Understanding the reasons for these
requirements will provide the user with
much greater capabilities to deal with
hazardous (classified) locations and any
abnormalities or issues that are not specifically covered in these locations.
Section 500.5 of the NEC provides
the classification of hazardous locations
based on the properties of the flammable
gases, flammable liquid-produced vapors,
combustible dusts and fibers/flyings that
may be present and the likelihood that a
flammable or combustible concentration
or quantity is present. Let’s concentrate
on flammable and combustible liquids
and gases. I’ll cover other combustible
material in a future article.
The chemical properties, chemical
concentrations and air circulation surrounding the chemical are extremely
important in determining the classification or area. If the chemical is a gas,
vapor or liquid that has reached its flash
point, determining the amount of air
circulation necessary to drop the concentration of the gas or vapor becomes
critical to rendering the area as unclassified or reclassifying the area.
When the amount of gas or vapor,
where mixed with air (normal concentra-
tion of oxygen in air is about 21 percent
with any concentration in excess of 22
percent considered to be highly oxygen-
ated) reaches the lower flammable limit
(LFL), the mixture of gas or vapor and
air becomes an ignitable concentration.
The ignitable concentration may be a
percentage value of low LFL to an upper
value of concentration called the upper
flammable limit (UFL). An example of
the LFL and UFL for hydrogen would
be a level of 4 to 75 percent. A level of 4
percent hydrogen with 96 percent air is
an ignitable concentration, and anything
over 75 percent is considered to be too
rich for ignition.
Another determining factor in hazardous area classification is whether the
gas or vapor molecular weight is heavier
than air, the same weight as air, or lighter
than air, where air is considered to be a
vapor density of 1. If the gas or vapor is
heavier than air (e.g., gasoline at a vapor
density of 3), the gas or vapor will fall
to grade level because it is three times
heavier than air. If the vapor density is
the same as air (vapor density of 1), the
gas or vapor will suspend close to its
release point and neither fall nor rise.
If the vapor density is less than 1 (e.g.,
hydrogen at a vapor density of 0.1), the
gas or vapor will rise above grade level.
The location of the concentration
Air circulation for hazardous locations
of gas or vapor is critical because dis-
persal or removal of the gas or vapor
at the point of concentration would be
best closer to ground or grade level for
heavier-than-air materials and at the
ceiling for lighter-than-air materials. A
level of 25 percent of the LFL of most
gases or vapors will ensure that an ignit-
able concentration does not exist.
An example of this 25-percent con-
centration level would be hydrogen
with an LFL of 4 percent concentration.
Reduced to a 1 percent concentration,
the hydrogen is not ignitable. Adding
heat to a gas or vapor or cooling the gas
or vapor can affect the molecular weight
of the material. A lighter-than-air gas,
such as hydrogen, can be super-cooled
to the extent that it changes from a gas
to a liquid and only reverts back to a gas
when it returns to normal temperature
or the flash point of the liquid hydrogen.
This extremely cold level for combustible
material is called “cryogenic material.”
An area’s ventilation is important in
determining its classification. A flat floor
can be ventilated with four air changes
per hour or 1 cubic foot per minute per
square foot of area to ensure the combus-
tible gas or vapor is adequately dispersed
or removed. Where the floor area has
pits, below-grade work areas, or sub-
floors, the ventilation should be within
12 inches of the pit floor, below grade and
subfloor work area.
Next month’s column will cover the
method of determining the airflow nec-
essary based on the molecular weight of
a chemical or a mixture of chemicals.
FOR MANY YEARS, the mystery of area classification has resided with a select group of
experts within the electrical, petrochemical and industrial sectors. Many electricians,
electrical engineers, fire inspectors and electrical inspectors have remained outside
of this group. Instead, designers and installers rely on National Electrical Code (NEC)
articles 500 through 517 for installation requirements regarding hazardous (
classified) locations. Rightfully so, but often, these industry personnel do not understand
the original basis for these requirements.
OD E is a lead engineering associate for Energy & Power Technologies at Underwriters
Laboratories Inc. and can be reached at 919.949.2576 and Mark. C. Ode@ul.com. IST