Combustible Dust Fire and Explosion Mitigation
Tools and Technology for Dust, Fire and Explosion Mitigation
In the expansive scope of airborne combustibles, wood dust is
typically a lower explosion severity dust as compared to others,
particularly when compared to those produced in the pharmaceutical
industry. Even though wood dust is considered moderate, the processes
utilized in wood pellet production are intrinsic to creating a highly
combustible fuel source. Unchecked, it could equate to a recipe for
disaster.
However, today’s cutting-edge processes, technology
and mitigation techniques are powerful tools to protect against and
combat the inherent fire and explosion hazards associated with wood
pellet production.
Through regulation standards and enforcement
bodies, such as OSHA and the National Fire Protection Association, the
challenges of managing these hazards are being met head on. But to truly
mitigate combustible dust and fire/explosion potential, the
responsibility falls on each individual facility.
Assessing the Threat
Conducting
a dust hazard analysis (DHA) has become the industry standard—and
requirement—against which all wood pellet manufacturers are measured.
The main goal of a DHA is to capture all the locations and equipment
where a combustible dust hazard exists. By examining the ignitions
source types to determine potential ignition sources within a given
facility, specific fire and deflagration risk scenarios can be
determined before an event occurs.
Once risk factors are identified, recommendations for additional safeguards and ignition control can then be implemented.
Existing
facilities must have conducted a DHA by September 2020. For new
building and facilities construction, and for processes and facilities
undergoing modifications, DHAs must now be completed as part of the
process. While the September deadline has come and gone, the
requirements stands despite the COVID-19 pandemic and all its associated
difficulties. Rumors of enforcement leniency on noncompliance exist,
but it has been business for assessment conductors, with some companies
providing flexible options during the coronavirus. “Remote DHAs are an
option for some people,” says Jeff Davis, director of engineering at
Conversion Technologies. “We’ve been working through a bunch of
different on-site options as well, minimizing contact and minimizing
time on site.”
“Hallam-ICS saw a lull as the pandemic first hit,”
adds Chris Giusto, director of combustible dust safety. “But a large
majority of our clients were considered essential businesses, and they
all had to find ways to continue operating safely,” he says.
Equipment
including bins, tanks and silos, hammermills, pulverizers and grinders,
dryers and dust collectors, conveyors, screw augers and bucket
elevators, sifters, screens and classifiers—are all pieces of equipment
that must be evaluated both internally and externally for dust and
particle buildup. In addition to equipment, general production areas
must also be assessed. Combustible dust atmospheres in the rooms and
buildings where equipment is housed, open processes, or from
malfunctions from closed equipment also need to be risk assessed.
“Once
risk areas are identified, evaluate each item for fire, deflagration
and explosion hazards,” Davis adds. “Identify credible ignition sources,
such as open flames. Examine all potential electrical hazards, as well
as mechanical sparks ... those generated from grinding, impact or
friction of processes. Also be aware of any hot surfaces, and locations
where self-ignition can be an issue. Each of these threat types has a
control measure to best mitigate the chances of fire or explosion.”
Once
equipment and risk areas are evaluated, it’s pertinent to review
existing safeguards and determine where additional controls are
required. Consider how fires and deflagration can potentially move
between pieces of equipment or buildings, under normal, abnormal and
upset conditions.
“The final step is to assess the DHA and
determine where additional safeguard and mitigation measures need
implementation,” Davis says. “Remember, all DHAs must be performed by a
qualified person, and each DHA must be reviewed and updated accordingly
every five years.”
Professionally conducted DHAs aside, it is
critical for facilities themselves to have a thorough understanding and
ability to recognize and manage the hazards imposed by wood dust
Combustible Dust Safety Cycle
Chris
Giusto serves as the director of combustible dust safety with
engineering and controls firm Hallam-ICS, which directs its clients to
follow what the firm calls the “combustible dust safety cycle.” It
encompasses a DHA, mitigation, management of change (MOC) and—not to be
overlooked—excellent housekeeping. “Once a DHA has been completed,
mitigation and controlling actions—such as explosion protection devices,
equipment upgrades, new procedures or training—should be taken,” Giusto
says. “This includes housekeeping.”
Ranked from most to least
effective, controls include elimination, substitution, engineering
controls, administrative controls and PPE. Eliminating a hazard is the
most effective way to control risk; as the reliance on personnel
increases to mitigate the controls, so does the risk factor. People are
more susceptible to human error and, thus, statistically less effective.
The
unfortunate situation with combustible dust is that elimination and
substitution are generally not applicable as mitigation options, because
that dust is an inherent part of the manufacturing of the finished
product. For this reason, engineer controls are leaned upon, including
explosion venting, suppression, isolation devices and fire protection
systems.
For example, a dust collection system isolates people
from the hazard by capturing the dust at the source and moving it to a
safer location. This engineering control is then supplemented with the
administrative control of housekeeping. It’s unwise to rely on
housekeeping as a primary defense, but rather, it should be used to
reinforce the engineering control of a properly functioning dust
collection system.
Because fuel and oxygen sources are intrinsic
to production processes, eliminating them is often not an option. In
this case, minimizing ignition risks is the most effective means to
mitigating the threats of fire, flash fire and explosion hazards.
In
short, MOC is what ensures all mitigations strategies remain effective,
according to Giusto. Any housekeeping implications should be
documented, noting accumulation levels. Changes here can highlight a
potential problem elsewhere before it becomes a safety hazard.
“MOC
is crucial, and is described as the best practices used to ensure that
safety, health and environmental risks and hazards are properly
controlled when an organization makes changes to their facilities,
operations or personnel,” Giusto adds. “The overreaching goal of the MOC
is to identify new potential hazards and mitigate them proactively.”
It’s
also noteworthy that, although changes to facilities, operations,
personnel or processes can eliminate or reduce hazards, they hold the
potential to create new ones.
“Housekeeping is the hub of the
combustible dust safety cycle,” Giusto continues. “Because good
housekeeping is integral to all aspects of combustible dust safety, it
must be considered and reviewed at every stage of the production cycle.
In fact, good housekeeping is critical in the avoidance of secondary
events, which are often the most destructive.”
A secondary event
occurs when a primary explosion creates a pressure wave that travels
faster than the actual flame front. This pressure wave disturbs fugitive
dust accumulations throughout the facility. The flame front, which
follows closely behind the pressure wave, ignites the newly airborne
dust cloud, causing a chain reaction with the potential to destroy an
entire operating facility.
As critical as housekeeping is, it must be considered a last line of defense.
So,
how much dust is safe? According to NFPA guidelines, no more than a
layered accumulation of 1/32-inch, which is approximately the thickness
of a standard No. 1 paperclip. Note that this is a general guideline,
and can vary by industry or material specifics. The NFPA also provides
additional guidelines and formulas for calculating the maximum layer
thickness that accounts for density variances of various dust types.
While
all these science- and math-based formulas offer good guidelines,
practical applications are often less black and white. “As a good rule
of thumb, if you can’t see the color of the surface, or distinguish
between two surface colors, then the dust layer is too thick,” Giusto
says. “You must remember that combustible dust is a fuel, and that no
level of dust accumulation is 100 percent safe. Aim for zero … removing
as much fugitive dust as possible. Get there as close as you can, as
often as you can, within your means and resources.”
Hazard
identification, protection strategies, training and awareness throughout
the entire manufacturing cycle are most effective when plant personnel
become involved at all levels. Management will become more aware of NFPA
requirements, plant personnel will become involved with mitigation
projects, and all employees will become more aware of hazards and how to
best protect against them, Giusto says.
Formal training is the most
effective means of speeding up this process, and NFPA 652 requires
hazard awareness training for all affected employees, helping personnel
identify hazards and conditions that can lead to hazards, and arming
employees with the information needed to be proactive in preventing
incidents.
Evolving Industry Standards
In
2015, NFPA 652 was released, dubbed the Standard on Combustible Dust. It
is designed to provide the basic principles and requirements for
identifying and managing the fire and explosion hazards of combustible
dusts and particle solids. This quickly became the gold standard, a
format by which all other standards have been revised to match—creating a
starting point for all industries to build from.
With biomass
specifically, hazard identification becomes exceptionally important, and
changes as the product goes through manufacturing processes. “On the
front end of the process, we often have a green or recycled product
coming in, which is typically being dried or milled into a much dryer
product ... which is going to be much more dangerous from an ignition
sensitivity and severity standpoint,” says Jason Krbec, sales
engineering manager of CV Technology. “And then when we put it back into
a pellet, we get a very different dust on the back end. As we go
through the biomass process, that hazard identification changes.”
It’s
noteworthy that NFPA 652 is now included in the International Fire Code
as a standard to comply with when equipment, processes and operations
involve dust explosion hazards. From there, industry- and
commodity-specific standards must be references, all the way through
state and local fire codes in order to maintain compliance. But despite
interagency collaboration and attempted standardization of regulations,
gaps still exist. The goal, of course, is to continue to consolidate
standardization efforts because it improves combustible dust safety, and
it can be challenging for companies with multiple industry functions to
comply. In addition, this would maximize and streamline safety
expertise.
In short, as Krbec describes it: A cleaner standard
equals less combustible dust incidents—and that is the ultimate goal.
This notion led to the infancy of NFPA 660: Combustible Dust Code (this
proposed title and document number are still evolving). Within the next
five years, the goal is to create a new, single standard resulting from
the consolidation of fundamental standards with all the industry- and
commodity-specific standards. It’s a process already underway, being led
by a special task group amongst the combustible dust committees. As
proposed, Chapters 1-9 would be fundamental to all industries; Chapter
10 dedicated to special fire protection requirements and 11-16 endemic
to industry-specific requirements (NFPA 664 specific to the wood
industry).
At this time, the new standard is slated to be issued effective sometime during 2024.
Identifying, Understanding Explosion Risk
When
it comes to explosion risk and mitigation methods for critical process
equipment in biomass facilities, knowing explosivity characteristics is
critical to protection design in engineered controls. Explosivity
parameters (dust cloud reactivity and concentration), ignition
characteristics and minimum safe oxygen concentration are all key
metrics to understand when planning protection efforts.
“There
are three major types of explosion prevention measures, according to Rob
Lade, chief technical officer for IEP Technologies. These are are
containment, venting and suppression.
Containment involves
isolating an entire facility to withstand an explosion. It’s effective
at limiting a chain incident, but it’s largely not applicable with a
large plant, according to Lade.
Venting consists of an engineered
panel that’s programmed to open at a predetermined pressure, allowing
the explosion to vent into a safe area. Venting does not extinguish
flames, so a secondary flame suppression system also needs to be in
place.
“It’s important to note that a vented explosion is a
rather energetic process,” Lade says. “In an explosion venting
occurrence, a fireball’s size on the exterior of the vessel is
approximately eight times the vessel volume—all of which must be vented
to a safe area. NFPA 68 details very descriptive standards for venting
sizing.”
Suppression utilizes an explosion detector connected to a
control panel, which releases an extinguishing agent into the vessel to
engulf the fire, mitigate the pressure and contain the explosion.
Deflagration isolation methods are also required for all prevention
measures, to minimize the spread of an event from one process vessel to
the next.
Types of applications typical to the biomass industry
include dust collection units, which are by far the most common
mitigation solution, followed closely by cyclone and cyclone separators.
Transfer points create an elevated opportunity for an incident
to occur, especially on conveyor systems, where internal and external
clutter can occur. Plus, it’s challenging to determine where the
ignition can take place. “In these situations, optical protection is
very nice because you can map pressures as a function of time,” Lade
adds. “With optical detection, you know where the flame position is when
it’s detected, and that gives much more information about the flame
location and the propagation points of that flame.”
Understanding Fire Hazards
Expansion
of the wood pellet and biomass energy sector has driven the need for
large-scale storage capacity, and therefore, the number of fire-related
incidents. “Between 2000 and 2018, 65 incidents have been reported,”
says Vahid Ebadat, CEO of Stonehouse Public Safety. “Of those, nine
occurred in 2017 alone, indicating that the frequency of these incidents
is on the rise. Of these incidents, 59% involved fire, and more than a
quarter of those incidents were confirmed to have been caused my
self-heating as the ignition source.”
One of the largest
incidents occurred in 2017, where a 500-ton pile of biomass fuels in
Thailand caught fire, due to accumulated heat. When a material is bulked
in mass volume, even a subtle ignition source can become a significant
problem. Most often, these sources are associated with the inherent
thermal instability of the material iself, overall bulk dimensions and
heating processes. Once self-heating begins, the potential exists for a
critical temperature to be reached, when the material smolders and
temperatures continue to rise. “Without air, combustion cannot take
place,” Ebadat says. “However, the bulk continues to heat, producing
gasses hazardous to inhalation. At this point, if air is introduced, the
trapped gasses are extremely susceptible to explosion.”
Operations
and processes prone to self-heating hazards include material drying and
heating, inadvertent sun heating, mechanical milling and grinding,
fugitive dust on a hot surface, and bulk storage. Because exothermic
reactions take time, it’s entirely possible for a hopper or bunker to
catch fire days, or even weeks, after being filled. Self-heating
measurements and identification within a given facility are often best
identified through heating a sample under controlled conditions to
determine the point at which temperature begins to increase, independent
of the external heat source. Once this metric is identified, mitigation
plans can be implemented.
“Keeping material temperatures at a
safe margin below the determined onset temperature for self-heating is
the best way to mitigate event risks,” Ebadat says. “Limiting storage
time, facility and equipment design that avoids ledges, corners and dead
zones, as well as limiting internal and external material deposits, are
all steps that can be taken to avoid the rise of material
temperatures.”
Like Ebadat, Timothy Heneks, director of
engineering services at Dustcon Solutions Inc., recognizes the uptick in
fire incidents within wood pellet manufacturing facilities. “According
to DustSafetyScience.com, 108 major fires and explosions were reported
within the wood industries between January 2019 and June 2020,” Heneks
says. “Of those events, 16 percent directly involved biomass wood
pellets.”
Surface layer fires involve material interaction with
an abundance of air, occluding on conveyors, coolers, baghouses, milling
areas and fugitive dust. On the other end of the spectrum, smoldering
fires react in areas of low oxygen, and generally occur within a pile,
such as in silos, storage heaps and domes. “Statistically speaking,
silos and other storage areas represent 36 percent of fires and
explosions taking place within wood pellet plants,” Heneks adds. “Dust
collection is second on that list, followed closely by mechanical
conveyors and then dryers.”
No Time like Now
While
the official deadline for conducting and documenting an official DHA
has come and gone, if not in compliance, the time to move forward is
now. And though NFPA 660 is still a handful of years away from
inception, it, too, will provide a clearer path forward in the
mitigation of fire and combustible dust, specifically as it pertains to
the wood pellet industry.
Until then—and beyond—the responsibility must be taken up by each manufacturing facility to address this ongoing cycle head on.
Author: Luke LeRoy
Pellet Mill Magazine Freelance Writer
www.biomassmagazine.com/pellet-mill-magazine
Printed in Issue 2, 2021 of Pellet Mill Magazine
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