DUST EXPLOSIONS HAPPEN BECAUSE WE BELIEVE...

Mary Kay O'Connor Process Safety Center

2010 International Symposium

DUST EXPLOSIONS

HAPPEN BECAUSE WE

BELIEVE IN UNICORNS

Paul R. AmyotteDepartment of Process Engineering and Applied Science

Dalhousie University, Halifax, Nova Scotia, Canada

[email protected]

OUTLINE

Introduction

Dust explosions – the problem

Unicorn or oryx?

A collection of myths

A set of realities

Some concluding thoughts

Acknowledgements

INTRODUCTION

Scope Prevention and mitigation of dust explosions

Motivation Protection of people, assets, production and

environment

Communication issues with respect to dust

explosions

Objective Explore the myths and realities associated with dust

explosion risk reduction

EXPLOSION PENTAGON

Confinement

Ignition Source

Fuel Oxidant

Mixing

UNICORN OR ORYX?

MYTH # 1 – FUEL

Dust does not explode

NFPA 68 (2007 Edition)

Dust

Any finely divided solid

< 420 μm in diameter

–40 mesh

Combustible Dust

Combustible particulate solid

Presents a fire/deflagration hazard in air

Regardless of particle size or shape

Will This Dust Explode?

Does Fly Ash Explode?

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500

Fly Ash Concentration [g/m³]

Vo

lati

le M

att

er

in M

ixtu

re R

eq

uir

ed

fo

r

Co

mb

us

tio

n [

%]

OPG Fly Ash

OPG Fine Fly Ash

OPG Coarse Fly Ash

Yes – Fly Ash Can Explode

REALITY # 1

Many industrial dusts are combustible and

present an explosion hazard

To conclude otherwise requires incontrovertible

evidence

Evidence from testing using standardized

equipment and following standardized

procedures is incontrovertible

Recall the Buncefield myth

Cold petrol (gasoline) does not explode

MYTH # 2 – FUEL

Dust explosions only happen in

coal mines and grain elevators

Polyethylene, Phenolic Resin,

Aluminum, Sugar,…

Frank (2004): Process Safety

Progress, 23, 175-184

Incident data from US CSB and FM Global

Wood, paper

Grain, foodstuffs

Metal, metal products

Coal, peat

Rubber

Pharmaceuticals

Dyes

Pesticides

Plastics, polymers

Sulphur

Linen, flax, wool

Plus…

REALITY # 2

Dust explosions occur in a wide range of

industries and industrial applications

Dust explosions involve numerous and

varied products

MYTH # 3 – FUEL

A lot of dust is needed to have an

explosion

C = (ρbulk) (h/H) Eckhoff (2003)

REALITY # 3

Millimeter thick (or thin) layers can lead to

combustible dust clouds

Head for the exit if you can see your footprints

in the dust on the floor

There’s too much layered dust if you can write

your initials in it

MYTH # 4 – FUEL

Gas explosions are much worse

than dust explosions

KL Cashdollar

Likelihood of Occurrence

Severity of Consequences

4

5

6

7

8

9

20 40 60 80 100

Volume Median Diameter [µm]

Pm

ax [

bar

(g)]

Air

2% C2H4

1% C6H14

2% C3H8

5% C2H4

REALITY # 4

Explosions of flammable gases and

combustible dusts are both hazardous

occurrences

A moot point as to whether one fuel type is

worse than the other

Need to consider the risk arising from the

hazard

Explosion likelihood → Prevention

Consequence severity → Mitigation

MYTH # 5 – FUEL

It’s up to the explosibility testing

lab to specify which particle size

to test

KSt = (dP/dt)max · V1/3

50

75

100

125

150

175

200

225

20 40 60 80 100

Volume Median Diameter [µm]

KSt

[b

ar(m

)/s)

]

Air2% C2H41% C6H142% C3H8Linear (Air)Linear (2% C2H4)Linear (1% C6H14)Linear (2% C3H8)

Wood Samples

Dust Particle Size Distribution KSt [bar·m/s]

Coarse 50 weight % < 1 mm

0.3 weight % < 75 μm9

Fine 93 weight % < 1 mm

35 weight % < 125 μm

16 weight % < 75 μm

130

REALITY # 5

Sample selection requires collaboration

between test facility and plant personnel

Key consideration is particle size distribution

Test facility alone cannot specify this

parameter

Need understanding of process conditions

during normal and upset conditions

MYTH # 6 – FUEL/

IGNITION SOURCE

Any amount of suppressant is better

than none

Inerting → Prevention

Suppression → Mitigation

Monoammonium

phosphate (MAPP)

Sodium Bicarbonate (SBC)

Rock dust or stone dust

Limestone

Dolomite

Time

Quan

tity

of S

upp

ressa

nt

Quantity of suppressant requiredto mitigate an explosion

Quantity of suppressant deliveredby high-rate discharge canisters

SEEP with Al and SBC (Inerting)

CCCCCC BBBB

B

B

B

B

E

E

EEE

E

E

J

J JJ

JJ

0

2

4

6

8

10

12

14

0 500 1000 1500 2000 2500

Concentration of Aluminum [g/m³]

C 60%

B 55%

E 50%

J 0%

Explosion Criterion

Coal Dust Inerting with Dolomite

0

1

2

3

4

5

6

7

0 20 40 60 80 100

Dolomite Concentration [%]

Pm

[b

ar(

g)]

REALITY # 6

A little is not good enough

Insufficient deployment of suppressant =

failed suppression in which resulting

overpressure can be greater than for

unsuppressed explosion

MYTH # 7 – MIXING

There’s no problem if dust is not

visible in the air Glowing 25-W light bulb cannot be seen

through 2 m of dust cloud at concentrations

> 40 g/m3

Primary vs Secondary Dust

Explosions (Eckhoff, 2003)

REALITY # 7

Relatively thin layers of dust deposited on surfaces can

easily provide fuel for secondary dust explosions even

if primary dust explosions inside processing units have

been effectively excluded

The accumulation of

combustible dust above

the suspended ceiling

is the most important

safety issue in the

West incident (CSB)

MYTH # 8 –

CONFINEMENT

Venting is the only/best solution

to the dust explosion problem

INHERENT SAFETY

PROCEDURAL (ADMINISTRATIVE) SAFETY

PASSIVE ENGINEERED (ADD-ON) SAFETY

ACTIVE ENGINEERED (ADD-ON) SAFETY

Hierarchical Approach

Inherent Safety

Moderation

Use hazardous materials in least

hazardous forms; run process equipment

with less severe operating conditions

REALITY # 8

Amyotte and Eckhoff (2010)

EXPLOSION PREVENTIONEXPLOSION MITIGATION

Preventing

Explosible Dust Clouds

Preventing

Ignition Sources

Process design to prevent undesired generation

of dust clouds and particle size reduction and

segregation

Inherent Safety – Minimization, Substitution,

Moderation, Simplification

Smouldering combustion in dust, dust

fires

Procedural Safety – may also involve

aspects of Inherent Safety or Engineered

Safety

Good housekeeping (dust removal/cleaning)

Mitigation with respect to secondary dust explosions; prevention with respect to primary

dust explosions

Inherent Safety – Minimization

Keeping dust concentration outside explosible

range

Inherent Safety – Minimization

Other types of open flames (e.g., hot

work)



Safety

Explosion-pressure resistant construction

Inherent Safety – Simplification

Inerting of dust cloud by adding inert dust

Inherent Safety – Moderation

Hot surfaces (electrically or

mechanically heated)



Safety

Explosion isolation (sectioning)

Inherent Safety – Moderation (e.g., unit segregation, product choke, etc.) if not using

mechanical devices. If mechanical devices are used to isolate plant sections,

classification would be Engineered Safety – Passive in the case of physical barriers, or

Engineered Safety – Active in the case of isolation valves.

Intrinsic inerting of dust cloud by combustion

gases

Engineered Safety – Active

Heat from mechanical impact (metal

sparks and hot-spots)



Safety

Explosion venting

Engineered Safety – Passive

VENTINGElectric sparks and arcs and electrostatic

discharges



Safety

Automatic explosion suppression


Partial inerting of dust cloud by inert gas


MYTH # 9 –

PENTAGON

The vocabulary of dust explosions is difficult to understand

It’s too technical for our people

Does one really need to be an ‘expert’ to understand the language of dust explosions?

Analogy with Gas Explosions

Pmax → bar(g)

(dP/dt)max → bar/s

KSt = (dP/dt)max · V1/3 → bar·m/s

MEC → g/m3

MIE → mJ

MIT → C

MOC (LOC) → volume %

LIT → C

REALITY # 9

Workers have a right to know about workplace

hazards

For some, it may be enough to simply know

that a non-zero KSt means there is a hazard

For others, possibly relate MIT to process hot-

surface temperatures and MIE to actual spark

energies

MYTH # 10 –

PENTAGON

Dust explosion parameters are

fundamental material properties

Explosion parameters are numbers

Numbers are data and it is tempting

to view these as constant physical

properties for a given material

But They Are Not!

Part of the problem is use of terms such as the cubic ‘law’ for KSt = (dP/dt)max · V1/3

…and the dust ‘constant’ for KSt

Explosion parameters such as KSt are strongly influenced by material characteristics (e.g., particle size) and test conditions (e.g., turbulence intensity)

Need to use standardized equipment and follow standardized procedures

ASTM, ISO, CEN

Is KSt a Constant?

REALITY # 10

Pmax and KSt (ASTM): The values obtained by this testing technique are specific to the sample tested and the method used and are not to be considered intrinsic material constants

Venting standards (e.g., NFPA 68) rely on data from such standardized tests

Also important to consider explosion parameters in a relative sense

Different MIE values for a material depending on moisture content

Different options for explosion prevention depending on value of MEC

MYTH # 11 –

PENTAGON

It makes sense to combine explosion parameters in a single index

Consequence Severity Pmax, (dP/dt)max

Likelihood of Occurrence MEC, MIE, MIT

USBM Indices (~50 Years Old)

Ignition Sensitivity = [MIT · MIE · MEC]Pittsburgh Coal

[MIT · MIE · MEC]Sample

Explosion Severity = [Pmax · (dP/dt)max]Sample

[Pmax · (dP/dt)max]Pittsburgh Coal

Explosibility Index = Ignition Sensitivity · Explosion Severity

Hertzberg (1987)

…the consensus is that the critical parameter

in obtaining a hazard evaluation is not the

explosivity properties of a dust relative to

some other dust (such as Pittsburgh seam

coal), but rather, their values relative to the

operating conditions within the industrial

system being evaluated

REALITY # 11

No single index for overall dust explosion risk

Data bases are available to ‘get a feel for the numbers’ in comparison to other materials

http://www.dguv.de/ifa/en/gestis/expl/index.jsp

Adopt risk-based approach to dust explosion prevention and mitigation

MIE → Electrostatic Ignition → Pmax/KSt → Venting

Need explosibility parameters for material in question

Material Safety Data Sheet (MSDS)

But there is a problem…

MSDS Analysis Using CSB Criteria

Hazard

Stated

Explicitly?

In Hazard

Information

Section?

Hazard

Warning

Repeated?

Dust

Explosibility

Data?

Reference to

NFPA

Standard?

Warning Against

Accumulation?

YES YES YES NO YES YES

NO NO NO NO NO NO

NO NO NO NO NO NO

YES YES YES NO NO YES


YES YES YES NO YES YES

YES NO NO NO YES YES


NO NO NO NO NO NO

YES YES YES NO YES NO

NO NO NO NO NO NO

NO NO NO NO NO NO

MYTH # 12 –

PENTAGON

It won’t happen to me

Sociologist Andrew Hopkins

Hopkins, A., Lessons from Longford. The Esso Gas Plant Explosion, CCH Australia Limited, Sydney, Australia (2000)

Hopkins, A., Safety, Culture and Risk. The Organisational Causes of Disasters, CCH Australia Limited, Sydney, Australia (2005)

Hopkins, A., Failure to Learn. The BP Texas City Refinery Disaster, CCH Australia Limited, Sydney, Australia (2009)

Safety Culture

Safety Culture

Reporting culture

Just culture

Learning culture

Flexible culture

Collective Mindfulness

High-reliability organizations (HROs)

Preoccupation with failure

Risk-Awareness

REALITY # 12

Are all accidents preventable? (OHS, 2000)

No…but we should proceed on the basis that they are

…Yes, you are required to plan and perform work as if all accidents were preventable

Different beliefs ↔ Same answer

Focus on organizational practices (Hopkins, 2009)

What can happen will happen if you believe it will not

CONCLUDING REMARKS

12 Myths ↔ 12 Realities

Other myths?

Many tools at our disposal

Experimentation

Phenomenological

modeling

Thermo-kinetic modeling

Computational fluid

dynamics modeling

Also need to focus on

communication

ACKNOWLEDGEMENTS

Various funding agencies and industrial

companies for financial support

Industrial colleagues for technical support

Research team members for doing the work

Trevor Kletz for providing the idea for the

paper (and the unicorn/oryx pictures)

Ashok Dastidar, Rolf Eckhoff and Trevor Kletz

for reviewing the manuscript

DUST EXPLOSIONS HAPPEN BECAUSE WE BELIEVE...

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