Dust Explosion Prevention and Protection a Practical Guide

Dust ExplosionspRisk to people, plant

d t tiand reputation

Mark Hoyle,Process Safety Group Pharmaceutical DevelopmentProcess Safety Group, Pharmaceutical Development

‘Foxconn confirms third death from explosion’ p(by Richard Lai..internet)

Initial investigation suspects combustible dust in ductwork to be the main causemain cause.Made major headlines in Europe/US (as ‘iPAD factory blows up!’)

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And many others! (All industries handling fine powder materials can be at risk))

Notice the major destruction to these installations. Why so much damage?

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damage?

Reactive Hazard Management Process C ll t i l i l d i t t ith thCovers all materials involved in process – starts with the ‘high level’ questions….

1. Do you handle REACTIVE materials?

2 Can you have REACTIVE interactions involving materials that2. Can you have REACTIVE interactions involving materials that you handle?

If 1 d/ 2 d YES ti it h d t b t ll d th h t tiIf 1 and/or 2 are answered YES – reactivity hazards must be controlled throughout entire lifetime of facility to avoid loss/incident

3. What DATA do you need to control these hazards?

4. What Safety precautions do you need to control these hazards?

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Dust Explosion Pentagon (v’s Fire Triangle)

OxidantOxidantFuel

(Dust)Ignition Source(Dust) Source

ConfinementDispersion

IgnitionIgnitionSourceSourceFuelFuel HAZARDHAZARD

Fire Triangle

Oxygen(Air)

D t P tFire Triangle Dust Pentagon

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How does Confinement influence the Dust H dHazard

• No confinement : ignition produces a flash fire capable of burn injuries and ignition of other combustible materials

• Partial Confinement : ignition produces a fireball and limited pressure rise in the enclosure and the venting of a flame outside the enclosurerise in the enclosure and the venting of a flame outside the enclosure

• Complete confinement : ignition produces full deflagration pressure (Pmax), which can destroy most buildings and process equipment ( l th d i d t ith t d thi )(unless they are designed to withstand this)

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Overview

1. General Prevention/Protection Protocol2. Secondary Dust Explosion Risky p3. Mitigation Methods4. Primary Dust Explosion Risk5 Mi i i M h d5. Mitigation Methods6. Quick reminder of dust tests and their application7 Useful information sources7. Useful information sources8. Key points

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Typical Protocol

• Hazard Assessment (Process + materials used)( )• Engineering Controls

Housekeeping (cleanliness of workplace)• Housekeeping (cleanliness of workplace)• Building Design• Explosion Protection• Worker Trainingg

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Dust Explosions• Any oxidisable fine powdered material can form a flammable dust cloud in

air. (When dispersed in appropriate concentration) (Definition in NFPA 6541

(2006) and NFPA 692 (2004))

• Most powder handling industries are at risk : Agricultural, Pharmaceutical, Chemical, Textile, Metal processing, Wood working, Paper works, recycling operations – almost anywhere that dust is generated.

• Assume it is flammable unless you have had it tested otherwise (MSDSs do not necessarily cover dust flammability information…GHS is addressing this with appropriate physical hazards data requirements)

• Dust does not disperse when released - like gas or vapour. It can settle on the ground as a powder layer and be dispersed again and again.

• This is what is generally termed a ’secondary’ dust explosion hazard• This is what is generally termed a secondary dust explosion hazard.

• This tends to do the most damage (as there can be a lot of fuel involved and propagation of the explosion throughout the facility can occur)

1 NFPA654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids 2NFPA 69: Standard on Explosion Prevention Systems

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Dust Explosion in the Work Place – ‘Secondary’ E l i DExplosion Damage• Dust settles on flat surfaces

• Dust is disturbed creating a cloud

• Dust cloud is ignited and explodesand propagates

KEY MESSAGE : Keep the workplace clean. Do not let dust/powder accumulate (remove the fuel that could support a secondary explosion)

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‘Typical’ Industrial Dust explosion Time-line

Initial explosion event inside

Building

event inside equipment

0 25 50 75 100 125 150 175 200 225 250 275 300 325Time in milliseconds (ms)

Initial explosion t i id

Building

event inside equipment

Shockwave (pressure from(pressure from ‘equipment’)

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Initial explosion

Building

event inside equipment Rebound shockwaves

(reflection)


Initial explosion

Building

Initial explosion event inside equipment

Dust clouds generated by rebound shockwaves

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Rupture/flame relief in

Building

D t l d t d b b d h krelief in equipment Dust clouds generated by rebound shockwaves


Initial explosion

Building

Initial explosion event inside equipment

Secondary Dust explosion – flame propagates through dust

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Rupture/flame relief in

Building

relief in equipment


BuildingRupture/flame relief in equipment

Explosion pressure (and flame) vents through the building


‘Typical’ Industrial Dust explosion Time-lineBuilding

Building collapses


Such an event is effectively over in less that 0.5 seconds. There is effectively little time for avoiding action.

Remember - even small layers can give rise to i ifi t d t l dsignificant dust clouds

General guidance in NFPA 654 – layer thickness limits= >0.4 mm over 5% of (<1900 2) A i t l th thi k f li !area (<1900 m2). Approximately the thickness of a paper clip!

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Toxicity/Bio activity does help within Pharma Industries• High toxicity/biologically active materials in Pharmacueticals makes containment of

material very high necessity/priority. • Significant powder accumulation in the workplace is simply not acceptable (from a

health perspective)• Obviously spillages can and do occur but the risk is dealt with by appropriate

protection and efficient clean-up.• If the powder material is not acknowledged as toxic/hazardous then such care with

dust in the workplace is sometimes not taken, for example food, textile and woodworking industries. (There can be a general lack of appreciation and understanding of flammability of the materials being used)understanding of flammability of the materials being used)

Sugar – 13 Fatalities, 42 injured

17 http://www.csb.gov/investigations/detail.aspx?SID=6

Hidden Areas / Spaces

• Many of the pictures seen earlier had significant accumulations of dust in ghidden areas (for example above false ceilings)

• Provide access to all hidden areas to allow inspection:

I t f d t id i d• Inspect for dust residues in open and hidden areas at regular time intervals

Cl d t id t l i t l• Clean dust residues at regular intervals.

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Control Dust levels / leakage / spills

• Make equipment as ‘dust-tight’ as possible.

• Where high levels of dust are expected consider extract and dust filters/collectors.

• Consider appropriate clean up methods for leaks / spills / slow dust build up in areas of the facilityspills / slow dust build up in areas of the facility.

• Maybe possible to consider water spray to wet y p p ydown / control dust movement in some applications (for example coal conveyers).

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Dust Accumulations/Spills - Clean up carefully

• Do not brush/blow powder about and create the very dust cloud risk you are trying to avoid (or simply displace it to somewhere else in th f ilit )the facility).

htt // ilfi k f k/

• Ideally use dedicated vacuum cleaners – which are not an ignition source themselves (ATEX approved versions).

http://www.nilfisk-cfm.co.uk/

• Alternatively wet down with water and clean up (if this is applicable)http://www.morclean.co.uk/ http://www.kerstar.co.uk/ http://www.nederman.co.uk/

• Alternatively, wet down with water and clean up (if this is applicable).

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Mitigation of Secondary dust explosions• Basis of safety with regards to dust

explosions external to equipment should be based on operation below the Under capacity

cyclonicminimum explosible concentration (MEC) – not enough dust to cause a fire/explosion hazard being present.

cyclonic separators

• There may be specific areas - defined b l ifi ti iby an area classification exercise –where all ignition sources need to be avoided.

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Primary Dust Explosion (inside equipment)Ignition Sources• They are many and varied, some may already exist in the equipment or be present due to

the operation. p

1. Hot surface heating pipes, electrical apparatus2. Flames and hot gases welding, exhaust gases3. Mechanical Sparks cutting, impact3. Mechanical Sparks cutting, impact4. Electrical Equipment electric sparks,/arcs5. Transient current/Cathodic protection stray current/short circuit to earth6. Static Electricity Spark Discharge7 Li ht i ( ifi t f l t t ti di h )7. Lightning (specific type of electrostatic discharge)8. Electromagnetic waves (HF) Induced heating9. Electromagnetic waves(optical range) Photoflash, laser10. Ionizing Radiation X-rays, UV raysg y , y11. Ultrasonics Cleaning/testing12. Adiabatic Compression/shock waves Heat of Compression13. Chemical Reaction, self-ignition Exothermic processes

f d tof dustsReference : EN1127-1 : 1997, Explosive atmospheres – Explosion prevention and protection Part 1. Basic concepts and methodology

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Hot Surfaces• Control surface temperatures to which powder/dust may come into contact appropriately.

(May need Minimum ignition temperature (MIT) / T5mm layer test / or other thickness layer test for the material(s) – depending on application / position).

• Hot Bearings

• Furnaces / Dryers

• Hot Steam / Heat Transfer Pipes

• Cutting Welding Ductwork cut whilst containing

• Halogen Light Bulb

Aluminium Dust

Bulb recovered from debris in coal dust collector hopper after

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coal dust collector hopper after explosion

Burning Material / Hot ParticlesHot particles / burning material produced by:-

•Frictional heating, for example from sanding/cutting•Radiant heating for example from curing of wood panels•Radiant heating, for example from curing of wood panels•Convective heating, for example in dryers

Flaming/Burning Smouldering (no flame)Flaming/Burning milk powder particles: 960oC – ignites dust cloud

Smouldering (no flame) milk powder particles: 700oC – does not ignite dust cloud.

(MIT of powder 410oC )

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Ref Gummer & Lunn, 2003

Examples of Explosions in Dust Collector

• Pellitizer

•Explosion in attached Dust Collector (damage to building as well)

•Residual burnt material found in duct – smouldering due to bl kblockage

Cyclone collector - door blown open. Ignition source - burning particulates from

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dryer.

Exothermic Process – Self-heating (in dryers)

• Powder accumulations remain in dryer ysufficiently long for oxidative self-heating to bring powder up to ignition temperature.

• Result can be either a fi d t l ifire or a dust explosion in dryer or downstream dust

ll tcollector.

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Impact/Friction Ignition

• During size reduction operations in various types of mills.

Typical examples:

• During mixing and blending if impeller is misaligned or deformed or has inadequate clearance, or tramp metal enters mixer.

• During grinding and polishing operations• During grinding and polishing operations.

• Tramp metal in a particle classifier or conveyor.

Hammermill (Sugar) : Decomposition/Ignition E id d b k h

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Evidence around broken hammer

Ignition/Friction in Blenders/Grinders

MIE (mJ) <1 1-3 3-10 10-30 30-100 100-300 300-1000 >1000

MAIT (oC) Do not process*

530 500 465 430 395 360 325

• On closing the mixer and provided it is filled to a level of 70 vol % or more the tip speed of theOn closing the mixer, and provided it is filled to a level of 70 vol.% or more, the tip speed of the mixer element no longer needs restriction.

• Tip speeds up to 10 m/s can be tolerated in a mixer with a product fill of less than 70 vol.%, provided the combination of material values shown above are present.provided the combination of material values shown above are present.

• * need an explosion protection methodology for powders of such low ignition energy – ideally inerting is the best option

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Prevention Avoid Flammable conditions if possible

• It is possible to avoid flammable conditions by removing one of the three l t d d Th t i /l th t ti f th f l (d t i

Avoid Flammable conditions if possible

elements needed. That is remove/lower the concentration of the fuel (dust in this case), remove the oxidant (oxygen in the surrounding air usually) or avoid all possible ignition sources.

XXOxidantOxidantXX

IgnitionIgnitionSourceSourceFuelFuel

HAZARDHAZARD XXXX• If none of these options can be achieved with sufficient reliance then it has to

be assumed that a dust explosion may occur. An appropriate protection

HAZARDHAZARD XXXX

method will then need to be implemented.

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Explosion ProtectionT i l th dTypical methods • Best approach may be dependant on material, connecting equipment and location. All

these need to be considered when adopting explosion protection designs.

• Basic methodologies are : Venting, Containment and Suppression (note various isolation methods may be used to stop propagation of explosion to connected equipment). Note: need to know the ‘explosion characteristics’ of the material(s) being handled)need to know the explosion characteristics of the material(s) being handled)

Suppression(extinguish explosion)

Venting(for weak structures – ‘safe’ materials)

Containment(strong enough to

Withstand explosion)Withstand explosion)

Typical Powder System could have the following

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Dust Testing + Applications

• Minimum Ignition Energy (MIE) – indication of ignition sources

• Minimum Ignition Temperature (MIT) Max surface temperature• Minimum Ignition Temperature (MIT) – Max surface temperature (equipment)

• T5mm Layer Test – Max surface Temperature (equipment)

• Diffusion/Air-over Layer tests – dryer temperatures

• Pmax and Kst (Explosion characteristics ) - Explosion Protection Design

• Minimum Explosible Concentration (MEC) – identification of flammable conditions

• Powder Resistivity/Decay – ability to retain electrostatic charge

•Limiting Oxygen for Combustion (LOC) – lowest oxygen concentration to support combustion - specify inerting conditions

•Note choose the test data you need for your specific application•Note – choose the test data you need for your specific application. (Unlikely to need absolutely all of the test data).

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Training

(E ti l) T i l t i b tibl d t l i• (Essential) Train employees to recognise combustible dust explosion risks and take preventative action (and/or how to alert management to take action).

• Safe work practices for their jobs• Overall plant program for dust control and ignition source control

• Training must be :

• Before (new) personnel start work• Periodically refreshed for existing personnel – reinforce the safety• Periodically refreshed for existing personnel – reinforce the safety

issue• Carried out when they are reassigned to another area/operation• When hazards or processes change (good ‘change-management’

processes needed to ensure this happens)• Be a managers responsibility (their performance ‘marked’ on this)• Encourage reporting of unsafe practices - good ‘safety culture’

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Key Point Summary• Keep plant environment as free from dust/powder layers as practicably

possible.

• If powder is spilt or dust layers form then clean them up as soon as is practicably possible.

• Clean up powder in an effective manner – that is do not simply re-deposit elsewhere or create the dust cloud you are trying to avoid.

• Do everything you can to avoid ignition sources in hazardous areas.

• If you cannot control/eliminate ignition sources reliably then consider an y g ythe most appropriate explosion protection method.

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European and US Standards in this areaEN 1127-1:2007 Explosive atmospheres, basic concepts and methodology Specifies methods for the identification and assessment of hazardous situations leading to explosion and the design and construction measures appropriate for the required safety.

EN 14491:2006 Dust explosion venting protective systems (specifies the basic requirements of design for the selection of a dust explosion venting protective system)

EN 14373:2005 Explosion suppression systems.

EN 14797:2006 Explosion venting devices (specifies the requirements for venting devices used to protect enclosures against the major effects of internal explosions).

EN 15089:2009 Explosion isolation systemsEN 15089:2009 Explosion isolation systems

NFPA 68:2007 - design, location, installation, maintenance, and use of devices and systems that vent the combustion gases and pressures resulting from a dust deflagration within an enclosure.

NFPA 69:2008 NFPA 69 - prevention of explosions by the prevention or control of deflagrations.

NFPA 654:2008 requirements for safety with respect to fire and explosion associated with theNFPA 654:2008 - requirements for safety with respect to fire and explosion associated with the manufacturing, processing, blending, pneumatic conveying, repackaging and handling of combustible particulate solids or hybrid mixtures.

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Other useful sources• http://www.dustexplosion.info/index.htm

• http://www.dguv.de/ifa/en/gestis/expl/index.jspGESTIS DUST EX (D b C b i d l iGESTIS-DUST-EX (Database Combustion and explosion characteristics of dusts

D t E l i P ti d P t ti A P ti l G id• Dust Explosion Prevention and Protection: A Practical Guide

• Dust Explosions in the Process Industries, Third Edition

El t t ti I iti f Fi d E l i• Electrostatic Ignitions of Fires and Explosions

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Documents

Dust Explosion Prevention and Protection a Practical Guide