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Safety Issues in Particle Handling:Dust Explosions

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Credit for Stealing Shamelessly

Our thanks to Bob Gravel and Karen Tancredi of DuPont for their permission to use a number of slides from their presentation to the CCPS Technical Steering Committee.

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®The Miracles of Science

Goal of this Presentation Provide a general overview of dust explosion fundamental

concepts, prevention/mitigation methods, and current regulatory trends in the United States

In light of several recent incidents there has been a flurry of activity around combustible dust safety; however, thousands of dust explosion incidents have been documented since 1785

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The Washburn Mill, Minneapolis, MN (1878)

US Chemical Safety Board Study

Combustible dust incidents in the US from 1980-2005:

- 281 events

- Wide range of products/industries

- Many different unit operations

- 119 deaths/718 injuries

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Distribution of Dust Events by Industry Distribution of Dust Events by Material Type

(Ref: U.S. Chemical Safety Board Report No. 2006-H-1)

Combustible Dust Events in US: 1980-2005

Note: Coal mines & grain handling facilities excluded from study

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Equipment Type % of Incidents

Dust Collector 52Impact Equipment 17Silos & Bins 13Dryers & Ovens 9Processing Equipment 6Conveyor 3

Dust Explosion by Equipment Type127 Incidents

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Consequences of Dust Explosions

Potential financial losses– Equipment– Liability– Fines– Lost product/production

Potential for personal injury or loss of life

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Importance of Awareness

25% of Causes are Unknown in Dust Explosions

36% of Incidents are Due to "Human Error“

Knowledge is essential to safe operation!

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Fundamental Concepts

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What is a “Combustible Dust”?Per NFPA-654 (2006 edition):

“A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.”

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Fire Triangle

FuelO

xida

nt

Ignition Source

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The Dust Explosion Pentagon

In addition to the traditional three components necessary for combustion, dust requires two additional conditions:• Fuel (combustible dust),

• Heat/Ignition (flame),

• Oxygen in air,

• Dispersion of dust particles

• Confinement of dust cloud

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Fundamental Concept of Dust Explosion

Log: DifficultTo Light, Burns Slowly

Kindling: Easier toLight, Burns Quickly

Dust: Lights Easily,Burns VERY Fast

(Ref: Dust Explosions in the Process Industries, R.K. Echhoff)

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Dust Combustion Consequences

Combustion in closed systems (e.g., vessel or room with few openings) results in pressure rise from confinement of the expanding hot gas and may result in sudden energy release (explosion) from mechanical/structural failure

Combustion in a relatively open area allows fireball expansion with little increase in pressure and poses a flash-fire hazard

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Key Dust Explosion Parameters

Minimum Explosible Concentration (MEC): “How much dust is needed to create a hazard?”

Dust Deflagration Index (Kst): “How fast will it burn?”

Limiting Oxygen Concentration (LOC): “How much oxygen is needed to support combustion?”

Minimum Ignition Energy (MIE): “How much energy does it take to make it ignite?”

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Minimum ExplosibleConcentration (MEC) and

Dust Deflagration Index (Kst)

20-Liter Test Vessel

PressureTransducer Port

PyrotechnicIgniters

Air/Dust Inlet

20-liter TestSphere

• MEC--Measure of the lowest dust cloud concentration capable of sustained combustion• Kst--Measure of the maximum burning rate of a dust cloud of ideal concentration under turbulent conditions• Determine via tests conducted in 20-liter spherical vessel

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A Common Question: “If I can see dust floating around in my work area do I have

a dust explosion hazard?”

The Answer:

Dust suspensions which you can see through are likely to be orders of magnitude below MEC; however, they may still pose a dust explosion hazard if they are allowed to accumulate in layers.

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Explosible Dust Concentration

Mass of Powder/Dust per unit Volume [g/m3]

(Ref: Dust Explosions in the Process Industries, R.K. Echhoff)

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Dust Classification by Kst

Explosion violence of dusts

classified by value of Kst:

St-1: 0 < Kst < 200 bar-m/s

St-2: 200 < Kst < 300 bar-m/s

St-3: Kst > 300 bar-m/s

Ref: Dust Explosions, W. Bartknecht

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Kst

Limiting Oxygen Concentration (LOC)

The minimum oxygen level required for combustion of a dust cloud at any concentration, evidenced by pressure rise or sustained flame propagation• Determined experimentally; typically 8-15%

• LOC depends on the inert gas used

• Nitrogen, CO2, water vapor, and combustion gases are commonly used inertants; the higher the molar specific heat the more effective the gas (i.e., less needed)

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Minimum Ignition Energy (MIE)

• The lowest capacitive spark energy capable of igniting a dust cloud of optimum concentration in a given number of laboratory trials

• Typically 10-100 mJ; flammable vapor MIEs usually < 1 mJ

• Threshold of perception ~1 mJ; ‘carpet spark’ in 2-5 mJ range

Dust Sample inBottom of Tube

Air to Disperse Sample

Glass TubeSpark

Electrodes

MIE Test Apparatus

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MIE vs. Particle Size

(Ref: Dust Explosions, W. Bartknecht)

Mean Particle Size, microns

Min

imu

m I

gnit

ion

En

ergy

, mJ

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Rate of Pressure Rise (Kst) vs. Particle Specific Surface Area(Ref: Dust Explosions, W. Bartknecht)

Surface Area of Dust

Rat

e of

Pre

ssu

re R

ise

Finer Particles

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The Bottom Line: Size Matters!

The finer the average particle size the greater the hazard since finer particles are easier to ignite and burn faster

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Product Moisture

Moisture can effect explosion properties; products usually dried to <2% moisture prior to testing

Testing at actual moisture levels may be warranted if it can be ensured that the level tested is an absolute minimum

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Prevention & Protection

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When to Protect...

Provide protection “…where an explosion hazard exists” (NFPA)

- Combustible dust clouds >25% of MEC present

- ‘Significant’ combustible dust layers present

- Must consider both normal & ‘abnormal’ operation

Consider both ignition source control and preventive/mitigative protection measures

Design of a system which eliminates all possibility of a dust explosion should be considered first (inherent safety)

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Types of Ignition Sources Involved in Incidents

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27%

18%

17%

15%

7%

7%

4%

3%2%

Unknown

Friction/mechanical

OverheatingFlames

Tramp material

Welding &Cutting

Static

Electrical

Other

(ref. Partner, B. Dust Explosions - Assessment, Prevention and Protection. IBC Symposium, November 1989)

Control of Ignition Sources

In general where a fuel and oxidizer are present control of ignition sources should not be relied on as a sole means of explosion prevention, although identification and elimination of such sources should be a high priority

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Sources and Hazards of Dust Deposits

Sources

• Settling of fugitive dust on horizontal surfaces

• ‘Caking’ of material on internal equipment surfaces

• Trapping of dust at stagnant points in system

Hazards

• Dust explosion from re-entrainment of dust

• Fire caused by self-heating of deposits

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Re-Entrainment Dust Hazard(Ref: Dust Explosions,W. Bartknecht)

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Re-Entrainment Hazard

May result from high air flow, mechanical shock, or from an impinging combustion event

Re-dispersion of relatively thin layers (1 mm) can result in dust clouds >MEC

In many cases the secondary explosion may be worse than the initiating event!

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West Pharmaceuticals Incident: January 29, 2003

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West Pharmaceutical Process

1) Rubber Batch Made in Mixer

5) Water Dried From Sheet

6) Sheet Folded In Box 4) Sheet Coated with

Anti-Tack Powder In Water Solution

3) Rubber Rolled Into Sheet

2) Batch Lowered & Dropped into Mill

(Source: US Chemical Safety Board)

Dust Layer

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Dust Generation Some dry powder became airborne during drying

Plant personnel did not recognize potential dust explosion hazard posed by this material

No dust explosion potential was found during inspections by OSHA, local officials, and insurance underwriters

Powder dust explosion properties very typical of ‘standard’ combustible dusts

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1) Minor Event Occurs Around Batch MixerDust Layer

2) Event Redisperses Dust Layer into Cloud

3) Dust Cloud Ignites And Propagates Through Building

The Event…

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Photo Courtesy U.S. Chemical Safety Investigation Board

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The Aftermath: Six Fatalities/38 Injuries

Ref: US Chemical Safety Board49

The Moral:

Cleanliness is truly next to Godliness in prevention of dust explosions! The three best ways to prevent dust explosions:

HOUSEKEEPING, HOUSEKEEPING, HOUSEKEEPING!

As a ‘rule of thumb’ accumulations of combustible dusts should not obscure the color of painted equipment (<1/32” thick)

Pay particular attention to overhead surfaces: finer particles are present here and they’re more likely to become airborne

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Fugitive Dust Control

Primary emphasis should be on dust containment by design and maintaining mechanical integrity of equipment

Provide adequate ventilation to capture fugitive emissions

Frequently clean deposits by non-dust producing methods (e.g., gentle sweeping or vacuuming rather than ‘blowing off’ deposits with compressed air)

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Other Considerations…

Damage-limiting construction (DLC) may be required for buildings if deposits cannot be adequately reduced:

- 1/32” deposit over 5% of floor area based on 75 lb./ft3

- Maximum area not to exceed 1000 ft2

- Include overhead horizontal (and possibly vertical) surfaces

DLC involves use of pressure-relieving and resistant walls

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Explosion inDust Collector

Pressure Pulse from Collector Redisperses Layer into Dust Cloud

And results in Flame Propagation in Duct

Deposits in Equipment & Ductwork can also be a Source for Secondary Events

Moral: Minimize stagnation points & provide adequate conveying velocity to keep solids in suspension

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Explosion inDust Collector

Solids in DrumRedisperse into

Dust Cloud

Back Propagation ofPressure Pulse from

Dust Collector

SecondaryExplosion

Pressure Pulses can also cause Secondary Events

Moral: Provide isolation device to prevent back propagation of pressure pulse

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Prevention of Spark Discharges

There can be no spark if conductive objects can’t become charged

Grounding of conductors will prevent charging

Personnel grounding recommended if MIE < 30 mJ and there is potential for exposure to combustible dust cloud

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Bonding & Grounding of all Conductive Objects should be the First Line of Defense!

Other Preventive & Mitigative Measures

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Preventive/Mitigative Strategies

Strategies may be preventive or mitigative in nature:

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- Preventive: remove either the fuel or oxidant leg of the ‘fire

triangle’ to prevent combustion (operate below MEC or inert)

- Mitigative: accept that an explosion may occur and institute

measures that eliminate the potential for injury and/or damage

(contain/vent/suppress)

Limit ‘Fuel’ Concentration

Operate at maximum ‘acceptable’ concentration at least 25% of MEC (NFPA-69)

Sometimes difficult to achieve in practice due to non-homogeneity of dust clouds

In many cases the average (bulk) dust concentration must be orders of magnitude below MEC for this strategy to be effective

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Inerting by Oxidant Reduction Addition of a non-oxidizing (inert) gas or operation under

partial vacuum may be used to prevent dust explosions by reducing the oxidant below a level where combustion is possible (LOC)

LOC used to establish safe operating limits; must be based on specific inert gas to be used

Do not overlook asphyxiation hazards posed by inerted vessels and processes!

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Containment

Design equipment to withstand internal explosion without catastrophic failure

Generally limited to smaller volume equipment due to cost; applicable only to code-designed and constructed vessels

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Explosion Venting

Use intentionally ‘weak’ elements to relieve the pressure & vent combustion event to a safe location to prevent catastrophic equipment damage or personnel injury

Use value of Kst along with appropriate nomographs and/or equations to size vent of proper area (e.g., NFPA-68)

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Length of Flame jet/Fireball

Reaction forces

Outbreak of fire

Pressure waves

Explosion Venting

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Effect of Vents on Explosion Pressures

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Time

Pre

ssu

re

Pstat

Pred

Pmax

Unvented Explosion

Vented Explosion

Pressure Relief Panel

Failure along scorelines at desired pressure

(Ref: Dust Explosions in the Process Industries, R.K. Echhoff)

Panel ‘As-Installed’

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Closures set torelease at Pstat

Vent Hatches(Ref: Dust Explosions in the Process Industries, R.K. Echhoff)

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Explosion Suppression

Use fast-responding system to detect incipient explosion and release extinguishing agent to terminate combustion (typically detect in <10 msec, suppress in <100 msec)

Either presence of flame and/or pressure rise may be detected

Agent may be extinguishing powder (e.g., sodium bicarbonate), water, inert gas, or halogenated compound

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SuppressorBarrier

Control Unit

Detector

Suppressor

Explosion Suppression System

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Suppression (cont’d)

Basis of design is generally vendor-specific but Kst is always needed

Systems must be periodically inspected to ensure operational integrity

Suppression systems only operate once; process must be interlocked to shut down upon activation

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Isolation

Must be used to prevent propagation of an event in one vessel to other attached pieces of equipment

Always necessary where containment is used and may also be needed where explosion is vented

Either passive or active methods may be used to prevent an explosion from propagating from its point of origin to other pieces of equipment

Process elements or dedicated special devices can be used to isolate the event

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Summary Comments Four of the most critical elements in preventing dust

explosions are proper equipment design, good housekeeping, attention to preventive maintenance, and proper grounding of conductive components

In general do not rely on the control of ignition sources as a sole means of protection where combustible dusts are or may be present

Proper training of personnel in the risks posed by combustible dusts and management of change are critical to safety but often overlooked

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Summary Comments…

Beware the “We’ve never had an Incident” Syndrome!

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Current Regulatory & Legislative Trends

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US Chemical Safety Board (CSB)

Conclusions from 2006 study on combustible dust hazards:

- Combustible dust explosions a serious hazard

- Existing OSHA standards not adequate for prevention

- Similar problem in grain handling greatly reduced by regulation

- Compliance with NFPA standards on combustible dust would have prevented most incidents

- Many MSDSs do not adequately address dust hazards

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Common Elements in Incidents

Workers, management and regulators unaware of hazard, leading to:

• Failure to conform to NFPA standards

• Inspections by numerous entities failed to identify dust hazard

• Unsafe accumulations of dust present (HOUSEKEEPING!)

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OSHA Combustible Dust National Emphasis Program (NEP)

Requires 3 annual audits of industries having more frequent/higher consequence dust explosion potential and 1 annual audit of lower risk industry

Reliance on NFPA standards (primarily NFPA-654)

Emphasis on housekeeping, ignition source control, preventive/ mitigative methods

Provision for citations under General Duty Clause as well as other regulations for PPE and hazards communication deficiencies

Outreach activities re. education/training

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Case History: Imperial Sugar CompanyPort Wentworth, Georgia

February 7, 2008

http://www.youtube.com/watch?v=LQZGWjVwN58

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The Incident

On February 7, 2008, at about 7:15 p.m., a series of sugar dust explosions at the Imperial Sugar manufacturing facility in Port Wentworth, Georgia, resulted in 14 worker fatalities.

Eight workers died at the scene and six others eventually succumbed to their injuries at the Joseph M. Still Burn Center in Augusta, Georgia.

Thirty six workers were treated for serious burns and injuries—some caused permanent, life altering conditions.

The explosions and subsequent fires destroyed the sugar packing buildings, palletizer room, and silos, and severely damaged the bulk train car loading area and parts of the sugar refining process areas.

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The Causes

The CSB investigation identified the following incident causes:

1. Sugar and cornstarch conveying equipment was not designed or maintained to minimize the release of sugar and sugar dust into the work area.

2. Inadequate housekeeping practices resulted in significant accumulations of combustible granulated and powdered sugar and combustible sugar dust on the floors and elevated surfaces throughout the packing buildings.

3. Airborne combustible sugar dust accumulated above the minimum explosible concentration inside the newly enclosed steel belt assembly under silos 1 and 2.

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The Causes (con’t.)

4. An overheated bearing in the steel belt conveyor most likely ignited a primary dust explosion.

5. The primary dust explosion inside the enclosed steel conveyor belt under silos 1 and 2 triggered massive secondary dust explosions and fires throughout the packing buildings.

6. The 14 fatalities were most likely the result of the secondary explosions and fires.

7. Imperial Sugar emergency evacuation plans were inadequate. Emergency notifications inside the refinery and packaging buildings were announced only to personnel using 2-way radios and cell phones. Many workers had to rely on face-to-face verbal alerts in the event of an emergency. Also, the company did not conduct emergency evacuation drills.

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Imperial Sugar Explosion: Wentworth, GA17 February 2008: 13 Fatalities

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Imperial Sugar Explosion: Wentworth, GA17 February 2008: 14 Fatalities

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Increased Attention Post-Imperial Sugar Incident

At the Federal level H.R. 5522 was recently passed by the U.S. House of Representatives and is now in Senate committee:

- Require interim regulation addressing specific areas of concern within six months after passage

- Final standard with federal rulemaking process with guidance from ‘relevant’ NFPA standards within 18 months of passage

- Amend Hazard Communication Standard to include ‘combustible dust’ as a physical hazard

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No Current Action

OSHA has just recently moved the new Dust Standard to its long-term agenda, meaning there will be no activity on the standard in 2012.

CSB, under its new chairman, Dr. Rafael Moure Eraso has issued a new call for the standard in the wake of the three incidents with fatalities at the Hoeganaes powdered metals plant in Gallatin, Tennessee.

However, OSHA has a housekeeping standard that, if enforced, would have prevented nearly 90% of all fatalities listed in the CSB report, including those at Imperial Sugar and Hoeganaes.

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No Current Action

CSB Chairperson Rafael Moure Eraso said, “The three accidents at the Hoeganaes facility were entirely preventable. Despite evidence released by the CSB and information that Hoeganaes had in its possession even before the first accident in January 2011, the company did not institute adequate dust control or housekeeping measures. Dust fires and explosions continue to claim lives and destroy property in many industries. More must be done to control this hazard. No more lives should be lost from these preventable accidents.”

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Questions?

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Thank You for your Attendance!

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Resources & References

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Dust Explosion References

Dust Explosions in the Process Industries, 3rd edition, R.K. Eckhoff, Elsevier, 2003

Dust Explosions: Course, Prevention, and Protection, W. Bartknecht, Springer-Verlag, 1989

Dust Explosion Prevention and Protection, J. Barton (IChemE), Butterworth-Heinemann, 2002

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NFPA Standards/Guidelines

NFPA-61, “Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities”

NFPA-68, “Guide for Venting of Deflagrations”

NFPA-69, “Standard on Explosion Prevention Systems” NFPA-77, “Recommended Practice on Static Electricity”

NFPA-484, “Standard for Combustible Metals”

NFPA-499, “Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas”

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NFPA Standards/Guidelines (cont’d)

NFPA-654, “Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids”

NFPA-655, “Standard for Prevention of Sulfur Fires and Explosions”

NFPA-664, “Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities”

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