Chemical Engineering July 2015 - 51

Table 1. Defining areas for Divisions
Class
Class I
Type of Material
Locations containing flammable
gases, flammable liquid-produced vapors,
or combustible liquid-produced
vapors
Class II
Class III
Locations containing combustible
dusts
Locations containing fibers and
flyings
ature) that is capable of igniting the
explosive atmosphere present
Determining hazardous areas in
a plant is normally performed by
experts from various disciplines. It
may be necessary for chemists, process
technologists, and mechanical
engineers to cooperate with an
explosion-protection expert in order
to evaluate all hazards. The possible
presence of a potentially explosive
atmosphere as well as its properties
and the duration of its occurrence
must be established.
Also understanding terms such as
minimum ignition energy (MIE), upper
and lower explosive limit (UEL/LEL),
flash point, and ignition temperature
in the evaluation of your hazardous
area will also provide a clearer direction
on how severe a hazardous area
might be.
In any situation involving an explosive
material, the risk of ignition
must be taken into account. In addition
to the nominal rating of materials
under consideration, parameters
related to the process involved are
especially important in the evaluation.
For example, the risk of explosion
may be caused by the evaporation
of a liquid or by the presence of
liquid sprayed under high pressure.
It is also important to know which
atmospheric conditions are present
normally and abnormally. The range
of concentration between the explosion
limits generally increases as
the pressure and temperature of the
mixture increases.
Divisions and zones
Once it has been determined that a
hazardous area exists, it now needs
to be classified. While the physical
principles of explosion protection
are the same worldwide and are
not differentiated, there are two different
and distinct models to define
your hazardous area -divisions and
zones - both of which are accepted
Class II
Class
Class I
Subgroup
Group A
Group B
Group C
Group D
Group E
Table 2. The breakDown of Classes inTo subgroups
Atmospheres
Atmospheres containing acetylene
Atmospheres containing hydrogen and flammable process gases with
more than 30 vol.% H2, or gases or vapors posing a similar risk level,
such as butadiene and ethylene oxide
Atmospheres such as ether, ethylene or gases or vapors posing a similar
risk
Atmospheres such as acetone, ammonia, benzene, butane, cyclopropane,
ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, propane
or gases or vapors posing a similar threat
Atmospheres containing combustible metal dusts, including aluminum,
magnesium, and their commercial alloys, or other combustible dusts
whose particle size, abrasiveness and conductivity present similar hazards
in the use of electronic equipment
Group F
Atmospheres containing combustible carbonaceous dusts, including
carbon black, charcoal, coal or coke dusts that have more than 8% total
entrapped volatiles, or dusts that have been sensitized by other materials
so that they present an explosion hazard
Group G
Atmospheres containing combustible dusts not included in Group E or
Group F, including flour, grain, wood, plastic and chemicals
Division
Class I
(gases and vapors)
In accordance with NEC
500.5 and CEC J18-004
Division 1
Areas containing dangerous
concentrations of flammable
gases, vapors or mist
continuously or occasionally
under normal operating
conditions
Division 2
Areas probably not containing
dangerous concentrations
of flammable gases,
vapors or mist under normal
operating conditions
Table 3. The Division MeThoD
Class II
(flammable dust or powder)
In accordance with NEC
500.6 and CEC 18-008
Areas containing dangerous
concentrations of flammable
dusts continuously or occasionally
under normal
operating conditions
Areas probably not containing
dangerous concentrations
of flammable dusts
under normal operating
conditions
and utilized worldwide.
In rather simple terms, we can differentiate
between the International
Electrotechnical Commission (IEC;
Geneva, Switzerland) (zones) and
the North American (division) procedures.
The differences lie in the categorization
of hazardous areas, the
design of apparatus, and the installation
technology of electrical systems.
The categorization of these areas is
carried out in North America in accordance
with the National Electrical
Code (NEC) NFPA 70, article 500.
The European Zone practice is described
in IEC/EN 60079-10.
So how does each work? First let's
start at the basics, and then we'll
cover each individually.
Defining the area
Hazardous location or area classification
methods specify the danger
of fire or explosion hazards that
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Class III
(flammable fibers or suspended
particles)
In accordance with NEC
500.5 and CEC 18-010
Areas containing dangerous
concentrations of flammable
fibers or suspended particles
continuously or occasionally
under normal operating
conditions
Areas probably not containing
dangerous concentrations
of flammable fibers or
suspended particles under
normal operating conditions
may exist due to flammable gases,
vapors, or liquids within a plant or
working environment. These are explained
by defining the type of hazardous
material present, severity of
the hazard, and probability of the
hazard. It may also depend on the
likelihood of the hazard, risk of an
explosion, and the boundaries of the
hazardous location.
This is usually determined by a
HAZOP (hazard and operability)
study and documented on a set
of electrical plot plans on record in
every plant.
For divisions, the type of material
is given by a class designation, as
shown in Table 1. These can be broken
down further into sub-groups,
as shown in Table 2.
Once we have determined the hazardous
material we are working with,
the probability of an explosion and
boundaries must also be taken in to
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