Chemical Engineering July 2021 - 31

ing to the area hazard classification,
which is based on the frequency of
dust being present and the characteristics
of the dust. The method
of determining area classifications
is detailed in the National Electric
Code, NFPA 70, Article 500 Hazardous
(Classified) Locations. Additionally,
it is important to note that effective
dust control and housekeeping
is key to limiting the potential for
external deflagrations. From a cost
standpoint, the best approach is to
limit fugitive dust through adequate
dust collection and by maintaining
a sealed process. Regular cleaning
of all horizontal surfaces, including
roofing supports, is a costly alternative
to keeping the dust contained
in the first place. Blowing down the
process with compressed air can
create a deadly combustible dust
cloud and must be avoided. Housekeeping
and dust control are detailed
in NFPA 652.
Within process equipment volumes,
unless it
is possible to fully
contain the maximum explosion
pressure, displace the oxygen or to
operate below the explosible dust
concentration, the primary method
of reducing the potential of a deflagration
is to monitor and prevent
potential ignition sources. Potential
ignition sources include:
* Overheated bearings
* Mechanical equipment failures,
such as impact sparks from a hammer
mill
* Rubbing or slipping conveyor belts
* Static electricity
* Foreign material entering the
process
* Exothermic chemical reactions
* External welding or other hot work
* Process overheating
Having identified potential ignition
sources, risk management solutions
can be applied, such as bearing temperature
monitoring. Testing the material
being handled to determine its
MIE and auto-ignition temperature will
be helpful in determining the best prevention
strategies.
Explosion protection
While methods such as monitoring
and controlling potential ignition
sources are key aspects of explosion
prevention, they do not completely
eliminate the risk. This
necessitates the application of
solutions to manage the pressure
and flame in the event of a
deflagration and prevent propagation
to adjacent process volumes.
NFPA 652 recognizes
the following passive and active
methods of protection:
* Reduction of oxidant concentration
*
Deflagration venting
* Deflagration venting via flameless
vents
* Deflagration pressure
containment
* Deflagration suppression
* Dilution with a non-combustible
dust to render the mixture
non-combustible
Of the methods of protection
listed, the most common are
explosion venting and explosion
suppression (Figure 1). We
will discuss these approaches
in more detail.
Dust explosion venting
During the early stages of a
dust or gas explosion, explosion
vents quickly open at a
predetermined burst pressure,
allowing the rapidly expanding
combustion gases to escape into
the atmosphere and limit the pressure
generated inside the process
equipment to calculated safe limits.
Venting is the most widely adopted
protection mechanism because it
provides an economical solution and
is often considered " fit-and-forget "
solution. However, it is important to
note that vents need to be regularly
inspected, according to guidance
contained in NFPA 68.
Rotary
Airlock
Flame Free Venting
Explosion
Vent
Explosion Suppression
Chemical Isolation
Barrier
Chemical Explosion
Suppression Cannon
Rotary
Airlock
Explosion on Venting
Explosion
Pressure
Sensor
Chemical Isolation
Barrier
Flame Free
Vent
Isolation Barrier
Flap Valve
FIGURE 1. A number of approaches can be used to mitigate
dust explosions, including isolation, venting and suppression
tionality of the vent. A vent that becomes
heavier in weight will open
slowly and less efficiently.
For decades, explosion vents have
traditionally been designed using a
" composite " approach that sandwiches
plastic film between more
resistant stainless-steel sheets with
holes or slots cut into them. These
vents are designed to open at typically
1 to 1.5 psi set pressure. With
this type of technology, over time,
the holes and slots in the stainlesssteel
sheets can admit particulate
matter and debris. The buildup of
solids can eventually affect the funcCHEMICAL
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A better solution is a single-section
explosion vent, comprised of a
solitary sheet of stainless steel in a
domed configuration. Perforations
around the perimeter aid opening at
the desired low-set pressure are protected
with gasket materials. The single-section
domed design produces
a vent that is more robust, lighter in
weight and largely eliminates the potential
for buildup or contamination.
Despite their popularity, explosion
vents will not work for every application.
With venting, the combustion
process releases a large ball of
flame into the atmosphere. While
this might be an acceptable consequence
for outdoor equipment, such
as silos, for applications within a
plant, it could endanger personnel or
equipment, and even lead to a secondary
explosion.
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