Dust Explosions-Safe Handling of Solids

7/24/2019 Dust Explosions-Safe Handling of Solids

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Dust Explosions

Safe handling of solids


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Dust Explosion Control

Introduction

Basic concepts of dust explosions

Ignition sources

Electrostatic ignitions

Deadly Dust II (Video)


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Solids Handling

The safe handling of solids in becoming more importantbecause the production and the processing of solids isincreasing.

ore chemicals are being produced and handled as solidsto eliminate reactions !ith "olatile and ha#ardous sol"ents.

Emphasis to produce products as po!ders ("ersus li$uids)to eliminate the need to handle empty containers.

ore chemicals are transported in reusable %super sac&s'


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Flammable gases & vapors

hen dealing !ith flammable

gases and "apors the

generally accepted ma*or

re$uirements for a fire or

explosion are fuel oxygen

and ignition.

In chemical industry they try

to eliminate or reduce one ormore of the sides of the

triangle.


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Explosive Dusts HexagonFor Dust explosions there is

a more complex situation

needed before an

explosion occurs. +uel , any dust such as

chemicals grain !ood dust

flour polymers lint etc.

oisture , !hen fuel contains ahigher moisture content then the

dust burning process is

extinguished.


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Explosive Dust Hexagon (cont)

Dust and -ir Suspension

/articles must be belo! a certain minimum si#e to

be able to be suspended.

/article loading (concentration) must be bet!een

certain limits

, 0o!er 12 to 32 g4m5

, 6pper 1 to 3 &g4m5

Dust loading must be fairly uniform to be explosi"e.


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Effects of suspension

In the upper picture a bin(!ith a "ent) that containsdust is ignited.

In the lo!er picture anadditional pile of dust !aslocated in the path of the"enting flame.

The dust becomesuspended and caused asecondary explosion.


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Prevention of Dust Explosions

Eliminate fuel

/re"ent dust suspensions

-dd moisture

7eep fuel belo! 0+0

8educe oxygen belo! 9:Eliminate ignition sources


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Flammable Dusts

-cetamide

-dipic -cid

-luminum

Barley

:arbon

:ellulose

:offee

:orn

Epoxy 8esin

Iron

il&

;ylon

/aper

/olystyrene

Starch

Steel

Sucrose

heat

ood


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Minimum gnition Energies

Dusts

-luminum =2 m>

:orn (1?moisture) ==2 m>Epoxy 8esin @=2 m>

il& /o!der A2 m>

Sugar 52 m>

Sulphur @=2 m>

Versus Vapors

-cetone = m>

-crolein 2.= m>Ben#ene 2.1 m>

:arbon Disulfide @2.= m>

eptane 2.1 m>

Toluene 2.1 m>


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Elimination of suspensions

Cood house&eeping , If dust is not lying around it

cannot get suspended !hich results in an explosi"e

situation. Dust on beams is especially a problem since an

explosion in one part of the facility !ill cause the dust to

be suspended and exacerbate the ha#ard.

9ften moisture is added to solids to minimi#e

suspensions and also to reduce explosi"ity.


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!eeping fuel belo" #F#

Venting of area and4or hooding used to collect dust. Thecollected dust is sent to a collections system such as abag house cyclone or electrostatic precipitator to remo"eand collect the dust.

/elleti#e solids to minimi#e amount of material in airsuspendable si#e range.

Inerting the area !here dust !ill be present to reduceoxygen to belo! 9:. 9ften not practical in largesystems. Inerting processes !ill be discussed at anothertime.


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Common gnition Sources

- 6nited 7ingdom study of dust explosions and

fires cited the follo!ing causes of ignition

sourcesechanical =?

9"erheating =?

9pen flames =A?Static Electricity ==?

elding ?

Electrical 5?

9ther 1F?


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Static Electricit$ Disc%arges

Static electricity is thee fourth largest cause of

ignition sources in dust explosions.

Because of the nature of solids the handling

and transportation of solids can actually be the

cause of the static electricity


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Introduction


Ignition sources


-ccumulation of charges

Electrostatic discharges



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Fundamentals of Static Electricit$

andling solids often leads to the accumulation of staticelectricity. This accumulation can lead to a spar& thatthen ser"es as an ignition source.

9ne method to pre"ent static electricity is to pre"ent theaccumulation of charge.

:harge -ccumulation

:ontact and +rictionalDouble layer

Induction

Transport


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Contact and Frictional C%arging

Dust transport

e.g. pneumatic transport of po!ders4solids

/ouring po!ders

e.g. pouring solids do!n chutes or troughs

Cears and belts

e.g. transporting charges from one surface to

another


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Double la$er c%arging

:aused by friction at interfaces on a microscopic

scale.

0i$uidGli$uid

SolidGli$uid

SolidGsolid

CasGli$uid

CasGsolid


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nduction c%arging 9ccurs !hen an isolated

conductor is sub*ect to a electricfield. :harges of different polarityare induced on opposite sides. If

an earthed electrode touches orapproaches the body then thecharges closest to electrode flo!a!ay lea"ing the body !ith a netcharge of opposite sign.

9ccurs by !al&ing across carpet. ;onconducti"e shoes are a

problem.


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C%arging b$ ransport

8esults from a charged dust li$uid or solid

particles settling onto a surface and transporting

their charges to this ne! surface.The rate of charge accumulation is a function of

the rate of transportation.

0ightening is an example of this type of chargingphenomenon.


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Introduction


Ignition sources


-ccumulation of charges

Electrostatic discharges



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Electrostatic gnitions

Static electric ignitions are the result of transferring the accumulatedcharges to another surface "ia a discharge.

The accumulated charge may be safely lea&ed a!ay to earth by

grounding. If energy of discharge exceeds IE then fire or explosion. Static electric Discharges

Spar&s

/ropagating brushBrush

:orona

:onical pile (aurer)


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Spar' disc%arges

Discharges bet!een t!o

conductors.

Very energetic !ithenergies ranging up to

=2 >oules.

:an ignite flammable

gases and dusts


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Propagating brus% disc%arge

9ccur bet!een a conductorand a nonGconducti"e lining.

Very energetic can be greater

than =22 >oules. a*or contributor to static

electricity ignitions.

If brea&do!n "oltage of liningis less than H &V thenpropagating brush dischargesare not possible becausecharge !ill pass through lining.


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rus% disc%arge 9ccurs bet!een nonGconductor

and a conductor.

Energetic @ Am>

;onconducti"e lining or surface

must ha"e a brea&do!n"oltage greater than H&V and athic&ness greater than 1mm.

:an ignite flammable "aporsbut rarely ignites flammable

dusts. ;onconducti"e coating can be

a layer of the po!dered solid


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Corona disc%arge

:orona discharge similar tobrush discharge but occurs

!hen electrode more pointed.

9ccurs o"er a longer period oftime than a spar& and maygi"e faint glo! and hissingsound.

:an cause ignition of

flammable gas mixtures !ithlo! IE.

6sually considered nonGincendi"e to dusts.


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Conical pile disc%arge (Maurer disc%arge) 9ccur bet!een sliding solids and

charged air. Vessels larger than = m5. ;onconducti"e particles !ith

resistance greater than =2=29hmm /articles larger than =mm diameter 8elati"ely fast filling rate greater

than 2.A &g4s

Energetic J = >oule :an ignite flammable dusts and

"apors


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Preventing Electrostatic gnitions

andout gi"es athought process

procedure to

pre"ent

electrostatic

ignitions from dust

or dust4flammable

"apor systems.


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Deadl$ Dusts

9pening scene is an actualexplosion captured by a TVphotographer filming an -d.

ade for grain handlers and dealsprimarily !ith grain silos.

To ma&e this "ideo rele"ant toother industries e"ery time the!or& grain is used substitute it!ith chemical dusts flour starchpharmaceutical dusts fiberspolymers plastics etc.

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Dust Explosions-Safe Handling of Solids