Transcript Document

HAZARDS OF COMBUSTIBLE DUST
PRESENTED FOR:
THE SUMMIT COUNTY SAFETY COUNCIL MEETING
SEPTEMBER 21, 2011
Presented by:
John L. Schmidt - Engineering Specialist,
FM Global - Cleveland Operations
PRESENTATION AGENDA:
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History Channel video
Video test demonstrations
Dust explosion losses
Definition (FM Global/NFPA)
5 elements of a dust explosion
Recognizing dust explosion hazards
Risk service testing
Room/building explosion hazards
Equipment/process explosion hazards
Ignition source control
Imperial Sugar Mill, Port Wentworth, GA; February 2008
Some Additional Dust Explosion Pictures, with the
Resulting Consequences…
“a picture is worth a thousand words”….
Dust explosion tests videos
FM Global Dust Explosion Testing:
• Open air fireball test
• Explosion bunker test
• Grinding process mock up test (with and without
explosion suppression)
• Large scale dust explosion test (bituminous coal and
cornstarch)
FM GLOBAL LOSS HISTORY (1983 – 2006)
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Woodworking – 64
Food – 26
Metals – 18
Chemical – 14
Pulp/paper – 12
Mineral - 11
Utility – 7
Plastics/Rubber – 5 each
Printing/Textile – 1 each
Others - 2
FM GLOBAL LOSS HISTORY (BY CAUSE):
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Friction – 50
Spark – 38
Chemical Action – 16
Hot Work – 13
Burner Flame – 10
Electricity/Static – 6 each
Overheating – 4
Hot Surface – 2
Unknown/No data - 21
FM GLOBAL LOSS HISTORY (BY DUST TYPE):
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Wood – 70
Food – 25
Chemical – 17
Metal – 15
Coal – 14
Plastic/Rubber – 13
Paper – 8
Others - 4
FM GLOBAL LOSS HISTORY (BY EQUIPMENT TYPE):
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Dust Collector – 66
Impact Equipment – 22
Silo – 8
Processing Equipment – 7
Oven – 5
Conveyor – 4
Grain Elevator – 4
Spray Dryer – 4
Dryer – 3
Boiler - 3
What is a combustible dust?
• FM Global: median particle size less than 500 microns
• NFPA: median particle size of less than 420 microns
HOW IS DUST TYPICALLY GENERATED?
Two ways :
– Finished product
– By-product
INTENTIONALLY MANUFACTURED DUSTS
• Flours; Cornstarch
• Pulverized Coal
• Carbon Black
• Metallic Stearates
• Metal Powders
BY-PRODUCT (NUISANCE TYPE) DUSTS
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Wood dust
Plastic dust
Rubber dust
Coal dust
Grain dust
Metal dust
THE “DUST EXPLOSION PENTAGON”
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Fuel
Oxygen
Ignition Source
Suspension
Confinement
Questions to Ask
• Is a dust generated in the process?
• Is a dust liberated from the process?
• Are dust accumulations present?
Dust Hazards Testing:
• FM Global testing for clients
• Variety of dust hazard tests
• Various outside labs for non-FM Global clients
• Approximate cost for hazards test range from $600/sample
(explosibility screening test) to $1700/sample (full Kst test)
Full Kst Testing
• Determine explosibility parameters (i.e. Kst; Pmax)
for explosion protection design
• Two important parameter from this test: Kst/Pmax
Testing apparatus (20L Sphere – Kst Test)
Kst Classifications
• Kst = 0 – “Nonexplosible”
• 1 < Kst < 50 bar-m/s – “Very Weakly” Explosible
• 50 < Kst < 100 bar-m/s – “Weakly” Explosible
• 100 < Kst < 200 bar-m/s – “Moderately” Explosible
• 200 < Kst < 300 bar-m/s – “Strongly” Explosible
• Kst > 300 bar-m/s – “Very Strongly” Explosible
Hazard Classifications
• ST1 Dust: Kst = 1 - 200 bar-m/s
• ST2 Dust: 201 < Kst < 300 bar-m/s
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ST3 Dust: Kst > 300 bar-m/s
Examples
• St-1 dusts: coal, coke, lampblack, etc.
• St-2 dusts: cornstarch; cellulose; wood flour; etc.
• St-3 dusts: aluminum; magnesium, etc.
Other Dust Tests
• Explosibility Screening
• Sieve Analysis
• “Hard-to-Ignite” Testing (internal to FM Global)
• Minimum Explosible Concentration (MEC)
• Minimum Ignition Energy
Dust Explosion Hazards
Management of change most critical!
Examples
• New equipment
• Process temperatures
• Product formulations
• Process changes
Important Dust Aspects
• Particle size
• Dust chemistry
• Moisture
Hazard Awareness
• Secondary dust explosion hazards most critical!
• FM Standard: > 1/16”
Hazard Awareness
• Identify sources of liberation
• Eliminate fugitive dust liberations/accumulations
Preventative Measures
• Housekeeping: vacuuming or sweeping (air blowdown should be
a last resort!)
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Perform frequently
Limit to small areas
Shut down all non-hazardous rated electrical equipment
Prohibit open flames and hot work operations
Ensure no hot surfaces exist
• Housekeeping NOT the end all solution for fugitive dust hazards!
• Elimination most important!
Preventative Measures
• Boxing in structural members
• Sloping of horizontal structural members
• If above not practical, then mitigation controls
using Damage Limiting Construction (DLC)
Equipment Hazards
• Processing equipment
• Storage equipment
• Material handling equipment
Equipment Hazards
• Two potentially hazards:
– inherent equipment explosion hazard
– A more serious secondary explosion hazard (if
excessive fugitive dust levels exist)
Prevention Techniques
• Phlegmatization
• Inerting
Mitigation Techniques
• Venting
• Suppression
• Containment
• Isolation
Venting
• Advantages: reliable passive protection approach; usually the
most economical and effective form of protection
• Disadvantages: not usually applicable for indoor applications,
unless venting to the outdoor s can be provided
• Can vent from the indoors to the outdoors, via a short vent duct
“Flameless” Venting
• Option to standard explosion venting (when indoor
venting only feasible option)
• Flame arrestor with rupture type membrane
• Retains burned/unburned dust; cool the combustion
gases; and no trace of flame exits the device
(FM APPROVED FIKE EXPLOSION QUENCH PIPE)
Suppression
• Active protection approach
– Advantages: indoor applications; no equipment damage
– Disadvantages: complex design; lower and upper volume
protection limitations; high associated installation/maintenance
costs
– Quick sensing of an incipient explosion; delivers extinguishing
agent quickly to suppress explosion
Containment
• Explosion resistant design (no deformation)
• Shock resistant design (possible deformation )
• Explosion isolation needed
“Pressure Piling”
• First vessel explosion pre-pressurizes second vessel
• Final explosion pressure directly proportional to initial
pressure
• Subsequent second vessel explosion is pre-pressurized
resulting in destruction of the vessel
Explosion Isolation
• Reduces potential for propagation between interconnected vessels
• Needed for interconnected vessels protected by explosion
containment
• May also be warranted for interconnected vessels protected by
explosion venting
Examples
• Chemical Blocking System
• Chemical suppressant injected into connecting pipe
upon detection
• Prevents flame front from breaking through and
propagating
• Used in conjunction with explosion suppression
systems
• Active isolation system
Examples
• Rotary Airlocks
– Explosion quenched within the gap between the
vanes and the housing
• Chokes
– Provides an accumulation of powder through which
explosion is unable to propagate
Examples
• Rapid Action Valves (gate or butterfly type)
– High pressure gas as the driving force
– Closes in milliseconds
– Active isolation device
– Enough distance to allow valve to close before flame
front arrival
(FIKE EXPLOSION ISOLATION VALVE)
Examples
• Flame Front Diverter
– Pressure wave moves ahead of the flame front
– Flame front cannot make the 180 degree turn after the
explosion vent is opened
– Works in both directions
– Passive isolation device
(FIKE BI-DIRECTIONAL EXPLOSION DIVERTER)
Examples
• Backblast Damper
– Check valve provided with an explosion vent
– Works only in one direction
– Passive isolation device
(FLAMEX BACKBLAST DAMPER)
Ignition Source Control
• Reduces frequency (not severity)
• Not a substitute for effective dust explosion hazard
elimination and/or mitigation techniques!
Ignition Source Control
• Hazardous location electrical equipment:
– Class II, Division 1
– Class II, Division 2
– Non-classified
Ignition Source Control
• Class II, Division 1 locations:
– combustible dust in the air under normal operating conditions
– explosible or ignitible mixtures
• Class II, Division 2 locations:
– combustible dust not normally in air
– ignitible mixtures, but may be as the result of infrequent
malfunctioning of equipment
Ignition Source Control
• Non-classified locations:
– a hazardous quantities of dust not foreseeably
– released, as a result of malfunctioning equipment
Ignition Source Control
• Electrical classification on rate of dust accumulation:
– > 1/8 in. (Division 1)
– < 1/8 in. (Division 2)
– Surface color discernible, and hazardous quantity of dust
cannot be foreseeably released: Non-classified
Ignition Source Control
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Hot Work Management
Smoking Control
Magnetic Separators
PM programs for conductive dust collector bags
Grounding and bonding
External bearings; belt alignment and zero speed
switches for bucket elevators
Ignition Source Control
• Spark Extinguishing System
– Detects and extinguishes an upstream spark or glowing ember
– Processes with high frequency dust explosion hazards
– Not the same as explosion suppression!
– Reduces the frequency of an explosion, not the severity!
Dust Hazard Summary
• Dust hazards pose potentially catastrophic consequences, (both a
property damage and business interruption) especially when
fugitive dust is allowed to accumulate.
• 1/16 in. accumulation over a greater than 5% of the floor area.
• Fugitive dust hazard:
– Usually what results in the secondary room explosion,
– Secondary explosion causes the greatest amount of damage and
business interruption.
Dust Hazard Summary
• Utilize either prevention or mitigation techniques to reduce the
equipement explosion hazard.
• Implement an effective management of change policy, to ensure
all potential dust hazards addressed during the design/conceptual
stages of the project.