by Pieter Coetzee

MIE (registered person)

Explosion Prevention Consultant

+27 79 507 3432

pieter.exp@gmail.com

Index

• Historical background

• Definitions

• Classification

• Related risks

• Facility evaluation

• Controls

– Prevention

– Mitigation

Index2

• Around 50 explosions are reported per year, ranging from small deflagrations to building destroying detonations which lead to large numbers of fatalities.

• They are usually associated with the grain and mining industries, however they can occur whenever a process uses particulate materials, either as feed stocks, intermediates or products.

Combustible Dust

Historical background

• SANS 60079-10-2 INTRODUCTION STIPULATES:

– Dusts, as defined in this standard, are hazardous because when they are dispersed in air by any means, they form potentially explosive atmospheres. Furthermore, layers of dust may ignite and act as ignition sources for an explosive atmosphere.

• DEFINITION - COMBUSTIBLE DUST

– finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, can burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures

Combustible Dust

Standard

Presence of dustResulting zone classification of

area of dust clouds

Continuous grade of release

20

Dust cloud likely to be present continuously or for

long periods

Primary grade of release

21

Dust cloud likely to be present occasionally in

normal operation

Secondary grade of release

22

Dust cloud unlikely to occur in normal operation,

but if it does, will only exist for a short period

Designation of zones depending on presence of dust

Example of zone classification in a filter system

red: zone 20green: zone 22 white: no zone (n. e. B.)

Filter system with top mounted fan, rotary valve with open dust discharge, protective measures for the prevention of combustion.

• The information should include items such as:

– ignition temperatures of dust layers,

– minimum ignition energy of a dust cloud,

– the dust group,

– explosive limits,

– electrical resistivity,

– moisture content,

– particle size

The properties used for the area classification

• Smouldering temperatureThe smoulder temperature describes the flammability behaviour of a flat dust layer on a hot surface.

– It is defined as the lowest temperature of a heated, free-standing surface which is capable of igniting a 5 mm thick dust layer.

– The thicker the layer the lower the smouldering temperature.

Combustible Dust

Ignition temperatures for common flammable dusts and fibers

Determination of the maximum surface temperature of dust layers of 5 mm to 50 mm

• Cloud ignition temperature

– This is the temperature from a hot surface that may ignite a dust cloud when the dust is suspended in a cloud.

– The ignition temperature is usually higher than the smouldering temperature.

– The maximum surface temperature of the apparatus must not exceed 2/3 of the minimum ignition temperature of the dust cloud.

Combustible Dust

Ignition temperatures for common flammable dusts and fibers

MaterialIgnition Temperature

Cloud LayerCoal Dust 380˚C 225˚CPolythene 420˚C (melts)

Methyl Cellulose 420˚C 320˚CStarch 460˚C 435˚CFlour 490˚C 340˚CSugar 490˚C 460˚C

Grain Dust 510˚C 300˚CPhenolic Resin 530˚C > 450˚C

Aluminium 590˚C > 450˚CPVC 700˚C > 450˚CSoot 810˚C 570˚C

Selection of EPE for dust areasThe permissible Ignition Temperature of the apparatus would be determined by firstly identifying the ignition temperature of the dust as a deposit, and then as a cloud as per SANS 61241-2-1:

– Smouldering temperature based upon 'Deposit' - Method A

– Ignition temperature based upon 'Cloud' - Method B

For example the following results may be drawn for brown coal :

• Method A: T max = Smouldering temperature -75 K e. g. brown coal 225°C -75°C = 150°C or,

• Method B: T max = 2/3 of the ignition temperature e. g. brown coal 2/3 * 380°C = 254 °C,

• In which case equipment should be selected that is limited to a Tmax 150°C rating.

MaterialIgnition Temperature

Cloud Layer

Coal Dust 380˚C 225˚C

• Apparatus is divided into Equipment groups:

– Group I for mines susceptible to methane (firedamp).

– Group II for explosive gases for locations other than mines

– Group III for dusts

• Dust groups defined as Group IIIA, IIIB and IIIC.

Combustible Dust

Dusts and fibres (Group III)

Dusts and Fibers (Group III) Surface

IIIA Combustible flying’s

IIIB Non-conductive dust

IIIC Conductive dust

Note: The equipment requirements increase from III A to III C

Are These Materials Explosive?

sugar metal

plastic wood

coal

medicines

Sources of dust

• Different dusts of the same chemical material can have different ignitability and explosibility characteristics, depending upon physical characteristics such as:

– particle size < 0.5mm (500μm)

• Hazard increases as particle size decreases

– shape (Flake ; round)

– moisture content (wet enough to prevent explosion)

• Any combustible dust with a Kst value greater than zero can be subject to dust deflagration. (Kst severity index)

• Even weak explosions can cause significant damage, injury and death. For example, sugar has a relatively low Kst but it fueled a tragic explosion in 2008 that killed 14 workers.

Combustible Dust Characteristics

ParticlesDusts may occur in the process stream and

cause a hazard, regardless of starting particle size of the material

may

break

into

or

Hazard increase

Particle Size of Common Materials

Common Material Size (microns)

Table salt 100

White granulated sugar 450 - 600

Sand 50+

Talcum (baby) powder 10

Powdered Milk 30 - 80

Flour 1 - 100

• Kst –Measures the relative explosion severity

• Pmax –Predicts the severity of the consequence

• (dp/dt)max–Predicts the violence of an explosion (maximum rate of pressure rise (bar/s) = Kst

= THE BIGGER THE NUMBER

THE BIGGER THE BOOM !

• MIE -Predicts the ease and likelihood of ignition

• MEC/LEL –Minimum amount of dust dispersed in air

• AIT -Auto Ignition Temperature

= THE SMALLER THE NUMBER

THE EASIER THE BOOM !

Combustible Dust Characteristics

Minimum ignition energy (MIE)

Coal dust >1000 mJ

Flour 300-1000 mJ

Sugar 10-30 mJ

Aluminium dust 100-300 mJ

Sewage sludge 30-100 mJ

Talcum (baby) powder 10 mJ

Powdered Milk 50 mJ

• Dusts sensitive to ignition by electrical spark

Minimum explosible concentration (MEC) and lower explosible limit

Sugar 125 g/m3

Sewage sludge 250 g/m3

Grain dust 125 g/m3

Flour 60 g/m3

Coal dust 30 g/m3

Aluminium dust 100 g/m3

• MEC is the lowest concentration of dust cloud that will allow combustion

Test results of some common dusts

Grain dust 89 bar.m/sec 9.3 bar g ST1

Coal dust 85 bar.m/sec 6.4 bar g ST1

Flour 63 bar.m/sec 9.7 bar g ST1

Sugar 138 bar.m/sec 8.5 bar g ST1

Wood dust 224 bar.m/sec 10.3 bar g ST2

Aluminium dust 515 bar.m/sec 11.2 bar g ST3

Sewage sludge 102 bar.m/sec 8.1 bar g ST1

• Kst value and Pmax are explosive properties measured in the laboratory to quantify the severity of a dust explosion.

• The “Explosion Severity Test” is a standard dust test used to quantity the maximum pressure of a dust cloud explosion (Pmax) and the speed of the pressure rise (KSt).

Dust explosion class* Kst (bar.m/s)* Characteristic* Typical material**

St 0 0 No explosion Silica

St 1 >0 and = 200 Weak explosion

Powdered milk,

charcoal, sulfur, sugar

and zinc

St 2 >200 and = 300 Strong explosion

Cellulose, wood flour,

and poly methyl

acrylate

St 3 >300 Very strong explosion

Anthraquinone,

aluminum, and

magnesium

The actual class is sample specific and will depend on varying characteristics of the material

such as particle size or moisture.

Examples of Kst Values for Different Types of Dusts

• The minimum amount of dust suspended in air that will support deflagration.

• It is estimated that this concentration (MEC) can reduce the visibility of a 25W bulb in a room to only 2meters of distance

• 50 g/m³ density in suspension

• A combustible concentration at or below 25% of the LEL is considered safe.

Minimum explosive concentration (MEC)/(LEL)

2 meters

Combustible dust

concentration suspended in

air > MEC

25 watts

Dust explosions – what can go wrong?

What's wrong ?

Dust explosions – bad housekeeping

Housekeeping?

• For a gas or vapour to ignite, three things are required:

– Fuel to burn

– Oxygen

– Ignition source (heat, spark, etc.)

Dusts ain't gases

• A dust explosion needs two additional elements - known as the "dust pentagon":

– Dispersion of dust particles in the right concentration, and

– Confinement of the dust cloud.

Dust explosion

Ignition

Source

Fuel (dust)Oxidant

Confinement

Dispersion

IMPORTANTNO DUST EXPLOSION OCCURS

if one or more elements are missing

• It is the effect of moving the cloud of dust from one place to another, may it be by mechanical processes (transport, ventilation, vibration, improper cleaning) or as the result of a primary explosion.

• Must be avoided at all times as it can be disastrous.

Dispersion

• Confinement means the dust is in an enclosed or limited space.

• Occurs when the fire occurs in a confined space, where the rapid change in temperature causes a rapid change in pressure.

• This change in pressure or shock wave can be more or less destructive depending on the magnitude of the explosion and how the structure is closed

Confinement

• There are many sources of ignition and a naked flame need not be one. Common sources of ignition include:

– electrostatic discharge;

– friction;

– sparks from machinery;

– sparks from electrical equipment;

– hot surfaces;

– fire.

Ignition sources

• Explosion. The bursting or rupture of an enclosure or a container due to the development of internal pressure from deflagration or detonation.

• Deflagration. Propagation of a combustion zone at a speed that is less than the speed of sound in the unreacted medium.

• Detonation. Propagation of a combustion zone at a velocity that is greater than the speed of sound in the unreacted medium.

Deflagration Vs. Explosion

Explosion

Deflagration

Detonation

• In the deflagration the flame advances as the material is consumed creating a fire front, with pressure changes

• Results in Explosion propagation

Deflagration

Copyright © ETC

Deflagration venting cont …….

• Maximum Pressure (Pmax). Themaximum pressure developed in a contained deflagration of anoptimum mixture.

• Reduced pressure(Pred). Themaximum pressuredeveloped in a ventedenclosure during a venteddeflagration.

• Static Activation Pressure(Pstat).Pressure that activates a vent closure when the pressure is increased slowly.

A protective measure that prevents unacceptably high explosion pressures by ensuringthat most of the [deflagration] takes place in a safe open area and not inside a building or dust handling enclosure.

Note: If designed and sized correctly, explosion vents reduce the maximum pressure (Pmax). of a deflagration to a safe level (Pred). that the equipment is designed to resist (Pes)

Pes

Equipment design

• Testing your dust to determine the KSt value & Pmax is essential for any type

of equipment design.

• These values are used by manufacturers to validate the design of protection

systems such:

• as spark detection,

• explosion venting,

• explosion suppression and

• explosion containment

Damage control

Some common techniques includes:

• Chemical Suppression

• Isolation of Equipment

• Explosion vent in open/safe area

Automatic fast

acting valve Explosion Vent

in open/safe

area

Suppression

Explosion

Results from the combination of these factors:

• fire

• dispersion of solid particulate material

• in a more or less enclosed space

+ + =firedispersion

Explosion

confinement

It is not inside, it’s on top!!!!!

• When combustible dusts ignite, there are often two explosions known as primary

and secondary explosions.

• The primary dust explosion is the first explosion. It occurs when there is a dust

suspension in a confined space that is ignited and explodes

• This secondary dust explosion is often more destructive than the primary one.

What are primary and secondary dust explosions?

What are primary and secondary dust explosions?

As this animation illustrates, secondary explosions can be far more destructive than primary explosions due to the increased quantity and concentrationof dispersed combustible dust.

• Dusts which have caused fires and explosions in the past are:

– MDF sandings (from final product sanding/parting saw area particularly)

– Foodstuff powders (sugar, flour, coffee, milk powder for example)

– Metal filings (aluminium, magnesium)

– Fibres (wool, cotton, paper)

– Trimmings (plastic from packaging plants)

– Industrial (coal, sulphur, carbon dust, shale dust)

– Medications (paracetamol for example)

Results of dust explosions (Imperial Sugar Port Wentworth, Georgia)

Copyright © ETC

14Killed

ManyHospitalized

February 7, 2008

Sugar Involved

• Use appropriate electrical equipment and wiring methods.

– Ex Certified for dust

• Control static electricity, including bonding of equipment to

ground.

– Static will support MIE

• Control smoking, open flames, and sparks.

• Separate heating systems from dusts.

• Adequately maintain equipment.

Ignition control

• Do not brush/blow powder about and create the very dust cloud risk you are trying to avoid

• Containment (Preferred)– Sealed Transfer Points

• Suppression– Water Based Suppression System

– Stone dust Suppression

• Collection– Dust Collectors System

– Dust Extraction System

• Clean-up– Housekeeping

• Combinations are most beneficial– Manage Dust at Point of Source

Combustible Dust

Methods for Managing Dust

Ex d

House keeping

Combustible Dust

Protection Concepts and specifications

Dust Protection (Electrical) Symbol

Typical IEC EPL

Typical Zone(s)

IEC Standard (status at July

2008)

Basic concept of protection

Enclosure

ta

tb

tc

Da, Db,

Dc

20

21

22

SANS 60079-31Standard protection for dusts,

rugged tight enclosure

Intrinsic safetyiaibic

DaDbDc

20

21

22SANS 60079-11

Similar to t, but with some relaxations if circuit inside is

intrinsically safe

Encapsulation

ma

mb

mc

Da

Db

Dc

20

21

22

SANS 60079-18Protection by encapsulation

of incendive parts

Pressurised p Db,Dc 21,22 SANS 60079-4 Protection by pressurisation of enclosure

Combustible Dust

Dust explosion demonstration – Talcum powder and Powdered milk

Product Particle

size

MIE St

Talcum (baby)

powder

5 - 10 µm 10 mJ St 2250 bar.m/sStrong explosion

Powdered

Milk

30 - 80 µm 50 mJ St 190 bar.m/sWeak explosion

Dusts ain't gases

Conclusion

photo credit: Eugene Kowaluk (LLE)