Page 17 - Dust Explosions in the Process Industries

xiv Dust Explosions in the Process Industries

7.20 Influence of adding inert dust to the combustible dust
on the ignitability and explosibility of dust clouds 542
7.21 Hazard classification of explosible dusts 543

8 Electrical Apparatuses for Areas Containing Combustible Dusts 549
8.1 Introduction 549
8.1.1 Background and objectives of chapter 549
8.1.2 Basic similarities and differences between dusts and gases 550
8.1.3 The “Atex 1OOa” directive gives unclear signals
with regard to dusts 551
8.1.4 Scope of the IEC standards on dusts 552
8.2 Classification of areas containing combustible dusts 553
8.2.1 What is area classification? 553
8.2.2 Definition of zones according to the three-zone concept 553
8.2.3 International standards 554
8.2.4 Need to revise area classification standards to include
dust fires as a hazard in its own right 557
8.3 Why different electrical apparatus design criteria are needed
for areas with combustible dust and explosive gas atmospheres 557
8.3.1 Influence of inertial forces on the movement of dust particles 557
8.3.2 Thermal hazards associated with accumulation of dust
layers inside electrical apparatus enclosures 558
8.3.3 Thermal hazards associated with accumulation of dust layers
on external surfaces of electrical apparatus enclosures 559
8.4 Enclosing potential ignition sources to prevent hazardous ingress of dust 560
8.4.1 The IP code for prevention of dust ingress 560
8.4.2 Design of apparatus to prevent ignition of dust clouds
and dust layers by hot enclosure surfaces 562
8.4.3 Other requirements to IP enclosures 564
8.4.4 International standards for design of enclosures for electrical
apparatuses for areas containing combustible dusts:
an overview 564
8.4.5 The IEC standard for pressurized electrical equipment
enclosures for areas containing combustible dust 565
8.4.6 Encapsulation by molding 567
8.4.7 Why the concept of flameproof enclosures is not relevant
for combustible dusts 567
8.5 Intrinsically safe electrical apparatuses 568
8.5.1 The original concept for gases and vapors 568
8.5.2 The situation with dusts 568
8.5.3 The new IEC Ex“iD” standard for dusts 570
8.5.4 Minimum ignition energy, a universal ignition sensitivity
parameter for the design of electrical apparatuses that are
intrinsically safe in the presence of explosive dust clouds 571
8.6 Summary and conclusions 574
8.6.1 “Atex 1OOa” directive 574
8.6.2 Area classification 574