1 Scope ……………………………………………………………………………………………………………………2 References …………………………………………………………………………………………………………….3 Definitions ………………………………………………………………………………………………………………4 General ………………………………………………………………………………………………………………..5 Instruments Connected to the Process ………………………………………………………………………6 Safe Practices ………………………………………………………………………………………………………..7 Surface Hot-spot Limit ……………………………………………………………………………………………..8 Appendix ……………………………………………………………………………………………………………….Figure 1 – Pressure Switch Installation, Single Process Barrier, Nonincendive, Double Seal and Vent …………………………………………………………………………………………………………………………….Figure 2 – Thermocouple Installation, Two Process Barriers, Nonincendive, Double Seal and
Vent …………………………………………………………………………………………………………………………….Figure 3 – Single Process Barriers …………………………………………………………………………………..Figure 4 – Two or More Process Barriers ………………………………………………………………………….Figure 5 – Purge Requirements For General Purpose Instrument Case In Class 1, Division 1,
Process Piped To Enclosure…………………………………………………………………………………………..Figure 6 – Purge Requirements For General Purpose Instrument Case In Class 1, Division 2,
Process Piped To Enclosure…………………………………………………………………………………………..Figure 7 – Purge Requirements For General Instrument Case In Non Hazardous Area, Process Piped To Enclosure ……………………………………………………………………………………………………….Table I – Hazardous Area Classifications and Electrical Circuit Type …………………………………..Table II – Safe Practices …………………………………………………………………………………………………Table III – Purge Requirements for Enclosures Subject to Internal Release of Hazardous
Materials………………………………………………………………………………………………………………………
1 Scope
1.1 This standard addresses the installation of electrical instruments, which are directly
connected to flammable process materials. Such instruments include pressure
switches (with or without diaphragm seals), thermowells, gas chromatographs,
transmitters and field controllers. Its purpose is to define proper practices, so that the
installation will:
a. Prevent ignition of the flammable materials under normal or abnormal
conditions.
b. Comply with applicable code requirements.
c. Provide protection consistent with the area classification; see 3.1, while
avoiding costly over design.
A related standard, SES R01-E04 addresses similar issues for devices, which do not
have direct process connections to hazardous materials.
1.2 Step-by-step procedures defining protection requirements are provided on Figures 5,
6, and 7.
1.3 This standard embodies the practices included in National Fire Protection Association
(NFPA) Standard 496 Chapter 8 and it’s other referenced chapters.
1.3.1 NFPA 496 Chapter 8 places no restriction on enclosure size, but does not
address analyzer houses or other rooms intended for human occupancy.
Note that the standard, SES R01-E04, does not address purging or
pressurizing occupied areas; see NFPA 496 Chapter 9 for analyzer rooms
with flammables piped into internal equipment. See also 8.7 below.
1.4 Purges may be required for instruments connected to the process in either hazardous
or nonhazardous areas. Both are covered in this standard.
1.5 This standard refers only to hazards created by flammable gases, vapors, and
volatile liquids. These are classified in NEC Article 500, as Class l, Groups A, B, C,
and D (see Table 1). The recommendations given do not apply to Class II or III
(hazards due to flammable dusts, fibers, or metal powders) which are not within the
scope of this standard.
1.6
It is recognized that hazards to personnel, corrosion problems, or other
environmental conditions shall be considered as part of a complete system design.
These problems are not within the scope of this standard.
1.7 This standard also gives recommendations to prevent the external surface of an
enclosure, which has power or line voltage wiring in it from reaching specific ignition
temperatures. This consideration applies to equipment in Division 1 areas protected
by purging or sealing.
1.8 This standard does not define all conduit/cable seal considerations
1.9 An Appendix, section 8, has been provided to explain the philosophy upon which this
standard is based. Wherever possible, this standard is based upon an analysis of the
particular physical conditions involved with minimum reliance on arbitrary rules.
2 References
Reference is made in this standard to the following documents. The latest issues,
amendments and supplements to these documents shall apply unless otherwise indicated.
SABIC Engineering Standards (SES)
R01-E03 Hazardous Area Instrumentation Protection – Intrinsically Safe
R01-E04 Hazardous Area Instrumentation Protection – Purged and Pressurized Enclosures:
Hazardous Areas
American National Standards Institute (ANSI) / National Fire Protection Association
(NFPA)
70 National Electrical Code
496 Purged and Pressurized Enclosures for Electrical Equipment
497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors
and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process
Areas
Underwriters Laboratories (UL)
913 Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III,
Division 1 (Hazardous) Locations
746C Polymeric Materials-Use in Electrical Equipment Evaluations
3
Definitions
For the purpose of understanding this standard, the following definitions apply.
3.1 A hazardous atmosphere for the purpose of this standard is a flammable mixture of
gases or vapors, or both, in air (oxygen concentration not above 21 percent).
3.1.1 Hazardous materials, in this standard, are those, which are flammable.
3.2 For the purpose of this standard, instruments include measuring, recording,
controlling, analyzing, and similar apparatus requiring the use of electrical energy for
normal operation.
3.3
Intrinsically safe equipment and wiring is equipment and wiring which is incapable of
releasing sufficient electrical energy or heat under normal or abnormal conditions to
cause ignition of a specific hazardous atmospheric mixture. (See NEC Article 504
and for additional detailed information, see UL 913.)
3.4 Nonincendive equipment and wiring is equipment and wiring which, in its normal
operating condition, would not ignite a specific hazardous atmospheric mixture.
Equipment and wiring having exposed surface temperatures above 80 percent of the
autoignition temperature in degrees Celsius of the specific hazardous material shall
not be classed as nonincendive. (See NEC Articles 501-3 and 501-4.)
3.4.1 The circuits may include sliding or make-and-break contacts releasing
insufficient energy to cause ignition.
3.4.2 Circuits not containing sliding or make-and-break contacts may operate at
energy levels potentially capable of causing ignition.
3.5
Ignition-capable equipment and wiring is equipment and wiring which, in its normal
operating condition, releases sufficient electrical or thermal energy to cause ignition
of a specific hazardous atmosphere.
3.6 For the purpose of this standard, process barriers are devices like bourdon tubes or
diaphragm seals, which serve to isolate the process from the instrument enclosure
internals. Additionally, properly installed conduit seal fittings are at times used as
secondary process barriers, see 8.4.2 and 8.4.3.
3.7 For the purpose of this standard, devices with a single process barrier are considered
to have no release under normal conditions, and unlimited release under abnormal
conditions. Devices with two or more process barriers are considered to have no
release under normal conditions, and limited release under abnormal conditions. See
5.4.1 c. and 5.4.2 c. for definitions.
4 General
4.1
Class I hazardous areas are classified as Division 1 or Division 2 in accordance with
the expectations of the presence of explosive or ignitable mixtures, see Table 1.
4.2
Division 1 locations may contain ignitable concentrations under normal operating
conditions. (See NEC Article 500-5 for detailed definition.)
4.3
Division 2 locations are those in which the atmosphere normally is nonhazardous but
may become hazardous due to equipment failures or failure of ventilating systems.
(See NEC Article 500-5 for detailed definitions).
4.4 Some of the methods listed in this standard are based upon classifying the volume
within an enclosure different from that of the area external to the enclosure.
4.5 Refer NFPA 497 for the classification of flammable liquids, gases, or vapors
and of hazardous (classified) locations for electrical installations.
4.6 Conflicts
Any conflicts between this standard, SESs and industry standards, engineering
drawings, and contract documents shall be resolved at the discretion of the Buyer.
4.7 Buyer Exceptions
Buyer exceptions to this standard will be detailed on the purchase order.
4.8 Vendor Exceptions
Vendor exceptions to this standard to be included in the quote.
5 Instruments Connected to the Process
5.1 Housed electrical instruments, which connect directly to the process, can become
containers of process fluids if the barriers between them and the process fail from
corrosion or metal breakage. The instrument enclosure then will become dangerous
from two standpoints; potential internal explosion, and the transport of process fluids
under pressure through the conduit or cable leaving the enclosure.
5.2 An instrument which seals the process fluid from an electrical component with a
barrier for example a bourdon or bellows only, is called a single-barrier device. An
instrument which seals with two process barriers is a double barrier device, for
example a diaphragm seal used in conjunction with a bourdon or bellows. The
probability of failure of both barriers at the same time is low. Therefore, the theory of
protection against leakage is based upon the probability that only a single failure
(broken or corroded barrier) will occur.
5.3 There are two approaches to safe installation of electrical instruments in hazardous
(classified) areas. One is the installation of devices which are approved for the
specific area classification (which represents our most common approach for devices
for example transmitters.) The other is to properly purge the device enclosure, which
allows safe use of equipment internal to the enclosure which is not approved for the
external area classification. The step-by-step procedures on Figures 5, 6, and 7
define the requirements for protection for both approaches. Note that “approved for
the area” includes items commonly referred to as “explosion-proof,” or those which
are non-incendive or intrinsically safe. SES R01-E03 details requirements for
intrinsically safe installations. The standard SES R01-E04 details the purging
requirements for instruments connected to flammable process materials, when that
approach is used.
5.3.1 For purged devices connected to flammable materials, the type of purge
required, X, Y, or Z, and the purge media (normally air or inert gas) is based
on an assessment of the process fluid leak rate into the enclosure under
normal and abnormal conditions; the specific electrical classification rating of
the enclosure and the equipment internal to it; and the classification of the
surrounding area. NFPA 496 Chapter 8 discusses these parameters, and is
included in the NEC by reference. See also Figures 5, 6, and 7.
5.4
In this standard, every enclosure is considered to have a normal and an abnormal
condition. In both conditions, the electrical equipment in the enclosure is assumed to
be operating properly.
5.4.1 Normal means the expected release of flammables within the enclosure,
occurs when the equipment (barriers, diaphragm seals, flow restrictors, etc)
is operating properly. The magnitude of this release may be:
a.
None; no release of flammables, or
b.
Limited; as defined in 5.4.1 c.
c. A limited release is defined in NFPA 496, Chapter 8, as one which
can be diluted by the purge system, with air, to below 25 percent of
the lower explosive limit. With inert gas purge, the dilution
requirement is to a concentration in the enclosure to a level of 5
percent oxygen or below, or to 50 percent or less of the oxygen
concentration required to form a flammable mixture, whichever is
lower.
5.4.2 Abnormal means the expected release of flammables within the enclosure
occurs when the barriers or flow restrictors are leaking or otherwise operating
abnormally. The magnitude of this release may be:
a.
Limited, as defined in 5.4.1 c., or
b.
Unlimited, as defined in 5.4.2 c.
c. An unlimited release is one whose magnitude is larger than the
limited release defined in 5.4.1 c.
5.5 NFPA 496 also defines types of purges:
a.
Type X
Reduces the classification of the enclosure internals from Division 1 to
nonhazardous.
b.
Type Y
Reduces the classification of the enclosure internals from Division 1 to
Division 2.
c.
Type Z
Reduces the classification of the enclosure internals from Division 2 to
nonhazardous
5.5.1 There are a variety of requirements which shall be met to achieve the intent
of the purge type, for example purge rates, purge media type or oil-immersed
contacts. Note that when the enclosure is located in a classified area, the
purge system shall maintain pressure in the enclosure to a minimum of 0.1
inches water. SES R01-E04 details these requirements, and describes in
4.3.2 a simple method to reliably indicate presence of the required internal
pressure. See also NFPA 496.
5.6 Figures 5, 6, and 7 define the purge types, if required, for enclosures subject to
internal release. Included as factors in the assessment of requirements are those
items listed in 5.3.1 above, and the effect that the number of barriers has on the
expected release rates, which applies to most commonly used devices, see 3.7.
Further, the requirements for nonincendive equipment is also included, see Table 2.
From this, the type of purge and purge media are then determined. Limited release
under normal conditions applies to devices like some gas chromatographs, which
may release some small amount of the sample gas into the enclosure, continuously
or periodically, see 5.10 and 5.11.
Example of use of the figures:
a. Given
(i)
Nonhazardous area
(ii)
Internal equipment is ignition capable
(iii)
Diaphragm seal pressure switch, thus two barriers.
b. Procedure
Nonhazardous area requirements are shown on Figure 7. Start with block 1.
Since the internal equipment is ignition capable, follow the “No” arrow.
Likewise for block 2, follow the “No” arrow. A double-barrier pressure switch
is considered to have no release for normal conditions, thus follow the
“None” arrow from block 3. See 3.7. A double-barrier pressure switch is
considered to have limited release for abnormal conditions, see 3.7, thus
follow the “Limited” arrow from block 6.
c. Result
Block 12 indicates “Z” purge required. Note that similar procedure for a
single-barrier switch would indicate “Z” with “Inert” required.
5.7 The first consideration is to protect against leakage by using more than one barrier.
When this is impossible, the next consideration is to provide the required additional
conduit seal, and proper vent and purge. This is to prevent buildup of flammable
concentrations of process fluid within the enclosure, and to dilute or inert any leak to
a concentration below the lower explosive limit. The additional seal prevents
propagation of flame or flammables, to other portions of the system. See Figure 1,
8.4.2.b, 8.6, and NEC 501-5 (f)(3).
5.8 When air is used for purging, the air pressure within the enclosure shall not exceed
the pressure of the flammable process material. This is to avoid accidental flow of the
purge air into the process, which could cause an ignitable mixture.
5.8.1 The commonly used purge rotameter (3-in. scale, 2 scfh maximum flow)
exhibits a pressure drop of not less than 0.2 to 0.3 inches of water at a flow
rate of 1.0 scfh. It should be installed to meter purge flow out of the enclosure
(not inlet flow), thus avoiding the effects of minor leaks from the enclosure.
When installed in this manner and metering a flow of not less than 1.0 scfh,
the internal enclosure pressure is then known to be not less than 11.1 inches
of water. Thus this installation would be suitable for process pressures above
this value.
5.9 Precautions to avoid excess enclosure pressure include ensuring there are no
restrictions between the flow indicator and the enclosure, and that normal wear of the
pressure regulator does not affect its output pressure. To this end, the flow indicator
shall not be equipped with a valve, and the regulator should be a “bleed” type (3port),
like
the
ordinary
instrument
air
pressure
regulator.
5.10
The purge flow indicator or other enclosure vents shall be located as far from the
purge inlet as practical, to allow effective removal of flammable materials.
5.11 Some analyzers utilize a detector with an open flame. Their enclosures shall be
considered to have internal equipment suitable for unclassified areas, when
determining purge requirements from Figure 5, 6, or 7. It should be assumed that
loss of flame is a normal occurrence and should be classified as a normal release,
unless such loss automatically stops the flow of the flammables to the device. The
flame shall be automatically extinguished upon failure of the purge system regardless
of purge type. See NFPA 496 8-2.5 and A-8-2.2.
5.12 A flow limiter is an acceptable method to limit abnormal release rates, such that the
expected flow under abnormal conditions does not exceed the dilution capability of
the purge system. This device shall be specifically designed to accomplish that task.
The most common use likely is with devices for example, gas chromatographs, that
normally have continuous or periodic flow of flammables into the enclosure. See
NFPA 496 A-8-2.2.
5.13 Instrument enclosures connected to pressurized sources of odorless, colorless inerts,
for example nitrogen, shall have conduit and cable seals installed similar to those
required for flammable service, if the conduit or cable is routed to an occupied
enclosed area. Nitrogen is a dangerous asphyxiant, and can cause unconsciousness
or death if inhaled in 100 percent concentration for only a few seconds.
6 Safe Practices
Protection is required for different types of electrical equipment according to the listing in
Table 2.
7 Surface Hot-spot Limit
7.1 A hazard exists when instruments are installed in Division 1 areas if sufficient power
is available to heat or burn through the enclosure. An abnormal event, for example a
short circuit or insulation failure, could raise an external portion of the enclosure to
the ignition temperature of the surrounding atmosphere.
7.2
Instrument enclosures shall be inspected for adequate mechanical strength, and for
resistance to damage or overheating in case of any failure or short of the electrical
components contained.
7.3 Currents in circuits contained in suitable enclosures that are located in Division 1
atmospheres shall be limited (as with fuses) consistent with 7.4, if the equipment is
protected by (a) explosion-proof enclosures, or, (b) purging.
7.4 Precautions shall be taken to ensure that a malfunction (short circuit) between the
power wiring and the enclosure walls shall not burn through the enclosure, or
otherwise raise the external surface temperature to 80 percent of the autoignition
temperature in degrees Celsius of the gas or vapor involved.
7.4.1 Precautions involve a combination of fuse type (i.e., quick blow, medium
blow), fuse rating, and thickness of enclosure wall.
7.4.2 Other materials of construction would be equally acceptable for instrument
enclosures if the equivalent mechanical ruggedness and resistance to
external overheating and burn-through are preserved. (See UL 746C for
information on polymeric enclosures.)
7.4.3 In the event that double-wall or box-within-box construction is used and the
purge is connected to the inside box, purging both, the fuse size for circuits
within the inner box may be increased by a factor of 10. The minimum
spacing between the two walls shall be at least ten times the wall thickness.
7.4.4 A double-sealed system may be arranged for analyzers using box-within-box
construction with adequate vent between them.
8 Appendix
8.1 This section contains additional background information on some of the items
contained in the text, and explains the philosophy upon which it is based.
8.2 The release of flammable gas or vapor into an enclosure is much more serious than
a similar release to the atmosphere. A small leak into the surroundings will most likely be diluted through normal air currents, etc, and stay safely below flammable
concentrations. A similar leak into the enclosure, in the absence of purging, will
slowly raise the concentration inside the enclosure until its atmosphere becomes
flammable. This rise in concentration is likely to be little affected by normal breathing
and diffusion of the enclosure. See NFPA 496 A-8-2.
8.2.1 Because of the confining nature of electrical equipment enclosures, it is
necessary to view normal and abnormal conditions in terms of much longer
time spans than is necessary for releases in the open. The probable
condition of the equipment after some years of service shall be considered,
including degradation of system components over time.
8.2.2 Materials and types of construction that degrade in service with age and that
are not likely to be maintained or replaced are likely to be a source of release
as defined in 5.4.1.
8.2.3 Threaded pipe joints, metal compression fittings, bourdon tubes and bellows,
and other reliable methods would normally be considered to have “normal
condition-no release.”
8.3 Refer to 3.4. NEC Article 501-3 (for Class I Division 2 environments) permits the use
of resistors, resistance devices, and rectifiers used in (or in connection with) meters,
instruments, and relays if no make-and-break contacts exist, and if the temperature
of the exposed surfaces does not exceed 80 percent of the autoignition temperature
in degrees Celsius of the hazardous atmosphere involved.
8.4 Refer to 5.1. Examples of enclosures connected to hazardous materials are found in
pressure, temperature, flow connections, analyzers, and sampling lines to
instruments. These may be easily recognized piping connections, thermocouple
conduit, or less obvious channels between stranded wires in flexible sheaths. The
number of process barriers between the process and the enclosure defines in part
the purge requirements for the enclosure.
8.4.1 Process barriers may consist of thermocouple wells, diaphragms, bellows,
bourdon tubes, conduit seal fittings or walls of analysis cells.
8.4.2 Conduit seal fittings may be used in two ways, as shown in this standard.
a.
First, they may be used as the second barrier (farthest from the
process) between the process and the enclosure, to establish a
double barrier system, see Figure 2 and 8.6. The intent is to further
prevent hazardous materials from entering the enclosure. Purge
requirements for the enclosure are those for two-or-more barrier
devices.
b. Second, for single barrier devices, a seal shall be installed in the
conduit between the enclosure and other enclosures, see Figure 1.
This provides a double seal to prevent passage of gases, vapors,
and flames from the enclosure through the wiring raceway to other
portions of the system. Note the enclosure still has only a single
barrier, and purge requirements are those for single barrier devices.
8.4.3 The combination of a single process barrier and a conduit seal, whether used
as a second barrier or as a double seal, shall have an adequate vent or drain
(or both) between them. This vent or drain shall be sufficient to allow the
escape of hazardous vapors or liquids if the primary barrier fails, and to make
any such leak visible or obvious to an observer. See Figures 1 and 2, and
NEC 501-5 (f)(3).
8.5 See 5.3.1 Purging may not be practical for some applications. There are some very
low range pressure switches, etc, where even the minimum required enclosure
pressure of 0.1 inches of water represents a substantial fraction of the device span.
Thus the apparent process pressure indicated by the device would be substantially in
error, since all gage pressure type devices actually indicate the pressure difference
between the process and the atmospheric pressure (now the enclosure pressure).
Figure 1 – Pressure Switch Installation, Single Process Barrier,
Nonincendive, Double Seal and Vent
8.6 An instrument connected through conduit to a bare wire thermocouple in a protecting
thermowell, without any break between the connection head and the enclosure,
would be a single-barrier system. Failure of the protecting thermowell could result in
the process fluid being communicated directly into the instrument case. Even if
insulated cable is used without conduit, failure of the thermowell may result in
process fluid being forced back to the instrument through the interstices of the cable.
However, if the system is vented at the connection head (the vent may be an open
nipple installed in the bottom of the head pointed toward the ground) failure of the
thermowell will cause the process fluid to discharge to the atmosphere. If the conduit
is sealed and vented in the conventional manner between the head and the
instrument, the system can be considered to have two process barriers, see Figure 2.
Figure 2 – Thermocouple Installation, Two Process Barriers,
Nonincendive, Double Seal and Vent
8.7 Refer to 1.3.1. SABIC Standards do not address occupied analyzer buildings with
flammable materials piped to their internal equipment.
8.8 Refer to 1.7 and 7.1. Because the exterior atmosphere has a high probability of being
hazardous at any particular time, it is essential that the probability of an internal
failure raising the temperature of the external enclosure surface to the ignition
temperature of the gas or vapor involved be negligible. This requirement can be met
by a correct selection of fuse size and type, and a suitable thickness of the case wall.
8.8.1 Tests show that at the instant a short circuit occurs, a tremendous surge
current, in the order of 1000s amperes, develops for a fraction of a cycle,
with the exact amplitude depending upon the capacity of the power source
and fuse type and rating.
8.8.2 External case temperatures are based upon actual tests made on metal
plates of the material and thickness listed, using Chase Shawmut OT (OneTime)
General
Purpose
type
fuses.
8.8.3 Quick Blow fuses of the Chase Shawmut Amp-Trap current limiting type
sufficiently reduced the current surge so that the external case temperature
generally was in the safe region despite the fuse rating.
8.8.4 The temperatures existing on the outside surface of a metal enclosure due to
the liberation of heat energy on the inside surface have been calculated
based upon the following assumptions:
a. Ambient temperature outside the enclosure is 30 °C.
b.
Temperature inside the enclosure is dependent upon the energy
released by electrical short (no sparking assumed) and upon the rate
of heat dissipation from the case.
c. A specific enclosure has been assumed having the dimensions of 18
in. x 14 in. x 12 in.
d.
The electrical contact is assumed to be by a 14 AWG wire.
e.
The temperature distribution around the point of contact is in
conformity with established laws of convection, conduction, and
radiation. Maximum temperature is achieved under steady-state
conditions.
f.
The combined radiation and convection coefficient for metal-to-air
heat transfer is assumed to be 2 pcu/(h) (ft2)(°C).
Note: At the ignition temperature for Group D gases, the heat
transfer coefficient from clean metal surfaces would be more nearly
in the region of 4 to 8 pcu per hour. The lower assumed value of 2
pcu per hour allows for a substantial dust loading on the outside of
the instrument enclosure.
g.
The inside of the enclosure was assumed to be covered 50 percent
with perfect insulating material.
h.
The current is supplied from a 115-volt circuit.
8.8.5 The following changes in the actual conditions will increase the rate of
energy release required to raise the external enclosure temperature.
a.
Liberation of the energy over an area larger than 0.064-inch diameter
circle, or at a location not directly contacting the internal surface.
b.
Liberation of the energy continuously available for an insufficient
interval of time to reach final temperature.
Improvement of the expected heat transfer to the atmosphere by:
(i) Applying any appreciable external air velocity.
(ii) Removing some of the expected external dust loading.
8.9 Refer to 5.13. Nitrogen is an insidious and dangerous material. Exposure is
undetectable to the individual and loss of consciousness usually occurs without the
individual being aware of anything abnormal.
8.10 For additional information, Table 8-2.4 from NFPA 496 is included as Table 3.
8.10.1 To determine the purge requirements according to Table 3:
a.
c.
Find the External Area Classification in Column (a).
b.
Find the Internal Equipment Type in Column (b).
c.
Determine the purge requirement for Limited Release under
Abnormal Conditions by using the appropriate Normal Condition in
Column (c).
d.
Determine any additional requirements from Column (d) if the
Abnormal Condition is Unlimited Release.
8.10.2 Note some gas analyzers, sample cells with windows, etc, likely will have
“limited release normal conditions;” see 5.4.1, and NFPA 496 Chapter 8 and
its Appendix
8.10.3 Example use of Table 3:
Figure 3 – Single Process Barriers
Figure 4 – Two or More Process Barriers
Figure 5 – Purge Requirements For General Purpose Instrument Case In
Class 1, Division 1, Process Piped To Enclosure
Figure 7 – Purge Requirements For General Instrument Case In Non
Hazardous Area, Process Piped To Enclosure
Table I – Hazardous Area Classifications and Electrical Circuit Type
HAZARDOUS LOCATIONS
Class I – Locations in which flammable gases or vapors are, or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Division 1
a. Hazardous concentrations exist continuously, intermittently, or periodically under
normal conditions.
b. Hazardous concentrations may exist frequently because of repair or maintenance
operations, or because of leakage.
c. Where breakage or faulty operation of equipment, or processes which might release
hazardous concentrations of flammable gases or vapors might also cause
simultaneous failure of electrical equipment.
Division 2
a. Where hazardous volatile liquids, vapors, or gases normally are confined within
closed containers; or closed systems from which they can escape only in case of
accidental rupture or breakdown of such containers or systems; or in case of
abnormal operation of equipment.
b. Hazardous concentrations normally are prevented by positive mechanical ventilation,
but might become hazardous through failure or abnormal operation of the ventilating
system.
c. Areas adjacent to Division 1 locations to which hazardous concentrations of gases or
vapors might be communicated.
Groups
A – Atmospheres containing acetylene
B – Atmospheres containing acrolein (inhibited), butadiene, ethylene oxide, hydrogen,
manufactured gases containing more than 30 percent hydrogen (by volume), or
propylene oxide.
C – Atmospheres containing materials for example acetaldehyde, carbon monoxide,
diethyl ether, diethylamine, hydrogen sulfide, tetrahydrofuran (THF), or unsymmetrical
dimethyl hydrazine (UDMH).
D – Atmospheres containing materials for example acetone, acrylonitrile, various alcohols,
ammonia, benzene, butane, cyclopropane, ethylene dichloride, gasoline, hexane,
isoprene, naphtha, natural gas, propane, propylene, styrene, vinyl acetate, vinyl
chloride, or xylenes.
Class II – Locations which are hazardous because of presence of combustible dust.
Class III – Locations where easily ignitable fibers or flyings are present, but not likely to be in
suspension in quantities sufficient to produce ignitable mixtures.
Hazardous Area Classifications and Electrical Circuit Type
NONHAZARDOUS LOCATIONS
Flammable vapors or gases are not present in air in concentrations sufficient to cause
explosive or ignitable mixtures.
ELECTRICAL CIRCUIT TYPES (See 3.3, 3.4, 3.5)
Intrinsically Safe (IS) For definition, see 3.3
Nonincendive (NI) for definition, see 3.4
Ignition Capable (IC) for definition, see 3.5
Table II – Safe Practices
Notes:
(1) Where protection is required it may be by purged or explosion proof enclosure.
(2) Purge required if enclosure has any “normal release”, see 5.4.1.
(3) The lowest cost acceptable installation is a general-purpose enclosure.
Table III – Purge Requirements for Enclosures Subject to Internal
Release of Hazardous Materials
Notes:
(1) Precautions shall be taken if unlimited release is large enough to alter the
nonhazardous external area.
(2) Gas analysers, sample cell window seals, etc, may have limited release under
normal conditions, and limited or unlimited release under abnormal conditions, see
5.6, 5.10, and 5.11.
(3)
Refer to 5.5 for definition of X, Y, and Z types of purges.