POWER March 2012 - 32

PLANT DESIGN
64 different industries; electric power generating
utilities experienced 28 incidents.
The U.S. Chemical Safety and Hazard Investigation
Board (CSB) investigates and
documents the causes of these industrial
dust explosions. Each of the CSB dust explosion
reports suggests a common cause:
Companies and their employees failed to
recognize the implicit danger of airborne
and accumulated dust.
The explosion at the Imperial Sugar plant
prompted the U.S. House of Representatives
to pass a bill requiring OSHA to adopt the
National Fire Protection Association (NFPA)
recommendations to prevent explosions
caused by combustible dust (see " Five Requirements
for a Dust Explosion " ), although
it never became law.
Today, OSHA's instruction No. CPL
03-00-008 is the guiding directive for controlling
dust in manufacturing facilities.
CPL 03-00-008 and NFPA 654 define the
conditions under which plants must immediately
remove dust accumulations that
are 1/32 inch thick. OSHA standard No.
29 CFR 1910.269(v)(11)(xii) requires the
elimination or control of ignition sources
when coal-handling operations may produce
a combustible atmosphere. NFPA
654, which includes a comprehensive list
of dust control, ignition sources, and damage
control provisions, is also an invaluable
reference.
A good understanding of what these instructions
require (and following through on
upgrades, as required) will make your next
OSHA inspection much less stressful. The
sidebar " Prepare for Inspections, Not Citations "
describes what happens during a typical
OSHA visit.
Prepare for Inspections, Not Citations
The U.S. Occupational Safety and Health
Administration's (OSHA's) area offices are
required to identify and inspect facilities
where employees may be exposed to
potential combustible dust hazards and
respond if a formal complaint or referral
is received. Facilities are selected using a
random numeric system or are chosen via
a Site-Specific Targeting Plan. OSHA's inspector
may elect to do one or more of the
following during a visit:
■ Conduct a preliminary walk around the
facility to observe areas of the plant for
dust accumulations. Areas to be viewed
include floors, structural members, conduit
and pipe racks, above the ceiling,
and on or around equipment.
■ Conduct an opening conference.
■ Take photographs or videos.
■ Collect samples of settled dust from
areas such as upper plant levels, roof
beams, tops of pipes and ductwork,
interior of dust collectors, and within
ducts.
■ Gather information about efforts to
abate the combustible dust hazard,
which is a factor in determining the employer's
good faith and penalty factors.
The OSHA inspector may issue citations
for housekeeping, general duty, deflagration/explosion
hazards, personal protective
equipment, electrical, warning sign,
egress, and fire protection violations. The
32
collection of air samples is not necessary
for a citation to be issued.
OSHA has initiated site visits to obtain
facility-specific information on combustible
dust recognition, prevention, and
protection programs, and to relay notable
findings and other facility feedback to
OSHA. In one such reported instance, plant
representatives expressed concern about
the amount of resources needed to comply
with OSHA dust standards and commented
that this would be particularly difficult for
older plants. They also expressed a concern
that new standards should not conflict
with environmental regulations and
that a new dust control system would be
required to be within permitted limits.
Ameren has experienced two OSHA
combustible dust inspections. In both
instances, the inspectors were primarily
interested in the silo fill areas-the
tripper area directly above the coal silos/bunkers.
The inspectors were satisfied
with the dust control measures
being used by Ameren. The company has
added a variety of dust control measures
to its plants in a continuing, multi-plant
improvement program. This spreads the
cost over several years, and the staff
gain valuable experience and confidence
with the equipment during that
time. Dust reduction results can also be
compared and verified over time so that
the most effective and economical technologies
are deployed, as needed.
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The Challenge with Handling Coal
Coal-fired power plants typically store coal
outdoors and normally avoid using enclosed
bucket elevators, making the design of a coalhandling
system unique and much different
than, for example, a grain-handling system at
an export terminal.
Most coal-fired plants have crushers, and
those in northern climates will often feature
enclosed transfer houses and conveyor galleries.
Day silos store one or more shifts' supply
of coal within the power block for operating
reliability. Consequently, devastating fires
and explosions such as those that occurred at
Imperial Sugar and at grain export terminals,
particularly during the 1970s, are an everpresent
threat at coal-fired plants that ignore
basic safety and housekeeping precautions.
At coal-fired plants, high-capacity conveying
systems handle thousands of tons of coal
per hour. When even a small fraction of this
tonnage is released and becomes airborne, an
unacceptable danger is present. The danger
escalates because the conveyed product is also
flammable and may reach explosive concentrations.
Airborne dust eventually settles on a
variety of surfaces and over time; thick layers
accumulate in less-visible or inaccessible areas.
These accumulations fuel the most devastating
events when a small initial explosion
shakes dust-laden equipment, piping, conduit,
ducts, and structures, thereby propelling
the dormant fuel into a dense, flammable
cloud that feeds a rapidly expanding fireball.
These secondary explosions typically cause
the majority of the damage in a plant.
Consider the bulldozer in Figure 2 that is
slowly pushing a blade of coal into a mostly
empty, below-grade reclaim hopper. As the
coal slides from the left into the hopper, it
displaces air within the hopper. As the photo
shows, the slow-flowing dry coal creates a
cloud of dust. The exiting displaced air floats
fine coal particles into a coal dust cloud that
drifts upward at the right side of the hopper.
Clearly, even relatively slow motions can create
a cloud of dust. Now consider the amount
of coal dust generated when a fast-moving
2. Quick and easy. Dust forms easily
when coal is slowly sliced by a bulldozer.
Courtesy: Energy Associates PC
POWER | March 2012
http://www.powermag.com
POWER March 2012

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