Explosions

ENGR 4355 – Industrial Safety Course 21994 (Spring, 2009)

Albert V. Condello, III

Professor, Safety Mgmt & Fire Protection Engineering

Department of Engineering Technology

Source

Chapter 11 Safety Engineering, 3rd Edition (ASSE)Gilbert Marshall, 2000

pp. 233-243

Aftermath – Effect of Explosions

Average 10 Explosions Per Year from 1980 to 2005

Learning Objectives

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

2. Comprehend that Combustible Dust Explosions do exist and are 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 has this as a hazard.7. Interpret consensus standards from NFPA and others

for recommendations8. Extrapolate the amount of effort necessary to protect

lives and ensure the health of the workers.

DefinitionsDeflagration – a flame spread rate of less than the speed of sound.(subsonic)

Explosion – a rapid release of high pressure gas into the environment.

Detonation – a flame spread rate that is above the speed of sound.(supersonic)

Combustible Dust - A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.

Explosive material/substance – those capable of causing an explosion influenced by confinement.

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

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

Definitions – Con’t.

• Minimum Ignition Energy (MIE) - The minimum ignition energy (MIE) of the sample is determined by suspending the sample in a Hartmann Lucite explosion chamber. To determine the MIE, the energy of the electrical spark used to ignite the dust is varied until the 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 various temperatures. The lowest temperature that ignites the dust is considered to be the MIT.

• Minimum Explosible Concentration - Minimum explosible concentration (MEC) of the sample is determined by suspending the sample in a 20-liter explosibility testing chamber and ignited with a 2500-joule chemical igniter. MEC is the lower concentration limit of explosibility for the dust. This limit is determined using test material that has been sieved through a 40-mesh sieve (425 μm particle size), dried, suspended in a 20-liter explosibility testing chamber. Approximately 200 grams of material with a particle size of 425 μm or less are needed for the MEC tests.

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

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 increased

internal pressure, sudden release also called an explosion.

NFPA 69 – Standard 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.”

NFPA Fire Protection Handbook

Defines an Explosion as:

“ a rapid release of high pressure gas into the environment.”

Commonality between definitions

• In either definition,

• The key word, “pressure”

• And its effects on the surrounding environment.

Other Situations - Explosion

• Explosions might result from a chemical reaction (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 (boiler explosion)

Reactive Hazard Definitions

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 an explosive stage.

• Controlling ventilation & humidity level above 25%

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

• Inerting – purging supply of oxygen when flammable atmospheres are detected with care being taken for those workers in the area – (BA available for Emergency Donning)

Design of Buildings• Relief of overpressure – break away & blow out

walls and window openings to minimize destructiveness (explosion release panel)

• Shielding personnel and equipment – deflection of shock wave so as not to pass unobstructived into another work area.

• Explosive proof wall or barrier

• Burst vessel/container disk – venting away to transmit pressure wave harmlessly to outside atmosphere

Explosives

• Ammonium nitrate• Aluminum and other metal powders• Tovex, water gels instead of

Dynamite – used for quarry operations

• Magazines – special buildings to storage lockers (OSHA 1910.109)

• Class I – not to exceed 50 pounds • Class II – 23 kilograms or greater

Dust Explosions

• Smaller the particles, the greater the potential for an explosion to 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 an ignition source, when air dispersed in the right concentration.

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

CTA Acoustics – Corbin, KY

Comparison of Foodstuff Silo Storage

• OSHA’s Grain Facilities Standard has successfully reduced the risk of dust explosions in the grain industry

OSHA Directive – Combustible Dust National Emphasis Program

CPL-03-000-006 (Effective Oct. 2007)

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

In some circumstances these dusts may cause a deflagration, other fires, or an explosion. 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

Excludes for OSHA Directive

• This directive does not replace the grain handling facility directive, OSHA Instruction CPL 02-01-004, Inspection of Grain Handling Facilities, 29 CFR 1910.272.

• In addition, this directive is not intended for inspections of explosives and pyrotechnics manufacturing facilities covered by the Process Safety Management (PSM) standard (1910.119).

• However, it does not exclude facilities that manufacture or handle other types of combustible dusts (such as ammonium perchlorate) covered under the PSM standard.

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 dust is at or above the minimum explosible concentration (MEC).

• There is an ignition source, such as an electrostatic discharge, spark, glowing ember, hot surface, friction heat, or a flame that can ignite the dispersed combustible mixture that is at or above the MEC.

Dust Explosion Pentagon

What is required

• A dust explosion requires the simultaneous presence of two additional elements—dust suspension and confinement (Figure 2).

• Suspended dust burns more rapidly, and confinement allows for pressure buildup.

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

Train Effect – Subsequent Explosions as Dust Disturbed

• Secondary dust explosions, due to inadequate housekeeping and excessive dust accumulations, caused much of the damage and casualties in recent catastrophic incidents.

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 does not contain sufficient deflagration venting capacity to safely release the pressures) such as a vessel, storage bin, ductwork, room or building. It must be noted that a small deflagration can disturb and suspend the combustible dust, which could then serve as the fuel for a secondary (and often more damaging) deflagration or explosion.

OSHA Poster

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

Current Best Practices

• To prevent Dust Explosions:

1. Methods of explosion containment2. Explosion suppression3. Explosion venting4. Suitable sitting of plant – minimize

effects5. Control of ignition sources or inerting

Dust Control Measures• The dust-containing systems (ducts and dust

collectors) are designed in a manner (i.e., no leaking) that fugitive dusts are not allowed to accumulate in the work area.

• The facility has a housekeeping program with regular cleaning frequencies established for floors and horizontal surfaces, such as ducts, pipes, hoods, ledges, and beams, to minimize dust accumulations within operating areas of the facility.

• The working surfaces are designed in a manner to minimize dust accumulation and facilitate cleaning.

NFPA 654 – Guidance on Dust Layer Characterizations & Precautions

Indicates that immediate cleaning is warranted whenever a dust layer of 1/32-inch thickness accumulates over a surface area of at least 5% of the floor area of the facility or any given room.

The 5% factor should not be used if the floor area exceeds 20,000 ft2, in which case a 1,000 ft2 layer 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 dust coverage 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 is approximately 5 % of the floor area and the equivalent surface area for steel beams can be as high as 10%.

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.

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.)

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

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

• The facility has an ignition control program, such as grounding and bonding and other methods, for dissipating any electrostatic charge that could be generated while transporting the dust through the ductwork.

• The facility has a Hot Work permit program.

• Areas where smoking is prohibited are posted with “No Smoking” signs.

• Duct systems, dust collectors, and dust-producing machinery are bonded and grounded to minimize accumulation of static electrical charge.

• The facility selects and uses industrial trucks that are approved for the combustible dust locations.

Prevention Measures

• The facility has separator devices to remove foreign materials capable of igniting combustible dusts.

• MSDSs for the chemicals which could become combustible dust under normal operations are available to employees.

• Employees are trained on the explosion hazards of combustible dusts.

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 relief venting 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 between pieces of equipment connected by ductwork.

• The dust collector systems have spark detection and explosion/ deflagration suppression systems.

• Emergency exit routes are maintained properly.

Equipment used when sampling

Equipment for collecting dust samples may include the following:

• Natural bristle hand brushes for collecting settled dust.

• Non-sparking, conductive dust pans (aluminum), for collecting settled 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.

OSHA - Salt Lake Technical Center

• Dust Samples are analysis to determine the explosibility and combustibility 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

Lab Results

• Lab results may contain some of the results listed below, but not all, depending on particular tests that are performed:

– Mesh size– Moisture content– Percent combustible dust– Sample weight– Explosion severity– Kst Value– MEC– Resistivity for metal dusts

Max Normalized Rate of Pressure Rise (dP/dT) – Kst Test

Kst is the Deflagration Index for dusts, and the Kst test results provide an indication of the severity of a dust explosion. The larger the value for Kst, the more severe is the explosion.

Kst is essentially the maximum rate of pressure rise generated when dust is tested in a confined enclosure. Kst provides the best “single number” estimate of the anticipated behavior of a dust deflagration.

Approximately 300 grams of "as received" sample material are needed for the Kst test. In this test, dust is suspended in the 20-liter explosibility testing chamber and is ignited using a chemical igniter. The 20-liter explosibility testing chamber determines maximum pressure and rate of pressure rise if the sample explodes.

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

Kst best “Single Number” Estimate – Anticipated Behavior

Kst Calculation

Reactive Hazard Mgmt Process

Summary

• Need to know your operations and what are the raw ingredients being 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 incorporate performance-based design for fire protection systems.

• Important that you as a safety professional become knowledgeable in recognition and control of combustible dust hazards and familiar with NFPA provisions

Discussion Questions

1. How is an explosive distinguished from an explosive 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 to correct the hazard?

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

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

Discussion Questions

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

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

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

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

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

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

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

Barton, John – Dust Explosion: Prevention and Protection, A Practical Guide, 1st Ed., 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 Dust Explosions and Fire (2006 ed.)

NFPA Publications Relevant to Combustible Dust Hazard Controls

Industries that may have Combustible Dusts

Industries – Con’t.

Industries – Con’t.

Measured Properties of Combustible Dust

For Additional Information

• Email: Condelloa@uhd.edu 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 77002-1001

Websites: http://www.uhd.edu/academic/colleges/sciences/engineeringtech/safetymanagement.htm http://www.uhd.edu/academic/colleges/sciences/engineeringtech/sfet/index.htm