Chemical Engineering July 2021 - 32

BS&B Systems
FIGURE 2. Explosion suppression equipment detects
an explosion in the first few milliseconds of
the event, then releases a flame-quenching substance
In
cases where a flame ball must
be avoided, flameless venting can be
deployed. Flameless vents incorporate
an enclosure covering the nonprocess
side of the explosion panel.
This enclosure provides a path to
atmosphere for the expanding hot
gases through a stainless-steel
mesh which absorbs the heat and
prevents the transmission of flame.
This style of vent adds considerable
weight, due to the size and construction
of the enclosure. External
supports may therefore be required.
When evaluating the cost of this solution,
explosion isolation needs to
be considered. This isolation is normally
incorporated into the design of
explosion-suppression systems. This
should be considered when comparing
flameless venting solutions to explosion
suppression.
Suppression equipment
For processes where an explosion
would ideally be prevented altogether,
suppression systems are
the optimal alternative. Explosion
suppression equipment detects a
dust explosion in the first milliseconds
of the event, signaling explosion
suppressors to rapidly release
a flame-quenching medium, such
as sodium bicarbonate, into the
process equipment (Figure 2). This
effectively stops the explosion in its
infancy and results in a reduced explosion
pressure that is safe for the
protected equipment.
For a process running 24 hours a
32
day, 7 days a week, a suppression
system can be desirable because
the speed of cleanup and refit allows
for a quick return to production.
In contrast, with venting or
flameless venting, the explosion
fully develops in the process equipment,
requiring cleanup, attending
to fire-related damages and other
consequences that take time before
the process can be brought back
into operation. A typical suppression
system consists of sensors and
several explosion suppressors that
propel an extinguishing agent into
the process equipment. Pressurized
nitrogen is typically used to provide
the motive power.
Explosion isolation
In the event of a deflagration, there
is a potential for the flame front to
propagate via interconnections between
equipment volumes, triggering
secondary explosions of increasing
severity. For this reason, where a
dust explosion hazard exists, NFPA
652 calls for isolation devices in accordance
with NFPA 69. Isolation
methods accepted by NFPA 69 include
the following:
* Chemical barriers
* Flap valves
* Float valves
* Pinch valves
* Slide-gate valves
* Material chokes (rotary valves)
Broadly, explosion isolation can
be categorized as passive or active,
per NFPA 69. A common example
of passive isolation is a flap valve,
which is most commonly installed
horizontally as a one-way valve on
the inlet duct to a dust collector. The
flap is open during normal operation,
latching closed against a seat
in response to the cessation of air
flow and a pressure wave traveling
in the opposite direction. Recent
advances in flap-valve design enable
some models to be installed
vertically and in ducts where the
direction of airflow is in the same
direction as the potential fireball.
This enables them to be used, for
example, on the exhaust duct of a
dust collector.
An example of an active isolation
method is chemical isolation,
which typically consists of an explosion
pressure detector that triggers
a chemical suppressor. Chemical
isolation is not limited to duct orientation
or air flow direction. Furthermore,
chemical isolation can be
used on rectangular ducts and casings
with moving internals such as
drag conveyors. Chemical isolation
does not restrict the pipe in any way,
eliminating concerns about pressure
losses.
NFPA dust standards
Plant owner/operators are required
to document the risk of dust explosions
at their faciltities in the form of
a Dust Hazard Analysis (DHA), the
scope of which is detailed in NFPA
652 (Standard on Fundamentals of
Combustible Dusts). The deadline
for implementing this DHA was September
7, 2020. The DHA is applicable
to existing plants, processes and
new projects. In NFPA 61, for the
food and grain industry, the deadline
has been extended to January
1, 2022 for existing plant and processes.
The main components of a
DHA are the following:
* Identifying where in the process
or facility the potential exists for fires
and explosions. This involves determining
material combustion characteristics,
identifying potential ignition
sources, identifying external dust
emissions, and noting the potential
for explosion propagation between
interconnected equipment volumes
and building compartments
* Identifying safe operating ranges
* Identifying existing protection strategies
and equipment
* Providing a plan for implementation
of any additional protection equipment
and strategies needed to manage
the risk
A range of safety measures are
available to process-plant owner/operators
in meeting the OSHA Combustible
Dust National Emphasis
Program requirements. These are
documented within the NFPA standards
61, 68, 69, 652, 654, 655 and
664. Compliance with these standards
ensures that your process
plant upholds a level of safety is acceptable
to employees, the general
public, the allied industry, state and
federal fire marshals, insurance companies,
and OSHA.
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