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Dust Explosions

Peter Wypych

wypych@uow.edu.au

Ingredients for Dust Explosion :1. Dust must be Explosible & Airborne

2. PSD can support Flame Propagation

3. Dust Cloud in contact with Ignition

4. Dust Cloud Atmosphere capable of supporting Combustion

5. Dust Conc > LEL (20 to 100 g m-3)

6. Dust Conc < UEL (2 to 6 kg m-3)

Rule of Thumb…

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Oxidant

Ignition Source

Dust Cloud

Particle Size

Shape

Conc

“Fire Triangle” modified for FlammableDust Clouds

EXPLOSIONDUST

Ingredients for Dust Explosion

Explosion Control Methods

Avoid Explosible Dust Clouds

Remove all possible Ignition Sources

Create Inert Atmosphere

Examples…

(1) Explosion Prevention

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(2) Explosion Protection

Venting

Suppression

Isolation

Containment

Combination of above

Examples…

Venting → Atmosphere

Venting → Duct → Atmosphere

Explosion Suppression

Explosion Isolation

Explosion Containment

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Gas vs Dust Explosions

Some Similarities → Many Differences :1. Combustion occurs at Surface of Material2. Particle Inertial Forces Significant3. Fuel Concentration Gradients → Different4. Unburnt Particles → Turbulence, Severity,

Shape and Inertia of Fireball5. Secondary Dust Explosions6. Solids/Powder Fires → Control Techniques7. Detonation (settled dust, 30barg, 2000m/s)

Gas vs DustExplosions

German MillExplosion

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Secondary ExplosionMr. P.E. Moore, Director of TechnologyKidde plc (supplier of fire/safety products)PBS Technology Forum, Chicago, May 2003…

"Only safe plant is one that is not operating"

News Story

Silo FiresPressure

Wave

(2)

Main Explosion ParametersMax “Unvented” Explosion Pressure, Pmax(closed vessel)Pmax ≥ 10 bar g → USUALLY >> Pdes(e.g. bins, filters, etc)Max Rate of Pressure Rise, (dP/dt)max→ Dependent on Dust Properties and Vessel Volume (V)More Convenient Parameter that Combines Effects of V and (dP/dt)max → Dust Explosibility Characteristics, Kst

where V = 20 litre or 1 m3 Standard Vessel [2]

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Main Explosion Parameters

Kst also Used to Classify Dusts…

FIGURE 2 Typical Pressure Traces of Dust Explosionsin a Closed Vessel

Main Explosion Parameters

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Explosion Test DataPSD, ρs, ρbl

Moisture and Temp (dust and air)

Kst and Pmax (preferably using a 1 m3 vessel)

(dP/dt)max and Dust Concentration

Lower Explosibility Limit, LEL

Upper Explosibility Limit, UEL

Min Ignition Temp → Dust Layer and Dust Cloud

Min Ignition Energy → Dust Cloud

Principles of Explosion Venting

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Brilex1 Brilex2

IndoorVenting

Rear vs Top VentingSafe Venting

Fundamental Design Parameters:Max Total “Dirty” Volume of Equipment, V(dP/dt)max

Kst

Pdes of Enclosure (weakest component)Opening Pressure of Vent Panel, Pstat

4 Main Methods to Estimate Av:Kst Nomograph MethodSt Group Nomograph MethodRadandt [5] Nomographs (Pmax ≤ 10 bar g)Scholl [6] Equation

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a = 0.000571 exp(2 Pstat)

b = 0.978 exp(-0.105 Pstat)

c = -0.687 exp(0.226 Pstat)

Kst Nomographs Approximated by:

Note: ensure correct units used (in paper)

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Kst Nomographs subject to following Limitations (cannot extrapolate):

Pstat = 0.1 to 0.5 bar gMinimum Pred = Pstat + 0.1 bar gMaximum Pred = 2 bar g10 ≤ Kst ≤ 600 bar m s-1

Pmax < 10 bar g (St 1 & St 2 Dusts)Pmax < 12 bar g (St 3 Dusts)1 ≤ V ≤ 1000 m3

L/D < 5 [3,4] or L/D < 2 [8] → ImplicationsVenting → Atmosphere (unrestricted)

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St Nomographs Approximated by:

Note: ensure correct units used (in paper)

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Radandt Nomographs Approximated by:

Scholl [6] Equation:

…Use with Care!!

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Worked Example No. 1Storage Bin = 16 m3 (L/D < 2)Dust Collector = 2 m3

Dust: Kst = 150 bar m s-1; Pmax = 9 bar gVenting Panel Pstat = 10 kPa gPress Strength of Bin = 45 kPa gPress Strength of Filter = 70 kPa g

Determine Av using all 4 Methods:V = 16 + 2 = 18 m3; Pdes = 45 kPa g (bin)Select [4] Pred = (2/3) Pdes = 30 kPa gKst = 150 bar m s-1 → Group St 1

Results

Hence → Av = 0.77 to 1.6 m2

Unless Reasons to do Otherwise → Should Select Kst value ≈ 1.5 m2

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Effect of Vent Ducts

Worked Example No. 2Sugar Beet (Pmax = 9 bar g; Kst = 150 bar m/s)

→ Group St 1

Existing Stand-Alone Dust Collector,V = 2.4 m3, L/D < 2, Pdes = 70 kPa g

Venting Panel Pstat = 10 kPa g

Filter Located ≈6 m from External Wall

Existing Vent Duct: D = 800 mm; L = 6.5 m

Effect of Vent DuctsCan we Increase Vent Duct to L = 12 m??Firstly → Need to check Existing DesignDetermine Av using Kst and St Group Nomographs → Vent Duct Diameter, D

Select Pred = 20 kPa g

Kst Method:Fig 2: Av = 0.55 m2; Eqn (2): Av = 0.49 m2

St Group Method:Fig 2: Av = 0.53 m2; Eqn (2): Av = 0.52 m2

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Effect of Vent DuctsSelecting Av = 0.5 m2 → D ≈ 800 mmExisting Design OK??(Nomograph Av for Venting → Atm)Effect of Vent Duct on Expl Pressure?

Calculate L/D (L/D = 8.1)

Use appropriate Design Chart → P’red

For Pstat = 1.1 bar abs & Kst = 150 bar m/s:

Relevant Design Chart → Fig 11…

Pred =1.2 bara

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Pred =1.2 bara

P’red =2.3 bara

L/D = 8.1

Effect of Vent DuctsHence → Vent Duct increases Expl Press to P’red = 2.3 bar abs = 130 kPa g>> Pdes = 70 kPa gHence → Rupture of Filter ExpectedWorking in “Reverse Order”:

P’red = 60 kPa g = 1.6 bar abs → Fig 11→ Max L/D = 3.8

Options??Increase Vent Duct DiameterRelocate Dust Collector (D = 800 mm, L = 3 m)

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Pred =1.2 bara

P’red =1.6 bara

L/D = 3.8