understanding dust explosions - the role of powder...

Understanding Dust Explosions -the Role of Powder Science and

TechnologyTechnology

Rolf K. EckhoffProfessor emeritus, University of Bergen,

Dept. of Physics and Technology, B NBergen, Norway.

Scientific/technical adviser,Scientific/technical adviser, Øresund Safety Advisers AB, Malmö, Sweden.

Øresund Safety Advisers AB

What is a dust explosionp

??? Øresund Safety Advisers AB

Controlled, vented wheat grain dust explosion experiment in a 500 m3 silo cell in Norway in 1980


p y

Destructive dustDestructive dust explosion experiment (maize starch) in a(maize starch) in a 500 m3 silo in Norway in 1982in 1982


Objective of present paper

h h d d di f.. to show that adequate understanding of dust explosion phenomena in the processdust explosion phenomena in the process industries requires knowledge of central t i f d i d t h ltopics of powder science and technology


Basic powder-technology-related topics that are essential for understanding

dust explosionsdust explosions

● Particle and powder characterization● Particle and powder characterization● Production of fine particles by crushing,

i di d b igrinding and abrasion● Powder mechanics (mechanical strength of ( g

powder deposits)● Particle segregation in powder bedsParticle segregation in powder beds● Dust cloud generation/powder

dispersion/powder fluidizationdispersion/powder fluidization


Powder-technology-related properties of dust l d i fl i th i i iti iti itclouds influencing their ignition sensitivity

and burning rate (explosion violence)g ( p )

● Distribution of primary particle sizes in● Distribution of primary particle sizes in dust/powder

● Particle shape characteristics● Degree of agglomeration of dust particles, i.e. g gg p ,

real `particle' size distribution in dust cloud● Dust concentration distribution in cloud● Dust concentration distribution in cloud● Degree of turbulence of cloud.


• In some special situations such as in air jet mills, explosive dust clouds areair jet mills, explosive dust clouds are generated in situ, i.e. the dust particles b d d i th i thbecome suspended in the air as they are producedp


• In most cases, explosive dust clouds are generated by re-entrainment and re-dispersion of powders and dusts thatre-dispersion of powders and dusts that have been produced at an earlier stage and allowed to accumulate as layers or heapsheaps


• Re suspension/re dispersion of• Re-suspension/re-dispersion of powder/dust occurs intentionally during pneumatic transport in pipes, by filling of silos and hoppers in fluidized beds insilos and hoppers, in fluidized beds, in spray dryers and other types of dryers, in mixers and blenders and in screening machinerymachinery


• Unintentional re-dispersion may be due p yto uncontrolled flow (“flooding”) in funnel-flow silos and hoppers bursting of sacksflow silos and hoppers, bursting of sacks and bags containing powder, or by sudden blasts of air generated by primary dust explosions elsewhere in the plantexplosions elsewhere in the plant


Factors influencing the ignitability andFactors influencing the ignitability and explosibility of dust clouds in air (I)

• Chemical composition of the dust including its• Chemical composition of the dust, including its moisture content

• Distributions of particle sizes and shapes in theDistributions of particle sizes and shapes in the dust, determining the specific surface area of the dust in the fully dispersed statedust in the fully dispersed state


M t i l th t i d t l iMaterials that can give dust explosions

• Natural organic materials (grain, wood, linen, sugar etc )sugar, etc.)

• Synthetic organic materials (plastics, organic pigments, pesticides, pharmaceuticals etc.).

• Coal and peatCoal and peat• Metals (aluminium, magnesium, titanium, zinc,

i t )iron, etc.)


Heats of combustion (oxidation) of various substances per mole O2 consumed


p 2

I fl f ifi f f l i i d fInfluence of specific surface area of aluminium powder of maximum rate of rise of explosion pressure during dust explosions in air in a standard 1 m3 closed bomb. From Bartknecht (1978)


Influence of mean particle diameteron minimum explosive dust concentration for three different dusts in the 20-litres USBM l dUSBM closed explosion bomb. F H t b dFrom Hertzberg and Cashdollar (1987)


Minimum electric sparkMinimum electric spark ignition energy (MIE) of clouds in air of three different powders, as functions of particle size From Bartknechtsize. From Bartknecht (1987)

Theoretical line for poly-ethylene from Kalkert and ScheckerKalkert and Schecker (1979)


I fl fInfluence ofchemistry(starch or protein) andprotein) andspecific surface area of natural organicorganic materials on maximum rate of pressure rise inpressure rise in closed 1.2 liter Hartmann bomb.

From Eckhoff (1977/1978)


Influence of chlorine in dust material molecule on maximum explosionexplosion pressure and maximum rate ofmaximum rate of pressure rise in 1 m3 standard ISO vessel, for various particle sizes.

From Bartknecht (1978)(1978)


Factors influencing the ignitability and explosibility of dust clouds in air (II)explosibility of dust clouds in air (II)

• Degree of dispersion (or agglomeration) of dust particles i e the effective specific surfacedust particles, i.e. the effective specific surface area available to the combustion process in the d t l d i th t l i d t i l it tidust cloud in the actual industrial situation


Illustration of a perfectly dispersed dust cloud consisting of primary particles only, and a cloud consisting of agglomerates. From Eckhoff (2003)


• One can define a theoretical dispersibility Dmax of a particular material as the mass that could ideally be dispersed into primary particles per unit of work, W, performed:

Dmax = 1/Wminmax min

• But no realistic dispersion process can be one• But no realistic dispersion process can be one-hundred per cent efficient. This may be accounted for by incorporating an efficiency factor K:for by incorporating an efficiency factor, K:

Dreal = K/ Wmin , 0 < K < 1


Inter-particle forces in powders include:● van der Waals' forces● Electrostatic forces● Electrostatic forces ● Forces due to liquid bridges and q g

capillary under-pressure


• In the dispersion of cohesive powderscomposed of very small particles (<5-10 µm) inter-particle forces play a major roleµm), inter particle forces play a major role, and inter-particle bonds cannot be broken

l h i l lunless the particle agglomerates are exposed to very large shear forces.exposed to very large shear forces.


• This means that complete dispersion into i ti l i l ibl i hi hprimary particles is only possible in high-

velocity flow fields, or if the particles are y , pexposed to high-velocity impacts


Cross section of nozzle for dispersing p gagglomerates of cohesive dust particles. From Yamamoto and Suganuma (1984)


Effective particle sizeEffective particle size distributions of an airborne talc dust after dispersal at different flow velocities through gorifices. Rw is the percentage by weight of the effective ‘particles’ that are larger than the size x. From Yamamoto and Suganuma (1984)


Max rates of pressure rise versus dust concentration in dust explosions in aMax. rates of pressure rise versus dust concentration in dust explosions in a 1.2-litre closed bomb with clouds in air of maize starch containing different

fractions of agglomerates. Eckhoff and Mathisen (1977/1978) Øresund Safety Advisers AB

Factors influencing the ignitability and explosibility of dust clouds in air (III)

• Dust concentration


Range of explosive dust concentrations for maize starch, compared with g p , ptypical range of concentrations relevant to industrial hygiene, and with a

typical density of dust deposits/layers. From Eckhoff (2003)


Influence of dust concentration on explosion rate and ignition sensitivity of dust cloud


g y

Influence of average dust concentration in a maize starch explosion in an experimental silo cell of volume 236 m3 height 22 m andin an experimental silo cell of volume 236 m3, height 22 m and

length-to-diameter 6, on the maximum explosion pressure generated in the silo. 5.7 m2 vent opening in silo roof. Ignition close to silo

bottom From Eckhoff (2003) Øresund Safety Advisers AB

bottom. From Eckhoff (2003)

Influence of averageInfluence of average dust concentration on the minimum electricthe minimum electric spark ignition energy (MIE) of clouds of an ( )anti oxidant in air, in the standard 1 m3 closed vessel. From Bartknecht (1979)


Factors influencing the ignitability andFactors influencing the ignitability and explosibility of dust clouds in air (IV)

Di t ib ti f i iti l t b l i th t l• Distribution of initial turbulence in the actual cloud

• Possibility of generation of explosion induced• Possibility of generation of explosion-induced turbulence in the still unburnt part of the cloud ( f )(location of ignition source important parameter)


Influence of initial turbulence on minimumturbulence on minimum electric spark ignition energy (MIE) of a dustenergy (MIE) of a dust cloud. Experiments with various dusts in a 20-litres closed explosion bomb. From pGlarner (1984)


Influence of initial turbulence on explosion rate of a dust cloud. Experiments with 420 g/m3 of lycopodium in air in a 1.2-litre closed explosion bomb. Bars: ±1

std dev From Eckhoff (1977) Øresund Safety Advisers AB

std. dev. From Eckhoff (1977)

Time of arrival of bituminous coal dust/air flame as aTime of arrival of bituminous coal dust/air flame as a function of distance from ignition point at closed end of gallery of length 260 m and diameter 3.2 m. Pressure at closed end as a function of time. Nominal average dust concentration 500 g/m3 (From Fischer, 1957)


The role of powder science andThe role of powder science and technology in dust explosion

prevention and control in practice


Inherently safe process designInherently safe process design

• Avoidance of undesired particle segregationAvoidance of undesired particle segregation


S ti h iSome segregation mechanisms

• Percolation of small particles through a bed f l b ib tiof larger ones by e.g. vibration

• Air current segregationAir current segregation• Segregation due to differences in density,

shape shape and surface properties of particlesparticles


Illustration of migration of a fraction of small particlesIllustration of migration of a fraction of small particles through a bed of larger particles (percolation). From

Eckhoff (1976a) Øresund Safety Advisers AB

Eckhoff (1976a)

I h tl f d iInherently safe process design

Example:• Application of powder mechanics to silo design

to avoid undesired segregation and dust cloudto avoid undesired segregation and dust cloud formation


Ill t ti fIllustration of uncontrolled flow, with “flooding” ofwith “flooding”, of bulk material in funnel flow causingfunnel flow causing generation of explosive dustexplosive dust clouds


Illustration of generation of gsmouldering nests in stagnant zones in funnel flow silos


Smooth controlled flow and re-mixing of segregated bulk material in mass flow il d i di hsilo during discharge

of stored bulk t i lmaterial


Casting of MgFeSi alloysCasting of MgFeSi alloysOld casting method:O d cast g et od• Large solidification flakes

Substantial segregation of Mg during solidification• Substantial segregation of Mg during solidification• Material with high Mg content most brittle and

created most fine dustTherefore:• More very fine dust then necessary was produced• More Mg in fine dust than average content• More Mg in fine dust than average content• Because MIE decreases with decreasing particle size

d i i M h fi d fand increasing Mg content, too much fine dust of very low MIE was generated


C ti f M F Si llCasting of MgFeSi alloys

New casting method:• Small solidification flakes • Minimal segregation i e less fine dust andMinimal segregation, i.e. less fine dust and

less Mg in finest dustTh fTherefore:• A better main productA better main product• Considerably reduced dust explosion

h dhazard


EXPLOSION PREVENTION

PREVENTING PREVENTING

EXPLOSION MITIGATION

EXPLOSIVE DUST CLOUDS

IGNITION SOURCES

Process design to prevent undesired

Smouldering combustion in dust,

Explosion-pressure resistant constructionp

generation of dust clouds and particle size segregation (“Inherent safety”)

dust fires

( y ) Intrinsic inerting of dust cloud by

Other types of open flames (e.g. hot

Explosion isolation (sectioning)y

combustion gases ( g

work) ( g)

Inerting of dust cloud by adding inert

Hot surfaces (electrically or

Explosion venting

y gdust

( ymechanically heated)

Keeping dust conc. outside explosive

Heat from mechanical impact

Automatic explosion suppressionp

range p

(metal sparks and hot-spots)

pp

Inerting of dust clouds by N2, CO2

Electric sparks and

Partial inerting of dust cloud y

and rare gases

parcs and electrostatic discharges

gby inert gas

Good housekeeping (dust removal/cleaning)


g)

Preventing explosive dust clouds

I i f d l d b ddi i d• Inerting of dust cloud by adding inert dust(mixing/segregation dispersion)(mixing/segregation, dispersion)

• Keeping dust conc. outside explosive range


Explosion mitigationExplosion mitigation


Explosion isolation (sectioning)

i.e. closure of connections between process unitsi.e. closure of connections between process units in time to prevent passage of propagating dust flame. Depends on flame propagation speed,flame. Depends on flame propagation speed, which in turn, for a given chemistry, depends on

- particle size- particle size- degree of dust dispersion - dust concentration - dust cloud turbulencedust cloud turbulence


Explosion isolationExplosion isolationIllustration of an explosion isolation system at a transfer point between two horizontal conveyors, based on strong reinforcedbased on strong reinforced concrete walls and an explosion proof rotary lock that is interlocked with pressure sensors detecting any abnormal pressure rise on either sideeither side.

From Bühler, Switzerland


Explosion ventingp g


Explosion venting

The maximum explosion pressure in a vented fdust explosion, Pred, is a result of two

competing processes, viz.:● Burning of the dust cloud within the enclosure which produces heat and henceenclosure, which produces heat and hence increases the pressure● Flow of unburnt burning and burnt dust cloud● Flow of unburnt, burning and burnt dust cloud out of the enclosure through the vent, which relieves the pressurerelieves the pressure


T pical ent panel that opens in a controlled aTypical vent panel that opens in a controlled way at a specified internal over-pressure


Explosion ventingExplosion venting

Influence of initial dust cloud turbulence on maximum pressure inInfluence of initial dust cloud turbulence on maximum pressure in vented maize starch explosions in a 64 m3 chamber. Dust

concentration 250 g/m3. Vent size 5.6 m2. From Tamanini (1989) Øresund Safety Advisers AB

g ( )

Explosion venting

R lt f t dResults from vented maize starch and wheat grain dust explosions in g pa 500 m3 silo cell. Comparison with P / tPred/vent area correlations used in various countries.

From Eckhoff (2003)


Explosion ventingExplosion venting

Results from vented i t h l i imaize starch explosions in

a 20 m3 silo cell, demonstration the markeddemonstration the marked influence of the mode of dust cloud generation on th i l ithe maximum explosion pressure Pred in the vented silo.vented silo.

From Eckhoff (2003)


Explosion venting

Influence of locationInfluence of location of the ignition point in a 236 m3 slim silo cell on the maximum explosion pressure P in the ventedPred in the vented silo. From Eckhoff (1990)


Fig 113Explosion venting:

Fig 113p gQuenching tube


Fig 114E l i ti Fig 114Explosion venting: Quenching-tubeQuenching tube


Explosion venting:p gQuenching tube


Explosion suppressionExplosion suppression


Fig 118Fig 118

Explosion pressure sensor for suppression systems Øresund Safety Advisers AB

Fig 119Fig 119

Explosion suppression system Øresund Safety Advisers AB

Fig 120Fig 120

L (45 l)Large (45 l)explosionexplosionsuppressorfor powder

tsuppressant


Explosion suppression

● Extinguishing agent permanently● Extinguishing agent permanently pressurized

● Large-diameter discharge orificeV f t i f l f i di t● Very fast opening of valve for immediate release of extinguishing agent by meansrelease of extinguishing agent by means of an explosive charge


Explosion suppression

Illustration of the mass of suppressant required and delivered as functions of time, for obtaining reliable, critical and failed suppression. From Moore (1987)


Explosion suppressionExplosion suppression

Illustration of how failed suppression can result ppfrom too late of suppressant injection, too low

injection rate and too small quantity ofinjection rate, and too small quantity of suppressant injected. From Moore (1987)


Secondary dust explosionsSecondary dust explosions


How thick does a dust layer have toHow thick does a dust layer have to be to produce hazardous volumesbe to produce hazardous volumes

of explosive dust cloudp

?? Øresund Safety Advisers AB

Potential dust explosion hazard of even very thin layers of combustible dustvery thin layers of combustible dust


We must understand:We must understand:

• Entrainment of dust particles from layers b air flo sby air flows

• Movement of dust particles suspended inMovement of dust particles suspended in air flows


Concluding statement

Hopefully this lecture has contributedHopefully this lecture has contributed to the appreciation of knowledge of

powder science and technology as an essential premise for a genuinean essential premise for a genuine understanding of dust explosionsg p


understanding dust explosions - the role of powder...

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