2. dust explosions git - georgia tech occupational safety ... · pdf filedust explosions (c)...

Dust Explosions

(c) All Rights Reserved - J.M.Cholin

Consultants, Inc. 1

Dust Explosions

Prepared for: Georgia Tech Research Institute

Presented by John M. Cholin, P.E., FSFPE, M.E.E.J.M.Cholin Consultants, Inc.Oakland, [email protected]

© Copyright 2008 All Rights Reserved

J.M.Cholin Consultants, Inc.2

Objectives

• Develop awareness of dust explosion

hazards and how to recognize them.

• Develop understanding of the process of

dust explosions.

• Develop awareness of the means

available for managing dust explosion hazards.



Did I Have a Dust Explosion Hazard?

Dust Explosions


Consultants, Inc. 2

Definitions and Core Concepts



Explosion

• The bursting or rupture of an enclosure or

a container due to the development of internal pressure from a deflagration.

(NFPA 654-2006)

• The explosion is the result.

• The deflagration is the process that produces the result.



Deflagration

• Propagation of a combustion zone at a velocity that is less than the speed of sound in the un-reacted medium. (NFPA 654-2006)– Can be thought of as an expanding ball of

flame that is rapidly consuming a cloud of dust.

– Usually deflagrations rapidly produce large increases in pressure when they occur within a confining enclosure.

Dust Explosions


Consultants, Inc. 3



Necessary Conditions for Dust

Deflagration

• Four conditions must be met:– Combustible particulate solid of sufficiently

small particle size to be deflagrable.

– Deflagrable particulate is suspended in air(or other oxidizing medium)

– Deflagrable particulate suspension of sufficiently high concentration.

– A competent igniter applied to the suspension where the concentration is sufficient for flame propagation.



Necessary Conditions for Dust Deflagration

Combustible Particulate

Deflagrable FractionOf the Particulate

Suspension of Deflagrable Particulate

Region of Sufficient Concentration

Sufficiently PowerfulIgnition Source applied to Region of Sufficient Concentration



Deflagration

Dust cloud

Dust Explosions


Consultants, Inc. 4



Deflagration



Deflagration



Deflagration

Dust Explosions


Consultants, Inc. 5



Deflagration



Deflagration



Deflagration

• Deflagrations liberate large quantities of heat very rapidly.– Increases the air temperature rapidly

– Causes the air and combustion product gases to expand rapidly

– Rapid heating and expansion causes large pressures to develop

– Large pressures cause structural failure

– Heat and structural failure cause personnel injuries

Dust Explosions


Consultants, Inc. 6




Deflagration• All four (4) conditions must be met in the same

place and at the same time.

– Deflagrable particulate.

– Suspension

– Sufficient Concentration.

– Competent igniter applied to the suspension where the concentration is sufficient for flame propagation.

• Consequently, deflagrations are rare events, but with devastating consequences.



Explosion Hazard

• An explosion hazard exists where ever it is probable that all 4 criteria for a deflagration will occur within a compartment or container at the same time during the life-time of the facility.– Combustible particulate solid of sufficiently small

particle size to be deflagrable.

– Deflagrable particulate is suspended in air (or other oxidizing medium)


– A competent igniter applied to the suspension.



Combustible Particulate Solids

• Any combustible solid material, composed

of distinct particles or pieces, regardless of size, shape or chemical composition.

(NFPA 654-2006)

• This definition includes dusts, fines,

fibers, flakes, chips, chunks or mixtures of

these.

• A definition for CPSs does not exist in the

model building codes.

Dust Explosions


Consultants, Inc. 7



Combustible Particulate Solids

Chunks

Flakes

Fibers

Chips

Fines

Dusts

“Combustible Dusts” are a sub-category of

“Combustible Particulate Solids”

CombustibleParticulateSolids



Combustible Dust

• Combustible Dust* “A combustible particulate solid that presents a fire or deflagration hazard

when suspended in air or other oxidizing medium over a range of concentrations, regardless of particle size or shape.”

• Dusts have been traditionally defined as a solid material 420 microns or smaller (material

passing a U.S. No. 40 Standard Sieve)



Combustible Dust

• A.3.3.4 “Dusts have been traditionally

defined as a solid material 420 microns or smaller (material passing a U.S. No. 40

Standard Sieve)…Flakes or fibers that do

not pass through the 420 µ sieve can still be hazardous… particles with surface area

to volume ratio greater than that of a 420 µ

sphere should be…deemed a combustible dust.”

Dust Explosions


Consultants, Inc. 8



Necessary Conditions for Deflagration

• The four criteria required for a deflagration

necessitate we define:

– Particle

– Suspension

– Concentration

– Ignition



Particulate Size

• The rate of combustion depends on particle size.

– Rarely are all particles the same size

– Usually shown as a distribution

• When the average particle size is sufficiently small a suspension of the particulates in air will

propagate a flame front.

– The generally accepted size criterion for flame propagation is 420 µ (passing U.S.#40 sieve).

– The smaller the particle size the more hazardous the particulate.



Combustible Dust?

420 µµµµ

1.3 mm

A particle that will pass a #40 Sieve

A flock fiber particle

12 µµµµ

Dust Explosions


Consultants, Inc. 9



Combustible Dust?

1.2 Dtex Flock on a U.S. # 40 Sieve



Combustible Dust

• Some particulates tend to agglomerate

into clumps.

– Agglomerating particulates can be more hazardous than the test data implies if the particulate was not thoroughly de-

agglomerated when testing was conducted.

– Agglomeration is usually affected by ambient humidity.



Agglomeration

Dust Explosions


Consultants, Inc. 10



Particle Size

• How fine is 420 micron?– Ordinary table salt passes through a 420

micron (U.S.#40) sieve.

– Ordinary granulated sugar is about 75% sub-420 micron, passing through the U.S. #40 sieve.

• A combustible particulate having the same fineness as ordinary table salt or granulated sugar should be expected to be capable of supporting a deflagration.



Particle Shape

• Particles with non-spheric or non-cubic

shape do not pass through a sieve as easily as spheric or cubic particles.

• This leads to under-estimation of small particle population.

• Particles with aspect ratio greater than 3:1 should be suspect.



Particle Shape

Wood dust for particle board

100 micron

420 micron

Dust Explosions





Particle Shape

• Flat flake or rod-shaped (fiber) particles

have more surface area per unit of mass and will provide a faster burning rate than

a sieve analysis suggests.

• Flakes or fibers that do NOT pass through

the 420 micron sieve can still represent a

deflagration hazard.



Hazardous Particulate

• The particle size distribution does NOT

stay the same throughout the process.

– Usually particle size gets smaller as the particulate is processed.

– “As Received” sample might not be indicative

of process particulate.

– Fugitive dust accumulations generally are the

finest (most hazardous) fraction of the process material.




• Generally the finest (most hazardous)

fraction of the accumulated fugitive dust settles highest in the compartment of

building.

Dust Explosions






Largest particles

Smallest particles

Every large compartment is a particulate separator, separating particles by mass.






– Particle

– Suspension

– Concentration

– Ignition



Suspension

• When solid particles are propelled upward into the air or poured through the air from above the particles separate and become surrounded by air.– This maximizes the surface available for

combustion, maximizing the rate at which combustion can occur.

– The particles will fall out of suspension, smaller particles falling more slowly than larger ones.

Dust Explosions





Suspension

• Suspension is controlled by particle size

and shape.

– Smaller particles are more easily suspended.

– Where particulate is allowed to escape a process, the smallest particles come to rest in

the highest portions of the building.






– Particle

– Suspension

– Concentration

– Ignition



Concentration

• When particles are suspended a concentration gradient will develop where concentration varies

continuously from high concentration to low concentration,

ConcentrationGradient

Ignitable

Non-ignitable

DustCloud

Dust Explosions





Concentration

• There is a minimum concentration that

must exist before a flame front will propagate.

– This concentration depends on particle size

and chemical composition.

– This concentration is measured in grams/cubic meter (ounces/cubic foot).

– The metric for concentration is the Minimum Explosible Concentration (MEC)



Combustible Concentration

Concentration, measured in mass per unit volume (g/m3)

0.001 0.01 0.1 1.0 10 100 1000 10k 100k

Dust suspensions are deflagrable over a range of concentrations

Industrial Hygiene Deflagrable Layer formation




? 2 to 3 meters6 to 9 feet

An ignitable dust concentration is almost totally light obscuring at a distance of 3 meters.

Dust Explosions






• A dust dispersion can come from a dust

layer.

• The concentration attained depends upon:

– Bulk Density of dust layer, g/m3

– Layer thickness and

– Extent of dust cloud




Concentration = (Bulk Density)(Layer Thickness)

(Dust Cloud Thickness)

1 mm (0.040”) thick layerWith ρbulk of 400 kg/m3

Concentration of 200g/m3

In cloud 2 meters thick






– Particle

– Suspension

– Concentration

– Ignition

Dust Explosions





Ignition

• Ignition occurs when sufficient energy per unit of time and volume is applied to a deflagrable

particulate suspension.

– Energy per unit of mass is measured as “temperature”.

– When the temperature of the suspension is increased to the “auto-ignition” temperature combustion begins.

– Ignitability is usually characterized by measuring the Minimum Ignition Energy (MIE).



Igniters

• The ignition source must provide sufficient

energy per unit of time (power) to raise the temperature of the particulate to its AIT.

Energy In

Energy Lost(Out)



Igniters

• Mechanical work on particulates

– Grinding

– Cutting

– Milling

• Mechanical work on materials that generate particulates

– Cutting

– Shaping

– Sanding

– Milling

Dust Explosions





Igniters



Igniters

This milling cutter attained a temperature of 600˚F, 100˚F above the ignition temperature of the wood



Igniters

• Frictional heat from conveying equipment

– Bearings

– Conveyors

– Fans

– Tramp Iron

Dust Explosions





Igniters

Mechanical Conveyors



Igniters

Mechanical Conveyors

The “Typical” Deflagration

Event

Dust Explosions





The “Typical” Explosion Event

• Must dust explosions occur as a series of

deflagrations leading to a series of explosions in stages.

– While a single explosion is possible it is the

exception rather than the rule.

– Most injuries are the result of the “secondary” deflagrations rather than the initial event.




Factory

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325

ProcessEquipment




ProcessEquipment

InitialInternal Deflagration

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325

Dust Explosions






ProcessEquipment


Shock Wave

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325




ProcessEquipment


Elastic ReboundShock Waves

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325




Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325

ProcessEquipment


Dust clouds causedby Elastic Rebound

Dust Explosions






ProcessEquipment

Containment Failure from InitialDeflagration

Dust Clouds Causedby Elastic Rebound

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325




ProcessEquipment

Secondary DeflagrationInitiated

Dust Clouds Causedby Elastic Rebound

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325




ProcessEquipment

Secondary DeflagrationPropagates through Interior

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325

Dust Explosions






ProcessEquipment

Secondary DeflagrationVents from Structure

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325




Secondary DeflagrationCauses Collapse and Residual Fires

Time, msec.0 25 50 75 100 125 150 175 200 225 250 300 325




• Most “explosion” events are a series of

deflagrations each causing a portion of the process or facility to explode.

• Primary deflagrations lead to secondary deflagrations, usually fueled by

accumulated fugitive dust that has been

suspended by the acoustic shock waves of the initial, primary, deflagration.

Dust Explosions






• The majority of the property damage and

personnel injury is due to the fugitive dust accumulations within the building or

process compartment.

• Control, limitation of elimination of

accumulated fugitive dust is CRITICAL

and the single most important criterion for a safe workplace.

Hazard Recognition



Explosion Hazard

• An explosion hazard exists where ever it is probable that all 4 criteria for a deflagration will occur at the same time during the life-time of the facility.– Combustible particulate solid of sufficiently small

particle size to be deflagrable.

– Deflagrable particulate is suspended in air (or other oxidizing medium)


– A competent igniter applied to the suspension.

Dust Explosions





Dust

ExplosionHazard?



Dust Explosion Hazard?




Dust Explosions








Dust Explosion

Hazard?



Dust Explosions










Dust Explosion Hazard?The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

Dust Explosions





Dust

Explosion Hazard?







Dust Explosions








Bag-House

Dust/Air into CollectorFilter bagsHigh Dust ConcentrationFiltered DustRotary Air LockDust out of Collector

Cleaned air outFanClean Air Plenum



Dust Collector

• After operating for a while the filter bags

become caked with dust.

• Accumulated dust reduces air flow and

conveyance efficiency.

• Most dust collectors have automatic bag

cleaning to shake or blow the dust down to the bottom bin.

Dust Explosions





Dust Collector

Pulse-jet CleanerClean Air SideTube sheetDirty Air SideFilter bags with Dust Cake



Dust Collector

Pulse-jet CleanerClean Air SideTube sheetDirty Air SideFilter bags with Dust Cake

A pulse of high pressure airblows the dust cake off thefilter bag.



Dust Collector

• The operation of the bag cleaning feature

produces a dust cloud within the dust collector.

• If burning material is present or introduced into the dust collector during the operation

of the bag or filter element cleaning cycle

a deflagration can result from.

Dust Explosions











Dust Explosions












• If you can write your name in the dust

there is probably sufficient dust present to blow the building away.

Dust Explosions












Dust Explosions






• The majority of the property damage and

personnel injury is due to the fugitive dust accumulations within the building or

process compartment.

• Control, limitation or elimination of

accumulated fugitive dust is CRITICAL

and the single most important criterion for a safe workplace.



Explosion Hazards

• Usually, an explosion hazard will contain

one or more of the requisite criteria as a normal operating condition.

– Sufficiently small particle size

– Particulate suspended

– Sufficiently high concentration.

– Competent igniter.



Explosion Hazards

• Dust explosion hazards exist where ever

combustible particulate solids are handled or produced.

• There is no alternative to pro-actively managing the hazard.

Dust Explosions



Explosion Hazard

Management



Explosion Hazards

• If you think you might have an explosion

hazard – you probably do!

• There is no alternative but to aggressively

manage the hazard to reduce the risk of personal injury to the lowest achievable

level.



NFPA Standards

• NFPA 654, Standard for the Prevention of

Fires and Explosions from the Manufacturing, Processing and Handling

of combustible Particulate Solids - 2006.

• NFPA 664, Standard for the Prevention of

Fires and Explosions in Wood Processing

and Woodworking Facilities -2007.

Dust Explosions





NFPA Standards

• NFPA 484, Standard for Combustible

Metals, Metal Powders and Metal Dusts -2007.

• NFPA 61, Standard for the Prevention of Fires and Explosions in Agricultural and

Food Products Facilities - 2007.



NFPA Standards

• As a general rule, all of the NFPA

Combustible Particulate Solids standards follow the same basic design rules and

requirements.

• The design requirements in NFPA 484

regarding the alkali metals (aluminum,

magnesium, zirconium and lithium) have some unique requirements.



Hazard Management

• There are two means to manage the

deflagration (explosion) hazard.

1. Prevent the deflagration

2. Control the deflagration to limit property damage and injury of the occupants

• All of the prescriptive criteria in NFPA 654 address either of these two goals.

Dust Explosions



NFPA 654Dust Explosion Hazard

Management Tool



Design Process

PHA

Performance-Based design Prescriptive Design

Achieve Safety Goals and Objectives of the Standard

Hazard Assessment

Dust Explosions






Deflagration• All four (4) conditions must be met in the

same place and at the same time.

– Deflagrable particulate.

– Suspension

– Sufficient Concentration.

– Competent igniter applied to the suspension

where the concentration is sufficient for flame propagation.



Hazard Assessment

• Hazard assessment begins with the

characterization of the dust.

– Particle Size Distribution

– Minimum Explosible Concentration (MEC)

– Minimum Ignition Energy (MIE)

– Minimum Ignition Temperature (MIT)

– Maximum Volume-Normalized Rate of Pressure Increase (KSt)



Particle Size Distribution

• The particle size distribution of a CPS must be known if the explosion hazard is to be

assessed.

• Particle size implies a specific surface area

(SSA) and affects the numerical measure of other parameters

– MEC

– MIE

– dP/dtmax and KSt

Dust Explosions






dP

/dt m

ax

[bar/

sec]

Decreasing Particle Size Increases dP/dtmax

EckhoffEckhoff

Aluminum Dust




• Particles greater than 420 – 500 µ in effective mean particle diameter are generally not

considered deflagrable.

• Most CPSs include a range of particle sizes in

any given sample.

• Process Hazards Analysis should anticipate and account for particle attrition and separation as

particulate is handled




• Usually determined by means of a “sieve

analysis”.

• Data presented in terms of the percent

passing progressively smaller sieves.

• Particles that have high aspect ratios

produce distorted, non-conservativeresults.

Dust Explosions






Particle Diameter, µµµµ

100 200 300 400 500

PercentOf

Particulate



Minimum Ignition Temperature

• Ignition Temperature can be measured

both as a suspended dust cloud as well as a layer.

• ASTM Standard Test Methods do not yet exist for these measurements.

• Layer and Cloud values are rarely the same.



Minimum Ignition Temperature

Cloud• Godbert-Greenwald Vertical Tube Furnace

Test Apparatus is usually used to obtain AITC.

• Dust dispersions are shot into the furnace tube at slowly increasing temperature until

flame is observed.

Dust Explosions





Auto-Ignition Temperature

Cloud

Typical Ignition Temperature data used to determine AITC

Cashdollar/Hertzberg



Auto-Ignition Temperature

Cloud

The Gobert-Greenwald Furnace




Minimum Explosible

Concentration• “The minimum concentration of combustible dust

suspended in air, measured in mass per unit

volume, that will support a deflagration.” NFPA 654-2006

• Annex advises that MEC is determined in accordance with the test procedure in ASTM 1515, Standard Test method for Minimum

Explosible Concentration of Combustible Dusts.”

Dust Explosions





ASTM E 1515

• Standard Test method for Minimum Explosible Concentration of Combustible Dusts.

• Uses a 20 liter sphere as the test vessel

• Uses 2500 or 5000 Joule pyrotechnic igniters for the ignition source.

• Measures pressure versus time to obtain Pmax and dP/dtmax.



ASTM E 1515

Typical 20-Liter SphereDust Test Apparatus



ASTM E 1515

Typical Recorder Traces for a ModerateDust Deflagration Test in a 20-Liter SphereUsing 2500 J Igniter.

Dust Explosions





ASTM E 1515

• MEC is deemed to be that concentration that produces a PR ≥ 2.0 and (dP/dt)V1/3 ≥ 1.5 bar-m/sec.

PR =Pex - ∆Pignitor

Pignition

MEC = 120 g/m3

ASTM



Minimum Ignition Energy

• The minimum “electrical energy

discharged from a capacitor, which is just sufficient to effect ignition of the most

ignitable mixture of a given fuel-mixture

under specific test conditions.”

ASTM E 2019




• MIE is relevant ONLY in the context of the Standard Test Method for Minimum Ignition

Energy of a Dust Cloud in Air, ASTM E 2019.

• A value for MIE allows the design professional to

compare particulates to known references and to infer the required energy input for ignition of the particulate under test.

Dust Explosions






• ASTM E 2019 employs the “Modified

Hartmann Apparatus”.

Cashdollar & Herztberg




Typically, MIE is least at a concentration slightly above the “stoichiometric concentration”

Eckhoff



Maximum Deflagration

Pressure (Pmax)

A “typical” 20-liter Sphere Test Apparatus


Dust Explosions





Pmax and dP/dtmax

ASTM



Maximum Deflagration Pressure

(Pmax)

Pressure is recorded as a function of time during the deflagration within the 20-liter sphere.

ASTM



Maximum Deflagration

Pressure (Pmax)

After a series of deflagrations at increasing concentrations have been tested, a maximum attainable pressure can be determined.

ASTM

Dust Explosions





Volume-Normalized Rate of

Pressure Increase, KSt

• KSt is the “Deflagration Index” or “Dust

Constant” of the particulate.

• KSt is the maximum rate of pressure

increase normalized to the volume in which the at rate was measured.

KSt = (dP/dtmax)(V1/3)



Volume-NormalizedRate of Pressure Increase, KSt

KSt is obtained from the measured dP/dtmax and the test vessel volume.

dP/dtmax

Eckhoff



Volume-NormalizedRate of Pressure Increase, KSt

• KSt is an important parameter for design.– The design of Deflagration Relief Venting

relies upon the numerical value of KSt

– The numerical value of KSt is used to calculate effective flame speed, Su, from relations similar to:

KSt = 4.84 [(Pmax/P0) – 1]PmaxSu

Bartknecht

Dust Explosions





KSt

• KSt provides the best “single number” estimate of the anticipated behavior of a dust deflagration.

• Combustible dusts are classified by the numerical value of KSt.

KSt Value ST Class

0 to 200 ST1

201 to 300 ST2

>300 ST3



KSt

• Dusts with values of KSt less than 100– Produce deflagrations with relatively slow

increase in pressure

– Generally produce less initial structural damage.

– Generally produce flame fronts that roll through the interior compartments, down corridors, through wall penetrations, etc.

– Usually ignite extensive secondary fires.



KSt

• Dusts with values of KSt between 100 and 200

– Produce deflagrations with greater rates of pressure increase

– Generally produce more initial structural damage.

– Generally produce flame fronts that knock down walls, lift roofs, blow out windows and doors.

– Often ignite one or more secondary fires.

Dust Explosions





KSt

• Dusts with values of KSt between 200 and 300– Produce deflagrations with high rates of pressure

increase

– Generally produce substantial initial structural damage.

– Generally produce pressure fronts that break-up walls, break-up roofs, blow out windows and doors.

– Can ignite secondary fires.



KSt

• Dusts with values of KSt above 300– Produce deflagrations with extremely high

rates of pressure increase

– Generally shatter the building, producing severe initial structural damage.

– Generally produce pressure fronts that shatter walls, blow out roofs, blow out windows and doors.

– Often do not ignite secondary fires.



Hazard Assessment

• With the deflagration metrics of the particulate quantified the hazard can be assessed.

• Each location in the production space is evaluated for the four (4) criteria requisite for a

deflagration.

– Where all 4 criteria are present as a normal operating condition an explosion hazard exists.

– Where all 4 criteria could be present under conditions of equipment failure, production upset, maintenance, etc. and explosion hazard exists.

Dust Explosions



Preventing the Initial

Deflagration



Preventing the Initial Deflagration

• Process Equipment

– Minimize volumes containing dust suspensions [7.1.1]

– Minimize conveying system length and volume [7.1.1]

– Maintain pneumatic conveying concentration below MEC. [7.3.2, 7.3.3]

– Prevent accumulation of particulate within conveying system ducts. [7.3.2, 7.3.3, 7.6]

– Prevent ingress of tramp (foreign) material into process stream. [7.3.1]




• Process Equipment, continued

– Minimize heat liberated when performing mechanical work on particulates. [7.6, 7.9,

7.10, 7.11, 7.12, 7.15, 7.16, 7.17, 7.18, Chapter 9 ]

Dust Explosions






• Building Interior– Separate dust handling operations from all other

operations. [6.2.2, 6.2.3]– Minimize the building volume where fugitive dust can

accumulate. [6.3.4, 6.3.6, 6.3.8] – Minimize area that can accumulate fugitive dust.

[6.3.3, 6.3.4] – Minimize dust escape from process equipment [7.6,

7.7, 7.8, 7.10.2, 7.11.1.2, 7.16.1, 8.1]– Minimize sources of dust suspension. [6.5, 7.10, 7.11]– Minimize ignition sources [6.6, 7.2.5.2, 7.9, 7.10, 7.11,

7.12, Chapter 9]



Electrical Classification

• All of the relevant dust standards adopt

the National Electrical Code (NEC), NFPA 70, by reference as a means to prevent

electrical ignitions.

– Hazardous Locations are distinguished by “Class”

– Class 1: Flammable Gases and Vapors

– Class 2: Combustible Dusts

– Class 3: Combustible Fibers and Flyings




• Each class of occupancy is divided into

divisions based upon the probability of an ignitable atmosphere occurring.

– Division 1: Ignitable atmosphere is present

continually, regularly or routinely

– Division 2: Ignitable atmosphere is present under conditions of equipment failure or

process upset.

Dust Explosions






• The NEC divides flammable and

combustible materials into Groups based upon the chemical make-up.

• Groups A, B, C and D are flammable gases and vapors

• Group E metallic dusts

• Group F carbonaceous dusts

• Group G most organic dusts



Managing the Initial Deflagration

• Process Equipment– Prevent deflagrations with process equipment

from venting into the building interior. [7.1.2, 7.13.1.1, 7.13.1.7 ]

– Prevent fires in process equipment from escaping into the building interior. [7.1.3, 7.2.2, 7.2.3, 7.10.1, 7.14 ]

– Prevent deflagrations from traveling to adjacent connected equipment. [7.1.4, 7.1.5, 7.2.3.1, 7.13.1.4]



Deflagration Relief Venting

• The vent size is determined by the volume of the containment vessel and the Kst of the dust, using

calculations per NFPA 68.

• The Kst of the dust depends upon the particle

size and chemical composition of the dust. It is determined by test per ASTM E1226.

Dust Explosions





Dust Collector

Filter Media

Dusty AirInto Collector

Return Air

Vent


(Explosion Venting)



Dust Collector

Filter Media


Return Air

Vent


(Explosion Venting)



Dust Collector

Filter Media


Return Air

Vent


(Explosion Venting)

Dust Explosions





Dust Collector

Filter Media


Return Air

Vent


(Explosion Venting)



Dust Collector

Filter Media


Return Air

Vent


(Explosion Venting)



Dust Collector

Filter Media


Return Air

Vent


(Explosion Venting)

Dust Explosions






• If the dust collector is located within the building the deflagration relief vents must be ducted to

the outside with short straight ducts of sufficient size.

• NFPA 68 provides the design parameters for vent ducts.

• When vents are ducted they must be of larger

cross-sectional area .



Dust Collector

Filter Media

Vent


(Explosion Venting)

Vent Duct

Wall



Deflagration Vents

• Normally Open– Louvered panels– Hanger door

• Normally closed– Rupture diaphragm– Frangible panel– Hinged panel– Frangible fastener– Spring-loaded fastener– Magnetic & friction latch

Dust Explosions





Deflagration Vents

• Each vent closure is rated for its static

release pressure, Pstat.

• Vent closures for high-strength enclosures

such as dust collectors, cyclones, bins, bunkers

– Pstat is determined by testing.

– Testing should include both the vent closure and the enclosure or vessel.



Deflagration Vents

• Vent closures for low-strength enclosures such as buildings are large panel closures.

– Documentation should include”

• Design Pred

• Pstat

• Enclosure dimensions

• Vent area

• Factor C for design

• Types of fasteners, spacing and quantity



Deflagration Vents

• Vent closures must be capable of

withstanding worst-case wind loads and maximum and minimum temperatures.

• Re-closing vent assemblies must be capable of withstanding vacuum forces

that can occur subsequent to a

deflagration.

Dust Explosions





Explosion Prevention

• NFPA 69 recognizes the following Explosion Prevention strategies:– Oxidant Concentration Reduction [Chapter 7]

– Combustible Concentration Reduction [Chapter 8]

– Pre-Deflagration Detection and Control of Ignition Sources [Chapter 9]

– Deflagration Suppression [Chapter 10]

– Active Deflagration Isolation [Chapter 11]

– Passive Deflagration Isolation [Chapter 12]

– Deflagration Pressure Containment [Chapter 13]

– Passive Explosion Suppression [Chapter 14]



Oxidant Concentration Reduction

• The process atmosphere is manipulated to

maintain the oxygen (or other oxidant) concentration below the Limiting Oxidant

Concentration (LOC) for the combustibles

present.

• Designed in accordance with Chapter 7 of

NFPA 69.




Exhaust

Cyclone SeparatorPurge Gas Supply

Feed Hopper

Concentration MonitoringInerted Mill

Open to atmospherePurged to reduced Oxidant concentration

Dust Explosions






• Design must address:– Required reduction in oxidant concentration– Variations in temperature, pressure and combustible– Adequacy of purge gas source– Compatibility of purge gas with process and materials– Operating controls– Inspection, testing and maintenance requirements– Hazards associated with purge gas leakage outside

process.




• LOC values determined per ASTM 2079

• LOC criteria also in Table C1 (a), (b), and (c) of NFPA 69.

• Purge gases include:– N2

– CO2

– Flue Gas

– Argon




• Where oxidant concentration is

continuously monitored:

– Design concentration shall be at least 2 volume percent below worst-case LOC, or

– Design concentration shall be <5% and

process operated at less than 60% LOC

Dust Explosions






• Where oxidant concentration is NOT

continuously monitored:

– Design concentration shall be more than 60% LOC or 40% of LOC if LOC is < 5%, or

– Operating concentration shall be checked on

regularly scheduled basis.



Combustible Concentration Reduction

• The process is designed and operated so

that the concentration of the combustible material is below the MEC.

• Designed in accordance with Chapter 8 of NFPA 69.




ExhaustCyclone SeparatorFeed HopperFanFeederConcentration MonitoringMill

Dust Explosions






• Where continuous concentration monitoring is NOT in place feed rate and air flow must be

designed to maintain concentration at or below 25% of the MEC.

• Where continuous concentration monitoring is in place feed rate and air flow can allow concentrations up to 60% of the MEC.



Spark Detection and Extinguishing Systems

• Systems using high-speed infrared

sensors to detect sparks and embers and special extinguishing units to quench them

before they ignite a deflagration.

• Designed in accordance with Chapter 9 of

NFPA 69, NFPA 72 and NFPA 15.



Dust Collector

Filter MediaReturn Air

Spark Detection and Extinguishment

Fan

ControlUnit

Pump

SolenoidValve

SparkDetectors

Vent

Dust Explosions





Dust Collector



Fan

ControlUnit

Pump

SolenoidValve

SparkDetectors

Vent



Dust Collector



Fan

ControlUnit

Pump

SolenoidValve

SparkDetectors

Vent



Dust Collector



Fan

ControlUnit

Pump

SolenoidValve

SparkDetectors

Vent

Dust Explosions





Dust Collector



Fan

ControlUnit

Pump

SolenoidValve

SparkDetectors

Vent




• Spark Detection and Extinguishing

systems are permitted as a means to reduce the frequency of deflagrations.

• Spark Detection and Extinguishing systems cannot be used in lieu of other

explosion prevention strategies.




• Spark Detection and Extinguishing

systems cannot be used:

– in processes that contain flammable gases.

– on transport ducts where the concentration exceeds the MEC.

– to quench deflagrations

Dust Explosions






• Design of the detection is governed by NFPA 72, Section 5.8.

• Design of the extinguishment is governed by NFPA 15.

• Distance between detectors and extinguishment per manufacturer’s instructions is based upon:– Conveyance system speed– Response time of detection– Response time of valve– Time required to establish water spray pattern



Deflagration Suppression

• Deflagrations suppression systems use

pressure sensors and high discharge rate extinguishing units to quench the

deflagration in the early stages of

propagation, preventing the attainment of damaging pressure.

• Designed in accordance with Chapter 10 of NFPA 69




Return Air

PressureSensors

ControlUnit

HRD ExtinguishingUnit


Dust Explosions






Return Air

PressureSensors

ControlUnit






Return Air

PressureSensors

ControlUnit






Return Air

PressureSensors

ControlUnit



Dust Explosions






• Reaction times are generally in the 30 to

40 millisecond time domain.

• Pressure sensors are generally calibrated

to 0.5 psi above ambient.

• Extreme care should be used when

Deflagration Suppression is used on vessels also equipped with Deflagration

Relief Vents.




• Each manufacturer of deflagration suppression equipment uses its own, proprietary computer

program to simulate the suppression of the deflagration.

• The program simulates the compartment and the propagation of the deflagration flame-front in 3-D space as a function of time increments.




T = 0

Dust Explosions






T = 10 mSec




T = 15 mSec

P = 0.5 psig




T = 20 mSecP = 0.7 psig

Dust Explosions






T = 40 mSecP < Pred




• Deflagration suppression systems are custom-designed for the particular compartment to be

protected.

• Each make has limitations on the quantity of

agent and distance it can be projected within the time-frame of the developing deflagration.

• Any change to the vessel or compartment

necessitates a review of the system to verify that it will still meet the design objectives.



Deflagration Isolation

• The isolation system prevents the

propagation of the deflagration along conveyance ducts from one vessel or

compartment to another.

• Designed in accordance with Chapter 11 &

12 of NFPA 69

Dust Explosions





Deflagration Isolation

• Isolation methods include:

– Rotary Valves

– Flame Arresters

– Fast-Acting Valves

– Flame-Front Diverters

– Chemical Isolation



Rotary Valves

• Rotary valves are often called rotary air-

locks.

• Not all rotary valves are suitable for

deflagration isolation.

• Only permissible for isolation of dust

deflagration hazards.



Rotary Valves

Sufficiently small gap to quench flame

At least 2 Vanes on each side closed

Dust Explosions





Flame Arresters

• Flame arresters prevent the propagation of

flame by absorbing the heat of the combustion reaction and quenching the

flame.

• Generally reserved for flammable gas

applications.



Flame Arresters

Westec



Fast-Acting Valves

• Fast-acting valves are used to close-off ducts or pipes upon the occurrence of a pressure increment from a deflagration.

• Must be tested for use with the combustible in question.

• Spacing between sensor and valve is based upon flame-speed and system response time.

• The duct must be able to withstand the pressure of the deflagration.

Dust Explosions





Fast-Acting Valves

• Design must address:

– Deflagration metrics of combustibles

– Volume and configuration of protected vessel

– Volume, length and cross-section of pipe

– Design velocity of conveyance air in pipe

– Location of Components

– Response and closure time of valve

– Detection speed.



Fast-Acting Valves

Vessel

Pressure or Flame-Front

Sensor

ControlUnit

Fast-actingValve

Duct



Flame-Front Diverters

• Design must address:– Deflagration metrics of combustibles

– Volume and configuration of protected vessel

– Type of deflagration management used for the vessel

– Volume, length and cross-section of pipe

– Design velocity of conveyance air in pipe

– Location of Components

– Flow characteristics of the diverter

– Probable ignition sources.

Dust Explosions






• The diverter assembly must be capable of

withstanding the anticipated deflagration pressures

• Diversion of flame front must be to a safe, outside location.

• Diverters shall be tested to verify capability.




• Flame front diverters are generally found with one of two (2) designs.

NFPA 69-2002



Chemical Isolation Systems

• Used to prevent deflagrations from propagating from one vessel to the next.

• Detect deflagration flame or pressure and discharge extinguishing agent into duct.

• Duct must be able to withstand the pressure of the suppressed deflagration.

Dust Explosions





Chemical Isolation Systems

Vessel

Pressure or Flame-Front

Sensor

ControlUnit

ChemicalIsolation Unit

Duct



Pressure Containment

• Entails the design of the vessel and

connected piping to withstand the maximum attainable deflagration

pressures.

• Cannot be used for mixtures that might

detonate.



Pressure Containment

• Deflagration pressure containment can only be used where one of the following are present:– Ducts are equipped with deflagration isolation,

– Ducts are vented to a safe outside location,

– All portions of the process are designed to withstand the excess pressure from additive deflagrations,

– At least one of the interconnected vessels is vented.

Dust Explosions





Flame-Arresting/Particulate-Retention Vent Systems

• New technology venting system that permits venting to the building interior.

– Includes:

• Vent panel,

• Flame quenching medium and

• Particle retention medium

– Listed for finite enclosure volumes

– Size of vent must be adjusted upward to account for reduced efficiency compared to traditional vent.



FA/DR Vent

Systems

NFPA 68-2002



FA/DR Vent Systems

Enclosure WallIntegral Internal Vent diaphragmCeramic dust filterStainless Steel Flame ArresterElectronic Signal Connection

Dust Explosions





FA/DR Vent Systems




FA/DR Vent Systems




FA/DR Vent Systems


Dust Explosions





FA/DR Vent Systems

• Limitations

– Since steam and Hot CO2 are emitted during venting personnel should not be permitted

near units during operations.

– The room must be capable of accepting vented gas without excessive increase in

temperature or pressure.



FA/DR Vent Systems

• Limitations

– Vented emissions might be toxic, including carbon monoxide, formic acid, formaldehyde,

etc.

– Possibility of combustible mixtures exterior to equipment.



Managing the Initial Deflagration

• Building Interior

– Vent rooms where fugitive dust accumulations create and explosion hazard. [6.4, 8.2]

Dust Explosions





Preventing Secondary Deflagrations

• Prevent escape of dust into building interior. [7.2.5, 7.3.2.3, 7.3.2.5, 7.6, 7.13, 7.16.1, 7.17.1, 8.1]

• Prevent deflagrations with process equipment from venting into the building interior. [7.1.2, 7.10.1, 7.2.3, 7.13, 7.16.3]

• Prevent fires in process equipment from escaping into the building interior. [7.1.3, 7.2.2, 7.2.3, 7.14 ]

• Minimize accumulated fugitive dust. [8.2]



Accumulated Fugitive Dust

• The single most important factor in

propagating a deflagration within a building.

• Dust layers trigger critical hazard management decisions

• See NFPA 499



Accumulated Fugitive Dust

Visible dust layer triggers Class 2, Division 2 Hazardous Location

Greater than 0.31 in (0.8 mm) triggers “Explosion Hazard”

Greater than 1/8th in. (3.2 mm) triggers Class 2 Division 1

Hazardous Location

Dust Explosions





Dust Collection Return Air Abort

• Return Air Abort prevents smoke,

combustion product gases and flame from entering the occupied space.

• Pneumatic conveying system air CANNOT be returned to the building interior of

automatic return air diversion is not in

place.



Dust Collection With Return Air Abort

DustCollector

Return Air DuctAbort Gate



Abort Gate

Dust Explosions






DustCollector

Return Air DuctAbort Gate



Dust Collection Without Return Air Abort

DustCollector

Return Air Duct




DustCollector

Return Air Duct

Radiant flux

Dust Explosions





Without Return Air Abort

• As the flame and hot gases fill the space

beneath the ceiling/roof deck sprinkler heads begin fusing.

• Often more heads fuse than the riser is designed to support.

• The excessive demand deprives entire facility of required delivered density and

fires are not controlled.




DustCollector

Return Air Duct

Radiant flux




• The diversion of return air to building

exterior is usually implemented with spark detection and a fast-acting abort gate.

Dust Explosions





Dust Collector

Filter Media


Return Air

Spark DetectorsAbort Gate

ControlUnit

NFPA 664-2002 Section 8.2.2.6 Minimum Compliance Design

Vent



Dust Collector

Filter Media


Return Air


ControlUnit


Vent



Dust Collector

Filter Media


Return Air


ControlUnit


Vent

Dust Explosions





Dust Collector

Filter Media


Return Air


ControlUnit


Vent



Dust Collector

Filter Media


Return Air


ControlUnit


Vent



Return Air Abort

• Upon actuation of the return air spark

detection the automatic bag cleaning is shut-down.

• DO NOT shut down the air movement as this can cause a deflagration

• If fire expands the sprinklers in the D/C will fuse and dissipate the heat generated by

the fire.

Dust Explosions



Maintaining a Safe Workplace




• Process system maintenance is a critical component of the safety program.

• Equipment maintenance:– Reduces ignition sources

• System upset

• Over heat of mechanical components

– Reduces dust escape

• Chapter 12 provides the absolute minimum-compliance maintenance criteria.– Each piece pf process equipment has its own maintenance

requirements

– Maintenance must be scheduled

– Record of maintenance must be kept, at least for 5 years.




• NFPA 654-2006, Chapter 12 provides the absolute minimum-compliance maintenance

criteria.

– Each piece pf process equipment has its own maintenance requirements

– Maintenance must be scheduled

– Record of maintenance should be kept, at least for 5 years.

– If a written record of maintenance does not exist there is no proof the maintenance was performed

Dust Explosions






If you fail to maintain, you

GUARANTEE failure.



Management of Change

• Change is a constant in the modern commercial environment

• Change must be MANAGED if safety is to be maintained.

• Before a contemplated change is implemented its impact on the fire/explosion safety of the process and facility must be investigated and documented. [4.3]



Training

Training is an environment,

not an event!

If you fail to train you

GUARANTEE

some one will screw-up!

Dust Explosions





Training

• Policies, operating procedures and training

materials must be in a non-volatile form (written down).

– A requirement for ISO 900X certification

– A requirement of OSH Act



Training

• Initial and regular refresher training should include:– The hazards of the workplace,

– Facility safety rules

– Safe operating procedures and WHY other ways are dangerous

– Role and importance of fire/explosion protection equipment

– Maintenance requirements

– Housekeeping requirements

– Emergency response plan



Summary

• If you can write your name in the dust

you can blow the place away!

– Both NFPA 654 and NFPA 664 provide dust layer depth criteria.

– NFPA 664-2007 establishes 3.2 mm (0.125”)

based upon bulk density of 20 lb/ft3.

– NFPA 654-2006 establishes 0.8 mm (0.032”)

based upon a bulk density of 70 lb/ft3.

Dust Explosions





Summary

• To attain a deflagration all four of these

criteria must be present in the same place and at the same time.

– Deflagrable Particulate

– Suspension in Air

– Sufficient Concentration

– Sufficiently Powerful Ignition source



Summary

• Most personnel injuries come from

secondary deflagrations fueled by accumulated fugitive dust.

• Fugitive dust accumulates in the high spaces within the compartment.

Thank you

John M. Cholin, P.E., FSFPE, M.E.E., PrincipalJ.M.Cholin Consultants, Inc.

101 Roosevelt Dr.Oakland, NJ 07436-2008, USA

[email protected]

2. dust explosions git - georgia tech occupational safety ... · pdf filedust explosions (c)...

Documents