Source: United States Department of Labor

Kingsly AmbroseDepartment of Agricultural and Biological Engineering

Small, dry, solid particles projected into the air by natural forces, such as wind, volcanic eruption, and by mechanical or man-made processes such as crushing, grinding, milling, drilling, demolition, shoveling, conveying, screening, bagging, and sweeping. Usually in the size range 1 – 100 µm, and they settle slowly under the influence of gravity (IUPAC, 1990)

Small solid particles, <75 µm in diameter, which settle out under their own weight but which remain suspended for some time (ISO 4225 – ISO, 1994)

Particles with aerodynamic diameter > 50 µm have terminal velocity >7 cm/s

Terminal velocity of a 1 µm particle is about 0.03 mm/s

Types of dust Mineral Metallic Chemical Organic Biohazards

Deposition mechanisms Sedimentation Inertial Impaction Diffusion (small particle <0.5 microns) Interception Electrostatic deposition

Dust Deposition

Particle size fractions Inhalable particulate fraction Fraction that can be breathed into the nose or mouth Hardwood dusts (nasal cancer) Grinding lead containing alloys (systemic poisoning)

Thoracic particulate fraction Enter lungs through airways Cotton; airway disease

Respirable particulate fractions Penetrate beyond the terminal bronchioles Crystalline silica; dusts from masonry drill bits, etc

Other Routes of Exposure Skin absorption Ingestion

Inhalation Cancer Heart disease Sytemic poisoning Lung disease Allergic responses (Farmer’s lung, wheat disease,

bagassosis, malt worker’s lung)

Hazard Group A – Skin and eye irritants Hazard Group B – Harmful substances Hazard Group C – Toxic substances – damages

skin (severe) Hazard Group D – Very toxic – Carcinogens, can

affect human fertility and unborn child Hazard Group E – More severe effects (dust

mostly with skin and eye contact rather than airborne)

Dustiness of the material High – visible dust clouds and remain airborne for

several minutes (cement, photocopier toner, etc) Medium – dust is seen but settles out quickly (soap

powder, sugar granules, etc) Low – pellet-like, non-friable solids (PVC pellets)

Control approach 1 – good housekeeping, ventilation, maintenance, training

Control approach 2 – Restricted access, protective clothing, RPE, specific trainings

Control approach 3 – Controlled access, written maintenance procedures

Control approach 4 – Specialist advice

Elutriators Particles separated according to their settling velocities

Cyclones Uses centrifugal force to separate

Impactors Separation efficiency depends on the air velocity

Filters Direct interception of particles

Elimination Substitution of materials Physical form Equipment or process modification Wet methods

https://www.youtube.com/watch?v=fcc6a0zmJ48

Grain Dust Explosions in the U.S.

Five year average of grain dust explosions in the U.S.

Grain Dust Explosions in the U.S.

Five year average of grain dust explosions in the U.S.

Grain Dust Explosions in the U.S.

Source: Ambrose and Sanghi, 2014, World Grain

OSHA’s Dust Definition:A fine powder/particulate solid material that is less than 420 µm, i.e. largest particles that will pass through a US 40 sieve.

• Two principal mechanisms of dust generation during bulk materials handling.

• The first mechanism involves dust generation during the free fall of the material and

• The second mechanism involves the release of entrained air during free fall, which then causes the dust emission

• The amount of dust emitted depends on flow rate, drop height, type of material, quality or grade (of grain), moisture content, degree of enclosure in the receiving area, and effectiveness of dust capture/collection systems.

2. A primary explosion disturbs the settled dust into a cloud

3. Dust cloud is ignited and a secondary explosion happens

1. Dust settles on surfaces

Dust

Adapted from U.S. Chemical Safety Board

Grain dust particles commonly range in size from less than 5 μm to more than 100 μm.

These dust particles have relatively low settling velocities in air from 0.001 to 0.25 m/s and result in dust cloud formation.

Particles smaller than 125 μm in diameter accounted for 76-86% of the total mass of corn dust and 43-54% of total mass of wheat dust.

• Explosion of dust clouds will not take place unless the dust concentration is within certain limits

• Corn starch – Minimum explosible concentration is 70 g/m3

• Rule of thumb – Zehr (1965)• If a glowing 25-W lightbulb is observed through 2 m of

dust cloud, the bulb cannot be seen at dust concentrations exceeding 40 g/m3

The smaller the work space, the smaller the tolerated dust accumulation presenting no dust explosion hazard (Bartknecht, 1989)

Δ is the acceptable height or thickness of dust, C is the lower explosible limit of dust, s is the density, l is the length, w is the width and h is the height

hwl

sC

1112

++=∆

Leading sources Unknown Welding or Cutting – any hot work Fire from any source Overheated bearings Friction from choked leg & rubbing pulley Tramp metal Friction sparks or other sparks Static electricity

CC1: No ignition; no self-sustained combustion CC2: Short ignition and quick extinguishing; local

combustion of short duration CC3: Local burning or glowing without spreading;

local sustained combustion but no propagation CC4: Spreading of a glowing fire; propagation

smoldering combustion CC5: Spreading of an open fire; propagating open

flame CC6: Explosible burning; explosive combustion

Kst Value: Maximum rate of pressure rise in 1m3 vessel when a

dust is ignited (cube root law)

(𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

)𝑚𝑚𝑚𝑚𝑚𝑚𝑉𝑉1/3 = 𝐾𝐾𝑠𝑠𝑑𝑑Explosion class Kst (bar.m/s) Characteristic Example

St 0 0 No explosion Silica

St 1 > 0 and ≤ 200 Weak explosion Powdered milk, Coal,sulfur, sugar, zinc

St 2 > 200 and ≤ 300 Strong explosion Cellulose, wood flour, grain dust

St 3 > 300 Very strong explosion Aluminum, magnesium

Ref: Eckhoff, 2004: Bartknecht, 1978, 1981; OSHA 2009

• Chemical properties • Dust particle size and shape

• Minimum ignition energy• Flammable limits • Deflagration Index, KSt.• Maximum Explosion Pressure• Ease of dispersion in air

Screening test (based on ASTM E1226-10 standard): This is a general classification of powders being either ‘explosible’ or ‘non-explosible’. This test determines the explosibility of dust clouds when exposed to an ignition source.

20 liter sphere apparatus

Minimum ignition energy (MIE) – electrostatic (based on ASTM E2019 standard): This test determines the minimum electrostatic or spark energy that is capable of igniting a dust cloud. This test characterizes the susceptibility of dust clouds formed from powders to ignition by electrostatic discharges or mechanical sparks.

Minimum explosible concentration (MEC, based on ASTM E1515 standard): This test determines the minimum concentration of dust-air mixture or the cloud that can propagate flame upon ignition. Though this tests could evaluate the lowest concentration required for a dust cloud to ignite, achieving a limit lower than MEC would not be always practical in a facility with continuous operation.

Minimum ignition temperature of a dust cloud (MITcloud, based on ASTM E2021 standard): This test determines the minimum temperature that is capable of igniting a dust cloud. This is an important test to assess the dust clouds that could form in heated operations, hot surfaces, and sparks.

Minimum ignition temperature of dust layer (MITlayer, based on ASTM E2012 standard): This test will determine the ignition temperature needed to ignite dust/powders in a layer of thickness ranging from 5 to 12.7 mm. As per National Electric Code (NEC), MITlayerand MITcloud defines the selection of Class II electrical devices that could be used in hazardous areas.

Material Allowable MIE range

Irganox 1010 1 – 5 mJ

Anthraquinone 1 – 11 mJ

Lycopodium 10 – 30 mJPittsburgh coal 30 – 140 mJ

Heat of combustion of grain dusts: 5500-7000 BTU/lb

Pressure from explosion range from 50 to 120 psi Structural damage as the pressure wave expands

radially from the explosion site. Severity depends on fuel availability, fuel location,

location of ignition source, and the vent area.

Over Pressure - psi

Corrugated asbestos 0.3

Windows 0.5

Wood framing 1 – 2

Concrete block 3 - 3.5

Brick walls 5 – 6

Sheet metal (legs) 1 – 2

Reinforced concrete 5 - 40

Failure Pressures

slab Span, ft

10 15 20

6” 21.9 7.9 3.0

8” 42.1 16.3 6.0

10” 67.5 29.5 14.2

Load limits – Concrete slabs

Source: Brasle, 1979, Guidelines for estimating damage from grain dust explosions

Source: BS&B Systems

Explosion Venting: Explosion flame, pressure and unburned mixture

are vented to a safe area.

Source: BS&B Systems Source: FIKE Corp.

Source: BS&B Systems

Source: NFPA 664