Chemical Engineering March 2015 - 83

4-in N2
Nitrogen supply system
(various options)
10 ft3
surge hopper
O2
sensor
Air operated
discharge valve
Air operated
vent valve
20 ft3
Air compressor
Surge tank
10 ft3
transporter
3-in Conveyline
To milling process
Figure 2. The pneumatic conveying system depicted here is equipped with traditional mechanical protection
systems for combustible dust, including explosion-detection systems and blowout panels
nitrogen, such as that generated by
a membrane or pressure swing adsorption
(psa) system - total nitrogen
operating costs can be reduced.
npFa 69 and 654 provide guidance
on determining the moc for a combustible
dust and the required nitrogen
purity for safe inerting.
Safety. asphyxiation risks are managed
with basic safety training and
simple engineering controls, such as
o2 monitoring and o2-concentration
control, as well as restricting inerting
to appropriate areas.
Practicality. maintenance on the nitrogen
system is typically handled by
the nitrogen provider and not by the
end user. nitrogen systems are in fact
not all that maintenance-intensive,
and unlike explosion detectors they
do not require quarterly inspections.
potential dust-related applications
of nitrogen inerting include closedloop
pneumatic conveying, hopper
storage, shredders, reactors, tanks,
vessels and enclosures. through
smart design, inerting systems can
eliminate the possibility of an explosion,
increase worker safety, and
provide a more cost-effective solution
relative to other mechanical-protection
systems.
Example 1. Plastic powder
plastic powder coatings require a
very fine powder to ensure a smooth
coating when applied. the powders
are created by pneumatically conveying
plastic pellets into a mill for
fine grinding. particle sizes less than
100 µm (140 mesh) are frequently required
for coating production.
polyethylene (pe) is the most common
plastic used in powder coating
manufacturing, but polyvinylchloride,
66
polyester and polyurethane compounds
are also common. at particle
sizes less than 100 µm (140 mesh),
these plastics are highly combustible.
in fact, pe powder is among the
most common fuels for combustibledust
explosions because of its broad
usage and its combustibility.
For example, fugitive pe dust
caused an explosion at the West
pharmaceutical plant in Kinston,
n.c., in 2003. according to the
proceedings of the 5th international
seminar on Fire and explosion hazards,
the pe was used in a slurry to
coat rubber parts, and fugitive pe
dust from dried slurries accumulated
in the suspended ceilings and
cooling air ducts. a spark from a
mill motor caused the dust to ignite,
sending a flame propagating through
the ducting. the ducting then overpressurized,
sending the explosion
front and burned dust into the
suspended ceiling, which caused a
larger explosion from the dust that
had settled in the ceiling.
N2
Nitrogen supply system
(various options)
Dust filter
10 ft3
From process
6-in Rotary airlock
10-hp blower
Air operated
discharge valve
Convey line
To milling process
Figure 3. The pneumatic conveying system depicted here is protected by nitrogen inerting
ChemiCal engineering www.Chemengonline.Com marCh 2015
10 ft3
hopper
hopper
Air operated
discharge valve
hopper
Dust filter
recovery line
this case is particularly noteworthy
because there was no previous accumulation
of dust in the facility, due
to top-class housekeeping practices.
the facility had recently upgraded its
dust-collection system and believed
that the dust in the ceilings was inert
based on previous laboratory testing.
indeed, investigative testing of the
dust in the dust collectors indicated
a largely inert dust. however, based
on investigations following the explosion,
only 280 lb of pe was required
to produce the magnitude of the explosion
felt during the incident. For a
10,000-ft2 ceiling, that amounts to a
layer of pe dust with a thickness of
only 0.013 in. - about the thickness
of four sheets of copy paper [4].
the explosion and subsequent fire
resulted in six fatalities and multiple
injuries. in the case of West pharmaceutical,
while it would be difficult to
inert the ceiling ducting, it is quite
possible that the mill itself could
have been inerted, preventing the
initial reaction. either way, the incident
highlights the significant danger
of pe powder.
Economic analysis
one of the main misconceptions
about nitrogen inerting is that it is
much more expensive than traditional
mechanical explosion-protection
systems. in reality, on a total
net-present-value (npV) basis, nitrogen
inerting systems are very comparable
to reactionary mechanical
systems and, in the case of even a
small incident requiring cleanup with
a mechanical system, the nitrogen
inerting system is often more economical.
the following example illustrates
how the economics could
recovery line

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Chemical Engineering March 2015

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