Static Electricity

Static Electricity

Understanding, Controlling, Applying

Günter Lüttgens, Sylvia Lüttgens, and Wolfgang Schubert

Authors

Günter LüttgensResearch and ConsultingAm Berg 2751519 OdenthalGermany

Sylvia LüttgensResearch and ConsultingAm Berg 2751519 OdenthalGermany

Wolfgang SchubertSCHUBERT GMD Publicly Appointed and Sworn Expert for Printing TechnologiesPrint-Machines Printability and Runnability Packaging PrintIndependent appraiser for ElectrostaticsWeidenweg 1504425 TauchaGermany

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v

Contents

About the Authors xiiiOpening Remark xvPreliminary Remark xviiPreface xix

1 Basics of Fire and Explosion: Risk Assessment 11.1 Basic Considerations on Fire and Explosion ( T1) 11.1.1 Fuel 21.1.2 Heat 21.1.3 Oxygen 31.1.4 Inerting Process 31.1.5 Heat versus Oxygen 31.2 Explosive Atmosphere 31.2.1 Explosion Limits with Flammable Liquids 31.2.1.1 Classification of Flammable Liquids 51.2.2 Explosion Limits with Combustible Dusts 61.2.3 Metal Dusts 61.3 Hybrid Mixtures ( P7) 61.4 Allocation of Explosion-Endangered Areas and Permissible

Equipment ( P6) 71.5 Permissible Equipment (Equipment Protection Level) 71.5.1 Classification of Equipment Protection Level That Is Currently

in the Introductory Stage 81.6 Ignition Sources 91.6.1 Hot Surfaces 91.6.2 Flames and Hot Gases (Including Hot Particles) 91.6.3 Mechanically Generated Sparks ( MGS) 101.6.4 Electrical Apparatus 101.6.5 Cathodic Protection 101.6.6 Static Electricity 101.6.7 Lightning 101.6.8 Electromagnetic Field 101.6.9 Electromagnetic Radiation 101.6.10 Ionizing Radiation 10

Contentsvi

1.6.11 Ultrasonics 111.6.12 Adiabatic Compression and Shock Waves 111.6.13 Chemical Reactions 111.7 Minimum Ignition Energy (MIE) 111.8 Imaginary Experiment to Assess the Hazardous Potential

of Flammable Liquids 15 PowerPoint Presentations 18 References 18

2 Principles of Static Electricity 192.1 Basics 192.2 Electrostatic Charging of Solids ( T2) 212.3 Triboelectric Series 242.4 Surface Resistivity 242.4.1 Influence of Surface Texture on Static Charging 282.5 Electrostatic Charging of Liquids ( T2, T8) 282.5.1 Charge Relaxation with Liquids 302.6 Charging by Gases 312.7 Electric Field 332.8 Electric Induction ( T3) 362.8.1 Specification of Electric Induction 362.8.2 Image Charge 372.9 Capacitance and Capacitor 38 PowerPoint Presentations 38 References 39

3 Metrology 413.1 Basics ( T7) 413.1.1 “Walking Test” as a Paradigmatic Example 413.2 Appropriate Metrology for Electrostatic Safety Measures 443.3 Comparison: Electrostatics/Electrical Engineering 443.4 Selecting the Suitable Measurement Methods 453.4.1 Electrical Resistance 453.4.2 Fundamental Remarks for the Realization of Resistance Measurements

( T4) 463.4.2.1 Volume Resistance and Deriving Volume Resistivity 463.4.2.2 Guard Ring Circuit to Measure the Volume Resistance 473.4.2.3 Surface Resistance and Deriving Surface Resistivity 483.4.2.4 Guard Ring Circuit to Measure the Surface Resistance 493.5 Assignment and Summary 493.5.1 Additive‐Depleted Surface 503.6 Conductivity of Liquids 513.7 Bulk Materials 523.8 Concerning the Use of Insulating Material in Endangered Areas 523.9 Measurement of Electrostatic Charges 523.9.1 Voltage Measurement with Electrostatic Voltmeters 533.9.2 Charge Measurement by Means of a Faraday Pail 54

Contents vii

3.9.2.1 Faraday Cage 553.9.2.2 Charge Measurement on Free‐Falling Objects 553.9.3 Measurement of Electric Field Strength 563.9.3.1 Induction Electric Field Meters 563.9.3.2 Errors When Measuring Field Strength 583.9.3.3 Further Types of Electric Field Meters 613.9.3.4 Further Applications of Induction Electric Field Meters 653.10 Other Measurement Applications 683.10.1 Measurement of Surface Charge on Moving Webs 683.10.2 Analysis of Protective Textile Clothing (Workwear) 683.10.2.1 Triboelectric Test Procedure 693.10.2.2 Test Procedures with Electrostatic Influence 693.10.3 Test Procedure to Determine Discharge Capacity (Charged Plate

Monitor) 713.10.4 Test Procedure for Paper 733.10.5 Electrostatic Charging of Powdery Bulk Materials 743.10.6 Electrostatic Charging with Fluids 753.10.7 Electrostatic Charges in Chemical Production 763.11 Capacitance 773.11.1 Capacitance Measurement (Charging Methods) 773.11.2 Measurement of the Permittivity Value 783.11.3 Charge Decay Measurement (Relaxation Time) 793.12 Themes around Air Humidity 813.12.1 Definitions about Climate 813.12.2 Fundamental Principles and Definitions 823.12.3 Methods of Measuring Atmospheric Humidity 833.12.3.1 Dew Point Hygrometry 833.12.3.2 Absorption Method 843.12.3.3 Hair Hygrometer 843.12.3.4 Psychrometer with Wet‐ and Dry‐Bulb Thermometers 843.12.3.5 Lithium‐Chloride Hygrometer 853.12.3.6 Capacitive Hygrometer 863.12.3.7 Resistive Hygrometer 863.12.4 Monitoring and Calibrating of Hygrometers 86 PowerPoint Presentations 87 Picture Credits 87 References 88

4 Gas Discharges 894.1 Mechanisms of Gas Discharges ( T5) 894.2 Electrostatic Gas Discharges 904.2.1 Detecting Gas Discharges by Means of Their High‐Frequency

Emissions 924.3 Types of Gas Discharges 944.3.1 Spark Discharge 944.3.2 One‐Electrode Discharges 954.3.2.1 Corona Discharge 96

Contentsviii

4.3.2.2 Brush Discharge 974.3.2.3 Cone Discharge (also Referred to as Powder Heap Discharge) 984.3.2.4 Propagating Brush Discharge 984.4 Consequences of Gas Discharges 1024.5 Listing of Traces Caused by Gas Discharges (  P11; T8) 1024.6 How Can Dangerous Gas Discharges Be Avoided? 1034.6.1 Spark Discharges (V4.1) 1044.6.2 Corona Discharges 1044.6.3 Brush Discharges and Super Brush Discharges 1044.6.4 Cone Discharges 1064.6.5 Propagating Brush Discharges 1074.6.5.1 Curiosity When Hydraulic Oil Gradually Flows Out of a Metal

Pipe 1074.6.5.2 Pores at Enameled Containers 1084.6.6 Simplified Overview of the Occurrence of Different Types of Gas

Discharges 1084.6.7 Assessment of Ignition Dangers Originating from Gas

Discharges 1084.6.8 Electrostatic Shock 111 PowerPoint Presentations 111 Picture Credits 111 Video Credits 111 References 111

5 Preventing Electrostatic Disturbances 1135.1 Electrostatics: When Sparks Fly 1135.2 Dielectric Strength 1175.3 Discharging Charged Surfaces 1185.3.1 Discharging on Material Webs 1195.3.1.1 Behavior of Composite Materials 1255.3.2 Discharging of Sheets 1275.3.3 Discharging Other Objects 1275.3.4 Discharging Granules and Similar Particles 1295.4 Potential Hazards Posed by Discharge Electrodes 134 Picture Credits 136 Video Credits 137 References 137 Further Reading 137

6 Description of Demonstration Experiments 1396.1 Preliminary Remarks 1406.2 Static Voltmeter 1416.3 Field Meter 1426.4 Van de Graaff Generator 1426.5 Explosion Tube 1426.6 Electrostatic Force Effects 1446.6.1 Rolling Pipes 145

Contents ix

6.6.2 Hovering Pipes 1466.6.3 Electroscope 1476.6.4 Depicting Electrical Field Lines (in a Classical Way) 1486.7 Charges Caused by Separating Process 1496.8 Charging of Particles 1506.8.1 Charging of Single Particles 1506.8.2 Charging of Many Particles (Granules) 1526.9 Electric Induction 1536.9.1 Basic Experiment 1536.9.2 Chimes 1546.9.3 Electric Induction on Isolated Conductive Parts 1556.10 Dissipating Properties 1576.11 Experiments with the Explosion Tube 1586.11.1 Electrostatic Charging of a Person 1586.11.2 Ignition Voltage 1596.11.3 Charging by Separation 1606.12 Gas Discharges 1606.12.1 Spark Discharges 1606.12.2 Corona Discharges 1616.12.3 Brush Discharges 1626.12.4 Model Experiment: Ignition by Brush Discharges 1626.12.5 Evidence of Ion Wind 1636.12.6 Super Brush Discharges 1636.12.7 Propagating Brush Discharges 1646.12.7.1 Ignition of Dust 1656.12.7.2 Short Circuit of a Double‐Layer Charge 1666.13 Fire and Explosion Dangers 1686.13.1 Flash Point 1686.13.2 Effects with Large Surfaces 1686.13.3 Rich Mixture 1696.13.4 Progressive Flame Front 1706.13.5 “Decanting” of Gasoline Vapors 1716.13.6 Oxygen Demand 1726.13.7 Extinguishing with Water 1736.13.8 Burning Handkerchief Does Not Burn Up 1746.13.9 Inflaming Solid Combustibles 1746.13.9.1 Gasification Process with Wood 1746.13.9.2 Inflaming a Dust Heap 175 Reference 175

7 Case Studies 1777.1 Strategy of Investigation 1777.1.1 Ignition Sources 1787.1.2 General Approach 1797.1.3 Hasty Consequence 1797.2 Ignitions Due to Brush Discharges 1807.2.1 Pouring Flaked Product into an Agitator Vessel 180

Contentsx

7.2.2 PE Liner Slipping Out of Paper Bag 1817.2.3 Ignition Caused by an Antistatic PE Bag 1827.2.4 Shaking Fine Dust Out of a PE Bag (Hybrid Mixture) 1837.2.5 Pumping Polluted Toluene 1857.2.6 Impregnation of a Glass Fiber Fabric 1867.2.7 Filling Pipe Blocked with Sulfur Leading to Ignition of Methanol 1877.2.8 Ion Exchanger Resin in Toluene 1887.2.9 Two Explosions in Big Storage Tanks 1897.2.9.1 Explosion in a Floating Roof Tank Followed by Fire (2014) 1897.2.9.2 Explosion Disaster Near Bitburg (1954) 1907.3 Case Studies Related to Propagating Brush Discharges 1927.3.1 Explosion in a Railcar Bulk Container 1927.3.2 Metal Drum with Inner Liner 1937.3.3 Plastic Drum with Inner Liner 1957.3.4 Failed Attempt to Eliminate Electrostatic Nuisances 1957.3.5 Fire in a Spray‐Bed Dryer 1977.3.6 Ignition in a Micronizer Jet Mill 2007.3.7 Explosion During Rotational Molding 2017.3.8 Explosion in a Mixing Silo for Plastic Granules 2027.3.9 Curiosity During Outflow of Liquid from a Metal Pipe 2027.4 Case Histories Related to Spark Discharges 2047.4.1 Powder Explosion in a Metal Drum 2047.4.2 Dust Removal from Pharmaceutical Pills 2057.4.3 Sparks at a Throttle Valve (V4.1) 2067.4.4 Filling n‐Hexane into Metal Drums ( P 15) 2077.4.5 Hose Filter 2087.4.6 Water Flowing Through PVC Hose 2107.4.7 Lost and Found 2117.4.8 Miraculous Earthing Clamp 2127.5 Ignition Caused by Cone Discharges 2127.6 Doubts with Electrostatic Ignitions 2137.6.1 Fire in a Polyethylene Drum 2137.6.2 Fire in a Solvent Cleaning Area 2157.6.3 Burst of a Glass Pipe 2187.7 Act with Relevant Experience 2197.7.1 Basic Information 219 PowerPoint Presentations 220 Video 221 References 221

8 Targeted Use of Charges 2238.1 Applications 2238.2 Examples of the Creative Implementation of Applications 2268.2.1 Adhesive Bonding – Blocking 2268.2.2 Adhesion of an Insert on a Variable Base 2278.2.3 Blocking a Number of Paper Webs or Film Webs in One Ribbon 2298.2.4 Adhesion of a Melt Layer on the Chill Roll 230

Contents xi

8.2.5 Avoiding Telescoping When Winding 2318.2.6 In‐Mold‐Labeling (IML)–In‐Mold‐Decoration (IMD) 2328.2.7 Oil Application on Metal Sheets 2348.2.8 Application of Liquid Media on Fast Moving Webs 2348.2.9 Drying of Fast Moving Substrates 2368.2.10 Gravure Printing and Coating Machine 2378.2.11 Reduction of Particle Mist in the Coating Process 2418.2.12 Use of Charging for Technical Measurement Processes 2438.2.13 Precipitation of Mixed Substances 2448.2.14 Electroadhesion 2478.2.15 Surface Treatment with Corona Systems 2488.3 Summary 251 Picture Credits 251 Video Credits 252 References 252

M Mathematics Toolbox 253M1 Energy W of a Capacitance 255M1.1 Minimum Ignition Energy WMIE 255M1.2 Power P 255M1.3 Electrical Efficiency η 256M2 Field E; Field Strength E ⃗ 256M2.1 Homogeneous Field between Plane Plates 256M2.2 Field of Point Charge 256M2.3 Permittivity ε 257M2.4 Field of Rod (Wire) Charge 257M3 Flux Density D⃗ (Earlier: Dielectric Displacement) 257M4 Frequency f 258M4.1 Wavelength λ 258M4.2 Circular Frequency ω 258M5 Inductance L 258M5.1 Inductance Ls of an Air Coil 259M6 Capacitance C 259M6.1 Rod (Wire) across a Conductive Area 259M6.2 Coaxial Cable/Cylinder Capacitance 260M6.3 Conductive Sphere in Space 260M6.4 Sphere Across a Conductive Area 260M6.5 Shunt of Single Capacitors 261M6.6 Plate Capacitor 261M6.7 Series of Single Capacitors 261M6.7.1 Series of Two Single Capacitors 261M7 Force F, F ⃗ 262M7.1 Force between 2 Point Charges (Coulomb’s law) 262M8 Charge Q 263M8.1 Moved Charge Qm 263M8.2 Charge of Electron Beam Qe 263M8.3 Surface Charge Density σ 263

Contentsxii

M8.3.1 Maximum Surface Charge Density σmax 264M8.4 Mass Charge Density Q 264M8.5 Volume Charge Density ρ 264M9 Potential φ 264M10 Voltage U 265M10.1 In a Homogeneous Electric Field 265M10.2 Voltage Gradient When Charging a Capacitor 265M10.3 Voltage Gradient When Discharging a Capacitor 265M10.4 Time Constant τ (of RC Circuit) 266M10.5 Kirchhoff ’s Loop Rule 266M10.6 Kirchhoff ’s Junction Rule 266M10.7 Breakdown Voltage of a Discharge Gap (Paschen’s Law) 267M11 Resistance R (Universal) 267M11.1 Resistance R0 (Object or Material) 267M11.2 Surface Resistivity ρS (Object or Material) 268M11.3 Volume Resistivity ρV (Object or Material) 268M11.4 Resistivity ρV of a Conductor (Wire) 268M11.5 Leakage Resistance RE (Object or Material) 269M11.6 Conductance G 269M11.7 Conductivity γ 269M11.8 Shunt (of Single Resistors) 270M11.8.1 Shunt of Two Single Resistors 270M11.9 Series (of Single Resistors) 270M11.10 Impedance of a Capacitance RC (AC Resistance) 271M11.11 Impedance of a Inductance RL (AC Resistance) 271

Annex 2751 Videos for download from www.wiley-vch.de 2752 PowerPoint Presentations 2752.1 Theory of Electrostatics (Visualized by Experiments) 2752.2 Practical Examples with “Freddy” (Electrostatic Hazards in Plant

areas) 276

Index 277

xiii

Günter Lüttgens was born in Berlin, 1933, and holds a master’s degree in electrical engineering. Since gradua-tion he mainly worked in the chemical industry in the field of electrostatics. He was primarily responsible for laboratory research, as well as plant safety, in the area of fire and explosion prevention. In 1998, he was nomi-nated by IEC as an expert for electrostatic test methods. For more than 25 years he gave lectures on static electri-fication and safety measures together with his wife Sylvia. He published several articles and specialist books. In

2013, he received the International Fellow Award by the European Working Party (EFCE) as a researcher and teacher in the field of “Static Electricity in Industry.”

Sylvia Lüttgens was born in Geroda, 1946, was gradu-ated a teacher, and tried to direct the interest of her stu-dents to Music and English. Then she learned about static electrification and that it could be the cause for many a fire or an explosion. So she has been working together with her husband Günter, carrying out experimental lec-tures (up to 2015) in seminars about electrostatics, giving practical proof of the theory. Besides, she is publishing articles and writing specialist books on this topic.

They actually compiled the first encyclopedia on static electricity 15 years ago, and the third edition was published in 2013.

Wolfgang Schubert was born in 1952. He studied print technology in Leipzig and is a trained printer. He became self‐employed in 1997 having previously worked in vari-ous managerial roles in the print industry and in sales and marketing for manufacturers of roll‐ and sheet‐fed printing presses. Since then he has also been working in the specialized field of electrostatics, in sales and mar-

keting and also in further education. He has coauthored the specialist publica-tion Static Electricity.

About the Authors

About the Authorsxiv

In May 2016, he was publicly appointed and inaugurated by the Leipzig Chamber of Commerce and Industry (IHK) as an expert in the fields of printing processes, printing presses, printability, runnability, and packaging printing. He also works as an expert in the field of electrostatics.

xv

Minds of Felix – our faithful companionMay I introduce myself; I am Felix the electrostatics specialist dog (see Figure

6.1 in Section 6). When my humans are carrying out seminars, I silently lie under the tables with the experimental devices until Sylvia prepares the one in which a plastic tumbler is flung into the air by an explosion in an explosion tube (see Section  6.11.1). Then, I run for the tumbler and noisily chew it apart, which makes the audience laugh.

When my humans Sylvia and Günter are working at the computer, I often lie on the sofa watching them. When I have had enough of it, I fetch my teddy and place it in front of them. Then, they throw it somewhere, and I have to find it. This happens several times but then I take my teddy and jump back onto the sofa again. My humans think I am doing this because I am bored, but this is not true! I feel sorry for them that they have to sit in front of the computer with lots of paper, clattering the keys, writing this specialist book. So I have to offer them some variety. I know what this is all about, and I am definitely responsible for my pack.

Opening Remark

xvii

In this specialist book, Videos and PowerPoint Presentations are referred to.The Videos are indicated with “V” and listed at the end of the relevant chapters

and may be downloaded from www.wiley‐vch.de/xxx.For better comprehension, at different places animated PowerPoint Presentations

are referred to with the symbol ( ). The letter T stands for theory and P for practice.

These presentations may be requested from the authors:G. & S. Lüttgens: elektrostatik@elstatik.deW. Schubert: ws@schubert‐gmd.de

Preliminary Remarks

xix

It gives me a great pleasure to preface this excellent reference book for engineers and technicians. Sylvia Lüttgens, Günter Lüttgens and Wolfgang Schubert are well known for their very didactic manuals, excellent presentations and so well prepared demonstrations such way that rather complicate phenomena seem simple.

This reference book on Electrostatic Hazards for engineers and technicians is, in my knowledge, the first one with very clear explanations, describing step by step the phenomenon with very didactic concepts and perfect pedagogic demonstrations.

Electrostatic Hazard is a very worrying problem in a lot of industrial processes, using liquids, granular material, powders, or foils etc. It concerns a wide range of industries: Chemical, Petroleum, Pharmaceutical industry, as well as the agricul-tural sector and electric power plants.

Indeed, in recent decades many industrial processes increasingly use electri-cally insulating materials. These materials such as polymers have emerged with the petroleum products industry and have played a growing importance in industry because of their lower costs like metals and their easier processing, manufacturing and use. This has partly led to the fact that these materials and products brought about electrostatic hazards and nuisances and have become an important concern. When speaking about electrostatics, everyone has in mind the spark which we sometimes feel by touching the door of a car after being parked; or small pieces of paper attracted to a plastic wall that has been rubbed before. In fact, electrostatic charging in general is the study of electrical phenom-ena when the charges are not moving (“static”). However, at present, the so-called electrostatic phenomena are those involving electrification processes whereby often charge accumulation due to the use of insulating materials and product takes place.

The electrostatic hazards are sources of dangers of electric discharges due to electrostatic phenomena. Under certain conditions, these discharges lead to ignitions causing fire or explosions. Electrostatic nuisances cause degradation of an industrial process due to electrostatic effects. Precondition for this is that the generated charge will be accumulated.

Preface

Prefacexx

Charge generation is, in principle, related to contact of material and separation thereafter as, e.g., friction, flow, transfer of solids, or liquids. The accumulation is the result of the storage or collection of such products or liquids in unearthed containers.

Unfortunately electrostatic hazards may result in fatal accidents, injuries, often serious, especially burns, property damage, often important or significant in that case, for example, of fire extension to nearby facilities.

This reference book has a very logical and scientific methodology, making these interrelations very clear and useful for engineers and technicians. Indeed, it starts with the situations with the Risk Assessment, explaining precisely when and where such risks come into being. Then the basics of Static Electricity are presented, developing all the concepts and equations which are needed to under-stand the different phenomena. In another chapter the metrology, needed to understand the different situations, is presented. The processes of the different gas discharges are then exposed as well as different methods to prevent electro-static disturbances. One important subject of the book is the presentation of very didactic descriptions of demonstration experiments and of case studies.

It would fall short of that goal to hold static electricity accountable only for dangers and nuisances, however, electrostatic mechanisms are used in many applications where one would not expect them as there are: photocopying tech-niques, car body lacquering etc. Widely used is static electricity in improvements of many different scopes of application like wetting, drying, printing etc. Therefore one chapter is dedicated especially to that task to motivate the curious reader to improve other technologies with the help of static electricity.

Finally a very useful mathematic toolbox is given at the end of the book, mak-ing an easy understanding of all equations needed to comprehend the different processes.

Each chapter provides a complete bibliography of what was stated.And in the good end, I have spent a pleasant time to read this very educational

and didactic reference book that I strongly recommend to any engineer and tech-nician who wants to learn on Electrostatics.

Prof. em. Gerard TouchardUniversity Poitiers,

Groupe ElectrofluidodynamiqueInstitute PPRIME

Poitiers, FranceOctober 2016

1

Static Electricity: Understanding, Controlling, Applying, First Edition.Günter Lüttgens, Sylvia Lüttgens, and Wolfgang Schubert.© 2017 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2017 by Wiley-VCH Verlag GmbH & Co. KGaA.

1

If static electricity was really static, as one may assume by its name, then it could be ignored. Only when it becomes more dynamic does it appear to be interesting and extend in our awareness from harmless electric shocks, sometimes felt when leaving a car, to the possibly fatal lightning strokes of a thunderstorm (for the latter, there is detailed information given in www. lightningsafety.noaa.gov).

However, our intention in this book is to demonstrate that the obviously weak electrostatic discharges are more or less capable of igniting combustible materi-als, thus causing hostile fire and casualties. It is probably because of its unpre-dictability that static electricity is often incorrectly blamed as a cause of fire and explosion when no other plausible explanation is at hand. So it seems logical to start with basics on fire and explosion.

1.1 Basic Considerations on Fire and Explosion ( T1)

In which way do fire and explosion differ from one another?Common to both is the manifestation of a flame, which always indicates a fast

combustion of fuel/air mixtures in the gaseous phase. The chemical reac-tion,  depending on the combustion heat of the fuel, leads to an increase in temperature.

Fire is characterized mainly by a stationary burning flame in an open atmos-phere, for example, a lighted candle. Therefore the reaction heat spreads into the surroundings without increase in pressure.

However, when an ignition occurs in a combustible atmosphere within an enclosed space, for example, a drum, a flame front runs through the entire space, starting from the ignition source. Under atmospheric conditions, the flame front extends at a speed of 10 m/s. Therefore the heating effect of the flame causes a pressure increase of about 10 bars, which diminishes during subsequent cooling. It is decisive that this short time pressure increase may cause a devastating dam-age called explosion.

The exothermic reaction of fuel in air occurs between the tiniest particles, that is, the molecules of fuel and oxygen. This is the case when prevailing fuel gas forms the required gaseous phase. With flammable liquids, this molecular fuel/oxygen

Basics of Fire and Explosion: Risk Assessment

1 Basics of Fire and Explosion: Risk Assessment2

mixture can easily be achieved by vaporization of the liquid. However, for solid fuels (dusts, but not metal dusts), it is necessary to break their chemical bonds so that hydrocarbon molecules are set free to react with oxygen. Therefore a consid-erable part of the ignition energy is used for melting, vaporizing, or cracking the dust particles to gaseous hydrocarbons. This is the reason why much more energy is always needed to ignite flammable dusts than is necessary to ignite flammable gases and vapors.

On the contrary, at metal dusts an oxidization at the particle surface takes place, which is exothermic as well.

Basically a fire or an explosion will occur when the following components coin-cide with time and volume, which is known as the “danger triangle” (see Figure 1.1):

● Fuel ● Oxygen ● Ignition source (heat)

This danger triangle is used worldwide mainly to show that three components are required to cause a fire, and if one of them is missing, combustion will not occur. Looking more into details, it is necessary to meet the additional condi-tions for each component.

1.1.1 Fuel

In this context fuel stands for the material that causes an explosive atmosphere. Although it is necessary to distinguish between gaseous, liquid, and solid fuels, a common feature between them is that combustion is sustained only within a certain explosion range, which is determined by the lower and upper explosion limits. For flammable liquids, the lower explosion limit is characterized by the so-called flash point (see Figure 1.2). Between the lower and the upper explosion limits, an explosive atmosphere always prevails.

1.1.2 Heat

In this context heat stands for the thermal energy needed to start an ignition, also called an ignition source (see Figure 1.4).

Fuel

Oxy

gen H

eat

W > MIEC > 10%

Within explosion limits

Figure 1.1 Danger triangle.

1.2 Explosiie Atmosppere 3

1.1.3 Oxygen

For all fuels, a minimum oxygen concentration (MOC) in air is necessary below which combustion cannot occur.

1.1.4 Inerting Process

It is worth mentioning the “MOC,” which is defined as the threshold of oxygen concentration below which combustion is impossible. It is expressed in units of volume percent of oxygen and is independent of the concentration of fuel (see Table 1.1). But it is to be noted that the MOC varies with pressure and tempera-ture and is also dependent on the type of inert gas.

1.1.5 Heat versus Oxygen

It has to be pointed out that there is an interrelation between the oxygen concen-tration and the energy of the ignition source: the higher the oxygen concentra-tion, the lower the need for ignition energy and vice versa.

1.2 Explosive Atmosphere

1.2.1 Explosion Limits with Flammable Liquids

In preventing fire and explosion in general, explosion limits are important. This can be explained by a simple experiment in which some lamp kerosene is poured

vol. (%)22

20

18

16

14

12

10

8

6

4

2

00 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

Upper explosion limit

Too rich

Vapor pressurecurve

Lower explosion limit

Liquid temperature °C

Upper explosion temperature °CLower explosion temperature °CFlashpoint

Explosive

Too lean

Figure 1.2 Vapor pressure/temperature curve of ethanol.

1 Basics of Fire and Explosion: Risk Assessment4

into a small coquille: when a lighted match is dipped into the liquid, it becomes extinguished. Obviously lamp kerosene is fuel!

However, when this experiment is repeated after the lamp kerosene is heated up to 45 °C, the lighted match causes an ignition, and the liquid continues to burn at its surface.

The explanation for the behavior of the lamp kerosene in the aforementioned experiment has to do with the vapor pressure of the liquid. Depending on the temperature of the liquid, a certain vapor pressure, and hence vapor concentration, is developed above the surface of the liquid. Figure 1.2 shows the vapor pressure temperature curve for ethanol and the relation between the vapor concentration at the surface of the liquid and its temperature. As ethanol is indicated by a flash point of 12°C the above mentioned experiment would lead to a flame already at room temperature.

By using the curve, temperatures can be assigned to the lower and upper explosion limits of a liquid. The temperature related to the lower explosion limit is called the flash point (°C) and is a simple and reliable way of defining the danger of flammable liquids in view of their ease of ignition. Liquids at a tem-perature lower than their flash point cannot be ignited. Therefore, the flash point ranks as the most important data when using flammable liquids and is listed in safety data sheets, for instance, indicating that they will not burn at room temperature.

In the example for ethanol, the explosion danger exists only within the explo-sion range, which is limited by the lower explosion temperature (12 °C) and the upper one (37 °C). After ignition, the flame spreads through the entire volume without any further fuel or air access. Also, it has to be taken into consideration

Table 1.1 Threshold of oxygen concentration for some gases and dusts with two kinds of inert gases (volume percent oxygen).

Gas or dust Nitrogen/air Carbon dioxide/air

Ethane 11 14Hydrogen 5 5Isobutane 12 15Methane 12 15n-Butane 12 15Propane 12 15PE-HD 16 —PE-LD 16 —Paper 14 —PMMA 16 —PP 16 —PVC 17 —

1.2 Explosiie Atmosppere 5

that ignition is not possible above the upper explosion temperature. The fuel/air mixture is, so to speak, too rich, because of a lack of oxygen. This effect is used, for example, in gasoline tanks for cars. They will never explode but may burn down when there is a leakage (access to air).

Below the lower explosion limit, the average distance between fuel molecules to each other in air is too large; hence, by means of radiation from the ignition source, no sufficient energy can be transferred to continue the ignition (the decrease of energy by radiation follows the square of the distance). Above the upper explosion limit, the concentration of fuel molecules is so high that there is no enough oxygen between them for a reaction to take place.

In this context, it has to be stated that all vapors of flammable liquids show a higher density than air; thus they will always accumulate at the bottom of a vessel.

1.2.1.1 Classification of Flammable LiquidsUntil 2009 the classification for flammable liquids depicted in Figure 1.3 was valid.

In 2009 flammable liquids were classified as hazardous substances and so have been covered in the United Nations Globally Harmonized System (GHS) of Classification and Labelling of Chemicals (UN 2013) [1].

The aim of the GHS is to have the same criteria worldwide for classifying chemicals according to environmental and physical hazards (see Table 1.2).

Now flammable liquids (see Table 1.3) are being classified according to their flash point (TF) and initial boiling point (TIBP).

°C6

F

F+

5

21

0

Label forflammable

liquids • Flammable: TF < 55 °C and > 21 °C• Highly flammable: TF < 21 °C [F]• Extremely flammable: TF < 0 °C [F+] and TIBP < 35 °C

53

Figure 1.3 System of flammable liquids (up to 2009).

Table 1.2 Criteria for flammable liquids.

Category Criteria

1 Flash point < 23 °C and initial boiling point ≤ 35 °C2 Flash point < 23 °C and initial boiling point > 35 °C3 Flash point > 23 °C and ≤ 60 °C4 Flash point > 60 °C and ≤ 93 °C

1 Basics of Fire and Explosion: Risk Assessment6

Table 1.3 Flammable liquids, classification, and labeling.

Hazard category

Pictogram Signal word Hazard statement Hazard statement codes

1 Danger Extremely flammable liquid and vapor

H224

2 Danger Highly flammable liquid and vapor

H225

3 Warning Flammable liquid and vapor H226

4 No pictogram

Warning Combustible liquid H227

Note: Aerosols should not be classified as flammable liquids.

1.2.2 Explosion Limits with Combustible Dusts

In contrast to gases and vapors, mixtures of solid fuels (combustible dusts) and air are inhomogeneous because of the effect of gravity on particles; for example, with dusts in air, the particle distribution is not constant with reference to time and space. In terms of safety, the explosion limits for dust/air mixtures are not as critical as those for vapor/air and gaseous/air mixtures.

For most combustible organic dusts, the lower explosion limit ranges between 20 and 50 g/m3. However, there are a few very sensitive dusts with a lower explo-sion limit down to 10 g/m3. For instance, a few millimeters of combustible dust settled on the floor may present an explosion hazard in the entire room when swirled up by a draft of air. To determine an upper explosion limit is difficult as it ranges in concentrations of 1 kg/m3 and above.

1.2.3 Metal Dusts

Finely dispersed airborne metallic dust can also be explosive in so far as the metal itself tends to oxidize.

In contrast to the aforementioned organic dusts, transfer into the gaseous phase is not necessary to ignite metal dusts because they react exothermally directly at their surfaces with the oxygen in air.

1.3 Hybrid Mixtures ( P7)

An increased ignition danger always exists when powder products are combined with combustible gases or vapors because the ignition energy of the latter is lower on most occasions. Furthermore it has to be taken into consideration that hybrid mixtures are already combustible when the concentration of the dust as

1.5 ­ermissiile EEqipment EEqipment ­rotection eiell 7

well as that of the gas is lower than their respective explosion limits. The needed energy to ignite hybrid mixtures is always lower than that of the pure dust cloud. Hybrid mixtures are to be expected, for example, when the powder is wet with flammable solvents.

1.4 Allocation of Explosion-Endangered Areas and Permissible Equipment ( P6)

In the ATEX 137 “Workplace Directive,” the minimum requirements for improv-ing the safety of workers potentially at risk from explosive atmospheres are laid down.

The plant management must divide areas where hazardous explosive atmos-pheres may occur into “zones.” The classification given to a particular zone and its size and location depends on the likelihood of an explosive atmosphere occur-ring and its persistence if it does.

An explosive atmosphere can be divided into zones according to IEC 60079-10-1 and 60079-10-2 [2]:

Zone 0: Area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is present continu-ously or for long periods or frequently

Zone 1: Area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is likely to occur in normal operation occasionally

Zone 2: Area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Zone 20: Area in which an explosive atmosphere in the form of a cloud of com-bustible dust in air is present continuously or for long periods or frequently for short periodsNote: Areas where piles of dust are present but where dust clouds are not present continuously, or for a long period, or frequently are not included in this zone.

Zone 21: Area in which an explosive atmosphere in the form of a cloud of com-bustible dust in air is likely to occur occasionally in normal operation

Zone 22: Area in which an explosive atmosphere in the form of a cloud of com-bustible dust in air is not likely to occur in normal operation but if it does occur will persist for a short period only

1.5 Permissible Equipment (Equipment Protection Level)

An equipment category indicates the level of protection provided by the equip-ment to be used according to zones ( T6).

Here, areas in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist prevails are indicated

1 Basics of Fire and Explosion: Risk Assessment8

with the letter G (gas). Correspondingly, areas in which an explosive atmosphere in the form of a cloud of combustible dust in air exists are indicated with the let-ter D (dust).

1.5.1 Classification of Equipment Protection Level That Is Currently in the Introductory Stage

As already discussed, explosive atmospheres are divided into zones based on the probability that such an atmosphere will occur. But experience has shown that in some situations, a risk assessment would give the plant operator more flexibility. On this account and to facilitate a dependable risk assessment approach to make equipment selection easier, “equipment protection levels” (EPLs) have been introduced. EPLs identify and characterize all equipments according to the igni-tion risk they might produce.

According to IEC60079-0:2011 [3], equipment for use in explosive atmos-pheres is classified into the following EPLs (with distinguishing signs such as M for mining, G for gases, and D for dusts).

EPL Ma: Equipment for installation in a mine susceptible to firedamp, having a “very high” level of protection, which has sufficient security that it is unlikely to become an ignition source in normal operation, during expected malfunc-tions, or during rare malfunctions, even when left energized in the presence of an outbreak of gas

EPL Mb: Equipment for installation in a mine susceptible to firedamp, having a “high” level of protection, which has sufficient security that it is unlikely to become a source of ignition in normal operation or during expected malfunc-tions in the time span between there being an outbreak of gas and the equip-ment being de-energized

EPL Ga: Equipment for explosive gas atmospheres, having a “very high” level of protection, which is not a source of ignition in normal operation, during expected malfunctions, or during rare malfunctions

EPL Gb: Equipment for explosive gas atmospheres, having a “high” level of pro-tection, which is not a source of ignition in normal operation or during expected malfunctions

EPL Gc: Equipment for explosive gas atmospheres, having an “enhanced” level of protection, which is not a source of ignition in normal operation and which may have some additional protection to ensure that it remains inactive as an ignition source in the case of regular expected occurrences

EPL Da: Equipment for explosive dust atmospheres, having a “very high” level of protection, which is not a source of ignition in normal operation, during expected malfunctions, or during rare malfunctions

EPL Db: Equipment for explosive dust atmospheres, having a “high” level of pro-tection, which is not a source of ignition in normal operation or during expected malfunctions

EPL Dc: Equipment for explosive dust atmospheres, having an “enhanced” level of protection, which is not a source of ignition in normal operation and which may have some additional protection to ensure that it remains inactive as an ignition source in the case of regular expected occurrences