This document discusses explosions caused by combustible dust, including definitions, characteristics, and criteria for dust explosions. It provides recommendations for preventing dust explosions, such as controlling ignition sources, inerting atmospheres, containment, suppression, and venting of explosions. Industries that commonly handle combustible dusts, like agriculture, chemicals, metals, and paper products are at risk. Current best practices include explosion containment, suppression, venting, siting facilities to minimize effects, and controlling ignition sources or inerting atmospheres. Dust control measures like dust collection system design and housekeeping programs are also recommended.
This document discusses explosions caused by combustible dust, including definitions, characteristics, and criteria for dust explosions. It provides recommendations for preventing dust explosions, such as controlling ignition sources, inerting atmospheres, containment, suppression, and venting of explosions. Industries that commonly handle combustible dusts, like agriculture, chemicals, metals, and paper products are at risk. Current best practices include explosion containment, suppression, venting, siting facilities to minimize effects, and controlling ignition sources or inerting atmospheres. Dust control measures like dust collection system design and housekeeping programs are also recommended.
This document discusses explosions caused by combustible dust, including definitions, characteristics, and criteria for dust explosions. It provides recommendations for preventing dust explosions, such as controlling ignition sources, inerting atmospheres, containment, suppression, and venting of explosions. Industries that commonly handle combustible dusts, like agriculture, chemicals, metals, and paper products are at risk. Current best practices include explosion containment, suppression, venting, siting facilities to minimize effects, and controlling ignition sources or inerting atmospheres. Dust control measures like dust collection system design and housekeeping programs are also recommended.
This document discusses explosions caused by combustible dust, including definitions, characteristics, and criteria for dust explosions. It provides recommendations for preventing dust explosions, such as controlling ignition sources, inerting atmospheres, containment, suppression, and venting of explosions. Industries that commonly handle combustible dusts, like agriculture, chemicals, metals, and paper products are at risk. Current best practices include explosion containment, suppression, venting, siting facilities to minimize effects, and controlling ignition sources or inerting atmospheres. Dust control measures like dust collection system design and housekeeping programs are also recommended.
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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, 3 rd 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 recommendations 8. Extrapolate the amount of effort necessary to protect lives and ensure the health of the workers.
Definitions Deflagration 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 Cont. 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 m particle size), dried, suspended in a 20-liter explosibility testing chamber. Approximately 200 grams of material with a particle size of 425 m or less are needed for the MEC tests.
Dust Deflagration Index (K st ) - 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.
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
OSHAs 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 elementsdust 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 containment 2. Explosion suppression 3. Explosion venting 4. Suitable sitting of plant minimize effects 5. 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 ft 2 , in which case a 1,000 ft 2 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) K st Test K st 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. K st 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 (K st ).
K st best Single Number Estimate Anticipated Behavior K st 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. References Eckhoff, Rolf K. - Dust Explosions in the Process Industries, 3 rd ed. Gulf Professional Publishing, 2003 ISBN 0-7506-7602-7
Barton, John Dust Explosion: Prevention and Protection, A Practical Guide, 1 st
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 Cont. Industries Cont. 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