BULK SOLIDS HANDLING

An I ntroduction to the Practice and Technology

C. R. WOODCOCK, DipTech, MSc, PhD, CEng, MIMechE

Formerly Chief Executive of Centre for Industrial Bulk Solids Handling, Glasgow Caledonian University

and

J. S. MASON, BSc, PhD, CEng, FIMechE, FIMarE, MIMinE Principal and Vice-Chancellor. Glasaow Caledonian Universitv

t1l! BLACKIE ACADEMIC cSt PROFESSIONAL

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First edition 1987 Reprinted 1993, 1995

© 1987 Chapman & Hall

Softcover reprint of the hardcover 1 st edition 1987

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ISBN-13: 978-94-010-7689-0 DOl: 10.1007/978-94-009-2635-6

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Preface

An understanding ofthe properties and the handling characteristics of liquids and gases has long been regarded as an essential requirement for most practising engineers. It is therefore not surprising that, over the years, there has been a regular appearance of books dealing with the fundamentals of fluid mechanics, fluid flow, hydraulics and related topics. What is surprising is that there has been no parallel development of the related discipline of Bulk Solids Handling, despite its increasing importance in modern industry across the world. It is only very recently that a structured approach to the teaching, and learning, of the subject has begun to evolve.

A reason for the slow emergence of Bulk Solids Handling as an accepted topic of study in academic courses on mechanical, agricultural, chemical, mining and civil engineering is perhaps that the practice is so often taken for granted. Certainly the variety of materials being handled in bulk is almost endless, ranging in size from fine dust to rocks, in value from refuse to gold, and in temperature from deep-frozen peas to near-molten metal. Almost everyone has seen a belt conveyor in operation-perhaps carrying grain on the local farm, or stone and rock from a nearby quarry-but how many would know that belt conveyors are now being developed to transport bulk solids at rates in excess of 30 000 tonnes per hour? The domestic vacuum cleaner is a familiar machine in which dust particles are conveyed through a pipe in a stream of air, but few people would appreciate that large lumps of coal and rock, and even fish, can be transported in a similar way. Examples of bulk solids handling can be found in almost every kind of industry and the problems associated with the design, installation and operation of plant for the storage and transport of materials in bulk are many and varied. No book can be a substitute for the technical skill acquired through long experience in the industry. Nevertheless, we have attempted to present here a foundation of knowledge, generally with a practical rather than an academic emphasis, upon which expertise in various specialized aspects of bulk solids handling can be developed subsequently.

Governments of many nations are now recognizing that the education and training of engineers in many fields should include some study of the technology of bulk solids covering the properties, storage, flow and transport of a wide range of materials in particulate or granular form. In the United Kingdom, for example, recent initiatives emanating from the Department of Industry have led to conferences, courses, and various publications aimed at promoting a greater awareness of the unique features of bulk solids. The School of Engineering at Thames Polytechnic, and in particular the staff of its

IV PREFACE

Bulk Solids Handling Unit, have been deeply involved in these initiatives from the outset and this book is the result of a clearly perceived need for an introduction to the subject that would identify and set out a structure for the area of study that is becoming known by the convenient, if not entirely accurate, title 'Bulk Solids Handling'. Many specialist treatments are already available: works on particle technology, hopper design, fluidization, dust control, pneumatic conveying, and others, can be found on library shelves and in most cases these are excellent and valuable works of reference for the experienced engineer. However, for the student and for the engineer who requires an overview of the emerging discipline of bulk solids handling, supported by an adequate coverage of fundamentals, this book should provide essential reading.

In common with most books of similar size and scope, this one should really be regarded as the product of a team effort. We, as authors and editors, would unhesitatingly acknowledge the contributions, both direct and indirect, of our colleagues in the Bulk Solids Handling Unit at Thames Polytechnic, notably Dr David Mills and Dr Alan Reed. In their various ways the academic and technician staff of the School of Engineering, together with many of our postgraduate and undergraduate students, have played some part in the events leading up to the conception, preparation and, ultimately, the production of this book. Although it is perhaps a little unfair to mention individuals by name, we do gratefully acknowledge the patient and tolerant efforts of Mrs Pam Colley in undertaking the massive task of typing the manuscript. Finally, our sincere thanks and appreciation goes to our respective families, especially to our wives Angela and Fran, for the patience, understanding and considerable fortitude that they have shown during the months that this book has been in preparation, and indeed during the many years that we have both been so deeply involved in the multitude of activities arising out of our interest in the fascinating subject of Bulk Solids Handling.

CRW JSM

Contents

PART 1 CHARACTERIZATION, FLOW AND STORAGE

1 The nature of bulk solids 1 1.1 Introduction 1 1.2 Sampling 3

1.2.1 Obtaining a gross sample 3 1.2.2 Preparing laboratory and test samples 4

1.3 Voidage and bulk density 7 1.4 Particle density 9 1.5 Particle size 10

1.5.1 Definition of 'size' and 'size distribution' 10 1.5.2 Measurement of particle size 15

1.6 Particle shape 25 1.7 Surface area 26 1.8 Particle hardness 28 1.9 Cohesion and adhesion 29

1.9.1 Angle of repose 31 1.9.2 Shear strength 33 1.9.3 The shear cell as a means of determining shear strength 35 1.9.4 Wall friction 39 1.9.5 Measurement of wall friction 40 1.9.6 Arching phenomena 41

1.10 Moisture content 43 1.11 Explosiveness 44 1.12 Notation 45

References and bibliography 46

2 Gravity flow of bulk solids 47 2.1 Introduction 47 2.2 Pressure distribution in a bulk solid 49

2.2.1 Bulk solid at rest 49 2.2.2 The effect of flow on the pressure distribution 52

2.3 Flow of bulk solids from hoppers 54 2.3.1 Introduction 54 2.3.2 Core flow 54 2.3.3 Mass flow 55 2.3.4 Obstructions to gravity flow 55 2.3.5 Predicting the solids discharge rate 56

2.4 Flow of bulk solids in chutes 64 2.4.1 Introduction 64 2.4.2 Flow patterns in straight inclined chutes 65 2.4.3 Flow patterns in curved chutes 68 2.4.4 Chute design 69

2.5 Flow of bulk solids in vertical pipes 74 2.5.1 Introduction 74 2.5.2 Mode of flow 76 2.5.3 Flow control-J-valves and L-valves 79

2.6 Notation 81 References and bibliography 82

vi CONTENTS

3 Dynamics of fluid/solids systems 84 3.1 Introduction 84 3.2 Flow through beds of fixed particles 84

3.2.1 Characteristics of flow in porous media 84 3.2.2 The prediction of pressure-drop across a fixed particulate bed 85

3.3 Settling behaviour of particles 91 3.3.1 Motion of a spherical particle settling in a stationary fluid 91 3.3.2 The settling of non-spherical particles 95 3.3.3 The settling of concentrations of particles (hindered settling) 98 3.3.4 Classification and sorting of particles 98

3.4 Fluidization 99 3.4.1 The fluidization process 99 3.4.2 The prediction of minimum fluidizing velocity 104 3.4.3 Entrainment of particles from a fluidized bed 109 3.4.4 The porous membrane, or distributor 109 3.4.5 The influence of particle size and density 110

3.5 Spouted bed behaviour 113 3.6 Gas/solids flow in pipes 116

3.6.1 Introduction 116 3.6.2 The flow of gas/solids suspensions in horizontal pipes 117 3.6.3 The flow of gas/solids suspensions in vertical pipes 122 3.6.4 Flow around 90° bends 124 3.6.5 The prediction of pressure-drop in flowing gas/solids suspensions 125

3.7 Liquid/solids flow in pipes 138 3.7.1 Flow characteristics of liquid/solids mixtures (slurries) 138 3.7.2 Non-Newtonian flow models for homogeneous suspension 139 3.7.3 The modelling of heterogeneous suspensions 148

3.8 Notation 150 References and bibliography 152

4 The design of storage bins and hoppers 154 4.1 Introduction 154 4.2 Hopper geometry 156

4.2.1 Shape 156 4.2.2 Overall dimensions 159

4.3 Outlet size and cone angle 162 4.3.1 Jenike's 'flow-no flow' criterion 162 4.3.2 Flow Functions and flow factors 165 4.3.3 Outlet dimension and cone angle 166

4.4 Period of storage and time consolidation effects 168 4.4.1 Caking 169 4.4.2 Testing for time consolidation 170 4.4.3 Practical ways of minimizing time consolidation 171

4.5 The effect of moisture 171 4.6 Overcoming space limitations 172

4.6.1 The use of low-friction linings 173 4.6.2 Changing hopper shape 175

4.7 Structural design 176 4.8 Control and measurement of discharge rate 178 4.9 Feeders 180

4.9.1 Introduction 180 4.9.2 Belt feeders 181 4.9.3 Apron feeders and rotary feeders 183 4.9.4 Rotary table feeders 184 4.9.5 Screw feeders 185 4.9.6 Vibratory feeders 187

4.1 0 Discharge aids 187 4.1 0.1 Introduction 187

4.10.2 Pneumatic methods 4.10.3 Vibrational methods 4.10.4 Mechanical methods

4.11 Notation References and bibliography

5 Dust control 5.1 Introduction 5.2 Dust as a hazard to health

5.2.1 Dust particle size

CONTENTS

5.2.2 Dust concentration limits 5.3 Dust suppression

5.3.1 Elimination of dust 5.3.2 Control of dust dispersion

5.4 Gravity and inertial separators 5.5 Air cleaners-cyclones

5.5.1 Principle of operation 5.5.2 Prediction of collecting efficiency 5.5.3 Prediction of pressure-drop 5.5.4 Cyclone selection

5.6 Air cleaners-wet washers or scrubbers 5.6.1 Principle of operation 5.6.2 Low pressure-drop wet washers 5.6.3 High pressure-drop wet washers

5.7 Air cleaners-filters 5.7.1 Mechanism of filtration 5.7.2 Filter media 5.7.3 Bag filters-design and selection 5.7.4 Filter cleaning

5.8 Air cleaners-electrostatic precipitators 5.9 Notation

References and bibliography

6 Explosion hazards 6.1 Introduction 6.2 Characteristics of dust explosions

6.2.1 Ignition 6.2.2 Explosibility limits 6.2.3 Expansion effects and explosion pressures

6.3 Measurement of explosion parameters 6.4 Explosion risks and system design

6.4.1 Minimizing sources of ignition and prevention of ignition 6.4.2 Containment 6.4.3 Explosion relief venting 6.4.4 Detection and suppression

6.5 Static electricity 6.6 Conclusion

References and bibliography

PART 2 MECHANICAL HANDLING

7 Belt conveyors 7.1 Introduction 7.2 Features of belt conveyors

7.2.1 Belt construction 7.2.2 Idlers 7.2.3 Drive arrangements

VB

188 192 198 200 201

203 203 204 204 208 208 208 209 211 213 213 215 218 218 218 218 220 222 224 224 226 227 230 232 233 233

235 235 238 238 239 240 241 246 248 249 250 253 256 258 258

260 260 261 261 265 268

viii CONTENTS

7.2.4 The power unit 7.2.5 Loading and discharge arrangements 7.2.6 Belt cleaners

7.3 Belt conveyor design 7.3.1 The bulk solid to be transported 7.3.2 Belt speed 7.3.3 Belt width 7.3.4 Belt tension 7.3.5 Idler spacing 7.3.6 Power requirements

7.4 Belt conveyor variants 7.4.1 The cable belt conveyor 7.4.2 Belt conveyors without idlers 7.4.3 Closed-belt or pipe conveyors 7.4.4 Sand wich belts

7.5. Notation References and bibliography

8 Bucket elevators 8.1 Introduction 8.2 Principal types of bucket elevator

8.2.1 Centrifugal discharge elevators 8.2.2 Continuous bucket elevators 8.2.3 Pivoted buckets 8.2.4 Profiled-belt elevators

8.3 Design and selection of bucket elevators 8.3.1 Design features 8.3.2 Loading 8.3.3 Discharge 8.3.4 Capacity 8.3.5 Driving power

8.4 Notation References and bibliography

9 Chain and flight conveyors 9.1 Introduction 9.2 Drag conveyors 9.3 En-masse conveyors

9.3.1 Design features 9.3.2 Performance calculations 9.3.3 Applications of en-masse conveying

9.4 Tubular drag conveyors 9.5 Apron conveyors 9.6 Aerial ropeways 9.7 Notation

References and bibliography

10 Screw conveying 10.1 Introduction 10.2 Principle of operation of screw conveyors 10.3 The enclosed screw or 'auger' conveyor

10.3.1 Constructional features 10.3.2 Prediction of the performance of an auger conveyor

10.4 The industrial screw conveyor or 'V-trough' conveyor 10.4.1 Constructional features 10.4.2 The conveyed product

271 272 273 274 274 275 277 281 284 284 289 289 290 291 293 295 296

298 298 300 300 301 303 304 305 305 308 309 312 315 316 317

318 318 318 321 321 323 325 327 328 331 333 334

335 335 337 338 338 340 342 342 344

10.4.3 Conveyor selection 10.4.4 Conveyor power

CONTENTS

10.4.5 Inclined screw conveyors 10.5 Vertical screw conveyors 10.6 Conclusion 10.7 Notation

References and bibliography

11 Vibratory conveyors 11.1 Introduction 11.2 Movement of a bulk solid in a vibrating trough

11".2.1 The motion of the trough 11.2.2 The motion of bulk material in the trough 11.2.3 Average conveying velocity 11.2.4 The influence of the design parameters 11.2.5 Two-phase trough motion

11.3 Design features 11.3.1 Drive mechanism 11.3.2 Mounting systems

11.4 Applications of vibratory conveying 11.5 Spiral elevators 11.6 Notation

References and bibliography

PART 3 PNEUMATIC AND HYDRAULIC TRANSPORT

12 Basic pneumatic conveying systems 12.1 Introduction 12.2 Modes of conveying-dilute-phase and dense-phase 12.3 Low-pressure pneumatic conveying systems

12.3.1 Positive-pressure systems 12.3.2 Negative-pressure (vacuum) systems 12.3.3 Combined negative/positive pressure systems

12.4 High-pressure systems 12.4.1 General features 12.4.2 Single blow tank systems 12.4.3 Twin blow tanks and continuously operating systems 12.4.4 Long-distance conveying

12.5 Low-velocity conveying and the use of supplementary air feeds 12.5.1 General features 12.5.2 Plug-forming systems 12.5.3 Plug-limiting systems 12.5.4 Air-injection and booster systems References and bibliography

13 Components of pneumatic conveying systems 13.1 Introduction 13.2 The air supply

13.2.1 General requirements 13.2.2 Fans and turbo-blowers 13.2.3 Roots-type blowers 13.2.4 Sliding-vane rotary compressors 13.2.5 Screw compressors 13.2.6 Reciprocating compressors 13.2.7 Vacuum pumps

IX

345 349 351 354 356 356 357

358 358 361 361 365 367 369 370 370 370 374 375 376 378 379

380 380 386 386 386 390 392 392 392 393 396 398 399 399 401 403 405 407

408 408 408 408 409 411 411 412 414 416

x

13.3 Feeding devices 13.3.1 Rotary valves 13.3.2 Screw feeders 13.3.3 Venturi feeders 13.3.4 Gate lock valves 13.3.5 Blow tanks

CONTENTS

13.3.6 Entrainment devices for vacuum systems 13.4 The pipeline -13.5 Disengaging and collecting devices 13.6 Notation

References and bibliography

14 Pneumatic conveyor design 14.1 Introduction 14.2 General design procedure

14.2.1 Conveying velocity and volumetric air flow rate 14.2.2 Solids mass flowrate and solids loading ratio 14.2.3 Pipeline diameter 14.2.4 Pressure-drop 14.2.5 Stepped pipelines 14.2.6 Selection of the air mover

14.3 Summary of preliminary design procedure for dilute-phase systems 14.4 Designing from available test data

14.4.1 Conveying characteristics 14.4.2 Scaling for pipe size and conveying distance

14.5 Notation References and bibliography

15 Air-assisted gravity conveying 15.1 Introduction 15.2 The flow of fluidised solids 15.3 Practical air-assisted gravity conveying 15.4 Design parameters for air-gravity conveyors

15.4.1 Slope of channel 15.4.2 Conveying distance 15.4.3 Width of conveying channel 15.4.4 Air requirement

15.5 Properties of bulk solids for air-gravity conveying 15.6 Air-float conveyors for horizontal and upward transport 15.7 Energy consumption of air-gravity conveyors 15.8 Notation

References and bibliography

16 Hydraulic conveying 16.1 Introduction 16.2 Components of a hydraulic conveying system

16.2.1 Pumps 16.2.2 Slurry preparation plant 16.2.3 The pipeline 16.2.4 De-watering equipment

16.3 System design 16.3.1 General design approach 16.3.2 Flow characreristics and pressure-drop

16.4 Recent development References and bibliography

17 Capsule transport 17.1 Introduction

417 417 425 426 427 428 431 433 436 436 436

438 438 439 439 441 442 443 446 446 447 448 448 449 454 454

456 456 458 461 465 465 466 466 468 470 472 475 476 476

478 478 481 481 485 486 487 489 489 490 491 492

494 494

CONTENTS

17.2 Capsule transport in a pneumatic pipeline 17.2.1 General features of a pneumo-capsule system 17.2.2 The capsules 17.2.3 The pipeline 17.2.4 The air supply 17.2.5 Loading and unloading stations

17.3 Capsule transport in a hydraulic pipeline 17.3.1 General features of a hydro-capsule system 17.3.2 The capsules 17.3.3 The pipeline 17.3.4 The water supply and pump system 17.3.5 Injection and ejection of capsules

17.4 Size of capsule fleet 17.5 Notation

References

Index

xi

498 498 499 501 502 503 504 504 504 505 505 508 508 510 510

513

... when you caD meaS\lfe what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the state of SCIENCE, whatever the matter may be. (Lord Kelvin, as Sir William Thomson, speaking on 'Electrical Units of Measurement' at the Institution of Civil Engineers, London, 3 May 1883.)

(Arab proverb, freely translated as: 'Experience without learning is better than learning without experience'.)