𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (0.278) 𝑇𝑖𝑛 = 𝐼𝑛𝑙𝑒𝑡 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝑎𝑖𝑟) 𝑝𝑜𝑢𝑡 = 𝐷𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑃𝑖𝑛= 𝐴𝑡𝑚𝑜𝑠𝑝ℎ𝑒𝑟𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑛𝑐𝑜𝑚𝑝 = 𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 [kJ/kg] [kg/s] [kJ/Kg.K] [K] [kPa] [kPa] [-]

As for the motor efficiency, the compressor efficiency can be obtained from the compressor specification sheet. If it is unavailable, a value of 0.8 can be assumed [32], [31]. With all the relevant information, the electrical power of the compressor motor can be calculated, as formulated in Equation 2-1.

From Equation 2-2 and Equation 2-4, the mass flow, discharge pressure and inlet air temperature have a direct relationship to the power usage of the compressor. This study will focus on methods to reduce the discharge pressure and mass flow rate of the compressors so as to reduce the overall power consumption.

The values of these parameters are determined based on the compressed air demand throughout a typical production day. Energy saving opportunities exist by reducing the

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Chapter 2 44

compressed air demand. The following section will focus on fundamentals and calculations at the compressed air demand side.

2.4.2 Auto compression

An increase in air pressure (due to the weight of the compressed air on itself) can be defined as auto compression [15], [31], [27], [59]. Due to the extreme depths of gold mines, the effect of auto compression plays an important role [31]. In theory, a deeper mining shaft will experience a greater effect of auto compression. The effect of auto compression can be calculated with the use of Equation 2-5.

Equation 2-5: Pressure increase due to auto compression [59]

𝒑𝟎 = 𝒑𝒊[𝟏 − 𝒈(𝒛𝒊− 𝒛𝟎) 𝑻𝟏𝑪𝒑 ] 𝟏 𝒌 𝑝0 = 𝐹𝑖𝑛𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑝𝑖 = 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑔 = 𝐺𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 (9.81) 𝑧𝑖 = 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒 𝑧0= 𝐹𝑖𝑛𝑎𝑙 𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒 𝑇1= 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑒𝑑 𝑎𝑖𝑟 𝐶𝑝= 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑒𝑑 𝑎𝑖𝑟 𝑘 = 𝑆𝑝𝑒𝑠𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑒𝑑 𝑎𝑖𝑟 (1.4) [kPa] [kPa] [m/s2_] [m] [m] [K] [kJ/kg.K] [kJ/kg.K]

To minimise the risk of pipes bursting, the effect of auto compression should be kept in mind. Auto compression at different discharge pressures and increasing mine depths are illustrated in Figure 2-9.