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

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)

• 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

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

• Wood dust• Plastic dust• Rubber dust• Coal dust• Grain dust• Metal dust

THE “DUST EXPLOSION PENTAGON”

• 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

• 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!)

– 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

• 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.

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.