Combustible Dust & Composites - acma

Report
Bob Lacovara, CCM, CCT-I
Convergent Composites
© Convergent Composites 2010
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U.S. Department of Labor
Occupational Safety & Health Administration
As a result of several catastrophic
dust explosion events OSHA issued
a national enforcement emphasis
on dust
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U.S. Department of Labor
Occupational Safety & Health Administration
A series of composites plants
have had routine inspections
where dust issues were cited
or dust was a discussion item
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What are the chances?
The risk of a composites plant
dust explosion has not changed,
nor have there been any recent
documented incidents.
However, the regulatory
emphasis elevates the technical
possibility of such an incident.
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Combustible Dust Policy Institute:
• 50% of dust explosions involved a dust collector
• 10% of combustible dust incidents involved
ductwork
• Few combustible dust incidents
have to do with housekeeping
• Most combustible particulate
incidents occur inside process
equipment
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What is a dust explosion?
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Combustible Dust is a
combustible particulate
solid that presents a fire or
explosion hazard when
suspended in air or the
process-specific oxidizing
medium over a range of
concentrations, regardless
of particle size or shape
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Explosion:
Rapid oxidation that results in a
sudden increase in gas volume
and temperature,
accompanied by
an overpressure
shock wave
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Deflagration:
Propagation of a combustion zone
through a fuel-oxidizer mixture at a
rate less than the speed of sound and
capable of producing a significant
increase in pressure.
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Defining the Terms
Flash Fire:
A rapidly moving, short duration
flame front resulting in
intense thermal radiation
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Defining the Terms
Dust Explosion:
In plain English - The ignition of airborne
particles resulting in an explosion or a
deflagration.
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How does a dust explosion occur?
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Dust Hazard Basics
Dust Explosion Characterization
Primary Event
An occurrence where standing
dust is dislodged forming a small
dust cloud
• Sudden flush of air over a
dust laden surface
• Mechanical vibration
that dislodges dust
• Small localized explosion
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Dust Hazard Basics
Dust Explosion Characterization
Primary Event
Secondary Event
An occurrence where standing
dust is dislodged forming a small
dust cloud
• Sudden flush of air over a
dust laden surface
• Mechanical vibration
that dislodges dust
• Small localized explosion
A large dust cloud is formed by
the primary event
• Ignition source ignites
dust cloud
• Large scale deflagration
occurs
• Catastrophic event
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The Safety Issue
How Does a Dust Explosion Occur?
Event occurs that dislodges
a small amount of dust
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The Safety Issue
How Does a Dust Explosion Occur?
Dust is exposed to
an ignition source
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The Safety Issue
How Does a Dust Explosion Occur?
Small dust cloud ignites
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The Safety Issue
How Does a Dust Explosion Occur?
Pressure wave dislodges
a large dust cloud
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The Safety Issue
How Does a Dust Explosion Occur?
Large dust cloud ignites
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The Safety Issue
How Does a Dust Explosion Occur?
Large Scale Deflagration Occurs
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What are the conditions required
for a dust explosion?
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Dust Explosion
Conditions for a Fire
Fuel
Ignition
Source
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Oxidant
(Oxygen)
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Dust Explosion
Conditions for a Dust Explosion
Fuel
Confinement
Mixing
Ratio
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Oxidant
(Oxygen)
Ignition
Source
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Dust Explosion
Mixing
Ratio
To ignite airborne dust the particle to air
concentration must be between the
upper and lower explosive level
(This ratio is different for various types of dust)
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Dust Explosion
Effects of Dust Concentration
• Explosive pressure is a
function of concentration
• Higher concentrations are
easier to ignite
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Dust Explosion
Effects of
Air Temperature & Humidity
• Temperature is generally not a factor
• Humidity may be a significant factor
• Hydroscopic nature of the dust
• High moisture content reduces potential
• Low humidity increases electrostatic
discharge potential (ESD)
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Dust Explosion
Hybrid Mixtures - Dust + Vapor
At dust concentrations below
the minimum explosive
concentration, and at vapor
concentrations below the LFL,
a flammable hybrid mixture
can occur
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How is dust characterized
in terms of hazard potential?
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Dust Characterization
Hazard Determination
Minimum Ignition Temperature (MIT)
Dust Layer / Dust Cloud
The lowest temperature
of a hot surface which
will cause a dust
cloud to ignite
and propagate
flame
Expressed in 0F
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Dust Characterization
Hazard Determination
Minimum Ignition Energy (MIE)
A measure of how sensitive an
explosible dust is to electrical
spark ignition. Gives guidance
on whether ignition by
electrostatic discharge
or process conditions
is likely to occur
in practice
© Convergent Composites 2011
Expressed in mJ
(milijoules)
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Dust Characterization
Hazard Determination
Minimum Explosible Concentration (MEC)
The lowest concentration of
dust or powder that will
ignite on contact with
an ignition source and
propagate a dust
explosion
Expressed in g/m3
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Dust Characterization
Dust Layer Thickness
• 1 mm thick - Potential for 10g/m3 concentration
• European standards test “smoldering” temperature
• 5 mm and 50 mm thick dust layers
• Thicker deposits require lower allowable
surface temperature
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Dust Characterization
OSHA Guidelines:
In the absence of definitive test data
OSHA will seek compliance for an
acceptable limit of 1/32 in. (0.79 mm)
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Dust Characterization
National Fire Protection Association
NFPA 654 - Use of Separation (6.2.3)
Allowable dust thickness is determined
with an engineering evaluation designed
to determine separation distances based
on the properties of materials handled,
type of operation, accumulations, building
design and surrounding exposures
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Uniform Downward Hemispherical Diffusion Model
Ceiling or Overhead Beam
Concentrated Dust Source
1 ft.
1,786 g/m3
2 ft.
223 g/m3
3 ft.
66 g/m3
4 ft.
5 ft.
14 g/m3
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28 g/m3
Dust Bulk Density = 0.359 g/cc
Thickness = 0.125”
Area = 1 ft2 35
Uniform Downward Linear Diffusion Model
Overhead Beams
Dust Bulk Density = 0.359 g/cc
Thickness = 0.125”
Area = 500 ft2
1 ft.
2 ft.
3 ft.
4 ft.
5 ft.
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Diffusion Models Spreadsheet
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Dust Explosion Prevention
General Principles
1. Reduce amount of dust (fuel) available
2. Avoid flammable atmospheres
3. Eliminate/control ignition sources
4. Provision against consequences of ignition
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Dust Explosion Prevention
Reduce Dust Generation
Make Less Dust
• Improve product design to eliminate
cutting/grinding
• Improve cutting/grinding process
• Use wet trimming/water jet cutting
• Improve QC - Less re-work
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Dust Explosion Prevention
Contain Dust Making Operations
Dust Capture
• Point capture at tool - Hoods, guards,
or suction devices
• Confine cutting-grinding-sanding to specific
dust making areas
• Operate in grinding booths/areas
• Segregate from general process
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Dust Explosion Prevention
Maintain Dust Collection Systems
Routinely clean duct work and plenums
• Eliminating residual dust build-up
reduces fire hazard
• Eliminating residual dust build-up
increases system efficiency
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Dust Explosion Prevention
Verify Grounding Continuity of
Flexible Hoses
Reduces static charge in system
Electrical Ground
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Dust Explosion Prevention
Properly Ground Bags and
Collection Equipment
Electrical Ground
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Dust Explosion Prevention
Isolate Dust Collection Equipment
From Personnel & Process
Best practice includes
reducing in-plant
exposure to
potential
hazards
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Dust Explosion Prevention
Housekeeping
Periodically clean:
• Beams, rafters, girts
• Dropped ceilings
• Storage areas
• Behind partial panel walls
• Interior roof surfaces - booths/offices
• Areas where dust may accumulate
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Dust Explosion Prevention
Revision of 654
Requires implementation of a planned
inspection process to determine dust
accumulation rates and determine
housekeeping frequency
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Dust Explosion Prevention
Revision of 654
Establishes a Hierarchy of Cleaning Methods
• Vacuum cleaning
• Sweeping
• Water wash down
• Compressed air blow down
(only after other methods & with
protected electrical equipment)
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Dust Explosion Prevention
Codes & Standards
Reference
OSHA
• PSM
regulation
• General Duty Clause 5(a)(1)
• 1910 Subpart S - Electrical installations
• 1910.94 – Ventilation requirements
• 1910.1200 - HazComm
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Dust Explosion Prevention
Codes & Standards
Reference
National Fire Protection Association
• General - NFPA 654
• Facility Design Codes
• NFPA 68-69-70-77-499-650
• Life Safety Codes
• NFPA 101
• Building Codes – IBC
• NFPA 5000
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Contact:
Bob Lacovara, CCM, CCT-I
Convergent Composites
[email protected]
215-257-1907
www.convergentcomposites.com
© Convergent Composites 2010
All rights reserved
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