Hydrocarbon Processing - December 2022 - 62

Plant Safety and Environment
(e.g., NFPA 6644
). Surprisingly, very
few facilities create an auditable system
of inspection and cleaning to meet this
requirement. Some facilities periodically
inspect and react accordingly, although
local accumulations may already have
significantly exceeded the identified
threshold thickness within this timeframe.
Other facilities simply clean on
a defined frequency, regardless of the
amount of dust accumulated within this
timeframe. Regrettably, some facilities
neither inspect for nor clean accumulated
dust on any defined schedule.
Although the NFPA standards do not
define an approach to achieve compliance,
the author's company believes that
the most effective method is to segment
the process areas into zones that are easily
inspected by a workgroup or operator,
ensuring no gaps between zones. An initial
inspection frequency should be established,
and the zone (including elevated
surfaces) cleaned as needed when the
threshold thickness is approached. For
zones that generate dust accumulations at
a steady rate, the inspection frequency can
be adjusted after a few inspections have
established the accumulation rate. The
keys to success are maintaining external
dust accumulation levels below threshold
levels, developing a reasonable inspection
frequency, documenting the inspections
and cleanings, and making the process
work intuitively without creating an undue
burden. An additional benefit to this
zoned approach is that the responsible
workgroup will likely begin to identify the
key equipment contributing to fugitive
dust emissions and seek methods to eliminate
the source of the emissions (FIG. 1).
The lack of dust explosibility data.
Most materials that can combust will
present a combustible dust fire and explosion
hazard if dust is created from processing
such materials. Dust generated from
combustible material should be assumed
to pose a combustible dust fire and explosion
hazard unless proven otherwise.
However, beyond identifying a combustible
dust hazard, a facility often does not
have sufficient information on the combustibility
characteristics of its dust.
Performing a simple Go/No-Go test
is fundamental to confirm that a hazard
exists. Once the determination is made
that the dust is combustible, additional
explosivity parameters may be useful to
quantify the hazard and develop effective
mitigation options. For example,
knowledge of the minimum explosible
concentration may be necessary to determine
if the process can generate enough
airborne dust to present an explosion
hazard. Likewise, knowledge of the minimum
ignition energy is essential when
determining the potential for static ignition
hazards, which may require operators
to be bonded/grounded when performing
certain operations.
For the most part, this information is
FIG. 1. Example of poor housekeeping.
well established for the variety of wood
species processed in industry. However,
particle size and moisture content play
a significant role in ignition sensitivity
and explosibility severity characteristics.
These parameters obviously will vary depending
on the type of wood processed,
the process involved (e.g., sander, rip saw,
computerized numerical control) and
locations in the process where dust concentrations
are greatest (e.g., dust collectors).
Having the correct data is essential
to appropriately evaluate the hazards and
associated risks of the process.
FIG. 2. Firefighter caught in a vented dust
deflagration fireball. (Photo courtesy of
OSHA.gov)
62 DECEMBER 2022 | HydrocarbonProcessing.com
The lack of rated deflagration isolation
device. NFPA standards state isolation
devices should be provided where an
internal explosion hazard exists to prevent
deflagration propagation between
connected equipment. Without isolation,
an explosion initiated in one piece of
equipment could propagate to connected
equipment. In addition to allowing the
explosion to propagate, this also results in
pressure piling in the downstream equipment,
which can significantly increase
the explosion pressures. The connected
equipment could fail even if protected
with a suitable explosion protection system
(e.g., explosion vent). Another concern
is exposing personnel working near
dust collection duct pick-up points or
hoods (e.g., bagging and hand-dumping
operations)
to the
flame propagating
through the connecting duct.
Many varieties and manufacturers of
deflagration isolation devices are often
sold as flame-front diverters, fast-acting
check valves, backblast dampers or backdraft
valves. Many observed installations
would not function as intended due to
a poor valve mating surface (i.e., would
not prevent flame propagation) or a
weak housing that would not withstand
the deflagration overpressure generated
in the connected equipment. The EN
16447 ATEX standard9
is the only international
testing protocol that evaluates
these devices under actual blast conditions
to ensure they will function properly
over the relevant explosibility parameter
range. When selecting a deflagration
device, always ensure the device meets
the ATEX certification and that the device
is rated for the explosibility characteristics
(e.g., KST
being handled. NFPA 6910
Lack of
, Pmax) of the dust
guides implementation
and other considerations relative
to isolation devices.
restricted access zones
around equipment equipped with
deflagration
vents. Restricted occupancy
zones must be established around
deflagration vents to reduce the likelihood
that personnel are in the vicinity if
a deflagration occurs in the vented equipment.
Several incidents have injured personnel
located near explosion vents due
to exposure to the flame and burning debris
from the vented deflagration. Equipment
controls and e-stops should be located
outside of these zones. NFPA 6811
provides guidance on fireball distances
from vented enclosures that can be used
to establish restricted access zones.
Of course, locations right at the edge
of the fireball are not necessarily safe due
to the high thermal flux; hence, an addi
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