Chemical Engineering July 2015 - 54

duit system. Therefore, the resulting
explosion pressures must be taken
into consideration. The conduit connections
must be constructed according
to specification and sealed
(that is, lead seals) with appropriate
casting compound. The housing is
not constructed gas-tight. Of course,
large openings are not permitted on
the enclosure, but small ones are inevitable
at any junction point. Some
of these gaps may serve as pressure
relief points. Escaping hot gases are
cooled to the extent that they cannot
ignite the potentially explosive atmosphere
outside the housing. Ignition
is prevented if the minimum temperature
and minimum ignition energy
of the surrounding potentially explosive
atmosphere is not reached. For
this reason, the maximum opening
allowed for a particular type of joint
depends on the nature of the explosive
mixture and width of the adjoining
surfaces (joint length).
The classification of a flameproof enclosure
is based on the gas group and
the maximum surface temperature
which must be lower than the ignition
temperature of the gas present.
Purging or pressurization. Purging
or pressurization is a protection
method based on the segregation
concept. This method does not allow
the dangerous air/gas mixture to penetrate
the enclosure containing electrical
parts that can generate sparks or
dangerous temperatures. A protective
gas - air or inert gas - is contained
inside the enclosure with a pressure
slightly greater than the one of the external
atmosphere (Figure 4).
The internal overpressure remains
constant with or without a continuous
flow of the protective gas. The
enclosure must have a certain degree
of tightness; however, there
are no particular mechanical requirements
because the pressure supported
is not very high.
To avoid pressure loss, the protective
gas supply must be able to
compensate during operation for
enclosure leakage and access by
personnel where allowed (the use of
two interlocked doors is the classical
solution). Because it is possible for
the explosive atmosphere to remain
inside the enclosure after the pressurization
system has been turned
off, it is necessary to expel the remaining
gas by circulating a certain
quantity of protective gas before re54
starting
the electrical equipment.
The classification of the electrical
apparatus must be based on the
maximum external surface temperature
of the enclosure, or the maximum
surface temperature of the internal
circuits that are protected with
another protection method and that
remain powered even when the protective
gas supply is interrupted.
The purging or pressurization
technique is not dependent upon the
classification of the gas. Rather, the
enclosure is maintained at a pressure
higher than the dangerous external
atmosphere, preventing the flammable
mixture from coming in contact
with the electrical components and
hot surfaces inside.
In the U.S., the term " pressurization "
is limited to Class II applications.
This is the technique of supplying
an enclosure with clean air or an
inert gas, with or without continuous
flow, at sufficient pressure to prevent
the entrance of combustible dusts.
Internationally, the term " pressurization "
refers to a purging technique
for Zones 1 and 2.
The divisional model of the purging
protection method is based on the
reduction of the classification inside
the enclosure to a lower level. The
following three types of protection (X,
Y, and Z) are identified in relation to
the hazardous-location classification
and the nature of the apparatus.
* Type X: reduces the inside of the
enclosure from Division 1 to a nonhazardous
state that requires an
automatic shutdown of the system
in case of pressure loss
* Type Y: reduces the inside of the
enclosure from Division 1 to Division
2
* Type Z: reduces the inside of the
enclosure from Division 1 to a nonhazardous
state, requiring alarm
signals only
Intrinsic safety. Finally, intrinsic
safety is based on the principle of
preventing an effective source of ignition.
The electrical energy is kept
below the minimum ignition energy
required for each hazardous area
(Figure 5).
The intrinsic safety level of an electrical
circuit is achieved by limiting
current, voltage, power and temperature;
therefore, intrinsic safety is
limited to circuits that have relatively
low levels of power. Of critical importance
are the stored amounts of energy
in circuits in the form of capacitance
and inductance. These energy
storage elements must be limited
based on the voltage and current
levels present in a particular circuit
or make-break component.
In normal operation and in the
event of a fault, no sparks or thermal
effects may occur that could lead to
the ignition of a potentially explosive
atmosphere. Intrinsically safe circuits
may therefore be connected and
disconnected by experts during operation
(even when live), as they are
guaranteed to be safe in the event of
a short circuit or disconnection.
Intrinsic safety is the only ignitionprotection
class that allows connectors
to be opened and intrinsically
safe apparatus to be removed and
replaced by an equivalent device in
a hazardous area. Because of the
level of freedom this brings, intrinsic
safety has become one of the most
important methods of protection in
the industrial automation industry.
Final remarks
Each method offers its own advantages
and disadvantages, and in most
cases no one method will be or can
be the only method used in a process
plant. Generally, this mixed system
does not present installation difficulty
if each of the protection methods is
appropriately used and is in compliance
with the respective standards.
No matter how you classify your
plant or which method of protection
you chose, it is always important
to remember that the method you
choose today may not necessarily
be the appropriate choice tomorrow.
Evaluate, choose and protect not
only to keep your plant safe, but to
keep your personnel safer.
n
Edited by Gerald Ondrey
Author
Robert Schosker is the product
manager/team lead for intrinsic
safety (IS), remote I/O, HART, signal
conditioners, power supplies
and surge protection at
Pepperl+Fuchs Inc. (1600 Enterprise
Parkway, Twinsburg, OH
44087; Phone: 330-425-3555;
Fax: 330-425-4607; email:
rschosker@us.pepperl-fuchs.
com). Since joining the company in 1995, Schosker has
been focused on technology and product-related support,
and is involved in a wide range of activities and
roles including certifications, sales, and marketing. He
has been the key lead in many IS and HART projects
resulting in the development of new products for intrinsic
safety and HART infrastructure. Schosker holds a
B.S.E.E. from the University of Akron.
ChemiCal engineering www.Chemengonline.Com july 2015
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Chemical Engineering July 2015

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