FLAMMABLE MIXTURE ANALYSIS FOR HAZARDOUS AREA CLASSIFICATION
Allan Bozek P.Eng, MBA
Vince Rowe, P.Eng
Member IEEE
EngWorks Inc.
1620, 49th Avenue SW
Calgary, Canada T2T2T7
Member IEEE
Marex Canada Ltd.
1550, 555 - 4th Avenue SW
Calgary, Canada T2P 3E7
Abstract - The properties of flammable mixtures as they
apply to a hazardous area classification analysis are
discussed. Mathematical formula and application rules of
thumb are provided to help estimate the relative density,
group classification, autoignition temperature, material
flashpoint and the flammability of a mixture. Application
guidelines are provided on how to apply the results in the
context of a hazardous area classification analysis.
Index Terms – Hazardous area classification, flammable
mixture analysis, density, group classification, autoignition,
flashpoint, flammability.
I.
INTRODUCTION
The vast majority of flammable materials handled in oil, gas
and petrochemical facilities consist of flammable mixtures of
various compositions.
Performing a hazardous area
classification risk assessment requires an understanding of
the behavior of flammable mixtures and how they might
influence the extent, group classification and autoignition
temperature of the classified area.
Unfortunately, very little guidance is given on mixture
analysis by the various industry recognized standards and
recommended practices on area classification. The search
for the proper chemical relationships and formula can be
frustrating. The mixture analysis must be performed with help
of chemical textbooks and other references not specifically
written for the purpose of a hazardous area classification
analysis.
The purpose of this paper is to provide guidance on how to
analyze the properties of flammable mixtures as they apply to
a hazardous area classification. The key flammable material
parameters and the test methods used to determine the
properties of pure flammable materials are discussed. In the
absence of actual test results, a series of mathematical
relationships and application rules of thumb are provided for
estimating the properties of flammable mixtures. Application
guidelines are also provided to assist in documenting the
results within the context of a hazardous area classification
analysis.
II. HAZARDOUS AREA CLASSIFICATION
OVERVIEW
A hazardous area classification analysis is a structured risk
assessment process used to determine the probability of a
flammable atmosphere being present during normal or
abnormal operations. The analysis divides the facility layout
into zones of risk which are then used to determine the
appropriate equipment and wiring methods for safe operation.
The probability that a flammable atmosphere exists is
dependent on the chemical properties of the flammable
materials present, as well as the grade and rate of release.
The extent of the hazard will be influenced by the behavior of
the flammable material under normal ambient pressure and
temperature conditions, the degree of ventilation within an
area, the geometry and velocity of the release and the
physical topography of an area.
There are several methods for classifying an area once the
flammable materials and physical parameters are identified.
The most common way is to use the direct example approach
where the facility is classified using diagrams that depict
typical situations.
This method of classification is
incorporated into most industry recommended practices for
hazardous area classification.
A second method uses the point source approach to
classify facilities. Potential release sources are identified and
a formal calculation is performed to determine the extent of a
classified area. The point source approach is more rigorous
and thus requires more detailed information on the nature of
the flammable materials to obtain a reasonable conclusion.
III. PROPERTIES OF FLAMMABLE MATERIALS
To assess the risk and the extent of the hazard, several key
properties of the flammable material must be determined.
They include:
A.
Vapor Pressure and Flashpoint
The hazard posed by flammable materials is influenced by
how easily it will mix with air and potentially form a flammable
mixture.
This is a function of vapor pressure and
temperature. Flammable gases have a very low vapor
pressure and as such are always in a gaseous state at
ambient temperatures. Flammable liquids in contrast, are in a
state of transition between a liquid and a gas. The hazard
posed by a flammable liquid is a function of how easily it
changes state from a liquid to a vapor at ambient
temperatures. The temperature at which a liquid provides
sufficient vapor to form a flammable mixture on the surface of
the liquid is termed its flashpoint. This is the key parameter
used to classify the potential hazard associated with
flammable liquids.
The flashpoint of a flammable material is determined
experimentally using two recognized methods as illustrated in
Fig. 1 and 2. The open cup flashpoint test procedure requires
heating the flammable material in an open cup and
periodically using an open flame to ignite the surface vapor.
When there is sufficient vapor to sustain combustion, the
flashpoint temperature is recorded.
The closed cup flashpoint test procedure is similar to the
open cup flashpoint test except the flammable material is
sealed within a closed cup environment. The closed cup
flashpoint test typically results in a lower flashpoint
temperature than observed with the open cup test. The
flashpoint temperatures most often quoted for flammable
materials in various publications are derived using the closed
cup method.
Flashpoint is used as a basis for categorizing the volatility
of flammable liquids. NFPA 30 [1] categorizes flammable
materials into three classes based on their flashpoint and
boiling point vapor pressure (Table I). Class I materials are
those that readily flash to atmosphere at ambient
temperatures and pressures. They are considered highly
volatile and require due consideration with respect to an area
classification.
Class II materials typically do not flash at temperatures that
would normally be considered “ambient”. From an area
classification perspective, Class II flammable materials are
considered when they are stored or handled at temperatures
above their flashpoint.
Class III materials are only a
consideration when they are exposed to very high process
temperatures and typically do not factor into a hazardous area
classification. In all cases, the properties of the flammable
material and the process conditions must be assessed prior to
defining the degree and extent of a classified area.
Part 15 (IP15) of the British Institute of Petroleum Model
Code of Safe Practice in the Petroleum Industry has a similar
classification for petroleum liquids based on flashpoint. The
petroleum classes defined in Table II are used in conjunction
with a series of typical diagrams that define the degree and
extent of a classified area.
IP15 takes into account the process conditions and adjusts
the extent of the classification based on if the material is
handled above or below its flashpoint temperature. It also
makes reference to situations where the fluid cannot be easily
classified by flashpoint and addresses the potential hazards
associated with flammable mists and sprays. Under these
circumstances, a judgment call must be made as to the fluid
category and the characteristics of the potential hazard. The
point source method of area classification is normally used in
these situations [2].
Thermometer
Flame
Bunsen
Burner
Fig. 1 Open Cup Flashpoint Test Apparatus
Test Cup
Thermometer
Agitator
Bath
Thermometer
Flammable
Liquid
Flame
Liquid Bath
Heating Vessel
Fig. 2 Closed Cup Flashpoint Test Apparatus
TABLE I
NFPA CLASSIFICATION OF FLAMMABLE/COMBUSTIBLE
MATERIALS
NFPA
Flashpoint (ºC)
Examples
Class
BP = Boiling Point
IA
< 22.8, (BP < 37.8)
Ethyl Ether, Pentane
IB
<22.8, (BP > 37.8)
Acetone, Gasoline
IC
≥22.8 and < 37.8
Naphtha, Xylene
II
>37.8 and < 60
Fuel Oil, Kerosene
IIIA
≥ 60 and < 93
Ethylene Glycol
IIIB
≥ 93
Asphalt, Transformer Oil
Open Cup
with
Flammable
Liquid
TABLE II
IP Class
0
I
II(1)
II(2)
III(1)
III(2)
Unclassified
B.
IP15 PETROLEUM CLASSES
Flashpoint (ºC)
Application
NA
<21
≥21 and ≤ 55
≥21 and ≤ 55
>55 to 100
>55 to 100
≥ 100
LPGs
Flam. at ambient conditions
Handled below FP
Handled above FP
Handled below FP
Handled above FP
Relative Vapor Density
The relative vapor density of a flammable gas/vapor is a
key parameter when using the direct example approach
method of area classification. The diagrams selected for a
particular situation are dependent on the relative density of
the gas/vapor as compared to air under standard ambient
temperature-pressure conditions.
For practical applications, a gas/vapor mixture that has a
relative vapor density of 0.8 is regarded as lighter-than-air and
a release would rapidly rise and collect in the upper levels of a
confined area. Gas/vapor mixtures that have a relative vapor
density greater than 1.2 are regarded as heavier-than-air and
will collect at ground level.
In theory, heavier-than-air
flammable mixtures can travel long distances and result in
large areas of potential hazard. Gas/vapor mixtures with a
relative vapor density of between 0.8 and 1.2 can exhibit
properties of both lighter-than-air and heavier-than-air
behavioral characteristics so both possibilities should be
considered [3] [4].
While gas/vapor density may have an influence on the
extent of a classification in enclosed areas under ideal
conditions, recent experiments using dispersion modeling
have shown that the relative density has little influence on the
extent of a hazard in a pressurized release. In these
circumstances, the angle of release and how close the
release point is to the ground has a larger effect. The extent
of a flammable region is determined more by the direction and
the velocity of the release than by the density of the gas or
vapor released [5].
C.
Group Classification of Flammable Materials
The group classification assigned to a hazardous location
determines the explosionproof or flameproof enclosure design
requirements and the performance specifications for
intrinsically safe circuits. The design of an explosionproof or
flameproof enclosures is dependent on flamepath tolerances
which are in-turn based on the Maximum Experimental Safe
Gap (MESG) distance defined for a given flammable gas or
vapor. The smaller the MESG defined for a material, the
longer the flamepath required to cool the hot gases
discharged from an explosionproof assembly during an
ignition event. Typically, the longer the flamepath required,
the more expensive the enclosure.
The group classifications for flammable gases/vapors are
determined experimentally based on the MESG determined
under test conditions.
The NEC group classifications for
flammable materials are based on tests performed by
Underwriter Laboratories using a Westerberg Explosion Test
Vessel (WETV).
The WETV consists of two chambers, one within the other,
and each filled with a stochiometric explosive mixture. The
mixtures are separated by an adjustable 25mm wide joint gap
assembly. The flammable atmosphere is ignited in the inner
chamber and allowed to propagate to the outside chamber.
The opening of the joint gap assembly is reduced until an
internal chamber ignition does not propagate to the external
chamber. The opening of the 25mm wide joint gap at that
point is the MESG. The group classification for the flammable
material is then assigned based on Table III.
The IEC group classifications are performed in a similar
fashion. The size and shape of the explosion chamber differs
from the WETV but the results are statistically similar. The
IEC group classification of materials as it relates to MESG is
also provided in Table III.
TABLE III
GROUP CLASSIFICATION CRITERIA
NEC Gas Grouping
IEC Gas Grouping
Group
MESG
MIC
Group
MESG
MIC
(mm)
ratio
(mm)
Ratio
A
Acetylene
IIC
≤0.50
≤0.45
B
≤0.45
≤0.40
>0.45
>0.40
>0.50
>0.45
C
IIB
≤0.75
≤0.80
≤0.90
≤0.80
D
>0.75
>0.80
IIA
>0.90
>0.80
The group classification for a flammable material can also
be based on the minimum ignition current (MIC) required to
ignite the material under stochiometric conditions within a
specified test apparatus. This is derived experimentally with
the results often expressed as a ratio to the minimum ignition
energy required for methane. Table III provides the group
classifications based on the minimum ignition current ratio
(MIC ratio) as compared to methane.
D.
Lower and Upper Flammable Limits
Flammable gases and vapors are flammable only when
they are between their lower flammable limits (LFL) and their
upper flammable limits (UFL).
For area classification
purposes, the LFL is of greater concern as the hazard can be
reduced by controlling the level of ventilation to dilute the
flammable gas to a concentration well below the LFL. The
LFL is a key parameter required for all hazardous area
classification ventilation and point source calculations.
Air
Flammable
Gas Mixture
Manifold
Hot Air
Ignition
Vessel
Air
Electrode
Manifold
Fig. 3 Flammable Limits Test Apparatus
The %LFL and %UFL values for pure substances are
derived experimentally using the apparatus described in
ASTM E 681-04 and illustrated in Fig. 3. The concentration
of the flammable material is gradually increased until an
ignition is observed. The % volume to air is recorded as the
lower flammable limit. The concentration is further increased
and ignited until the mixture no longer ignites. The %
concentration is then recorded as the upper flammable limit.
E.
Autoignition Temperature (AIT)
The autoignition temperature of a flammable material is
required to determine maximum safe operating surface
temperature for equipment installed in a hazardous location.
The maximum surface operating temperature for electrical
equipment is defined by a temperature code. The temperature
code is used to determine if the equipment item is suitable for
installation based on the AIT defined for the hazardous
location.
There are a number of definitions for autoignition
temperature in current use. For example, API RP500 defines
AIT as: “The minimum temperature required, at normal
atmospheric pressure, to initiate or cause self sustained
combustion (independent of any externally heated element).”
[3]
API RP505 defines AIT as: “The lowest temperature of a
heated surface at which, under specified conditions, the
ignition of a flammable substance in the form of a gas or
vapor mixture with air will occur.” [4]
The API RP505 definition is a modification of the definition
in IEC 60079-4 which is: “Ignition temperature – The lowest
temperature at which ignition occurs when the method
prescribed in this standard is used.” [7]
The test method used in IEC 60079-4 consists of heating a
200ml Erlenmeyer flask in a hot air furnace as illustrated in
Fig. 4. The flask is heated to the desired temperature, at
which point a liquid or gaseous sample is injected into the
flask. If no flame is observed within 5 minutes, the test is
repeated until the minimum temperature at which ignition
occurs is determined. The process is repeated until a degree
of correlation exists between several identical tests. This
correlated value is then defined as the autoignition
temperature for the flammable material.
ASTM E659 Standard Test Method for Autoignition
Temperature of Liquid Chemicals defines AIT as: “The
minimum temperature at which Autoignition occurs under the
specified conditions of test.” [8] The test method described in
standard ATSM E659 is similar to the test procedure used in
IEC 60079-4 with the exception of the volume and the shape
of the flask used.
Published AIT tables are usually based on the laboratory
procedures outlined in IEC 60079-4 and ASTM E659. The
prescribed methods are relatively simple and the results are
repeatable, however, they are only accurate for the conditions
under which the testing is performed.
Autoignition
temperatures can be influenced by the volume, shape and the
material composition of the test apparatus, the method and
rate of heating, the flame detection method as well as the
percent gas/vapor/air composition of the sample [9].
The published AIT tables typically reference the lowest AIT
observed under controlled laboratory conditions. Industry
experience however indicates that the actual minimum
temperature at which ignition occurs is much higher. API RP
2216 Ignition Risk of Hydrocarbon Liquids and Vapors by Hot
Surfaces in Open Air concluded that the minimum
temperature (referred to as the minimum hot surface ignition
temperature) should not be assumed unless the surface
temperature is approximately 360°F (182°C) above the
published AIT [10].
It should be noted that there is no standardized test for hot
surface ignition temperature. The temperature at which hot
surface ignition occurs is not a fundamental fluid property and
is influenced by a number of factors including ambient
conditions as well as the size, geometry and properties of the
hot surface itself. Hot surface ignition data cannot easily be
extrapolated to different situations and the use of a general
rule of thumb based on a minimum autoignition temperature
can be very inaccurate [11]. Fig. 5 illustrates the relationship
between vapor pressure, upper and lower flammability limits,
autoignition and hot surface ignition temperature.
Flame
Arrestor
Mirror
Viewing Line
Syringe
200 ml Erlenmeyer
Inflatable
Reservoir
Thermocouple
Heater Elements
Electric Furnace
To Instrument
Monitors
To Heater Controls
Fig. 4 IEC 60079-4 Autoignition Temperature Test Apparatus
V
a
p
o
r
P
r
e
s
s
u
r
e
Mixture
Vapor
Pressure
Curve
Published
Autoignition
Temperature
Upper Flammable Limit
Autoignition
Zone
Flash Point
Temperature
Hot Surface
Ignition Zone
(Curve may shift
based on less ideal
conditions)
Lower Flammable Limit
Mixture/Surface Temperature
Fig. 5 Autoignition and Hot Surface Ignition Temperature as a
Function of Temperature and Vapor Pressure
IV. INDUSTRY REFERENCES
Most area classification standards and recommended
practices do not incorporate a list of flammable material
properties. A secondary publication must usually be
referenced to obtain the necessary data. The one notable
exception is NFPA 497 Recommended Practice for the
Classification of Flammable Liquids, Gases, or Vapors and of
Hazardous (Classified) Locations for Electrical Installations in
Chemical Process Areas which
incorporates a list of
flammable material properties within the context of the
document [12].
Historically, API 500 and 505 has referenced NFPA 325
Guide to Fire Hazard Properties of Flammable Liquids, Gases
and Volatile Solids as the recommended source of information
for flammable material properties.
The document was
originally intended for fire prevention purposes and does not
incorporate information on MESG, MIC ratio or minimum
ignition energy data required for determining the group
classification of a flammable material.
As of 1998, NFPA 325 is no longer in print and has been
superseded by the NFPA Fire Protection Guide to Hazardous
Materials [13]. The document combines information from nine
different NFPA reference documents on hazardous materials.
With NFPA 325 no longer being published, the 2007 edition
of API 500 and 505 now make reference to NFPA 497 as the
recommended source of information for flammable material
properties. NFPA 497 provides a complete list of chemical
parameters including the NFPA flammable material
classification, flashpoint, autoignition temperature, vapor
pressure, density, group classification and the MESG and
MIE/MIC for a list of commonly found flammable materials
used in chemical process facilities. The list, although not
comprehensive, includes most flammable materials commonly
found in upstream production, transportation and refinery
facilities.
Area classifications performed using IEC 60079-10 [14] are
referred to IEC 60079-20 Electrical Apparatus for Explosive
Gas Atmospheres – Part 20 Data for Flammable Gases and
Vapours [15] for a list of flammable material properties. IEC
60079-20 was created specifically for hazardous area
classification purposes and includes a complete set of data for
performing an analysis.
There are minor differences between the data published in
NFPA 497 and IEC 60079-20. The differences are related to
the variation in test procedures and apparatus used. TABLE
IV provides an overview of the differences between the
reference publications for a cross sample of flammable
materials.
Other sources of flammable material property information
include MSDS (Material Safety Data Sheets US OSHA) and
WHMIS (Workplace Hazardous Materials Information System
– Canada) material data sheets and a number of chemical
reference texts that tabulate the properties of flammable
materials. MSDS and WHMIS data can be valuable because
they often provide information on chemicals and products not
normally listed in industry chemical references.
They
however omit MIC and MESG information required for the
group classification of a flammable material.
TABLE IV
FLAMMABLE PROPERTIES COMPARISON BETWEEN
NFPA 497 and IEC 60079-20
Flammable
AIT
Ref.
%LFL
%UFL
MESG
Material
ºC
NFPA
5
15
630
1.12
Methane
IEC
4.4
17
537
0.92
NFPA
1.5
7.8
243
0.93
Pentane
IEC
1.4
7.8
258
0.93
NFPA
4
75
520
0.28
Hydrogen
IEC
4
77
560
0.28
NFPA
4
44
260
0.90
Hydrogen
Sulfide
IEC
4
45.5
270
0.89
V. PROPERTIES OF MIXTURES
In a perfect world, a hazardous area classification would be
completed using the information provided in the various area
classification standards and recommended practices and the
properties listed in the hazardous material references. All
process materials would be of a homogenous nature with their
properties and flammable characteristics well defined.
Unfortunately, real world process facilities combine process
streams with different materials thus affecting the properties
and characteristics of the combined mixture.
This would not be an issue if the tests for flashpoint,
MESG, %LEL and autoignition could be done for a mixture
the same way it is done for a pure substance. This is
however impractical in the context of plant design where the
cost and effort associated simulating and testing each
process stream does not justify the end result. Some
mathematical means of estimating the properties of a
flammable mixture is required to properly assess the nature of
the hazard and define the extent of the classified zones.
Fortunately there are some mathematical relationships that
can be used to help predict the properties of flammable
mixtures.
A.
Base assumptions
To estimate the properties of a flammable mixture, a
number of key assumptions are required.
The first
assumption is that the mixture is in a gas or vapor state upon
release and it does not exist as mist or spray. The properties
of flammable mists and sprays are extremely difficult to model
and must be treated as a unique case for area classification
purposes [5].
The second assumption is that the flammable materials
remain in mixture. The behavior of ideal vs. non-ideal
mixtures will impact the accuracy of this assumption to a
degree but not enough to render the approximation invalid.
B.
Estimating Mixture Relative Density
As previously discussed, the relative density of a mixture
must be estimated to determine if it is heavier-than-air, lighterthan-air or of neutral buoyancy.
The information is
necessary to determine what direct example approach
diagram is appropriate for a particular situation. The relative
density of a mixture is calculated based on the molar weight
of the mixture as it compares to the molar weight of air. The
molar weight of air is 28.96 grams/mole at 0°C at sea level.
The following formula is the basis for the calculation:
n
RDair =
where
∑ Mi,xi
i
(1)
28.96
RDair = Relative density of the mixture compared to air
Mi = molar weight of the individual constituent
xi = corresponding mole fraction
The results are then compared to the values in TABLE V
and then applied to the appropriate area classification
diagram.
TABLE V
RELATIVE DENSITY CATEGORIZATION
RDair < 0.8
0.8 ≥ RDair ≤ 1.2
RDair > 1.2
Lighter than air
Neutral buoyancy
Heavier than air
1
MESGmix=
Consider the mixture illustrated in TABLE VI. Calculate the
molar weight of the mixture and determine if it is lighter or
heavier than air.
TABLE VI
EXAMPLE: ESTIMATING RELATIVE DENSITY
Material
Methane
Ethane
H2S
Nitrogen
Heptane
Octane
Total
Mi
16.04
30.07
34.08
28.02
100.2
114.22
xi
50%
10%
15%
15%
5%
5%
100%
Mi x i
8.02
3.0
5.11
4.2
5.0
5.71
31.04
The molar weight calculated for the mixture is 31.04. When
divided by the molar weight of air (28.96) the mixture has a
relative density of 1.07 indicating the mixture has neutral
buoyancy. This information would then be used to judge the
extent of the transient vapor zone associated with the
appropriate area classification diagram.
It should be noted that most industry references for
flammable materials do not provide information on the molar
weight of materials. A chemistry textbook must be referenced
to source this information.
C.
system will counteract as far as possible, the effect of the
disturbance on the system. This principle is applied to
estimating the MESG and subsequent group classification of
a flammable mixture.
The mathematical relationship for
estimating MESG using Le Chatelier’s principle is as follows:
Estimating the Group Classification of Mixtures
Determining the group classification of a mixture is
essential for determining the appropriate hazardous location
electrical equipment specifications.
For most upstream
production and downstream refinery facilities handling
hydrocarbon mixtures, the group classification is typically
group D under the division system and group IIA under the
zone system of area classification.
The two flammable materials that most often influence the
group classification of a hydrocarbon processing facility are
H2S and hydrogen. API RP500, RP505 and IP15 provide
some guidance in determining the group classification of
facilities handling H2S and hydrogen. Generally, a group C
or IIB group classification is not justified unless H2S
comprises at least 25% of the flammable mixture stream. A
group B or IIC classification is not justified unless the
hydrogen component of a flammable stream exceeds 30%.
For most hydrocarbon processing facilities, the H2S and
hydrogen rule of thumb guidelines are sufficient to determine
the group classification. For chemical processing facilities
handling flammable mixtures other than hydrocarbons, the
group classification is more complex. In these situations, Le
Chatelier’s principle can be applied.
Le Chatelier studied the influence of pressure, temperature
and concentration on systems at equilibrium. Le Chatelier
postulated that when a system at equilibrium is disturbed, the
∑
i
where
(2)
xi
MESGi
MESGmix = Estimated MESG of the mixture
MESGi = MESG of the individual constituent
xi = corresponding mole fraction
Consider the example of a flammable mixture consisting of
50% Diethyl Ether, a Group C, (IIB) classified material as
illustrated in TABLE VII.
TABLE VII
ESTIMATING MESG EXAMPLE
Material
Diethyl Ether
Methanol
Nitrogen
Isopropyl Ether
Methyl Ether
MESG (mm)
0.83
0.92
0.94
0.84
xi
50%
20%
15%
10%
5%
Group
C, (IIB)
D, (IIA)
D, (IIA)
C, (IIB)
Applying Le Chatelier’s principle to the mixture:
1
MESGmix =
0.5
0.83
+
0.2
0.92
+
0.15
∞
+
0.1
0.94
+
0.05
0.84
MESGmix = 1.014
Using TABLE III, the gas mixture is thus classified group D,
(IIA).
It is interesting to note that the MESG value obtained for
the mixture using Le Chatelier’s principle has a value greater
than all MESGs for each of the individual material
components. This is due to the effect nitrogen gas has on
calculating the MESG of the mixture. Inert gases will
influence the MESG of a mixture by reducing the oxidant
quality of the mixture. Le Chatelier’s principle will tend to
overemphasize the effect of the inert gas on the MESG of an
inert-flammable gas mixture and will produce odd results
when used with mixtures that contain a large percentage of
inert gas.
The same is true of mixtures that contain oxygen in excess
of a nominal 21% concentration. The higher concentration of
oxygen increases the volatility of the ignition producing higher
pressures which in turn render the MESG test values for the
individual components invalid. Materials such as carbon
monoxide or carbon disulfide that do not incorporate hydrogen
bonds can also result in erroneous results. Laboratory test
results has shown that Le Chatelier’s principle is reasonably
accurate provided the following situations are avoided:
[16][17]
1.
3.
4.
Flammable mixtures where a significant portion of the
gas is an inert.
Mixtures that incorporate oxygen as one of the
components.
Mixtures that contain greater than 5% carbon monoxide.
Mixtures that contain acetylene.
D.
Estimating LFL and UFL of Mixtures
2.
Estimating the lower flammable limit of a mixture is required
for point source calculation methods and for determining the
level of adequate ventilation in an enclosed area using the
fugitive emissions calculation procedure described in
appendix B of API RP 500 and 505. Upper flammable limits
are seldom used as a basis for area classification.
The LFL of a mixture may be estimated by applying a
derivation of Le Chatelier’s principle. The derivation uses only
the flammable components of the mixture with their mole
fractions adjusted to their relative percentage of the
flammable mix as follows:
1
%LFLmix=
∑
%LFLi
i
where
(3)
xiflam
%LFLmix = Estimated LEL of the mixture
%LFLi = %LFL of the individual constituent
xiflam = Corresponding mole fraction on a
flammable bases
Consider the flammable mixture example previously used
for estimating MESG.
TABLE VIII
EXAMPLE: ESTIMATING %LFL
Material
Diethyl Ether
Methanol
Nitrogen
Isopropyl Ether
Methyl Ether
xi
50%
20%
15%
10%
5%
100%
xiflam
58.8%
23.5%
11.8%
5.9%
100%
LFL
1.7%
6.0%
Le Chatelier’s principle provides flammability limits that are
close to the experimental values of simple hydrocarbon
mixtures with sufficient accuracy for most area classification
analysis. The approximation is valid with mixtures in air at
ambient pressures [18].
E.
Flammability of Mixtures
There are often situations where gas or fluid mixtures
contain flammables in low concentrations. The question
arises whether the mixture as a whole is flammable and
therefore must be considered as part of an area classification.
To determine the flammability of a mixture, an assessment
must be made to determine if the mixture contains sufficient
flammables to exceed the lower flammable limit of the mixture
as a whole. The calculation procedure is relatively easy for
mixtures in a gaseous state. It is more complex for fluid
mixtures where the flammable components may flash to
atmosphere based on ambient, operating or storage
temperatures.
1) Flammability of a Gas Stream: The flammability of
a process gas stream may be determined by first estimating
the %LFL of the mixture using Le Chatelier’s principle and
then comparing the %LFL to the total %flammables contained
in the mixture. If the %flammables exceed the %LFL, the
mixture is considered flammable [19]. Consider the example
in TABLE IX where a relatively small percentage of
flammables make up a gas mixture:
TABLE IX
EXAMPLE: DETERMINING THE FLAMMABILITY OF A GAS
STREAM
Material
Methane
Butane
Ethane
Air
% Vol
3
1.5
1
94.5
100%
xiflam
54.5%
27.3%
18.2%
Estimating the %LFL for the mixture using Le Chatelier’s
principle:
1
X 100 = 3.2%
%LFLmix =
1.4%
3.4%
LFL
5.0%
1.9%
3.0%
0.545 0.273
+
0.05 0.019
+
0.182
0.03
The flammable percent volume of the mixture is:
1
X 100
%LFLmix =
0.588 0.235
+
0.017 0.06
+
0.118
0.014
+
0.059
0.034
%LFLmix = 2%
The UFL of a mixture can also estimated using Le
Chatelier’s principle in a similar way.
%Flammix = 3% + 1.5% + 1% = 5.5%
Since the %Flammix exceeds the %LFLmix the mixture is
considered flammable and must be considered within the
context of an area classification.
2) Flammability of a Process Fluid Stream:
Determining the flammability of a process fluid stream
requires estimating the flashpoint of the mixture. A common
industry practice for estimating the flashpoint of mixtures is to
use the lowest published flashpoint for any of the pure
components. If the flashpoint temperature for the single
lowest pure component in the mixture is exceeded at ambient,
process or storage handling conditions, the mixture is
considered flammable.
This assumption is thought to provide a conservative value
for the flashpoint of a mixture however; further research has
shown that this assumption is not always valid. Mixtures can
exhibit lower flashpoints than any of the pure component
values depending on if the mixture is classed as ideal or nonideal.
An ideal mixture is defined as a mixture where all molecular
interactions are the same as if the individual components
were in their pure state at the same pressure and temperature
as the mixture [20]. The mixture will follow the properties of
Raoult’s law which allows the thermodynamic properties of
the mixture to be predicted based on the properties of the
individual components.
Using this principle, the vapor pressure and flashpoint of a
mixture can be estimated diagrammatically as illustrated in
Fig 6. The vapor pressure or the flashpoint of a mixture is
directly influenced by the mole fraction of the individual
constituents of the mixture.
PsatB
PsatA
Vapour Pressure
Tf,B
Flashpoint
xi
xi
(mole fraction)
(mole fraction)
Tf,A
Fig. 6 Vapor Pressure and Flashpoint Behavior of Ideal
Mixtures
Mixtures that exhibit ideal behavior include water-alcohol
and heptane-hexane hydrocarbon mixtures.
Non-ideal mixtures deviate from Raoult’s law in a positive
or negative manner depending on the molecular attraction of
the mixture components. Fig. 7 illustrates the behavior of a
mixture that deviates in positive manner from Raoult’s law.
Note that under certain conditions, the flashpoint of the
mixture may be less than the flashpoint of either component in
the mixture.
Most mixtures exhibit non-ideal behavior to a degree.
Methanol hydrocarbon mixtures are highly non-ideal and have
been shown to exhibit flashpoint temperature lower than
either component. This complicates the development of a
simple rule of thumb for determining the flammability of a fluid
mixture. Unless a mixture can be positively confirmed to
exhibit ideal behavior in accordance with Raoult’s law, the
mixture must be assumed to be flammable with the flashpoint
lower than what is published for any of the flammable
components.
The only reliable way of determining the
flashpoint and hence the flammability of a fluid mixture is by
test procedure.
Actual Vapour
Pressure
PsatA
PsatB
Tf,B
Ideal
Ideal
Actual Flashpoint
xi
xi
(mole fraction)
(mole fraction)
Tf,A
Fig. 7 Vapor Pressure and Flashpoint Behavior of Non-Ideal
Mixtures
F.
Autoignition Temperature
Estimating an AIT for a mixture is a challenging problem
due to the complex nature of the autoignition-combustion
process. There are no established mathematical relationships
to help in this regard. The only way to determine the AIT of a
mixture is by laboratory test.
If AIT data for a mixture is not available and the situation
precludes testing, often the lowest AIT of any pure component
in the flammable mixture is used as the AIT. This typically
results in a very conservative AIT value and may not reflect
the true nature of the hazard.
A rule of thumb often used to estimating the AIT of
hydrocarbon mixtures is to take the lowest AIT of any
component exceeding 5% (by volume) of the total mixture.
This is based on the assumption that the autoignition
temperature of a mixture will be influenced more by the
properties of the major components than by the minor
components in the mixture. This assumption has not been
verified and additional research in this area is required.
Rules in the NEC and the CEC require the maximum
surface operating temperature of electrical equipment to be
less than the ignition temperature of the specific gas or vapor
in a hazardous area. While the rules do not specify the use of
AIT’s (as opposed to hot surface ignition temperature), the
use of published AIT’s has been standard practice.
In some situations it may make sense to use a temperature
other than the published AIT based on documented
engineering judgment. A real life example involves the
installation of MI heat tracing on high temperature process
piping in heavy oil upgrader facilities. In these situations, it is
common for process piping to operate at elevated
temperatures in the range of 325°C in areas where diluents
(predominantly hexane - AIT 215°C), are present.
Historically, the heat trace design would have been limited to
a maximum surface operating temperature of 215°C on a pipe
that normally operates at 325°C. This results in multiple
passes of a lower wattage electric heat trace to limit the
maximum surface operating temperature of the tracer to less
than 215°C.
Recent upgrader facility designs have made documented
engineering decisions to allow the maximum design
temperature of the heat trace to be as high as the operating
temperature of the hot piping. This allows more flexibility in
the design of the heat trace system and typically reduces the
number of tracer circuits required to meet the process piping
heating requirements.
VI. APPLICATION GUIDELINES
The hazardous area classification for a facility is usually
documented using layout drawings and details that define the
degree and extent of the classified areas. The drawings also
provide pertinent information on the group classification and
autoignition temperature defined for the facility. For complex
facilities that incorporate a variety of flammable mixture
streams, the drawings are usually supplemented by an
engineering document that summarizes the basis for the area
classification. The hazardous area classification study report
should include:
1)
A description of the process and the flammable
materials handled.
2) A list of the codes, standards, recommended
practices and material references used in the
analysis.
3) A list of assumptions used to classify the facility
including the basis for normal and abnormal
operations. This would also include a section on
flammable mixtures and how their material properties
were estimated.
4) A hazardous materials worksheet that documents
the properties of the flammable materials handled
within the facility.
The flashpoint, group
classification, autoignition temperature, LFL/UFL and
hazard classification is summarized for each process
stream.
5) A source of release worksheet that identifies the
most likely release points within a facility.
6) The basis for the group classification and
autoignition temperature for the overall facility.
7) A summary of any fugitive emission and ventilation
calculations.
8) Recommendations for gas detector placement
including information on what flammable gases are
to be detected and the calibration procedure required
for gas detector maintenance.
9) A list of recommendations for operation and
maintenance activities performed in classified areas.
10) The area classification layout drawings should
incorporate a note that references the study report
as the bases for the area classification design.
The hazardous area classification study report provides
essential information for understanding the basis for the
original hazardous area classification and for planning any
future modifications to the area classification design for the
facility.
the molar weight of air at ambient temperatures and
pressures. Determining the group classification, estimating
the LFL, UFL, and predicting the flammability of a gas mixture
can be done using Le Chatelier’s principle.
The complex nature of the autoignition process precludes
the use of a simple mathematical model or industry rule of
thumb that can be used to estimate the AIT of a flammable
mixture. Additional research is required to develop a basis for
an industry accepted practice for estimating the autoignition
temperature of flammable mixtures.
X.
REFERENCES
[1]
NFPA 30, Flammable and Combustible Liquids Code,
National Fire Protection Association, Quincy, MA., 2003.
[2]
Institute of Petroleum, Model Code of Safe Practice –
Part 15, Area Classification Code for Installations
Handling Flammable Fluids, 3rd Edition, Energy Institute
(2003), Portland Press.
[3]
ANSI/API RP 500,
Recommended Practice for
Classification of Locations for Electrical Installations at
Petroleum Facilities Classified as Class I, Division 1 and
Division 2, American Petroleum Institute, Washington,
DC, 1998.
[4]
ANSI/API RP 505, Recommended Practice for
Classification of Locations for Electrical Installations at
Petroleum Facilities Classified as Class I, Zone 0, Zone
1, and Zone 2, American Petroleum Institute,
Washington, DC, 1998.
[5]
Propst, John, E., Volatility and Mists—Electrical Area
Classification's Important Variables, IEEE Transactions
on Industry Applications, Vol. 36, No. 2, March/April
2000.
[6]
ASTM E 681-04, Standard Test Method for
Concentration Limits of Flammability of Chemicals,
American Society for Testing and Materials,
Conshohocken, PA, 2004.
[7]
IEC 60079-4, Electrical Apparatus for Explosive Gas
Atmospheres Part 4: Method of Test for Ignition
Temperature, International Electrotechnical Commission,
Geneva, Switzerland 1975.
[8]
ASTM E659-78, Standard Test Method for Autoignition
Temperature of Liquid Chemicals, American Society for
Testing and Materials, Conshohocken, PA, 2005.
[9]
NFPA 325, Guide to Fire Hazard Properties of
Flammable Liquids, Gases and Volatile Solids, National
Fire Protection Association Quincy, MA., 1994.
VII. CONCLUSION
Performing a hazardous area classification risk analysis
requires an understanding of how flammable mixtures
behave. In an ideal world, the properties of a flammable
mixture would be determined by test procedure; however, this
is impractical in most cases. Methods are required to
estimate the properties of flammable mixtures to support a
hazardous area classification analysis.
Predicting the relative vapor density of a mixture is a simple
calculation that compares the molar weight of the mixture to
[10] API RP 2216, Ignition Risk of Hydrocarbon Liquids and
Vapors by Hot Surfaces in Open Air, American
Petroleum Institute, Washington, DC, 2003.
[11] Colwell, J.D., Reza, A., Hot Surface Ignition of
Automotive and Aviation Fluids, Fire Technology, 41,
105-123, 2005.
[19] Crowl, Daniel, A., Understanding Explosions, American
Institute of Chemical Engineers, New York, New York,
2003
[12] NFPA 497, Recommended Practice for the Classification
of Flammable Liquids, Gases, or Vapors and of
Hazardous (Classified) Locations for Electrical
Installations in Chemical Process Areas, National Fire
Protection Association Quincy, MA., 2004.
[20] Perry’s, Chilton, Kirkpatrick, Perry’s Chemical Engineers
Handbook, McGraw-Hill Inc. New York, New York, 1963.
[13] Fire Protection Guide to Hazardous Materials, National
Fire Protection Association, Quincy, MA., 2002.
[14] IEC 60079-10, Electrical Apparatus for Explosive Gas
Atmospheres – Part 10 Classification of Hazardous
Areas, International Electrotechnical Commission,
Geneva, Switzerland, 2002.
[15] IEC 60079-20, Electrical Apparatus for Explosive Gas
Atmospheres – Part 20 Data for Flammable Gases and
Vapours, related to the use of Electrical Apparatus,
International Electrotechnical Commission, Geneva,
Switzerland, 2000.
[16] Briesch, Edward, NEC Group Classification of Mixtures,
American Institute of Chemical Engineers, 34th Loss
Prevention Symposium, Atlanta, Georgia, March 8, 2000.
[17] Report on Proposals NFPA 497, Report of the
Committee on Electrical Equipment in Chemical
Atmospheres, F2007.
[18] Hristova, M., Tchaoushev, S., Calculation of Flash Points
and Flammability Limits of Substances and Mixtures,
Journal of the University of Chemical Technology and
Metallurgy, 41, 3, Pg 291-296, 2006.
H. VITA
Allan Bozek, P.Eng., MBA, is a Principal with EngWorks Inc.
providing consulting engineering services to the oil and gas
industry. He is a registered professional engineer in the
provinces of Alberta and British Columbia, Canada and has
been a member of the IEEE since 1989. Allan’s areas of
expertise include hazardous area classification, power
systems design, protective relaying and grounding for large
scale industrial and petrochemical facilities.
Mr. Bozek
graduated from the University of Waterloo in 1986 with BASc
in Systems Design Engineering and a MBA from the
University of Calgary in 1999. Allan may be contacted at
ABozek@EngWorks.ca
Vince Rowe retired from Shell Canada in 1994 and continues
to be active in the Oil and Gas industry as a consultant. He
provides training on electrical installations and maintenance in
classified locations. He is a partner with Marex Canada
Limited and a member of the steering committee for the
Canadian Electrical Code. He is Chairman of the Hazardous
Location and Electric Heating sections and one of the
initiators of the Objective Based Industrial Electrical code for
Canadian Industry. Mr. Rowe graduated from the University
of Manitoba in 1960 with a Bsc in Electrical Engineering.
Vince may be contacted at Vince.Rowe@shaw.ca