Classifying Explosion Prone Areas for the Petroleum, Chemical and Related Industries

425 Pages·1995·5.33 MB·English

by  W.O.E. Korver (Auth.)

ABOUT THE AUTHOR
W.O.E. Korver has over fifteen years experience in
electrical design and engineering for chemical and
petrochemical plants, and over ten years experience
in field construction, as well as many years
experience in electrical design and engineering for
fossil and nuclear power plants.
He has been involved in classifying hazardous
locations for the chemical and petrochemical
industries for over 30 years.
Mr. Korver has a Master's degree in electrical power
engineering. He is currently employed by the Jet
Propulsion Laboratory, California Institute of
Technology, Pasadena, California, as a Safety
Engineer. His primary responsibility is to execute
and monitor safety requirements for electrical power
installations, explosion-prone areas and other safety
related matters.
PREFACE
The intent of this publication is to provide an in-depth understanding of the
factors that influence the classification of a hazardous location. One factor, in
combination with one or more other factors, will have an impact on the level of
danger and its hazardous boundaries. These factors and their influences will be
explained in detail in this publication, and once their impact is understood, the
classification of a hazardous location becomes a straightforward procedure.
The intent of the classification of a hazardous location is to provide safety for
personnel and equipment. The intent is also to achieve an economical electrical
installation which will provide an acceptable level of safety for personnel and
equipment at the lowest possible cost. To accomplish this, it is necessary to
analyze in detail the environmental conditions of the location and the
characteristics of the source of hazard. The engineer who is involved in preparing
the area classification must understand all of the details which will have an impact
on his decision to classify the area Division 1, Division 2, or Nonhazardous.
Without the knowledge of the environmental conditions, and the characteristics of
the source of hazard, he most certainly will give the location a safety level much
too high, which is not economically justified, or a level too low, which is unsafe.
It is this approach that must be avoided.
In nine out of ten cases, a hazardous location is classified much too
conservatively. The reason for this conservative approach is a lack of knowledge
and a misunderstanding of the actual concept of safety and danger. In the
majority of cases, hazardous areas are classified Division 1 when the location
could have been classified Division 2, and areas which are classified Division 2
could have been classified nonhazardous. In other cases, the location is classified
nonhazardous when it should have been classified Division 1 or Division 2. It
must be kept in mind that a location classified Division 1 requires explosive proof
equipment, which equipment will range in price from two to four times the cost
of general purpose electrical equipment, some of which are allowed in Division
2 locations. Therefore, it is important to strive to a classification of a lower but
acceptable level of safety, but one which is commensurate with an acceptable risk
and which reduces the cost of electrical installation.
vii
INTRODUCTION
The degree of danger in the atmosphere of a hazardous location needs to be
determined prior to selecting an acceptable electrical equipment installation.
If maximum safety is the predominant factor in determining the type of
electrical installations, the cost of electrical equipment will be extremely high. If
low cost of electrical installation is the predominant factor, safety to personnel and
equipment may be unacceptably low.
It is, therefore, necessary to find a point of balance at which the cost and
safety requirements are both satisfied and acceptable. The purpose of this
publication it to establish such a point, enabling both the cost of electrical
installation and the safety of personnel to be satisfied. To establish such a point,
it is necessary to evaluate the characteristics of the flammable products, along with
the conditions under which the product must operate. By listing this information
on appropriate forms, the evaluation of the degree of hazard and its boundaries
can be correctly performed and, as a result, the proper electrical equipment can
be selected under the provisions of the NEC.
Tables and illustrations have been developed to assist the engineer in
establishing the degree of danger and its boundaries for locations with flammable
products.
This publication has been divided into three sections with an appendix.
Section 1 will discuss the flammable and combustible principles of hazardous
products and other pertinent information associated with an area classification.
Section 2 will discuss the environmental conditions in hazardous locations. A
number of specific illustrations are included in this section. Section 3 will discuss
the application procedure for classifying NEC Class I locations. A number of
examples are included in this section. Following these sections is an appendix
listing properties of flammable liquids, gases, and vapors.
The application of the information explained herein is mainly for flammable
liquids, vapors, and gases which are processed, handled, stored, and/or transferred.
A small portion of this publication explains the classification of coal handling
facilities.
Where there may still be cases that are difficult to solve, sound engineering
judgment should be applied in compliance with the content of this publication.
CHAPTER 1
FLAMMABLE AND COMBUSTIBLE PRINCIPLES OF HAZARDOUS
PRODUCTS
A. General
A hazardous product, as described herein, is a product that has the capability
of being easily ignited. The hazardous product may consist of flammable liquid,
flammable gas, or combustible dusts. When these vapors, gases, or dusts are in
the right proportion with air, they will explode when ignited with a sufficient
amount of heat. Sources of sufficient heat shall be mainly considered as coming
from electrical equipment, although in rare cases, sources that are not electrical
are considered also.
B. Flammable and Combustible Liquids
.1 Classification of Flammable and Combustible Liquids
According to the standard and basic classifications of flammable and
combustible liquids, NFPA 321, flammable and combustible liquids are grouped
into three classes: Class ,I Class II, and Class III. These classes refer to the
flammability class of a flammable product.
A Class I liquid is a flammable liquid with a closed-cup flashpoint below
100~ at a vapor pressure not exceeding 40 psi. A Class II liquid is a combustible
liquid with a closed-cup flashpoint at or above 100~ but below 140~ A Class
III liquid is a combustible liquid with a closed-cup flashpoint at or above 140~
Class I liquids when released to the atmosphere in large quantities may
produce large volumes of gases, especially when the more volatile types (such as
gasoline, propane, propylene, ethane, ethylene, butane, etc.) are released. These
types of flammable substances should be treated very conservatively since they
may cover large areas before reaching safe concentrations.
Class II liquids will produce vapors in their explosion range close to their
point of release when heated above their flashpoint. Some of the kerosene and
diesel fuels, for example, are not much of a hazard because they produce very
Flammable dna Combustible Principles of Hazardous Products 5
small quantities of vapor when heated above their flashpoints.
Class III liquids are generally not considered for area classification because
the extent of the hazardous area is very small and only close to the point of
release.
.2 Flashpoint
The flashpoint of a flammable or combustible liquid is a condition in which
sufficient amounts of vapors are produced by the liquid to form an ignitable
mixture with air or oxygen at the surface of the liquid. The flashpoint is
measured by a standard ASTM in which the temperature of the liquid is slowly
increased and by periodically exposing the vapor space above the liquid to an
ignition source. When the vapor first flashes or bums, the temperature of the
liquid is called the flashpoint. In other words, the flashpoint is the temperature
of the volatile liquid above which there is a danger of fire or explosion.
Flammable vapors may also be present at temperatures below the flashpoint since
evaporation will also take place below the flashpoint. These vapor concentrations
with temperatures below flashpoint are below the lean limit and will not ignite,
therefore are not considered dangerous. Flashpoint is usually a few degrees below
the LEL.
.3 Ignition Temperature
The ignition temperature is the minimum temperature necessary to ignite a
combustible mixture, thus causing an explosion or fire.
.4 Vapor Density
Vapor density is the weight of a volume of vapor or gas without the presence
of air compared with the weight of an equal volume of dry air at the same
pressure and temperature. A figure less than 0.75 indicates a vapor is lighter than
air, and a figure that is greater than 0.75 is heavier than air. Between heavier- and
lighter-than-air densities, there is a grey area where the gases and vapors seem
to be undecided which way to go. Gases and vapors that have densities between
0.75 and 1.0 may travel along the floor first before rising. For example, if an
airborne gas or vapor with a density between 0.75 and 1.0 is not instantly caught
by ventilating air, it may act as a heavier-than-air flammable gas or vapor. If it
is instantly caught by ventilating air it will behave as a lighter-than-air flammable
gas or vapor. Vapor density is calculated by dividing the molecular weight of a
vapor by 29, where 29 is the composite molecular weight of air. The molecular
weight of Naptha Petroleum, for example is 72.5 and its vapor density is
72.5/29=2.5.
6 Classifying Explosion-Prone Areas
Heavier-than-air gases or vapors will travel along the floor, covering large
horizontal areas when the temperature is at or above Ilashpoint. Lighter than air
gases or vapors have the tendency to rise, thereby covering only small horizontal
areas when-the temperature is at or above tlashpoint.
.5 Explosion Range of Flammable saG or Vapor
The presence of a flammable gas or vapor in the air is not sufficient to cause
an explosion. An explosion will occur only when the gas or vapor has mixed with
air or oxygen in a ratio in which the gas or vapor concentration is within certain
limits. These limits are known as the lower (LEL) and upper (UEL) explosion
limits and are expressed in terms of percentage by volume of gas or vapor in air.
Between the explosive limits the range is known as the explosive range. This
range may vary from a few percent to 100% as shown in Fig. 1-1A.
Below the LEL the mixture is too lean for combustion, because there are
insufficient gas or vapor molecules. Above the UEL the mixture is too rich for
combustion because there are too many gas or vapor molecules. However, within
the LEL and UEL range, combustion is possible and the flame will spread
throughout the mixture when it is ignited. This is known as flame propagation,
and if the flame propagation is very rapid, it is popularly called an explosion. For
example, the LEL of hydrogen gas is 4%, and the UEL is 75%. When the
mixture of gas/air contains a concentration of gas of less than 4%, the resulting
mixture is too lean (not enough fuel) for combustion. Should the mixture contain
more than 75% of gas in air, the resulting mixture is too rich (too much fuel and
not enough oxygen) for combustion. The mixture can only cause an explosion if
the gas concentration is in between 4% and 75%, as shown in Fig. l-lB.
The maximum rate of explosive pressure developed by hydrogen gas is at a
point within the explosion range, specifically at a point between 4 and 75%.
A different case is presented by volatile liquid fuels slowly vaporizing into
the air. Initially, the fuel vapor concentration will be below the lean limit. At
this point the mixture cannot be ignited. As the vaporization progresses with time,
the fuel vapor concentration in the air will reach the lean limit, at which point
combustion is possible. When the lean limit is exceeded and if the mixture is
ignited, the flame will propagate through the mixture. The ignitable limits are
based on normal atmospheric temperature and pressure. There may be variations
in the explosive limits at temperatures and pressures above or below normal.
An increase in temperature of the mixture will cause the flammable range to
shift downwards and a decrease in temperature will shift it upwards. Under most
conditions an electrical spark discharge will ignite a flammable mixture. The
minimum amount of energy required to ignite a flammable mixture varies with the
fuel. Therefore, there is an absolute minimum below which ignition will not take
place. The minimum arcing energy to ignite hydrocarbon-air mixtures varies
Flammable and Combustible Principles of Hazardous Products 7
100
100
TOO RICH
90
80
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GROUP A GROUP B GROUP C GROUP D
ACETYLENE HYDROGEN ETHYLENE PROPANE
GAS GAS GAS GAS
FIG. 1-1A EXPLOSIVE RANGE OF NEC CLASS I FLAMMABLE PRODUCTS
8 Classifying Explosion-Prone Areas
DIPAR EXPANSION YLRALUPOP
CALLED NOISOLPXE ~
--EXPLOSION EGNAR
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THERE ERA TNEICIFFUSNI
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FIG. 1-1B. FLAMMABLE VAPOR CONCENTRATION NI TERMS OF PERCENTAGE VAPOR IN AIR
Flammable dna Combustible Princiopfl es Hazardous stcudorP 9
from 0.017 to 0.3 millijoules. Hydrogen gas, for example, can be ignited by 0.017
millijoules.
.6 Explosion Hazard as a Function of Temperature and System Pressure
Temperature and pressure are important aspects in classifying a hazardous
location.
In classifying a hazardous location, it is necessary to consider the temperature
and the pressure in the system of the process equipment containing a flammable
liquid. Temperature and pressure have a great impact on the quantity of
flammable vapors released to the atmosphere. System temperature and system
pressure are closely related--- the higher the liquid temperature; the higher the
pressure in the system. The larger the quantity of vapors a flammable liquid is
capable of releasing into the atmosphere; the greater the hazard. A hazardous
condition exists when the temperature is above flashpoint. Systems at
temperatures below flashpoint are not considered hazardous.
When a temperature above flashpoint is applied to a flammable liquid in a
closed containment, a pressure increase in the system is developed. The higher
the pressure in the containment, the greater the possibility of a rupture. If the
closed containment is under pressure and ruptures, flammable liquid will start to
evaporate. The evaporation rate of the liquid is a function of the vapor pressure
and the temperature and the liquid discharge rate. The liquid discharge rate, in
turn, is a function of the system pressure and the size of the rupture opening.
Systems under pressure are expressed in terms of low, moderate and high. Low
pressure is considered less than 100 psi; moderate pressure ranges from 100 to
500 psi; and high pressure is above 500 psi.
Generally, Class I, Class II, and Class III liquids have different evaporation
rates because of their different vapor pressures. For a given temperature, the
vapor pressure of a Class I liquid will be higher than that of Class II and Class
III liquids, and the vapor pressure for a Class II liquid will be higher than that of
Class III liquids. Consequently, these vapors will cover different floor distances
when released into the atmosphere.
Vapors from a Class I liquid will spread out farther than the vapors from
Class II and Class III liquids, and the vapors of a Class III liquid will stay closer
to the point of release.
These conditions are illustrated in the example Table 1.1, where heavier-
than-air vapors are released to the atmosphere from a liquid spill.
01 Classifying Explosion-Prone Areas
Table 1.1. Vapor Pressure Versus Vapor Traveling Distances
Hash Liquid Vapor Vapor
Liquid Point Temp. Pressure Traveling
Class ~ F* ATM Distance
I 50 200 0.45 Large
II 100 200 0.12 Small
III 140 200 0.048 Minimal
Class I and Class II flammable liquids are considered dangerous when
temperatures are above flashpoint. The danger of a Class III flammable liquid is
only considered at the surface of the liquid when the temperature is above
flashpoint and, therefore, these vapors will not render any significant hazard.
Flammable liquids are also capable of evaporation at temperatures below
flashpoint. These vapors, however, are not considered explosive.
The temperature of the surrounding air may provide an additional limit to the
vapor travel distance. If the temperature of the surrounding air is lower than the
temperature of the liquid, the vapors will cool and form a mist, thereby reducing
the vapor traveling distance.
.7 Extent of Hazard as a Function of Molecular Weight
It is expected that the vapor traveling distances are equal when the vapor
pressures are kept the same. This is not necessarily true because of differences
in molecular weights.
When the vapor pressures of different classes of vapors are kept the same,
as shown in Table 1-2, vapors from a Class I liquid will generally cover a larger
area than the vapors from Class II and Class HI liquids, and vapors from a Class
II liquid will cover a larger area than the vapors of a Class III liquid. The
difference in areas covered is due to varying molecular weights. For example, if
the molecular weight of Class I vapors is lighter than Class II vapors and Class
HI vapors, and the molecular weight of Class II vapors is lighter than Class HI
vapors, the vapor traveling distances are not the same. When the vapor pressure
is kept the same, vapors from a lighter Class I liquid will generally cover a larger
area than the vapors from heavier Class II and Class III liquids, and vapors from
a lighter Class II liquid will cover a larger area than the vapors from a heavier
Class III liquid. This is shown for heavier-than-air vapors in Table 1-2.
Table 1-2 shows that if the vapor pressure is the same, but the molecular
weight of the vapors vary, vapor distances will also vary. Since the density of a
vapor is the weight of a volume of pure vapor compared to the weight of an equal