Chemical Engineering February 2021 - 41

vent a fire or explosion. Rather they
say that if you follow their guidelines
then, historically, you are usually safe
from fires and explosions. They are
recommending one way to stay safe,
not the only way. If you can provide
adequate mitigative measures to
prevent one or more of the above
three conditions from being present,
then area electrical classification is
not the only possible approach.
NFPA 497 is clear in 5.2 saying
" The decision to classify an
area as hazardous is based on the
possibility that an ignitible mixture
could occur. " API 500 in section 6.1
echoes this guidance. " The decision
to classify a location is based on the
probability that flammable gases or
vapors may be present. ... It is noted
that the occurrence of flammable
material liberation from some of the
above apparatus is so infrequent
and at such a small rate, that it is not
necessary to consider it as a source
or to classify adequately ventilated
nonenclosed areas containing such
apparatus. " If you provide mitigative
measures to reduce the possibility
that an ignitable mixture could occur
to less than a credible possibility,
then the area does not need to be
electrically classified.
Realistic mitigation measures
The key to this is what mitigative
measures can realistically be applied.
Limit the amounts. The first approach
might be to limit the amount
of flammable and combustible materials
to a de minimis amount. While
44 mL of flammable liquid is enough
to catch fire, the realistic amount of
damage one shot glass of liquid can
produce has not suggested that we
electrically classify bars. Yet a bar
involves open transfers, open containers,
frangible containers, and a
very high probability for spills and
human error. Despite this, we know
- from experience - that bars do
not catch fire due to flammable and
combustible liquid handling with any
degree of frequency. While no code
addresses how much flammable or
combustible material is enough to
be of concern, consider that pint,
quart, and even gallon containers
of flammable or combustible materials
are sold, handled, and used
in homes, businesses and stores
with incredible frequency with relatively
low consequences. Hence, I
suggest that quantities of 1 gallon
(~4 L) or less are probably small
enough to be considered de minimis.
Note, however, that when one
talks about the quantity of flammable
or combustible material present,
one must include all the flammable
or combustible material in the given
area; not just an individual container.
So, while I would not worry about a
single 1-gal container of hexane in
a laboratory, I would be more concerned
if it were one of a dozen.
This limiting approach may be feasible
for some research applications
but, in most cases - particularly
those handling flammable gases or
utilities - it may not be very practical
nor useful.
Ventilation. A second approach
might be to provide so much ventilation
that the flammable or combustible
material is captured at the source
and the extent of any ignitable area
is negligible (Figure 2). Note that I
carefully avoided the term " adequate
ventilation " , which is used extensively
in all the codes.
Adequate ventilation is defined in
NFPA 30 section 17.1.1 as " a rate
sufficient to maintain the concentration
of vapors within the area at or
below 25 percent of the lower flammable
limit (LFL). " NFPA 497 in 3.3.1
defines it as:
A ventilation rate that affords six air
changes per hour, 1 cfm per square
foot of floor area (0.3 m3/min/m2),
or other similar criterion that prevents
the accumulation of significant
quantities of vapor-air concentrations
from exceeding 25 percent of
the lower flammable limit (LFL).
API 500 in section 3.1.38 uses
similar language:
Ventilation (natural or artificial) that
is sufficient to prevent the accumulation
of vapor-air or gas-air mixtures
in concentrations above 25% of their
lower flammable (explosive) limit,
LFL (LEL).
API 500 expands upon this definition
in section 6.3.2 where it gives
practical guidance in this area. However,
API 500 limits the use of ventilation,
allowing ventilation to lower
the electrical classification of a location
from Division 1 to Division 2 but
not be used to lower the electrical
classification of a location from classified
to unclassified.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM FEBRUARY 2021
FIGURE 2. Ventilation is an important mitigation
measure. This pilot plant is skid mounted, and can
be easily moved into a hood
Some organizations feel this guidance
prohibits ventilation as a mitigative
measure. I disagree. API 500
tacitly assumes that there is always a
credible possibility of the presence of
a flammable or combustible material
in ignitable quantities in most areas
of a petrochemical facility, a not
unreasonable
assumption.
Hence
using ventilation as a mitigative measure
is not realistic since, in most
cases, the adjacent areas could still
be a source of flammable or combustible
material. Ventilating an area
in this case would simply keep drawing
in other ignitable mixtures rather
than capturing a local release so designing
a failsafe ventilation system
in these circumstances is probably
not feasible.
However, most research applications
- not all but most - usually
are small enough in size and area
and limited enough in the quantity of
flammable or combustible materials
that providing ventilation designed
to capture any releases at the local
source and keeping the ignitable
area to a negligible extent is feasible.
Hence, API 500 does not prohibit
using ventilation as a mitigative measure
in many research applications.
It, for example, specifically excludes
laboratories in 6.3.1.2.3 due to the
nature of their operations and their
high ventilation rates.
NFPA 497 is clearer on this point,
noting in 5.5.1 that:
Experience has shown that the
release of ignitible mixtures from
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