Dust Explosions

Dust Explosions
p
Risk to people, plant
and
d reputation
t ti
Mark Hoyle,
Process Safety Group
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 cause
cause.
Made major headlines in Europe/US (as ‘iPAD factory blows up!’)
2
And many others! (All industries handling fine powder materials
can be at risk))
Notice the major destruction to
these installations. Why so much
damage?
3
Reactive Hazard Management Process
Covers all
C
ll materials
t i l involved
i
l d in
i process – starts
t t with
ith the
th
‘high level’ questions….
1. Do you handle REACTIVE materials?
2 Can you have REACTIVE interactions involving materials that
2.
you handle?
If 1 and/or
d/ 2 are answered
d YES – reactivity
ti it h
hazards
d mustt b
be controlled
t ll d th
throughout
h t entire
ti
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?
4
Dust Explosion Pentagon (v’s Fire Triangle)
Oxidant
Fuel
(Dust)
Ignition
Source
Dispersion
Fuel
HAZARD
Fire Triangle
5
Ignition
Source
Confinement
Oxygen
(Air)
D tP
Dust
Pentagon
t
How does Confinement influence the Dust
H
Hazard
d
• 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 enclosure
• Complete confinement : ignition produces full deflagration pressure
(Pmax), which can destroy most buildings and process equipment
( l
(unless
th
they are d
designed
i
d tto withstand
ith t d thi
this))
6
Overview
1.
2.
3.
4.
5
5.
6.
7
7.
8.
7
General Prevention/Protection Protocol
Secondary
y Dust Explosion
p
Risk
Mitigation Methods
Primary Dust Explosion Risk
Mi i i Methods
Mitigation
M h d
Quick reminder of dust tests and their application
Useful information sources
Key points
Typical Protocol
• Hazard Assessment ((Process + materials used))
• Engineering Controls
• Housekeeping (cleanliness of workplace)
• Building Design
• Explosion Protection
• Worker Training
g
8
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’
secondary dust explosion hazard.
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
9
Dust Explosion in the Work Place – ‘Secondary’
E l i Damage
Explosion
D
• Dust settles on flat surfaces
• Dust is disturbed creating a cloud
• Dust cloud is ignited and explodes
and 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
Building
Initial explosion
event inside
equipment
0
25
50
75
100
125
150
175
200
225
250
275
300
325
Time in milliseconds (ms)
Building
Initial explosion
eventt iinside
id
equipment
Shockwave
(pressure from
‘equipment’)
0
25
50
75
100
125
150
175
200
Time in milliseconds (ms)
11
225
250
275
300
325
‘Typical’ Industrial Dust explosion Time-line
Building
Initial explosion
event inside
equipment
0
25
50
Rebound shockwaves
(reflection)
75
100
125
150
175
200
225
250
275
300
325
Time in milliseconds (ms)
Building
Initial explosion
event inside
equipment
0
25
50
Dust clouds generated by rebound shockwaves
75
100
125
150
175
200
Time in milliseconds (ms)
12
225
250
275
300
325
‘Typical’ Industrial Dust explosion Time-line
Building
Rupture/flame
relief in
equipment
0
25
50
D t clouds
Dust
l d generated
t d by
b rebound
b
d shockwaves
h k
75
100
125
150
175
200
225
250
275
300
325
Time in milliseconds (ms)
Building
Initial explosion
Secondary Dust explosion – flame propagates through dust
event inside
equipment
0
25
50
75
100
125
150
175
200
Time in milliseconds (ms)
13
225
250
275
300
325
‘Typical’ Industrial Dust explosion Time-line
Building
Rupture/flame
relief in
equipment
0
25
50
75
100
125
150
175
200
225
250
275
300
325
Time in milliseconds (ms)
Building
Rupture/flame
relief in
equipment
Explosion pressure (and flame) vents through the building
0
25
50
75
100
125
150
175
200
Time in milliseconds (ms)
225
250
275
300
325
‘Typical’ Industrial Dust explosion Time-line
Building
Building collapses
0
25
50
75
100
125
150
175
200
225
250
275
300
Time in milliseconds (ms)
Such an event is effectively over in less that 0.5 seconds.
There is effectively little time for avoiding action.
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Remember - even small layers can give rise to
significant
i ifi
t dust
d t clouds
l d
General guidance in NFPA 654 – layer thickness limits= >0.4 mm over 5% of
area (<1900 m2).
) Approximately
A
i t l the
th thi
thickness
k
off a paper clip!
li !
16
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)
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
g
accumulations of dust in
hidden areas (for example above false
ceilings)
• Provide access to all hidden areas to
allow inspection:
• IInspectt for
f dust
d t residues
id
in
i open and
d
hidden areas at regular time intervals
• Clean
Cl
d
dustt residues
id
att regular
l intervals.
i t
l
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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 facility.
facility
• Maybe
y p
possible to consider water spray
p y to wet
down / control dust movement in some applications
(for example coal conveyers).
19
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 ffacility).
the
ilit )
8
htt //
http://www.nilfisk-cfm.co.uk/
ilfi k f
k/
• Ideally use dedicated vacuum cleaners – which are not an ignition
source themselves (ATEX approved versions).
http://www.morclean.co.uk/
http://www.kerstar.co.uk/
http://www.nederman.co.uk/
• Alternatively,
Alternatively wet down with water and clean up (if this is applicable).
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
minimum explosible concentration (MEC)
– not enough dust to cause a
fire/explosion hazard being present.
• There may be specific areas - defined
b an area classification
by
l
ifi ti exercise
i –
where all ignition sources need to be
avoided.
21
Under capacity
cyclonic
separators
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.
2.
3.
4.
5.
6.
7
7.
8.
9.
10.
11.
12.
13.
Hot surface
Flames and hot gases
Mechanical Sparks
Electrical Equipment
Transient current/Cathodic protection
Static Electricity
Li ht i
Lightning
Electromagnetic waves (HF)
Electromagnetic waves(optical range)
Ionizing
g Radiation
Ultrasonics
Adiabatic Compression/shock waves
Chemical Reaction, self-ignition
off dusts
d t
heating pipes, electrical apparatus
welding, exhaust gases
cutting, impact
electric sparks,/arcs
stray current/short circuit to earth
Spark Discharge
(
(specific
ifi ttype off electrostatic
l t t ti di
discharge)
h
)
Induced heating
Photoflash, laser
X-rays,
y , UV rays
y
Cleaning/testing
Heat of Compression
Exothermic processes
Reference : EN1127-1 : 1997, Explosive atmospheres – Explosion prevention and protection Part 1. Basic concepts
and methodology
22
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
• Halogen Light Bulb
23
Ductwork cut whilst
containing
Aluminium Dust
Bulb recovered from debris in
coal dust collector hopper after
explosion
Burning Material / Hot Particles
Hot particles / burning material produced by:•Frictional heating, for example from sanding/cutting
•Radiant heating
heating, for example from curing of wood panels
•Convective heating, for example in dryers
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 )
Ref Gummer & Lunn, 2003
24
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 k
blockage
Cyclone collector
- door blown
open. Ignition
source - burning
particulates from
dryer.
25
Exothermic Process – Self-heating (in dryers)
• Powder accumulations
remain in dryer
y
sufficiently long for
oxidative self-heating
to bring powder up to
ignition temperature.
• Result can be either a
fi or a dust
fire
d t explosion
l i
in dryer or
downstream dust
collector.
ll t
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Impact/Friction Ignition
Typical examples:
• During size reduction operations in various types of mills.
• During mixing and blending if impeller is misaligned or deformed or has
inadequate clearance, or tramp metal enters mixer.
• During grinding and polishing operations
operations.
• Tramp metal in a particle classifier or conveyor.
Hammermill (Sugar) : Decomposition/Ignition
E id
Evidence
around
d broken
b k hammer
h
27
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,
mixer and provided it is filled to a level of 70 vol
vol.%
% or more
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.
• * 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
elements
l
t needed.
d d Th
Thatt iis remove/lower
/l
the
th concentration
t ti off the
th fuel
f l (dust
(d t in
i
this case), remove the oxidant (oxygen in the surrounding air usually) or avoid
all possible ignition sources.
X
Oxidant
X
Fuel
HAZARD
X
Ignition
Source
• 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
method will then need to be implemented.
29
Explosion Protection
T i l methods
Typical
th d
• 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
explosion characteristics
characteristics’ of the material(s) being handled)
Venting
(for weak structures – ‘safe’ materials)
Containment
(strong enough to
Withstand explosion)
Suppression
(extinguish explosion)
Typical Powder System could have the following
31
Dust Testing + Applications
• Minimum Ignition Energy (MIE) – indication of ignition sources
• 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.
application
(Unlikely to need absolutely all of the test data).
32
Training
• (E
(Essential)
ti l) Train
T i employees
l
to
t recognise
i combustible
b tibl dust
d t explosion
l i
risks and take preventative action (and/or how to alert management to
take action).
•
•
•
Training must be :
•
•
•
•
•
•
33
Safe work practices for their jobs
Overall plant program for dust control and ignition source control
Before (new) personnel start work
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’
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 redeposit elsewhere or create the dust cloud you are trying to avoid.
• Do everything you can to avoid ignition sources in hazardous areas.
• If y
you cannot control/eliminate ignition
g
sources reliably
y then consider an
the most appropriate explosion protection method.
34
European and US Standards in this area
EN 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 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 the
manufacturing, processing, blending, pneumatic conveying, repackaging and handling of
combustible particulate solids or hybrid mixtures.
35
Other useful sources
• http://www.dustexplosion.info/index.htm
• http://www.dguv.de/ifa/en/gestis/expl/index.jsp
GESTIS DUST EX (D
GESTIS-DUST-EX
(Database
b
C
Combustion
b i and
d explosion
l i
characteristics of dusts
• Dust
D t Explosion
E l i P
Prevention
ti and
d Protection:
P t ti
AP
Practical
ti l Guide
G id
• Dust Explosions in the Process Industries, Third Edition
• Electrostatic
El t
t ti Ignitions
I iti
off Fires
Fi
and
d Explosions
E l i
36