Che415 Explosion Hazards

8/12/2019 Che415 Explosion Hazards

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Advanced Particle Processes -Fire and Explosion Hazards

Chunyan Fan

Department of Chemical EngineeringSemester 1, 2014


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OUTLINE

Introduction

Combustion Fundamentals

Combustion in Dust Clouds

Control of the Hazards


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Introduction - History of Industrial Explosions

National Fire

ProtectionAssoc. formed

(NFPA)

First recorded millexplosion, flour

dust, Italy

1785 late 1800s

Studies of f lour

mill explosions

begin in US

1896


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Introduction Early timelines of events

Occupational Safety

& Health Adm.formed (OSHA)

Coal dust mine

explosion, UT

1900

1922

NFPA creates

Explosive DustCommittee

1970

246 dead

early 1900s

Studies of coal

dust explosions

begin in US


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Introduction Combustible Dust Explosions History

January 29, 2003 - West

Pharmaceutical Services,

Kinston, NC Six deaths, dozens of

injuries

Facil ity produced

rubber stoppers andother products for

medical use

Plastic powder

accumulated abovesuspended ceiling

ignited


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February 20, 2003 CTA

Acoustics, Corbin, KY

Seven Workers died

Facil ity produced

fiberglass insulation

for automotive

industry

Resin accumulated

in production area

that got ignited


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October 29, 2003 - Hayes

Lemmerz Manufacturing

Plant Two severely burned

(one of the victims

died)

Facility

manufactured cast

aluminium

automotive wheels

Accumulated

aluminium dust


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Malden Mills

Methuen, MADecember 11, 1995

37 Injured

Nylon Fibre

Jahn Foundry

Springfield, MAFebruary 26, 1999

3 dead 9 Injured

Phenolic resin dust


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Ford River Rouge:

SecondaryFebruary 1, 1999

6 dead, 36 injured

Coal Dust Explosion

Rouse Polymerics

Vicksburg, MSMay 16, 2002

5 dead, 7 injured

Rubber Dust


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Introduction Combustible Dust

What Combustible Dusts are explosible?

Metal dust such as aluminium

and magnesium

Wood dust

Coal and other carbon dusts

Plastic dust

Biosolids Organic dust such as sugar,

paper, soap, and dried blood

Certain textile materials

Flour Dust;

Sawdust;

Dust Explosion at Imperial Sugar


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IntroductionWhich Industries have Potential Dust Explosion Hazards?

Food products

24%

Lumber & wood products

15%

Chemical manufacturing

12%Primary metal industries

8%

Rubber & Plastic products

8%

Electric services

8%

Other

7%

Fabricated metal products

7%

Furniture & Fixtures

4%Equipment manufacturing

7%


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Introduction

Finely divided combustible solids, or dusts, dispersed

in air can give rise to explosions in much the same

way as flammable gases.

In the case of flammable gases, fuel concentration,

local heat transfer conditions, oxygen concentration

and initial temperature all affect ignition and resulting

explosion characteristics.

In the case of dusts, however, more variables are

involved (e.g. particle size distribution, moisture

content) and so the analysis and prediction of dustexplosion characteristics is more complex than for the

flammable gases.


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OUTLINE

Introduction





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Combustion Fundamentals - 1

Classic Fire Triangle

Remove any element

eliminates the

possibility of fire


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Dust Explosion Pentagon

Remove any element

prevents explosion,

but not necessarily

fire!


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Flames

Stationary flame (for example a candle flame or gas

stove flame)

unburned fuel and air flow into the flame

front as combustion products flow away

from the flame front.

A stationary flame may be from either premixed fuel and air,or by diffusion of air into the combustion zone.

Explosion flame

the flame front passes through a homogeneous premixed

fuel air mixture.

The heat released and gases generated result in either an

uncontrolled expansion effect or, if the expansion is

restricted, a rapid build-up of pressure.

Bunsen

burner


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Explosions and Detonations-1 Explosion flames travel through the fuelair mixture at velocities

ranging from a few metres/second to several hundreds of metres/

second and this type of explosion is called a deflagration.

Flame speeds are governed by many factors including the heat of

combustion of the fuel, the degree of turbulence in the mixture and the

amount of energy supplied to cause ignition.

It is possible for flames to reach supersonic velocities under some

circumstances. Such explosions are accompanied by pressure shockwaves, are far more destructive and called detonations.

The increased velocities result from increased gas densities generated

by pressure waves. It is not yet understood what conditions give rise to

detonations. However, in practice it is likely that all detonations begin as

deflagrations.


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Explosions and Detonations-2

Deflagration. Propagation of a combustion zone at a speedthat is less than the speed of sound in the unreacted medium.

Detonation. Propagation of a combustion zone at a velocitythat is greater than the speed of sound in the unreacted

medium.

Explosion. The bursting or rupture of an enclosure or acontainer due to the development of internal pressure from

deflagration.


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The Typical Explosion Event- 1


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Ignition, Ignition Energy, Ignition Temperature-1

Ignition is the self-propagation of a combustion reaction

through a fuel air mixture after the initial supply of energy.

Ignition of a fuel-air mixture can be analysed in a mannersimilar to that used for thermal explosions.

Volume -

Surface area -

Volumetric concentration of fuel -

Heat transfer coefficient -

Fuel air mixture

The rate of heat loss to surroundings, is:


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The rate of heat loss to surroundings, is:

The variation of the combustion reaction rate

with temperature will be governed by the

Arrhenius equation. For a reaction which is

first order in fuel concentration:

- pre-exponential coefficient the reaction activation energy

ideal gas constant

- molar density of the fuel

(Eq. 1)

(Eq. 2)


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The rateat which heat is adsorbed by the fuel-air mixture in

the element is:

1

, - molar specific heat capacities of fuel and air

,

- molar densities of the fuel and air

(Eq. 3)

If is the rate at which heat energy is fed into the

element from outside, then the heat balance for the element

becomes:

Zexp

1

(Eq. 4)


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Heat balance for the element:

Zexp

1 2

1

3 4

For steady state: term(3)= 0

Analyse the heat balance graphically

Plot the rates of heat loss to the surroundings (term 4) and the

rate of heat generation by the combustion reaction (term 2) asa function of temperature.


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At an initial element temperature Ti the rate of heat loss from theelement is greater than the rate of heat generation and so the

temperature of the element will decrease until point A is reached.

Any initial temperature between TB and TA will result in the element

cooling to TA . This is a stable condition.


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If the initial temperature is greater than TB , the rate of heat

generation will be always greater than the rate of heat loss to the

surroundings and so the element temperature will rise,

exponentially. Thus initial temperatures beyond TB give rise to an

unstable condition. TB is the ignition temperature, Tig , for the fuel

air mixture in the element.

Ignition energy is the energy that we must supply from theoutside in order to raise the mixture from its initial temperature Ti

to the ignition temperature Tig. Since the element is continuously

losing energy to the surroundings, the ignition energy will actually

be a rate of energy input, Q input .


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This raises the heat generation curve by an amount Qinput ,

reducing the value of Tig. The conditions for heat transfer from

the element to the surroundings are obviously important in

determining temperature and energy.


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There are cases where the heat loss curve will be always lower

than the heat generation curve. Under such circumstances the

mixture may self-ignite; this is referred to as auto-ignition or

spontaneous ignition.


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In many combustion systems there is an appreciable interval

(from milliseconds to several minutes) between arrival at the

ignition temperature and the apparent onset of ignition. This is

known as the ignition delay.

Minimum Ignition Temperature (MIT). The lowest temperature at

which ignition occurs.

Lower the particle size Lower the MIT Lower the moisture content - Lower the MIT

Minimum Ignition Energy (MIE). The lowest electrostatic spark

energy that is capable of igniting a dust cloud.

Energy Units (millijoules) Decrease in particle size and moisture content decreases MIE

An increase in temperature in dust cloud atmosphere - decreases

MIE


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Flammability Limits-1

From the analysis it can be seen that below a certain fuel

concentration ignition will not occur, since the rate of heat

generation within the element is insufficient to match the rate of

heat loss to the surroundings (Tigis never reached).

This concentration is known as the lower flammability limit CfLof

the fuel air mixture.

At CfL

the oxygen is in excess.

Lower Flammable Limit (CFL). The lowest concentration of a

combustible substance in an oxidizing medium.


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Flammability Limits-2 For fuel concentration increase beyond the stoichiometric ratio,

the oxygen is limiting and so the amount of fuel reacting per unit

volume of mixture and the quantity of heat released per unit

volume decrease with fuel concentration.

A point is reached when the heat release per unit volume of

mixture is too low to sustain a flame.

This is the upper flammability limit, CfU. This is the concentration

of fuel in the fuel air mixture above which a flame cannot bepropagated.

Upper Flammable Limit (Cfu). The highest concentration of a

combustible substance in an oxidizing medium that will propagate a

flame.

Within the range of from CfL to CfU, a flame can be propagated, this

range will widen (CfLwill decrease and CfU will increase) as the initial

temperature of the mixture is increased. In practice, flammability limits

are measured and quoted at standard temperatures (usually 20C).


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Minimum oxygen for combustion-1 At the lower limit of flammability there is more oxygen available than is

required for stoichiometric combustion of the fuel.

For example, the lower flammability limit for propane in air at 20C is

2.2% by volume.

For complete combustion of propane according to the reaction:

five volumes of oxygen are required per volume of fuel propane.

5 3+4O

In a fuel air mixture with 2.2% propane the ratio of air to propane is:

100 2.2

2.2 44.45

and since air is approximately 21% oxygen, the ratio of oxygen to propane

is 9.33. Thus in the case of propane at the lower flammability limit oxygen

is in excess by approximately 87%.


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Minimum oxygen for combustion-2

It is therefore possible to reduce the concentration of oxygen in

the fuel air mixture whilst still maintaining the ability to

propagate a flame.

When the effect on the ability of the mixture to maintain a flame is

minimal until the stoichiometric ratio of oxygen to fuel is reached,

the oxygen concentration in the mixture under these conditions is

known as the minimum oxygen for combustion (MOC).

Minimum oxygen for combustion is therefore the stoichiometricoxygen equivalent to the lower flammability limit. Thus

For example, for propane, since under stoichiometric conditions five

volumes of oxygen are required per volume of fuel propane,

2.2% 5 11%


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OUTLINE

Introduction




C i i C 1


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Combustion in Dust Clouds - 1

Fundamental Specific to Dust Cloud Explosions-1

The combustion rate of solid in air will in most cases be limited by

the surface area of solid presented to the air.

If the particles of solids are small enough to be dispersed in airwithout too much propensity to settle, the reaction rate will be

great enough to permit an explosion flame to propagate.

For a dust explosion to occur the solid materials of which the

particles are composed must be combustible, i.e. it must reactexothermically with the oxygen in air.

To make the combustion fundamentals applicable to dust

explosions, need to add in the influence of particle size on

reaction rate.

C b i i D Cl d 2


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Fundamental Specific to Dust Cloud Explosions-2 Assuming the combustion reaction rate is determined by the surface

area of the solid fuel particles (assumed spherical) exposed to the air,

the heat release term (2) in Eq. (4) becomes:

6

- particle size; - molar density of the solids fuel

The rate of heat generated by the combustion reaction is inverselyproportional to the dust particle size. Thus the likelihood of flame

propagation and explosion will increase with decreasing particle size.

Qualitatively, this is because finer fuel particles:

More readily form a dispersion in air; Have larger surface area per unit mass of fuel;

Offer greater surface area for reaction;

Consequently generate more heat per unit mass of fuel;

Have greater heat-up rate.


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C b ti i D t Cl d 4


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Characteristics of Dust Explosions -2

Most test include an assessment of the following

explosion characteristics:

Minimum dust concentration for explosion;

Minimum energy for ignition;

Minimum ignition temperature;

Maximum explosion pressure;

Maximum rate of pressure rise during explosion;

Minimum oxygen for combustion.

Classification test A test for explosibility in the test apparatus,classifying the dust as able or unable to ignite and propagate a flame in

air at room temperature under test conditions.

C b ti i D t Cl d 5


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Apparatus for Determination of Dust Explosion Characteristics-1

All devices include:

A vessel(open or closed);

An ignition source(electrical spark or electrically heated wire coil);

A supply of air for dispersion of the dust.

Vertical Tube apparatus: The simplest

Classification test

Minimum dust concentration

for explosion

Minimum energy for ignition Minimum oxygen for combustion

Comb stion in D st Clo ds 6


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Apparatus for Determination of Dust Explosion Characteristics-220 Litre Sphere:

Maximum explosion pressure

Maximum rate of pressure rise during explosion

These give an indication of the severity of explosion and enable the designof explosion protect equipment.

Combustion in Dust Clouds 7


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The vessel pressure is reduced to about 0.4 bar before the test so that

upon injection of the dust, the pressure rises to atmospheric.

Ignition is by a pyrotechnical device with a standard total energy

(typically 10 kJ) positioned at the centre of the sphere. The delay

between dispersion of the dust and initiation of the ignition source has

been found to affect the results. Turbulence caused by the air injection

influences the rate of the combustion reaction



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Godbert-Greenwald furnace

Godbert-Greenwald furnace

apparatus:

Determine the minimum ignitiontemperature

It includes a vertical electrically

heated furnace tube which can

be raised to controlledtemperatures up to 1000C.

When ignition occurs, the

furnace temperature is lowered

in 10C steps until ignition doesnot occur.



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Applications of the Test Results-1 The minimum dust concentration for explosion is used to give an

indication of the quantities of air to be used in extraction systems for

combustible dusts.

The minimum energy for ignition is measured primarily to determine

whether the dust cloud could be ignited by an electrostatic spark.

The minimum ignition temperature indicates the maximum temperature

for equipment surfaces in contact with the powder.

The maximum explosion pressure is usually in the range of 8-13 bar

and is used mainly to determine the design pressure for equipment

when explosion containment or protection is opted for as the method of

dust explosion control.



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Applications of the Test Results-2 The maximum rate of pressure rise during explosion is used in the

design of explosion relief.

It has been demonstrated that the maximum rate of pressure rise in a dust explosion is

inversely proportional to the cube root of the vessel volume, i.e.

The value ofis found to be constant for a given powder. The severity of dust explosion

is classified according to the St class based on thevalue.

The minimum oxygen for combustion (MOC) is used to determine the

maximum permissible oxygen concentration when inerting is selected

as the means of controlling the dust explosion.

Dust explosion classes based on 1m3

test apparatus

OUTLINE


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OUTLINE

Introduction




Control of the Hazards - 1


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Dust Control

Ignition Source Control

Damage Control



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The main approaches in approximate order of decreasing strategic

component:

Change the process to eliminate the dust;

Design the plant to withstand the pressure generated by any explosion;

Remove the oxygen by complete inerting;

Reduce oxygen to below MOC;

Add moisture to the dust;

Add diluent powder to the dust;

Detect start of explosion and inject suppressant;

Vent the vessel to relive pressure generated by the explosion;

Control dust concentration to be outside flammability limits;

Minimize dust cloud formation;

Exclude ignition sources.



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Control of the Hazards 3

Ignition Sources Control

Electrical equipment

Static electricity control Mechanical sparks & friction

Open flame control

Design of heating systems & heated surfaces

Use of tools & vehicles

Maintenance



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Explosion Relief Venting

Explosion Relief venting is the process of relieving the explosion

products (pressure and flame) from the plant to a safe location.

The principle of explosion venting is to discharge the vessel contentsthrough an opening or vent to prevent the pressure rising above the

vessel design pressure.

Advantage: relatively simple and inexpensive

Limits:

Cannot be used when the dust, gas or combustion products are toxic.

Very strong explosible dust with rate of pressure rise is greater than 600bar

m/s would be very difficult to vent (Lunn, 1992).

Venting of explosions to inside a building is not usually acceptable.



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Explosion Suppression

Explosion suppression is detecting an explosion at an early stage and

suppressing it with a suitable suppressant (inert gas or powder).

The suppression systems triggered by the pressure rise accompanyingthe start of the explosion have response times of the order of a few ms,

and are able to effectively extinguish the explosion (within 0.08sec).

Advantage:

Extinguishing the flame Reducing the risk of ejecting toxic/or corrosive materials to the atmosphere

Process equipment does not need to be located in an area suitable for

explosion relief venting

Disadvantage: More expensive to install and maintain than explosion relief venting.

Some suppression systems are not suitable for powders with high

explosion severity (above 300 bar m/sec).



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Containment

Where plant vessels are of small dimensions it may be

economic to design them to withstand the maximum pressure

generated by the dust explosion.

All interconnected pipes, flanges, covers, etc. should

withstand the maximum explosion pressure of the dust being

handled.

If an explosion-resistant vessel fails, the pressure effects will

be more severe than if an extremely weak vessel fails as aresult of dust explosion.

Inerting

Nitrogen and carbon dioxide are commonly used to reduce

the oxygen concentration of air to below the MOC.



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Minimize Dust Cloud Formation

Use of dense phase conveying as an alternative to dilute phase

Use of cyclone separators and filters instead of settling vessels for

separation of conveyed powder from air

Avoiding situations where a powder stream is allowed to fall freely

through air

Good housekeeping practice should be ensure.

Example-1


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p

A fine flammable dust is leaking from a pressurized container at a

rate of 2 litre/min into a room of volume 6m3 and forming a

suspension in the air. The minimum explosible concentration of the

dust in air at room temperature is 2.22% by volume. Assuming that

the dust is fine enough to settle only very slowly from suspension,

(a)what will be the time from the start of the leak before explosion

occurs in the room if the air ventilation rate in the room is 4m3/h,

and

(b)What would be the minimum safe ventilation rate under these

circumstances?

Example-2


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Solution:(a) Mass balance on the dust in the room:

assuming constant gas density,

0.12 4

where 0.12 is the leak rate in m3

/h,is the volume of the room andis the dust concentration in the room at time.

Rearranging the integrating with the initial condition, 0at 0,

1.50.12 4

0.12

Assuming the explosion occurs when the dust concentration reaches

the lower flammability limit, 2.22%

2.02

Example-3


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Solution-continue:

(b) To ensure safety, the limiting ventilation rate is that which gives a room

dust concentration of 2.22% at steady stage (i.e. when 0).

Under this condition,

0 0.12

hence, the minimum ventilation rate, 5.4 .

Che415 Explosion Hazards

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