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:
• 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
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)
• 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):
• 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):
• Wood – 70• Food – 25• Chemical – 17• Metal – 15• Coal – 14• Plastic/Rubber – 13• Paper – 8• Others - 4
FM GLOBAL LOSS HISTORY (BY EQUIPMENT TYPE):
• 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
INTENTIONALLY MANUFACTURED DUSTS
• Flours; Cornstarch
• Pulverized Coal
• Carbon Black
• Metallic Stearates
• Metal Powders
BY-PRODUCT (NUISANCE TYPE) DUSTS
• Wood dust• Plastic dust• Rubber dust• Coal dust• Grain dust• Metal dust
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
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
• 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
Hazard Awareness
• Identify sources of liberation
• Eliminate fugitive dust liberations/accumulations
Preventative Measures
• Housekeeping: vacuuming or sweeping (air blowdown should be a last resort!)
– 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
• Two potentially hazards:
– inherent equipment explosion hazard
– A more serious secondary explosion hazard (if excessive fugitive dust levels exist)
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
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
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
Examples
• Backblast Damper
– Check valve provided with an explosion vent
– Works only in one direction
– Passive isolation device
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
• 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
• 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!
Ignition Source Control
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.