Dust ExplosionsImplementation of the European ATEX Directives

Nico VerslootTNO Prins Maurits Laboratory

O What is a dust explosion? S Fast burning of a mixture of a flammable dust with air

O Conditions for an explosionS Flammable dustS Oxygen (usually from air)S Ignition source confinement

explosivemixture

ignitionsource

DUST EXPLOSIONS

O Dust explosions in confined spacesS Because of limited volume, pressure will increaseS If pressure exceeds equipment design pressure:

equipment will be destroyed

DUST EXPLOSIONS

t0 : point of ignition

p : explosion pressure

dp/dt : rate of pressure rise

p

r

e

s

s

u

r

e

time

DUST EXPLOSIONS

O Dust explosion characteristicsS Explosion possibilityS Ignition sensitivityS Explosion effects

O Determined by standard or special tests

O Necessary for prevention and protection measures

DUST EXPLOSIONS

O Particle size (distribution) S Usually a flammable dust becomes explosive at

particle sizes smaller than 0.5 mm (500 m)S Special attention to coarse fractions with fine dust!

O Explosion limitsS Lower Explosion Limit (LEL)

Below LEL no explosion because not enoughflammable dustFor dusts usually around 30 g/m3

S Minimum Oxygen Concentration (MOC)Below MOC no explosion because not enoughoxygen

EXPLOSION POSSIBILITY

O Minimum Ignition Energy (MIE) S Smallest amount of energy of a spark just capable of

igniting an optimal dust/air mixtureO Minimum Ignition Temperature (MIT)

S Minimum temperature at which a mixture of dust and air spontaneously ignites in contact with a hot surface

O Smouldering TemperatureS Minimum ignition temperature of a dust layer (5 and

15 mm)Demonstration determination of MIE

IGNITION SENSITIVITY

O Maximum Explosion Overpressure(Pmax) S Pressure to be expected at dust explosionS For dusts usually around 8 to 11 bar

O Maximum Rate of Pressure Rise(dp/dt)maxS Measure of how fast the explosion reaches PmaxS Related to KSt value via Cubic Law:

KSt = (dp/dt)max V1/3

EXPLOSION EFFECTS

EXPLOSION EFFECTS

O Based on KSt different classes fordusts

Class St1: 0 < KSt < 200 barm/sClass St2: 200 < KSt < 300 barm/sClass St3: KSt > 300 barm/s

O Avoiding formation of explosivemixtures

O Avoiding ignition sources

O Explosion protection (minimisingexplosion effects)

PREVENTION & PROTECTION

O Dust-proof equipmentO Local extraction of dustO Avoid horizontal surfacesO Good Housekeeping (no dust layers

of more than 0.1 mm)O Removal of oxygen by e.g. nitrogen

or carbon dioxide

AVOIDING EXPLOSIVE MIXTURES

AVOIDING EXPLOSIVE MIXTURES

O Trivial ignition sourcesS Welding, cutting, burning, smoking, fires

O Operational ignition sourcesS ProcessS EquipmentS Malfunctioning of equipment

AVOIDING IGNITION SOURCES

O Measures via e.g. working permitsS Remove (or protect) flammable dust or other

flammable materials at the work place S Avoid transportation of flammable materials to the

working place (via pumps, pipes and transporters)S Assure that equipment is in good conditionS Fire extinguishers within reachS No smoking allowed at the work place

TRIVIAL IGNITION SOURCES

O Hot surfaces S At a maximum of 75 C below smouldering

temperature, or,S At a maximum of 2/3 of the Minimum Ignition

TemperatureS Depends on which one is the lowest!

O Open fires and hot gasesS No explosion venting in the working place! S Direct heating

OPERATIONAL IGNITION SOURCES

OPERATIONAL IGNITION SOURCES

O Mechanical sparksS Reduction of rotation velocities

below 1 m/s : low risk on ignition1 10 m/s : investigate ignition sensibilityabove 10 m/s : high risk on ignition

O Electrical equipmentS Dust-proofS Maximum surface temperatureS CE certification (ATEX 95)

O Static electricityS EarthingS Prevent electrical charging of materialsS Risk depends strongly on Minimum Ignition Energy

O Chemical exothermic reactionsS CoolingS Reduction of storage time of bulk materialsS Temperature measurements in storageS Measurement of carbon monoxide

OPERATIONAL IGNITION SOURCES

O Explosion resistant equipmentS Equipment can withstand explosion pressure

O Explosion ventingS Explosion panelsS Bursting discs

O DecouplingS Flame quenchersS Quick closing slide valves

O Explosion suppressionS Explosion is suppressed by injection of fire

extinguishing powder

EXPLOSION PROTECTION

EXPLOSION PROTECTION

O Two new directivesS ATEX: Atmosphres ExplosivesS Due to harmonisation of European trade marketS Directives will have to be incorporated into laws of

member states and therefore become law!S As per July 1, 2003

O DirectivesS 1999/92/EC (ATEX 137)S 94/9/EC (ATEX 95)

THE ATEX DIRECTIVES

O Directives become active inS European Economic Area (European Member States)

Austria ItalyBelgium LuxembourgDenmark NetherlandsFinland PortugalFrance SpainGermany SwedenGreece United KingdomIreland

S European Free Trade AssociationIcelandNorwayLiechtenstein

THE ATEX DIRECTIVES

O Directive 1999/92/EC (ATEX 137)S On minimum requirements for improving the safety

and health protection of workers potentially at risk from explosive atmospheres

O Obligations of the employerS Prevention of and protection against explosionsS Assessment of explosion riskS Classification of places where explosive

atmospheres may occur (area classification)S Use of certified equipment and explosion protectionS Providing an Explosion Protection Document

1999/92/EC ATEX 137

O Assessment of explosion riskS Likelihood that explosive atmospheres will occur and

their persistenceS Likelihood that ignition sources, including

electrostatic discharges, will be present and becomeactive and effective

S Installations, substances used, processes, and theirpossible interactions

S Scale of the anticipated effectsS Places which are or can be connected via openings

to places in which explosive atmospheres may occurshall be taken into account

1999/92/EC ATEX 137

O Area classificationS Classification into zones according to Annex I of the

DirectiveS Practical guidelines:

S NPR7910-2 (The Netherlands)S Marking of the zones at point of entryS Three zones:

S 0, 1 and 2 for gas explosionsS 20, 21 and 22 for dust explosions

1999/92/EC ATEX 137

O Zones for potential danger on dustexplosionsS Zone 20

S A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, orfor long periods or frequently

S Zone 21S A place in which an explosive atmosphere in the form of a

cloud of combustible dust in air is likely to occur in normaloperation occasionally

S Zone 22S A place in which an explosive atmosphere in the form of a

cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only

1999/92/EC ATEX 137

O Area classificationS Normal operation

S Installations are used within their design parameters

S Continuously, long periods or frequently (zone 20)S Guidance by NPR7910-2: more than 1000 hours/year

S In normal operation occasionally (zone 21)S Guidance by NPR7910-2: within 10 and 1000 hours/year

S Not likely or short periods (zone 22)S Guidance by NPR7910-2: less than 10 hours/year

1999/92/EC ATEX 137

O Explosion Protection DocumentS Explosion risks have been determined and assessedS Adequate measures will be taken to attain the aims

of the directiveS Areas classified into zones 20, 21 and 22S Minimum requirements set out for non-hazardous

places with equipment required for the safe operation of equipment located in hazardous places

S Workplace and work equipment, including warningdevices, are designed, operated and maintained withdue regard to safety

1999/92/EC ATEX 137

O Explosion Protection DocumentS Arrangements made for the safe use of work

equipmentS Document has to be revised when the workplace,

work equipment or organisation of the workundergoes significant changes, extensions orconversions

1999/92/EC ATEX 137

O Directive 94/9/EC (ATEX 95)S On the approximation of the laws of the member

states concerning equipment and protective systemsintended for use in potentially explosiveatmospheres

O Implies CE certificationS To be done by Notified Body (except for category 3

equipment)S All new equipment sold and/or installed after July 1,

2003, must comply with the ATEX 95 directive!

94/9/EC ATEX 95

O Equipment groupsS Group I : (in underground parts of) minesS Group II : other places

O Equipment categoriesS Group I

S Category M1 : with a very high level of protectionS Category M2 : with a high level of protection

S Group II S Category 1 : with a very high level of protectionS Category 2 : with a high level of protectionS Category 3 : with a normal level of protection

G added for gas, D added for dust

94/9/EC ATEX 95

O Levels of protectionS Category 1 very high

S Two independent means of protection, or,S Safe even when two faults occur independently

of each otherS Category 2 high

S Suitable for normal operation and frequentlyoccurring disturbances, or,

S Equipment where faults are normally taken intoaccount

S Category 3 normalS Suitable for normal operation

94/9/EC ATEX 95

O Link between ATEX 137 and ATEX 95S ATEX 137 gives category of equipment according to

ATEX 95 to be used in the different zones

S Zone 20: category 1D equipment

S Zone 21: category 1D or 2D equipment

S Zone 22: category 1D, 2D or 3D equipment

ATEX 137 AND ATEX 95

9999 II 2 D EEX p IIB T4 135 CCE marking

ID NoBo

explosion protection

group II, category 2, dust

explosion protectionelectrical equipment

EN standards

temperature class

CE MARKING

Notified Bodyinvolved!

94/9/EC ATEX 95

THE ATEX DIRECTIVES

Comply with ATEX 137At modifications, extensions and changesof the work places fullycomply with ATEX 137

Fully comply with ATEX 137 before June 30, 2006

Work places

Comply with ATEX 95Comply with ATEX 137, Annex II, part A

Equipment

First use afterJune 30, 2003

First use untilJune 30, 2003