Dust Explosions - Lecture

8/10/2019 Dust Explosions - Lecture

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Source

Chapter 11

Safety Engineering, 3rdEdition

(ASSE)Gilbert Marshall, 2000

pp. 233-243


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AftermathEffect of Explosions


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Average 10 Explosions

Per Year from 1980 to 2005


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Learning Objectives

1. Know the underline conditions require to have an explosion.

2. Comprehend that Combustible Dust Explosions do exist andare a challenge for many industries

3. Apply the recommendations to design performance-based fire

protection systems.4. Analyze the 5 necessary components for Dust Explosions &

recognize ways to mitigate.

5. Synthesize recommendations for innovative approach.

6. Evaluate whether or not a particular industrial situation hasthis as a hazard.

7. Interpret consensus standards from NFPA and others forrecommendations

8. Extrapolate the amount of effort necessary to protect lives andensure the health of the workers.


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DefinitionsDeflagrationa flame spread rate of less than the speed of sound.(subsonic)

Explosiona rapid release of high pressure gas into the environment.

Detonationa flame spread rate that is above the speed of sound.(supersonic)

Combustible Dust - A combustible particulate solid that presents a fire or deflagrationhazard when suspended in air or some other oxidizing medium over a range of

concentrations, regardless of particle size or shape.

Explosive material/substancethose capable of causing an explosion influenced byconfinement.

Hybrid Mixture - A mixture of a flammable gas with either a combustible dust or acombustible mist.

Minimum Explosive Concentration (MEC) - The minimum concentration of combustibledust suspended in air, measured in mass per unit volume that will support a deflagration.


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DefinitionsCont.

Minimum Ignition Energy MIE) - The minimum ignition energy (MIE) of the sample isdetermined by suspending the sample in a Hartmann Lucite explosion chamber. Todetermine the MIE, the energy of the electrical spark used to ignite the dust is varied untilthe MIE is determined.

Minimum Ignition Temperature MIT) - Minimum ignition temperature (MIT) is determined by

using the Godbert-Greenwald furnace. Dust is discharged through this furnace at varioustemperatures. The lowest temperature that ignites the dust is considered to be the MIT.

Minimum Explosible Concentration - Minimum explosible concentration (MEC) of the sampleis determined by suspending the sample in a 20-liter explosibility testing chamber andignited with a 2500-joule chemical igniter. MEC is the lower concentration limit ofexplosibility for the dust. This limit is determined using test material that has been sievedthrough a 40-mesh sieve (425 m particle size), dried, suspended in a 20-liter explosibilitytesting chamber. Approximately 200 grams of material with a particle size of 425 m or less

are needed for the MEC tests.

Dust Deflagration Index (Kst) - test results provide an indication of the severity of adust explosion.


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Characteristics of an Explosion

Commonly begins with the ignition of a fuel

that burns very rapidly.

Produces a large and sudden release of gas

An explosion need not involve a fire.

When a container bursts from increasedinternal pressure, sudden release also called

an explosion.


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NFPA 69Standard on Explosion

Prevention Systems

Defines an explosion as:

the bursting or rupture of an enclosure

or container due to the development of

internal pressure from a deflagration.


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NFPA Fire Protection Handbook

Defines an Explosion as:

a rapid release of high pressure gas into

the environment.


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Commonality between definitions

In either definition,

The key word, pressure

And its effects on the surroundingenvironment.


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Other Situations - Explosion

Explosions might result from a chemicalreaction (combustion of a flammable

gas mixture) From over-pressurization of an structure

or enclosed container/vessel

By physical means (bursting of a tank) By physical/chemical means (boilerexplosion)


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Reactive Hazard Definitions


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Designing Facilities for Use of

Explosive Materials Pressure rate-of-rise detectors can activate a device or system

to extinguish a potential explosion before it reaches anexplosive stage.

Controlling ventilation & humidity level above 25%

Fixed monitoring for mass and size fraction using light-scatteringlaser photometers providing real-time aerosol mass readings.

Inertingpurging supply of oxygen when flammableatmospheres are detected with care being taken for thoseworkers in the area(BA available for Emergency Donning)


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Design of Buildings Relief of overpressurebreak away & blow out walls

and window openings to minimize destructiveness(explosion release panel)

Shielding personnel and equipmentdeflection ofshock wave so as not to pass unobstructived intoanother work area.

Explosive proof wall or barrier

Burst vessel/container diskventing away to transmitpressure wave harmlessly to outside atmosphere


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Explosives

Ammonium nitrate

Aluminum and other metal powders

Tovex, water gels instead of Dynamiteused for quarry operations

Magazinesspecial buildings to

storage lockers (OSHA 1910.109) Class Inot to exceed 50 pounds Class II23 kilograms or greater


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Dust Explosions

Smaller the particles, the greater the potential for an explosionto occur.

Primary areas in process industries inside process equipment

such as conveyors, dryers, mills, mixers, and storage silos.

Many materials can explode it they come in contact with anignition source, when air dispersed in the right concentration.

Combustible powders (metals) difficult to avoid danger of dustexplosions in processes where being handled.


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CTA AcousticsCorbin, KY


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Comparison of Foodstuff Silo

Storage

OSHAs Grain Facilities Standard has

successfully reduced the risk of dustexplosions in the grain industry


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OSHA DirectiveCombustible

Dust National Emphasis ProgramCPL-03-000-006 (Effective Oct. 2007)

Contains policies and procedures for inspecting workplaces that create or handlecombustible dusts.

In some circumstances these dusts may cause a deflagration, other fires, or anexplosion. These dusts include, but are not limited to:

Metal dust such as aluminum and magnesium.

Wood dust

Coal and other carbon dusts.

Plastic dust and additives Biosolids

Other organic dust such as sugar, paper, soap, and dried blood.

Certain textile materials


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Excludes for OSHA Directive

This directive does not replace the grain handling facilitydirective, OSHA Instruction CPL 02-01-004, Inspection of GrainHandling Facilities, 29 CFR 1910.272.

In addition, this directive is not intended for inspections ofexplosives and pyrotechnics manufacturing facilities covered bythe Process Safety Management (PSM) standard (1910.119).

However, it does not exclude facilities that manufacture orhandle other types of combustible dusts (such as ammonium

perchlorate) covered under the PSM standard.


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Criteria that must be met before

a Dust Deflagration can occur The dust has to be combustible.

The dust has to be dispersed in air or another

oxidant, and the concentration of this dispersed dustis at or above the minimum explosible concentration(MEC).

There is an ignition source, such as an electrostatic

discharge, spark, glowing ember, hot surface, frictionheat, or a flame that can ignite the dispersedcombustible mixture that is at or above the MEC.


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Dust Explosion Pentagon


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What is required

A dust explosion requires the simultaneouspresence of two additional elementsdustsuspension and confinement (Figure 2).

Suspended dust burns more rapidly, andconfinement allows for pressure buildup.

Removal of either the suspension or theconfinement elements prevents an explosion,although a fire may still occur.


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Train EffectSubsequent

Explosions as Dust Disturbed

Secondary dust explosions, due to

inadequate housekeeping and

excessive dust accumulations, causedmuch of the damage and casualties in

recent catastrophic incidents.


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Criteria that must be met for an

Dust Explosion to occur

The above criteria for deflagration must be present.

The combustible mixture is dispersed within a

confined enclosure (and the confined enclosure doesnot contain sufficient deflagration venting capacity tosafely release the pressures) such as a vessel,storage bin, ductwork, room or building. It must benoted that a small deflagration can disturb andsuspend the combustible dust, which could thenserve as the fuel for a secondary (and often moredamaging) deflagration or explosion.


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OSHA Poster


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Industries that handle

Combustible Dusts Agriculture

Chemicals

Textiles

Forest and furniture products

Metal processing Tire and rubber manufacturing plants

Paper products

Pharmaceuticals

Wastewater treatment

Recycling operations (metal, paper, and plastic.) Coal dust in coal handling and processing

facilities


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Current Best Practices

To prevent Dust Explosions:

1. Methods of explosion containment2. Explosion suppression

3. Explosion venting

4. Suitable sitting of plantminimize effects5. Control of ignition sources or inerting


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Dust Control Measures

The dust-containing systems (ducts and dustcollectors) are designed in a manner (i.e., no leaking)that fugitive dusts are not allowed to accumulate inthe work area.

The facility has a housekeeping program with regularcleaning frequencies established for floors andhorizontal surfaces, such as ducts, pipes, hoods,ledges, and beams, to minimize dust accumulationswithin operating areas of the facility.

The working surfaces are designed in a manner tominimize dust accumulation and facilitate cleaning.


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NFPA 654Guidance on Dust

Layer Characterizations &

PrecautionsIndicates that immediate cleaning is warranted whenever a dust layer of

1/32-inch thickness accumulates over a surface area of at least 5% ofthe floor area of the facility or any given room.

The 5% factor should not be used if the floor area exceeds 20,000 ft2, inwhich case a 1,000 ft2layer of dust is the upper limit.

Accumulations on overhead beams, joists, ducts, the tops of equipment,and other surfaces should be included when determining the dustcoverage area.

Even vertical surfaces should be included if the dust is adhering to them.Rough calculations show that the available surface area of bar joists isapproximately 5 % of the floor area and the equivalent surface area forsteel beams can be as high as 10%.


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What is 1/32 of an inch?

When observe areas of the plant for dust

accumulations of greater than 1/32 of

an inch

It is the approximately equal to the

thickness of a typical paper clip.


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Likely Areas for Dust

Accumulations

Within a plant are: structural members

conduit and pipe racks

cable trays

floors

above ceiling

on and around equipment

(leaks around dust collectors and ductwork.)


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Ignition Control Measures Electrically-powered cleaning devices such as vacuum cleaners, and

electrical equipment are approved for the hazard classification forClass II locations.

The facility has an ignition control program, such as grounding andbonding and other methods, for dissipating any electrostatic chargethat could be generated while transporting the dust through theductwork.

The facility has a Hot Work permit program.

Areas where smoking is prohibited are posted with No Smokingsigns.

Duct systems, dust collectors, and dust-producing machinery arebonded and grounded to minimize accumulation of static electricalcharge.

The facility selects and uses industrial trucks that are approved for

the combustible dust locations.


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Prevention Measures

The facility has separator devices to removeforeign materials capable of ignitingcombustible dusts.

MSDSs for the chemicals which couldbecome combustible dust under normaloperations are available to employees.

Employees are trained on the explosionhazards of combustible dusts.


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Protection Measures

The facility has an emergency action plan.

Dust collectors are not located inside of buildings. (Some exceptions)

Rooms, buildings, or other enclosures (dust collectors) have explosion reliefventing distributed over the exterior wall of buildings and enclosures.

Explosion venting is directed to a safe location away from employees.

The facility has isolation devices to prevent deflagration propagation betweenpieces of equipment connected by ductwork.

The dust collector systems have spark detection and explosion/ deflagrationsuppression systems.

Emergency exit routes are maintained properly.


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Equipment used when sampling

Equipment for collecting dust samples may include thefollowing:

Natural bristle hand brushes for collecting settled dust.

Non-sparking, conductive dust pans (aluminum), for collectingsettled dust.

Non-spark producing sample container.

Non-spark producing funnel for filling sample containers.

Non-spark producing scoops for removing dust from cyclone

containers or other ventilation equipment.


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OSHA - Salt Lake Technical

Center

Dust Samples are analysis to determine the explosibility andcombustibility parameters of the dust samples submitted

Percent through 40 mesh

Percent moisture content

Percent combustible material

Percent combustible dust

Metal dusts will include resistivity

Minimum explosive concentration (MEC)

Minimum ignition energy (MIE)

Class II test Sample weight

Maximum normalized rate of pressure rise (dP/dt)Kst Test

Minimum ignition temperature


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Lab Results

Lab results may contain some of the resultslisted below, but not all, depending onparticular tests that are performed:

Mesh size

Moisture content

Percent combustible dust

Sample weight

Explosion severity

Kst Value

MEC

Resistivity for metal dusts


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Max Normalized Rate of Pressure

Rise (dP/dT)Kst

Test

Kstis the Deflagration Index for dusts, and the Kst test resultsprovide an indication of the severity of a dust explosion. Thelarger the value for Kst, the more severe is the explosion.

Kstis essentially the maximum rate of pressure rise generated

when dust is tested in a confined enclosure. Kst provides thebest single number estimate of the anticipated behavior of adust deflagration.

Approximately 300 grams of "as received" sample material areneeded for the Kst test. In this test, dust is suspended in the 20-liter explosibility testing chamber and is ignited using a chemicaligniter. The 20-liter explosibility testing chamber determines

maximum pressure and rate of pressure rise if the sampleexplodes.

These parameters are used to determine the maximum normalizedrate of pressure rise (Kst).


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Kstbest Single Number

EstimateAnticipated Behavior


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KstCalculation


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Reactive Hazard Mgmt Process


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Summary

Need to know your operations and what are the raw ingredientsbeing used as well as any byproducts and scrap.

Need to test the physical and chemical properties to determine if

the hazardous substances are reactive.

Utilize available guidance for hazard control and incorporateperformance-based design for fire protection systems.

Important that you as a safety professional become

knowledgeable in recognition and control of combustible dusthazards and familiar with NFPA provisions


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Discussion Questions

1. How is an explosive distinguished from anexplosive material?

2. What is a forbidden or unacceptable explosive

material?3. If you learn that explosive material was being used

in your plant, what is the first thing you would do tocorrect the hazard?

4. Why should explosive materials be stored andcarried in small containers?

5. Why should a plastic container not be used forexplosive liquid dispensing?


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Discussion Questions

6. Why is it necessary to ground a metal container prior to pouringa explosive liquid?

7. What would you incorporate in a design for a system that willforce an inert gas into a container as explosive liquid is beingdrawn out of the container?

8. Explain how you would design the four rivets that are used tohold a 1 m x 1 m explosion-release panel in place if the panel isto release at an overpressure of 3.5 kP(0.5 psi)

9. How can we evaluate the explosion potential of a substance byusing the NFPA 704M Hazard Symbol?

10. Give an example of an explosion occurring without combustion.


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ReferencesEckhoff, Rolf K. - Dust Explosions in the Process Industries, 3rded. Gulf

Professional Publishing, 2003 ISBN 0-7506-7602-7

Barton, JohnDust Explosion: Prevention and Protection, A Practical Guide, 1stEd., Gulf Professional Publishing, 2002 ISBN 0-7506-7519-5

NFPA 654 Standard for the Prevention of Fires and Dust Explosions from the

Manufacturing, Processing, and Handling of Combustible Particulate Solids(2006 Edition)

NFPA 68 Guide for Venting of Deflagrations (2002 Edition)

NFPA 69 Standard on Explosion Prevention Systems

Explosive Identification Guide, Mike Pickett, Delmar 1999,

FM Global, Data Sheet No. 7-76, Prevention and Mitigation of Combustible DustExplosions and Fire (2006 ed.)


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NFPA Publications Relevant to

Combustible Dust Hazard

Controls


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Industries that may have

Combustible Dusts


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IndustriesCont.


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IndustriesCont.


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Measured Properties of

Combustible Dust


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For Additional Information

Email: [email protected]

Office Phone: (713-221-8089)

Fax: (713-221-2712)

Professor Albert V. Condello, III FSI-III, TCFP Master Instructor

University of Houston Downtown

College of Science and Technology

Department of Engineering Technology

One Main Street, Suite N-717

Houston, TX 77002-1001Websites: http://www.uhd.edu/academic/colleges/sciences/engineeringtech/safetymanagement.htm

http://www.uhd.edu/academic/colleges/sciences/engineeringtech/sfet/index.htm
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Dust Explosions - Lecture