Identifying and Managing the Hazards of Combustible Dust

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Identifying and Managing the Hazards of Combustible Dust

Presenter:Brian Edwards, PE

Director of EngineeringConversion Technology Inc.


Scope of Discussion

Introduction to Combustible Dust Characterizing Dust Hazards Conducting a Dust Hazard Analysis of

a Facility Dust Hazard Management and

Control


IntroductionWhat do we mean when we talk about combustible dust?

Combustible Dust is a “particulate solid that presents a fire hazard when suspended in air … regardless of size or shape.” - NFPA

Layman’s terms: if it can burn, and it hangs in air (even briefly), it is likely a combustible dust.

Combustible Dust can be: dry food, plastics, wood, rubber, textiles, pesticides, pharmaceuticals, dyes, coal, and metals, among many other substances.


Why is Combustible Dust a Hazard?

The rate of combustion of a material depends on particle size, and smaller particles have increased burning rates.

Normally slow burning material can create flash fires if particles are sufficiently small and suspended in air. Image from U.S. Chemical

Safety Board


Greatest Hazard of Combustible Dust

Dust can accumulate on surfaces, with the smallest particle sizes accumulating in hard to reach places (overhead surfaces), often unnoticed.

Primary explosions can dislodge the accumulated dust, and create a much larger, SECONDARY EXPLOSION.

Image from OSHA


Fires, Deflagrations, and Explosions

Hayes Lemmerz, Huntington, IN - Photo From US Chemical Safety Board


Definitions FIRE: The rapid oxidation of a material in the chemical

process of combustion, releasing heat, light, and various reaction products.

DEFLAGRATION: Propagation of a combustion zone at a velocity that is less than the speed of sound in the un-reacted medium. (From NFPA 654)

EXPLOSION: An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases.


Deflagrations

For a free-air deflagration to occur, four conditions must be met:

Having all four conditions come together simultaneously is relatively rare in most facilities.

However, if rooms or pieces of equipment have the potential for dust concentrations to reach the MEC, then a deflagration hazard exists.

Small combustible particles are present (fuel) The particles are suspended in the air in sufficient

quantity Available oxygen Ignition source (energy)


Deflagrations


Deflagrations


Deflagrations


Deflagrations


Deflagrations


Dust Explosions Explosion hazards exist when all four conditions needed for

a deflagration can occur within an enclosure – whether it is a room, building, container, or piece of equipment.

Most dust explosions originate inside pieces of equipment or containers. Explosions often occur in the following types of equipment (in order of frequency): Dust Collectors (account for over 50% of incidents) Dryers / Ovens Conveyors / Elevators Silos / Bins


Explosion Pentagon

Oxygen

Containment of Dust Cloud

FuelIgnition Source

Dispersion of Dust Cloud


Containment of Dust Cloud

The dust cloud must be contained for an explosion to occur.


Start of an Explosion

When you have a confined dust cloud, all that is missing is an ignition source (assuming oxygen is present).

Ignition Source


Process Equipment Explosion












Explosion Chain Reactions Industrial dust explosions can create a chain-reaction resulting in

more explosions and flash fires.

Often times, piece after piece of connected equipment will have an explosion.

Flame fronts can travel through ductwork from equipment to equipment, or pressure waves can disturb settled dust creating a hazardous atmosphere throughout a building.

These SECONDARY EXPLOSIONS that occur in the building are often the most damaging.


Deflagration Propagation






Secondary Explosions

The most significant hazard associated with combustible dust is a secondary explosion that occurs throughout a building, fueled by disturbed, accumulated dust.

Image from OSHA


To determine the hazards associated with a dust, we must know a few things:

Is the dust combustible / explosible?

How sensitive is the dust to ignition?

How violent is a dust explosion from this material?


The first step is relatively obvious: identify all potential dusts HANDLED and GENERATED at the facility.

Review SDS for the materials and contact the vendors for any additional data.

Review textbooks, online databases and NFPA standards.

NFPA standards that provide select material properties include: NFPA 61, 68, 69, 499, and 652

Conduct laboratory testing.


Dust Explosibility Classification Test Dust is placed in a 20 L sphere at increasing concentrations

and tried to ignite. If it ignites and produces a sufficient increase in pressure, it is considered combustible.

Results are “Go” / “No-Go”.

Specific Test Methods Minimum Explosible Concentration (MEC) – ASTM 1515 Minimum Ignition Energy (MIE) – ASTM 2019 Minimum Ignition Temperature (MIT) – ASTM 1491 Maximum Deflagration Pressure (Pmax) and Rate of

Pressure Rise (dp/dt and Kst) – ASTM E1226 Resistivity – ASTM D257


Prior to sampling, develop a plan.

Two types of materials to consider for testing: Fugitive dust Process dust

Lab work can be expensive, so consider which samples are most representative.


Facility Dust Hazard Analysis The first step in ensuring safety is to

conduct a dust hazard analysis of the facility.

The analysis should identify:

Drawing from a Dust Hazard Analysis

The types of combustible dust present; Processes which use, consume, or

produce combustible dusts; Open and hidden areas where

combustible dusts may accumulate; How dust may be dispersed in the air; Potential ignition sources; Potential effects of a combustible dust

incident, and mitigating safeguards.


Identify All Processes That Use, Consume or Produce Combustible Dust

Develop a list of process lines where combustible dust could be present.

Itemize all pieces of equipment, paying special attention to: Dust collectors Pneumatic conveyance systems Bins, tanks, and silos Particle size reduction equipment Dryers and ovens Conveyors, screw augers and bucket

elevators Sifters, screens, and classifiers


Identify Areas Where Dust Can Accumulate

Conduct an inspection of all areas of the facility where dust is handled, processed, and generated. Identify the extent of dust accumulation.

Identify pieces of equipment that are sources of fugitive dust.

Pay special attention to areas that are hidden or not frequently visited, such as: Overhead piping and beams Drop ceilings Interior overhead surfaces (control

rooms, mezzanines, etc.)


Identify Ways Dust Can Be Dispersed in the Air

Two major ways for dust to be dispersed: EQUIPMENT and DISTURBANCE OF ACCUMULATED DUST.

Identify equipment and activities that typically release dust into the work environment.

Also, identify equipment that could release dust due to a malfunction or failure.

For areas where dust commonly accumulates, identify how that dust could be disturbed, such as from cleaning activities, process malfunctions, or primary explosions.


Identify Ignition Sources

Ignition occurs when sufficient energy (heat or electrical energy) is applied to the suspended dust cloud.

Common sources of ignition include: Open Flames Hot Surfaces Static Electricity Powered Industrial Trucks Self Ignition

Mechanical Sparks Frictional Heat Hot Work Foreign Objects Electrical Equipment


Once you have identified where deflagrations and/or explosions can occur, you need to develop What-If scenarios.

Points to consider:

What types of equipment and occupancies are near the location of the hazard?

What pieces of equipment are connected via ducts or conveyors to units where an explosion hazard exist?

What is the potential chain of events that could occur? What engineering controls are in place to mitigate the

hazard, and what additional controls could be added?


Quantify the Risk

A Risk Matrix can be used to compare the likelihood and severity to determine if the level of risk is acceptable.


Using the Results of the DHA

The completed risk matrix will identify the areas and equipment that are at unacceptable levels of risk.

Recommendations designed to reduce the likelihood and/or severity of fires and explosions should be developed for everything that is unacceptable.

The priority of the recommendations should be based on a balance of which items present the greatest risk and which recommendations are most feasible.


Hazard Management Building Design Fugitive Dust Control Ignition Source Control Deflagration and Explosion

Protection Housekeeping Maintenance / Inspections Personal Protective

Equipment Hazard Communication and

Training


Fugitive Dust Control Fugitive dust control includes

reducing the amount of dust emitted from equipment as well as collecting dust that is emitted.

You must ensure you have the right equipment and that they are properly maintained.

Dust collection systems are important, but only if installed correctly and properly maintained.

Properly balanced air flow in dust collection systems is imperative.

Dust Collection System


Secondary Engineering ControlsSecondary controls are used to reduce the effects of a deflagration.

Deflagration and Explosion Protection

Containment Isolation

Explosion Suppression System


Housekeeping Housekeeping schedules should be sufficient

to prevent excess dust accumulation.

Pay attention to hidden and hard to reach areas.

Methods should focus on limiting the generation of dust clouds.

Portable vacuum cleaners should be rated for use with combustible dust.

Central Vacuum Systems can be very useful, but need to be designed to handle combustible dust.

Compressed air should only be used with extreme caution.

Central Vacuum SystemImages from

www.vac-u-max.com

Explosion Proof Vacuum


Questions?

Brian Edwards, [email protected](770) 263-6330 x 103

For Additional Information:

Documents

Identifying and Managing the Hazards of Combustible Dust