Hydrocarbon Processing - December 2022 - 63

Plant Safety and Environment
tional safety distance should be considered
(FIG. 2). Restricted zone warning
signs and barriers should be established
to prevent unauthorized personnel access.
Permits should be used to evaluate
potential hazards encountered by personnel
working within the restricted zone,
specify appropriate personal protective
equipment and limit the time to which
personnel are exposed to this hazard.
Lack of design basis information to
support explosion protection design.
When designing and implementing explosion
protection systems, it is imperative
to know the strength of the protected
equipment. For example, the maximum
explosion pressure in equipment fitted
with explosion vents can significantly
exceed the pressure at which the
vents open, and the equipment may fail
catastrophically if not designed to withstand
the maximum pressure. Similarly,
if chemical suppression is implemented,
the activated suppression system pressurizes
the equipment as it injects the suppressant,
and hence the equipment will
fail if not designed appropriately.
In one instance, the author's company
assessed the capacity of a rectangular
baghouse with an explosion suppression
system and determined that the baghouse
would fail at the roof-to-shell seam
without additional structural reinforcement
(FIG. 3). Activation of the suppression
system, even inadvertently, posed a
significant hazard to the operators.
The lack of design basis system flow
velocities. NFPA standards prescribe
minimum flow velocities through dust
collection systems to prevent dust from
falling out of suspension and accumulating
inside the duct. Dust accumulations
can lead to a hidden fuel source that may
be ignited by a spark drawn into the system.
Duct fires can present a challenge to
extinguish and often can damage equipment
beyond repair. In some cases, the
burning material can be pulled into the
dust collection system and initiate a deflagration.
Maintaining a minimum flow
velocity of 4,000 fpm is recommended
by NFPA standards to prevent the fallout
of dust in dust collection ductwork-the
actual required velocity is a function of
the dust properties. In some cases, facilities
have created peepholes in the ductwork
into which a flowmeter can be periodically
introduced to monitor the flow
velocity and validate system integrity. Intelligent
dust collection systems are also
available that include in-line flow velocity
sensors that control the fan speed to
maintain the minimum designated velocity
and balance the system when sections
are taken out of service.
The lack of duct inspection hatches.
The placement of inspection hatches
along the dust collection system duct allows
for periodic inspection for dust accumulation,
although the NFPA standards
do not explicitly require such hatches.
The author's company recommends that
inspection hatches are located the equivalent
of approximately 20 duct diameters
along the main duct. This criterion may
be different for individual small-diameter
trunks sprouting from the main duct. Regardless
of specific inspection hatch spacing,
the intent is that all sections of the
ductwork can be inspected and cleaned
sufficiently through the hatches.
Spark suppression system nozzles
located too far
from the spark
source. Dust collection ductwork connected
to high-frequency spark generating
equipment (e.g., sander, grinder, hog)
should be equipped with a spark suppression
system to minimize or eliminate
sparks entering the dust collector, which
could trigger a collector fire or deflagration.
The spark suppression system typically
incorporates a sensor (i.e., infrared,
rate of pressure rise) that detects the
spark, along with spray nozzles located
downstream at a distance determined
by flow velocity. The separation distance
between the detector and suppressors is
critical: too close and the spark may get
beyond the spray before it is activated;
too far and the spray may halt before the
spark arrives. The location of the spark
suppression system is also critical. The
FM Global Loss Prevention Data Sheet
7-7612
recommends that the system be
as close to the final piece of spark-generating
equipment as possible. Otherwise,
if dust has accumulated in the duct between
the spark-generating equipment
and the spark suppression system, such
accumulations can be ignited. Of course,
spark suppression systems only reduce
the likelihood of ignition, and additional
explosion protection is required.
It is also noted that NFPA standards
FIG. 3. Failure of baghouse roof-to-shell seam
under suppression injection pressure.
require that horizontal ductwork be capable
of supporting the weight of the system
plus the weight of the duct half-filled
with water (where sprinkler protection is
provided in the duct) or the material being
conveyed, whichever has the higher
density. The author's company knows
at least one case where a large duct collapsed
due to water accumulation from
fire suppression activities.
Takeaway. The identified common hazards
have all been causal factors in actual
combustible dust fire or explosion incidents
that resulted in the loss of life or
severe injury, in addition to the loss of
capital assets and business interruption.
Performing a robust DHA should identify
these hazards and, using the guidance
provided in relevant NFPA standards, determine
prevention and mitigation strategies
that can be implemented to manage
the risk associated with combustible dust
fire and explosion hazards.
LITERATURE CITED
1
NFPA 61, et al., " Standard for the prevention of fires
and dust explosions in agricultural and food processing
facilities, " 2020.
2
NFPA 484, et al., " Standard for combustible metals, "
2022.
Complete literature cited available online at
www.HydrocarbonProcessing.com.
PHILIP PARSONS is a Principal
Engineer for BakerRisk and resides in
San Antonio, Texas. He has
performed more than 200 dust
hazard analyses and numerous
incident investigations across a wide
range of industries worldwide.
Parsons is a committee member of NFPA 652 " Standard
on the fundamentals of combustible dust, " NFPA 654
" Standard for prevention of fire and dust explosions
from the manufacturing, processing, and handling of
combustible particulate solids " and NFPA 61 " Standard
for the prevention of fires and dust explosions in
agricultural and food processing facilities. "
Hydrocarbon Processing | DECEMBER 2022 63
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