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Understanding dust properties to determine the cause of the explosion

Dr. Scott G. Davis

617-407-3300

GexCon US

Bethesda, Maryland

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Contents

• Introduction

• Explosive atmospheres

• Ignition sources

• Housekeeping

Conditions required for a dust explosion

• Flammable material.

• Material exists as a dust

cloud with correct particle

size distribution and

concentration.

• Ignition source.

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Conditions necessary to have a dust explosion

• Explosive atmosphere

• Ignition sources

• Necessary to understand common preventive measures in order to determine how the explosion occurred

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Contents

• Introduction

• Explosive atmospheres

• Ignition sources

• Housekeeping

Was there an explosive atmosphere?

• Was house-keeping at sufficient level: was flammable material removed regularly?

• Were any measures taken to keep dust concentrations in equipment below the lowest explosible concentration (MEC) (like aspiration)? Did they fail?

• Were other measures taken to make dust non-flammable such as adding non-flammable dust such that the mixture becomes non-explosive?

– For example – rock dust in mines

• Was inerting applied? Introducing inert gas in order to reduce the oxygen content to below the ”limiting oxygen concentration” (LOC). Was the system well-designed?

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House-keeping: removal of flammable material

• Dust accumulation is often difficult to prevent from happening due to process considerations or due to poor process design (partially open equipment)

• Dust layers external to the actual process shall be removed regularly

• Poor house-keeping is often the cause for serious dust explosions: Imperial Sugar, West-Pharmaceutical

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Control of dust concentration in equipment by e.g. aspiration

• Experience indicates that explosions occur in process vessels where dust concentrations are normally below the lowest explosive concentration.

• In investigations one therefore needs to evaluate plant running disturbances.

• Can dust accumulations occur in equipment (poor design of dust collection system)?

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Addition of non-flammable material

• Possible option but difficult in normal process industry

• Is used in mines

– Mix stone dust, limestone and other non-flammable materials with coal dust on the mine floor

• Over 50% non-flammable is necessary

• Upper Big Branch mine explosion was that serious due to insufficient stone dust mixed with coal dust in mine galleries

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Inerting (complete or partial)

• Addition of inert gas in order to reduce oxygen content in the atmosphere to values below “Limiting Oxygen Content”.

– Typical Gases: Nitrogen, carbon dioxide, steam, “combustion products”, noble gases

– In accident: was oxygen concentration below limiting oxygen concentration: typical limiting value: maximum 10 – 14% oxygen (in nitrogen)?

– In case of concentrations exceeding limiting oxygen concentration: are available ignition sources able to ignite atmosphere?

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Inerting

• Minimum ignition energy MIE for dust increases as oxygen content decreases

• i.e. the energy required to ignite the dust cloud increases.

Important in cause determination

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Smoldering combustion still possible even when operating below limiting oxygen concentration for dust explosions:

Difficult to inert dust in bulk

Layer may smolder and possible ignition source (when opening hatch)

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Practical example of how to design proper inertingsystem

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Important for evaluating inadequate inerting systems• Possible air pockets

– Is inert gas in areas where good mixing with air is obtained

– Add inert gas throughout the whole flow volume

• Possible entry of air through openings in the equipment or during product filling or loading

– Look for possible openings in equipment on negative side of the fan

• Oxygen sensors and system design

– Are they located near the outlet

– Functionality – test the sensors, calibration, maintenance

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Contents

• Introduction

• Explosive atmospheres

• Ignition sources

• Housekeeping

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Identification of possible ignition sources

• EN 1127-1 describes 13 different ignition sources with different properties and energies

• In addition it is important to know how easily the products can be ignited

– Minimum ignition energy for a dust cloud (MIE)

– Minimum ignition temperature for dust cloud (MIT)

– Minimum ignition temperature for dust layer (MITdl)

– Self-ignition temperature for dust in bulk (AIT/SIT)

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Ignition sources

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European standard EN-1127-1 (*NFPA 664, 654)

describes 13 possible types of ignition sources

• Hot surfaces* Radio frequency waves

• Flames and hot gases* Electromagnetic waves

• Mechanical sparks* Ionizing radiation

• Electrical apparatus* Ultrasonics

• Static electricity* Adiabatic compression

• Exothermic reactions Stray electric currents* & self-ignition* Lightning*

For dust explosions, those made bold are the most common and should be considered first.

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Most frequent ignition sources for dust explosions

• Smoldering nests/self-heating

• Open flames

• Hot surfaces

• Mechanical friction

• Electric equipment

• Electrostatic discharges

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Smoldering nests/self-heating

• Heat generation or “exothermicity” occurs in most materials.

• If heat generation > heat loss over a certain period ignition occurs

Self-ignition

In order to identify the likelihood of self-ignition it is necessary to know:

• Temperature

• Volume

• Self-ignition temperature of the product for the volumes in question

• Induction times (time delay to ignition) for the product for a given volume and temperature

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…. example

• Self-heating in a silo

• Can arise if dust is stored too long

• Can cause an explosion during emptying of silo: dust or gas explosion

• CO generation in top of silo due to incomplete combustion

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Self-heating usually occurs…

• In organic products

• For materials with high fat content

• High moisture content

• At high storage temperatures (e.g. in a dryer)

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Open flames

• Ignition depends on the size and temperature of the flames

• Open flames

• Hot work

Open flames / “hot work”

Was hot work being carried out at the moment of the incident?

Were adequate procedures in place and were these respected? Not respecting the procedures is typically the reason for “hot work” causes

Strict procedures for hot work:1. Work permit2. Controlled shut-down3. Cleaning – removal combustible material4. Moving of work object to safe area if possible5. Cooling of work object prior to remounting and start-up 6. Dampen combustible/flammable material

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Hot surfaces

• Ignition efficiency depends on size and temperature of the hot surface

• Large surface area and high temperature is more hazardous than a small surface with lower temperature

For investigations

• Hot surfaces seldom ignite dust clouds directly

• Usually a smoldering nest is created in a dust layer which then ignites a dust cloud

Hot surfaces, intended

• Heater elements

• Dryers

• Steam ducts and other pipes

• Electrical equipment

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Hot surfaces, unintended

• Motors

• Bearings

• Blowers and fans

• Mechanical transport systems

• Light bulbs and light fittings

• Mills and crushers

• Mixers

• Hot work

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Suspected hot surface

• For equipment that generates heat, the maximum surface temperature should not exceed 2/3 of the minimum ignition temperature of dust clouds in degrees Celsius.

• Similarly, the maximum surface temperature of horizontal surfaces should not exceed a temperature equal to the minimum ignition temperature of dust layers (in degrees Celsius) – 75 ºC

• Must therefore know the minimum ignition temperature of the dust for both dust layer and dust cloud

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Mechanical friction

• Arises for example when to objects collide due to impact or friction

• Two potential ignition sources can occur:

• Mechanical sparks (frictional sparks)

• Hot surfaces

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Example

• Collision generates heat

• Hot fragments can be torn off

• Fragments can be oxidized and become hot

• Hot areas arise in the area(s) around the collision(s)

• Collisions can occur once only

• Collisions can occur repeatedly

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Where can mechanical friction occur

• Mixers, rotary feeders, screw feeders

• Mills and crushers, rotary sifters, fans

• Dryers (spray dryers)

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Factors affecting the likelihood

• Material properties (moving v. struck object)

• Speed, force, angle, duration of collision

• Single v. multiple impacts

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Incendivity/ignition energy

• Mechanical sparks have low energy content.

• They can ignite dusts with lowignition energy and low ignitiontemperature

• Spark generation creates turbulence, -

• Turbulence increases minimum ignition energy of a dust cloud

• It makes it more difficult to ignite a dust cloud

Mechanical friction

• Estimating the probability of ignition from mechanical friction is possible based on MIE and MIT

• In addition, the following is often used (for steel):

– V < 1 m/s no danger of ignition

– V = 1 – 10 m/s danger of ignition depending on situation

– V > 10 m/s always danger of ignition

• Tests at GexCon have shown ignition at V < 1 m/s at high contact pressure and long contact duration

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Could it be a mechanical spark: assessment based on MIE and MIT

• V = 1 – 10 m/s

• Grinding: 20-50 ms

• Friction: 0.5-2 s

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Special case: Thermite reactions

• Light-metals + rust light-metal oxide + iron + energy

example

2Al + Fe2O3 Al2O3 + 2Fe + energy

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Potential ignition sources

• Electrical arcs/sparks

• Hot surfaces

Can be prevented if the equipment is correctly designed, constructed, installed and maintained in accordance with relevant Standards.

• Use explosion safe equipment in classified hazardous areas

• Looking for an ignition source can therefore include search for non-approved equipment used in classified areas

Electric equipment

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Choice of electric equipment: hazardous area classification

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In practice, it is very difficult to avoid the occurrence of explosive atmospheres inside equipment

• Equipment shall not cause ignition of this explosive atmosphere (residual risk for ignition is minimized).

• Area classification shall enable the correct choice and installation of equipment in the process plant (equipment density & proximity, max. temperature) depending on the probability of occurrence of an explosive atmosphere.

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• Guidance documents

• NFPA 70 (2008): National Electrical Code (NEC)

• NFPA 499 – “Classification of combustible dusts and of hazardous (classified) locations for electrical installations”

• Classification of Locations for proper selection of electrical equipment where fire or explosion hazards may be present due to flammable gases or vapors, flammable liquids, combustible dust, or fibers or flyings

• Hazardous (Classified) Locations• Classes I, II, and III

• Divisions 1 and 2

• Classes I,II, and III

• Material Groups A through G

Electric equipment: Hazardous area classification

Choice of approved electric equipment

• Class II Combustible Dusts (Division 1 and 2): Equipment, wiring, connections, etc. as described in Article 502 of NFPA 70 (2008): National Electrical Code (NEC)

• Class III Easily ignitable fibers or flyings (Division 1 and 2): Equipment, wiring, connections, etc. as described in Article 503 of NFPA 70 (2008): National Electrical Code (NEC)

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Static electricity

• Electrostatic sparks and discharges are often pointed out as the ignition source if one cannot explain the incident

• Electrostatic sparks• From conductive materials

• May have very high energy

• Electrostatic discharges • From non-conductive materials

• Normally low energy content (except propagating brush discharge)

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Electrostatic sparks and discharges

What are the conditions necessary for electrostatic sparks and discharges to occur?

• Charge must be:

• Generated

• Accumulated

• Discharged

Accumulation of charge occurs when charge generation exceeds charge loss.

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Charge generation

• Charge exchange usually occurs between product and process vessels.

• Two different materials (product and process vessel) that are in contact with each other are SEPARATED – triboelectric effect

• If the two materials have different ability to hold onto electrons, then a charge difference occurs between them.

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Examples of locations where charge can be generated

• Mills and crushers

• Various dryers

• Pneumatic transport

• Dust extractor systems

• Central vacuum systems

• Sifters and sieves

• During filling and emptying of containers and bags

• Non-conductive floor covering

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Examples of “processes” where charge can be generated

• Emptying of containers and bags

• Walking on non-conductive flooring

• Removal of synthetic clothes and materials

• Non-conductive conveyor belts over rollers

• Removal of plastic foil

• Removal of plastic sacks from metal containers

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Different electrostatic discharges

• Electrostatic spark

• Corona discharge

• Brush discharge

• Cone discharge

• Propagating brush discharge

• All these discharges have different energies which should be compared to the MIE of the dust in question to see whether the discharge can be the ignition source

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Electrostatic sparks

• Discharge occurs between two electrically conducting bodies

• “All” available energy is discharged

• Yield sparks having high energy-content

• Energy can be calculated from capacitance and potential difference (E = ½ CV2), in practice max 1 J

• Can be avoided by grounding equipment to same electrical potential. Checking whether this was the case after an incident is an important way of excluding or pointing out these sparks as an ignition source.

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Theoretical spark energy of electrostatic sparks (compare to MIE)

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E = ½ CV2, where C = capacitance and V = voltage

Checking of grounding

• Check for ground of all electrically conducting parts of the plant to the same electrical potential

• Check for grounding of personnel if MIE < 30 mJ

• Resistance to ground should lie in range 10 –100

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Electrostatic spark, examples

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Example, cyclone

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Electrostatic discharges due to flow in pipe across flange

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Flexible hoses

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Filter bags

Grounding of antistatic (“conductive”) filter bags is important

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Example - truck

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Example – tipping / emptying

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Corona discharge

• Occur when sharp/pointed conducting materials approach charged non-conducting materials

• Low energy-content

• Can lead to ignition of very ignition-sensitive gases like acetylene and hydrogen

• NOT considered able to ignite dust clouds

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Brush discharge

• Usually occur when rounded conducting materials approach charged non-conducting materials

• Only a limited part of the available energy is discharged

• Equivalent energy-content < 4 mJ (determined using flammable gas)

• Experiments at Gexcon show that dust cannot be ignited*

• Brush-discharges therefore unlikely ignition source

*Larsen, Ø., Hagen, J.H. & van Wingerden, K. (2001). Ignition of dust clouds by brush discharges in

oxygen enriched atmospheres. Journal of Loss Prevention in the process industries, 14: 111-122

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Brush discharge, example

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Cone discharge

• Can arise when strongly-charged non-conducting powder is filled into a silo

• High charge density occurs along the top of the powder heap

• Significant discharges occur along the surface of the powder

• Energy-content depends on the size of the silo and powder properties

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Cone discharge

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Investigating cone discharge as an ignition source

• Difficult to avoid

• Confirm degree of filling on day of incident

• Was inerting of atmosphere used?

• Energy in cone discharge (compare to MIE)?WE=5.22D3.36d1.462

With:

• WE = equivalent energy cone discharge (mJ)

• D = diameter silo (m)

• d = median particle diameter dust (mm)

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Propagating brush discharge

• Can occur through non-conducting coatings on the inside of conducting tanks or pipes

• Coating must have very low conductivity and high dielectric properties in order to produce propagating brush discharges

• Energy < 1 J (in practice)

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Can propagating brush discharges have been the ignition source? Can they arise?

• There shall be a non-conducting coating with a breakdown voltage > 4 kV (coating thickness 10 µm) to > 8 kV (coating thickness 200 µm)

• Strong charging process can occur in pneumatic transport lines and cyclones

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Electrostatic discharges, summary

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Thank you very much for your attention

sgdavis@gexcon.com

+1 617-407-3300