Chemical Engineering July 2021 - 30

Feature Report
Part 1
Explosion Protection:
Considerations for
Combustible Dusts
Managing the risk from combustible dust explosions involves a number of strategies for
preventing explosions, as well as isolating, venting and suppressing explosions if they occur.
Complying with applicable NFPA standards is a pathway toward minimizing risk
Clive Nixon
BS&B Pressure Safety
Management
IN BRIEF
DUST EXPLOSION
SCIENCE
FIRE AND EXPLOSION
PREVENTION
EXPLOSION PROTECTION
DUST EXPLOSION
VENTING
EXPLOSION
SUPPRESSION
EQUIPMENT
EXPLOSION ISOLATION
NFPA DUST STANDARDS
CONCLUDING REMARKS
C
ombustible dust explosions present
an ongoing risk to plants within the
chemical process industries (CPI)
that handle combustible powders
or bulk solids. While dust explosions are not
as common as fires, if an explosion occurs,
the potential for extensive damage can be an
order of magnitude higher. Managing dust explosion
risks involves a number of strategies,
including reducing the likelihood of an explosion
through prevention activities, as well as
implementing measures to isolate, vent and
suppress explosions if they do occur.
Safety measures established by the U.S.
Department of Labor's Occupational Safety
and Health Administration (OSHA; Washington,
D.C.; www.osha.gov), through the
OSHA Combustible Dust National Emphasis
Program, provide a good basis for combustible
dust safety. The program recommends
following practices in applicable National
Fire Protection Association (NFPA; Quincy,
Mass.; www.nfpa.org) standards. This article
presents an overview of NFPA standards that
are relevant for combustible dusts, along
with considerations for managing combustible
dust risks.
Dust explosion science
Dust explosion hazards exist in a wide range of
industries, including chemical, petrochemical,
grain, food, wood, paper and pharmaceutical
industries. If the material being processed
is combustible and capable of generating
dust, then there is a potential dust explosion
hazard. Common process equipment that
presents an explosion risk include dust collectors,
filter receivers, pneumatic conveyors,
mechanical conveyors, size-reduction equipment,
dryers, bins and storage silos.
A deflagration is an expanding flame front
travelling through a fuel-air mixture at a velocity
lower than the speed of sound. The
30
resulting hot expanding gases rapidly pressurize
any containing vessel, causing it to
explode, unless it is capable of withstanding
the full explosion pressure. This pressure
can exceed 100 psi. The maximum explosion
pressure is referred as Pmax, and is
commonly expressed in the units barg. Pmax
is one of several combustion characteristics
referenced for protection system design.
Others include the following:
* Pmax (barg) - Maximum explosion pressure
* Kst (bar.m/s) - An explosibility index determined
from measuring the rate of pressure
rise with respect to time in a test vessel
* Minimum ignition energy (MIE; mJ) - The
minimum ignition energy needed to ignite the
fuel-air mixture
* Auto-ignition temperature (°C) - The lowest
temperature at which a dust layer or dust
cloud will spontaneously ignite. These are
determined by two different test apparatuses
* Minimum explosible concentration (MEC;
g/m3) - The minimum concentration of a
combustible dust in air that will support a
deflagration
These explosibility characteristics are affected
by the physical characteristics of the
material. For example, grinding a material
and reducing the particle size can increase
the rate of combustion and increase the Kst
value. It is therefore important that the specific
material being handled is tested. If published
data are used, care should be taken
that the data correspond to a material that is
representative of the material being handled.
Fire and explosion prevention
Deflagrations can occur both within process
equipment and from the ignition of dust external
to the process. To reduce the potential
for deflagrations external to the process,
electrical components, such as lights and
motors, should be suitably rated accordCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM JULY 2021
http://www.osha.gov http://www.nfpa.org http://WWW.CHEMENGONLINE.COM

Chemical Engineering July 2021

Table of Contents for the Digital Edition of Chemical Engineering July 2021

Contents
Chemical Engineering July 2021 - Cover1
Chemical Engineering July 2021 - Cover2
Chemical Engineering July 2021 - Contents
Chemical Engineering July 2021 - 2
Chemical Engineering July 2021 - 3
Chemical Engineering July 2021 - 4
Chemical Engineering July 2021 - 5
Chemical Engineering July 2021 - 6
Chemical Engineering July 2021 - 7
Chemical Engineering July 2021 - 8
Chemical Engineering July 2021 - 9
Chemical Engineering July 2021 - 10
Chemical Engineering July 2021 - 11
Chemical Engineering July 2021 - 12
Chemical Engineering July 2021 - 13
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Chemical Engineering July 2021 - 15
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Chemical Engineering July 2021 - 17
Chemical Engineering July 2021 - 18
Chemical Engineering July 2021 - 19
Chemical Engineering July 2021 - 20
Chemical Engineering July 2021 - 21
Chemical Engineering July 2021 - 22
Chemical Engineering July 2021 - 23
Chemical Engineering July 2021 - 24
Chemical Engineering July 2021 - 25
Chemical Engineering July 2021 - 26
Chemical Engineering July 2021 - 27
Chemical Engineering July 2021 - 28
Chemical Engineering July 2021 - 29
Chemical Engineering July 2021 - 30
Chemical Engineering July 2021 - 31
Chemical Engineering July 2021 - 32
Chemical Engineering July 2021 - 33
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Chemical Engineering July 2021 - 35
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Chemical Engineering July 2021 - 46
Chemical Engineering July 2021 - 47
Chemical Engineering July 2021 - 48
Chemical Engineering July 2021 - Cover3
Chemical Engineering July 2021 - Cover4
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