Dust explosion prevention and protection

Preface Acknowledgements Membership of the Advisory Panel I, Background to dust explosions 1. l Introduction 1.2 Approach to handling dust explosion hazards 1.3 Selection of a basis for safety Logic diagram 1 omg III V .,s VII 2. Determination of dust ignitability, flammability and explosibility characteristics 8 2.1 Explosibility classification 8 2.1.1 The standard explosibility tests 12 2.1.2 Tests at elevated temperatures 12 2.1.3 Further explosibility tests 12 2.2 Explosibility characteristics 13 2.3 Other explosibility and ignitability characteristics 14 2.3.1 Explosion limits 14 2.3.2 Minimum ignition energy 14 2.3.3 Minimum ignition temperature of a dust cloud 15 2.3.4 Limiting oxygen concentration 15 2.4 Dust layer flammability characteristics 16 2.4.1 Flammability 16 2.4.2 Burning behaviour 16 2.4.3 Minimum ignition temperature of a dust layer 17 2.5 Where to go for tests 18 Logic diagram 2 and notes 20 3, Control of ignition 16 3.1 Introduction 26 3.2 Ignition sources 263.2.1 Flames 3.2.2 Hot surfaces 3.2.3 Spontaneous heating 3.2.4 Thermite ignition 3.2.5 Friction (impact and rubbing) and mechanical sparks 3.2.6 Electricity 3.2.7 Electrostatic discharges 3.3 Assessment of ignition hazards and risks 3.3.1 Risk assessment 3.3.2 Zoning 3.4 Examples of ignition prevention measures 3.4.1 Size reduction processes 3.4.2 Pneumatic conveying lines 3.4.3 Screw conveyers 3.4.4 Drag link or en masse conveyors 3.4.5 Belt conveyors 3.4.6 Bucket elevators 3.4.7 Dryers 3.4.8 Storage bins 3.4.9 Dust filters Logic diagram 3 27 29 31 33 33 34 34 37 38 38 41 42 42 43 43 43 44 44 45 45 48 4. Inerting 4.1 Outline of principles 4.1.1 Diluent dust addition 4.1.2 Use of inert gas 4.1.3 Inerting gases 4.2 Inerting as a basis of safety 4.3 LOC for preventing explosion 4.3.1 Use of steam for inerting 4.4 Supply, distribution and monitoring of inert gas 4.5 Peak demands for inert gas 4.6 Examples 4.6.1 Grinding installation 4.6.2 Spray drying installation 4.6.3 Application of steam inerting in a sewage sludge drying plant 4.7 Inerting/venting and inerting/suppression combinations 4.8 Worked example of the calculation method for inerting Logic diagram 4 and notes 50 50 50 50 51 51 52 53 54 56 57 57 58 59 60 60 64S. Explosion containment 5.1 Introduction 5.2 Explosion containment as a basis of safety 5.3 Explosion containment in linked vessels 5.4 Guidance on explosion containment in linked vessels 5.4.1 General 5.4.2 Notes for guidance 5.4.3 Worked examples Logic diagram 5 and notes 66 66 67 67 67 67 69 69 72 6. Explosion suppression 6.1 Introduction 6.2 Suppression as a basis for safety 6.3 Methods of detection of an incipient explosion 6.4 Control systems 6.5 Methods of injecting suppressant 6.6 Suppressant materials 6.6.1 Common suppressants 6.6.2 Performance 6.7 Design procedures 6.7.1 Background 6.7.2 Design methodologies 6.8 Applications 6.8.1 Sulphur mill and bucket elevator 6.8.2 Two leg belt and bucket elevator 6.8.3 Bucket elevator protected by a combination of suppression and venting 6.8.4 Spray drying plant 6.8.5 In-line hopper/collector 6.8.6 Product feed, classification and collection system 6.8.7 Bag filter unit 6.8.8 Vent-suppression combination system 6.8.9 Waste sludge drying system Logic diagram 6 and notes 75 75 78 78 81 82 87 88 89 90 90 92 95 99 99 102 102 103 104 105 106 108 109 7. Explosion venting 7.1 Introduction 7.2 Venting as a basis of safety 7.3 Sizing of vents for single enclosures 7.3.1 Enclosures with Pre~t >/ 0.1 bar 7.3.2 Buildings 7.3.3 The Factory Mutual Research Corporation (FMRC) method of vent sizing 116 116 118 119 119 121 1257.4 Conclusions Logic diagram 7 and notes 126 133 8. Explosion venting of linked vessels 8.1 Introduction 8.2 Guidance on explosion venting of linked vessels 8.2.1 Simple rules 8.2.2 More detailed guidance 8.3 Other considerations 142 142 142 142 144 146 9. Explosion venting - supplementary design considerations 163 9. l Introduction 163 9.2 Plant layout 163 9.3 Safe discharge area 164 9.3.1 Use of deflector plates 165 9.3.2 Use of combined suppression/venting 166 9.3.3 Use of flameless venting devices 167 9.4 The use of vent ducts 168 9.4.1 The draft European Standard (prEN) method 168 9.4.2 The Institution of Chemical Engineers (IChemE) method 169 9.4.3 The FMRC method 174 9.4.4 Discussion 175 9.4.5 Allowed vent duct designs 177 9.4.6 Ducts with areas larger than the vent 179 9.4.7 Recoil forces 180 10. Explosion venting - special dust cloud conditions 182 10.1 Introduction 182 10.2 Inhomogeneous dust clouds of relatively low turbulence 183 10.2.1 Pneumatic filling of vented vessels 183 10.2.2 Filling of a silo by free fall from a cyclone 192 10.2.3 Conclusion 192 10.3 The effects of high turbulence 193 10.3.1 The AFNOR Nomographs 193 10.3.2 The Proust Nomograph 196 I I. Explosion venting - design of vent closures 199 1 l. l Introduction 199 11.2 Vent closures 199 11.2.1 Criteria 199 11.2.2 Type 200 11.2.3 Shape 20111.2.4 Disposition 11.2.5 Location 11.2.6 Installation and maintenance of vent covers 11.3 Testing of vent closures 11.3.1 Venting efficiency 201 202 202 203 203 12. Explosion venting - safe discharge area 12.1 Introduction 12.2 Relationships for determining the pressure and flame effects outside vents 12.2.1 Venting to an open area 12.2.2 Guidance on maximum flame length 12.2.3 Guidance on maximum explosion pressure 12.2.4 Pressure decay with distance 12.3 Venting towards an obstruction 12.3.1 Pressure 12.3.2 Flame effects 12.3.3 Pressure effects on buildings 12.4 Alternative approaches when predicted safe areas are impractical 206 206 207 207 207 208 209 209 209 210 210 211 13. Explosion venting in industrial plant 212 13.1 Introduction 212 13.2 Examples of explosion venting applied to an industrial plant 212 13.2.1 Spray dryer system 212 13.2.2 Milling system 217 13.2.3 Spin flash dryer 219 13.3 Venting of specific plant items 223 13.3.1 Bucket elevators 223 13.3.2 Recovery cyclones 224 13.3.3 Dust collector (filter/separator) 225 13.3.4 Spray dryers 226 13.3.5 Grinders and mills 228 13.3.6 Venting of storage bins 228 13.3.7 Venting ofpowder blenders 228 13.3.8 Equipment that is difficult to vent 228 14. Explosion propagation, protection of pipelines and isolation techniques 232 14.1 Introduction 232 14.2 Explosion propagation and protection of pipelines 232 14.2.1 Protection against explosion propagation in pipelines 239 14.3 Prevention (or limitation)of explosion propagation (isolation techniques) 239 14.3.1 Explosion relief diverter 239 14.3.2 Rapid action valves 240 14.3.3 Suppressant barriers 241 14.3.4 Material chokes 242 14.4 Prevention of re-ignition 243 14.5 Preventing the transfer of smouldering material and burning particles into an explosible dust cloud 244 Appendices 246 1 Incidents 246 2 Legislation 249 3 Descriptions of explosibility apparatus 254 4 Dust fires 261 5 Kst Nomographs 264 6 - - Estimates for reduced explosions 268 A) Estimates of reduced explosion pressures for straight vent ducts 268 B) Estimates of reduced explosion pressures for vent ducts with a single sharp 45 ~ bend 288 C) Estimates of reduced explosion pressures for vent ducts with a single sharp 90 ~ bend 308 D) Estimates of the reduced explosion pressures for St 3 metal dusts 328 Glossary 342 Index 348