ProSoft's “Phased Migration” Solutions - Control Design

DESIGN
OF ALL THE REASONS TO CONSIDER INTRINSIC SAFETY (IS) FOR YOUR OPERATIONS,
THE FIRST ONE IS THAT THESE INSTALLATIONS REDUCE THE OVERALL RISK
INTRINSIC SAFETY (IS) MAKES IT POSSIBLE TO PERFORM LIVE
maintenance at any point in the control loop because, by its nature,
IS always keeps the amount of available energy on the wire pair below
the ignition point for the gases/environment in which it’s installed.
There are two aspects to determining the hazardous area requirements
for an installation: area classification (the type of gas present and
the likelihood of it being present) and the temperature classification
(maximum surface temperature of the device or apparatus).
Figure 1 shows how area classifications are determined for North
America using the class-and-division principle. The division is based
on the likelihood of a specific type of gas being present at any point
in time. As a rule of thumb, Division 2 assumes the potentially
explosive gas is present one hour/year, and Division 0 assumes the
gas is always present.
The second aspect of hazardous area control is the temperature
rating. Figure 2 shows how the type of gas present determines the
required “T-rating.”
Fortunately, the majority of the hydrocarbon industry needs to
meet only the T1 or T2 temperature limitations for the majority of
its facilities, and this is often why the T-rating is overlooked when
specifying and purchasing instruments. When we think about
temperatures, we tend to be more concerned with the ambient
temperature range in which the device can continue to operate.
Intrinsic safety is entity-based, meaning all the components need
to be considered as a single entity. The devices in the loop also
need to be treated as “simple apparatus” as defined in ISA–60079-
11 (12.02.01)–2009 “Explosive Atmospheres—Part 11: Equipment
Protection by Intrinsic Safety,” which is summarized below:
Simple apparatus are defined as those devices in the following
three categories:
1. Passive components, including items such as switches, junction
boxes, resistors and simple semiconductor devices that neither
store nor generate energy. Sensors that use catalytic reaction
or other electrochemical mechanisms are not normally simple
apparatus.
2. Stored energy sources consisting of single components in simple
circuits with well-defined parameters; for example, capacitors or
The IS Alternative
OF EXPLOSION THROUGH HUMAN ERROR
BY IAN VERHAPPEN
inductors, whose energy storing values should be considered when
determining the overall safety of the system.
3. Generated energy sources; that is, thermocouples and photocells
that do not generate more than 1.5 V, 100 mA and 25 mW.
In addition to the above, the following (taken from the ISA
standard) also applies to simple apparatus installations:
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and/or current-limiting and/or suppression devices.
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available voltage or current, for example dc-dc converters.
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temperature-classified.
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other electrical circuits, the whole shall be assessed according to the
requirements of ISA–60079-11 (12.02.01)–2009.
Surface of
tank contents
10ft
Below grade trench
Derived from API Recommended Practice 500A
5ft radius
around vent
HOW AREA CLASSIFICATIONS ARE DETERMINED
Figure 1: The division is based on the likelihood of a specific
type of gas being present at any point in time.
2012 � Q3 � INDUSTRIAL NETWORKING
Division 1
Division 2
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