100 Introduction to Fire Prevention
Abstract
This section covers the Company’s policy on fire protection and discusses the principles of combustion. (Refer to Section 200 for application of the principles.)
Chevron Corporation
Contents
Page
110
Fire Prevention Policy at Chevron
100-2
111
Corporate Policy 530
112
Incident Free Operation
113
Principles of Fire Protection
114
Process Safety Management
115
Responsibilities of Personnel
120
Principles of Combustion
121
Definitions
122
Fire Triangle
123
Fuel
124
Flammable Limits
125
Source of Ignition
126
Spread of Flame
127
Explosion Hazard
128
Gas Testing
130
References
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110 Fire Prevention Policy at Chevron
This section gives the Company’s policy on fire protection, lists the principles of fire
protection, and discusses how effective fire prevention includes good management
of process hazards. Responsibilities of personnel are also given.
111 Corporate Policy 530
Corporate Policy 530, dated October 5, 1989, for Safety, Fire, Health and the Environment, sets the standard for fire prevention efforts in the Company. The policy
applies to Company operations worldwide and states:
It is the policy of Chevron Corporation to conduct its business in a socially
responsible and ethical manner that protects safety, health and the environment. The goal is to be a leader within the industry by emphasizing innovation
and encouraging creative solutions, both of which will improve our competitive position. To that end, the Company will:
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Integrate safety, fire, health and environmental protection into every aspect
of its business activities.
Comply with all safety, fire, health and environmental laws or regulations
without regard to the degree of enforcement.
Seek opportunities to participate in the formulation of safety, fire, health
and environmental legislation, regulation or policy issues that may significantly impact our business. Work actively with the appropriate governmental agencies to ensure timely, reasonable and cost effective solutions
for issues wherever possible.
Encourage employees to initiate and maintain an open dialogue within the
Company and with the public or its agents regarding safety, fire, health and
environmental matters. This includes recognizing and responding as appropriate to Company and community concerns about such matters.
Follow relevant standards, good engineering practices and principles of
risk management to ensure Chevron's safety, fire, health and environmental protection activities are conducted responsibly. This specifically
includes the design and siting of new facilities or modifications to existing
facilities.
Exhibit socially conscious leadership and demonstrate exemplary safety,
fire, health and environmental performance.
Conserve Company and natural resources by careful management of emissions and discharges and by eliminating unnecessary waste generation.
This also includes wise use of energy in our operations. Discretionary environmental, health and safety expenditures should be managed prudently to
enhance Chevron's long-term competitive position.
Conduct scientific hazard and risk assessments, as needed, to identify,
characterize and safely manage any present or future potential hazards of
Company products and operations.
Maintain a product stewardship program by taking the appropriate steps to
understand the health effects of Chevron products and to inform
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employees, contractors, distributors, customers and the public with regard
to the proper handling, use and disposal of these products.
Assess potential safety, fire, health and environmental liabilities prior to
the sale, lease, transfer or purchase of real property. Work to resolve any
problems created by past operations, practices or handling of oil or
hazardous materials.
Ensure conformity with this policy by a comprehensive compliance
program including audits.
112 Incident Free Operation
The 1995 study on achieving Incident Free Operation (IFO) identified that safety,
reliability, and human performance are the keys to profitable operations
•
Safety must come before production
•
There needs to be a focus on system reliability which leads to predictable operations
•
Improvements to human performance through leadership direction and
balanced accountability is needed to achieve IFO
Tenets
As part of IFO, Refining identified 10 Tenets which establish their values and principles guiding their daily decisions. The Tenets are based on two common-sense
principles:
•
Do it safely or not at all.
•
There is always time to do it right.
The 10 Tenets are:
Chevron Corporation
1.
Never operate equipment or tanks outside of design or environmental limits.
2.
Always move to a safe, controlled condition, and seek assistance when a situation is not understood.
3.
Always operate with safety devices armed.
4.
Always follow all safe work practice procedures and act to stop unsafe conditions and actions.
5.
Always produce a quality product for your customer. Never transfer off spec
product.
6.
Never contaminate or compromise a dedicated product system.
7.
Always report environmental/safety compliance information accurately and on
time.
8.
Always address abnormal conditions and clarify/understand procedures before
proceeding.
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100 Introduction to Fire Prevention
9.
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Always follow written procedures for high-risk or unusual situations.
10. Always involve people with expertise and first-hand knowledge in decisions,
improvements, and changes that affect procedures and equipment.
113 Principles of Fire Protection
The five principles below augment Corporate Policy 530. Fire protection programs
should be built on these principles.
•
It is generally better to spend resources on fire prevention than on fire suppression.
•
Facilities having fire potential are designed, built, and maintained in a manner
compatible with applicable industry codes and consensus standards.
•
Fire protection is an integral part of the design, construction, maintenance, and
operation of a facility.
•
Fire protection programs are maintained and kept up to date by periodic inspection to ensure effective performance.
•
Those responsible for implementing fire prevention measures must know the
philosophy behind modern petroleum fire prevention programs and must understand the fundamentals of ignition and combustion. The Fire Protection
Manual tries to give this needed information.
114 Process Safety Management
Fire protection includes concern for any release having the potential to ignite.
Because of this, a good fire protection plan for facilities having potential for fire
will include, in addition to specific fire protection programs outlined in this manual,
applicable elements of process safety management described below.
The objective of process safety management is to prevent accidental release of toxic
or flammable substances that have the potential for impacting the public. This
means having effective management controls in key areas of design, construction,
operation, and maintenance. The same elements of management control effective in
preventing large, accidental releases are equally effective in preventing small incidents.
A process safety management program includes:
Employee participation. Facilities must involve employees in the design, implementation and use of the process safety management elements.
Process safety information. Facilities must have all current and necessary information describing the hazards of the materials being handled, as well as applicable
process and mechanical design information, such as Piping and Instrumentation
Diagrams (P&IDs).
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Process hazards analysis. An analysis, using an accepted, recognized method, such
as a Hazard and Operability Study (HAZOP), needs to be performed in order to
identify potential hazards, assess the likelihood and consequences of a failure, and
identify changes that could mitigate the risks involved.
Management of change. Many serious incidents have resulted from changes made
to a processing unit after initial designs are completed and the plant has been operating. All changes should be reviewed and all hazards associated with change
should be identified and controlled.
Operating procedures. Written operating procedures need clear instructions for all
phases of operation, including startup, shutdown, and interim activities (e.g., catalyst decoking). The procedure should contain operating limits, and steps to avoid or
correct undesirable process deviations.
Safe work practices. Practices need to be set up to ensure the safe operation, maintenance and modification of our facilities as well as the control of materials and
substances that could affect process safety.
Training. All personnel operating and working in the facility must be trained
adequately. Training must be in accordance with accepted practices and standards
for the duties and responsibilities of personnel.
Mechanical integrity and quality assurance. Process equipment needs to be
designed, fabricated, installed, and maintained in a manner consistent with the
service requirements. This includes maintenance, testing, and inspection procedures. It also includes positive material identification of alloy products.
Pre-startup safety review. Pre-startup reviews must be conducted to confirm that
construction complies with the specifications and that safety, operating, maintenance, and emergency procedures are in place and adequate.
Emergency response and control. An emergency response plan needs to be established in accord with the requirements of OSHA 29 CFR 1910.38a, “Employee
Emergency Plans and Fire Prevention Plans.” The plan should include emergency
procedures and hypothetical drills.
Incident investigation. Incidents that result in, or could have caused, a catastrophic
release need to be investigated in order to learn from the incident, prevent a recurrence, and help prevent similar incidents. This information should be disseminated
in a timely fashion to the rest of the Corporation.
Audit of the Process Safety Management Systems. All elements of process safety
management outlined above need to be audited periodically to ensure they are being
effectively implemented.
For more information on process safety management, refer to the Refinery Process
Safety Management Guidance Document.
115 Responsibilities of Personnel
Effective fire prevention requires giving the following responsibilities to personnel.
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Management Responsibility
Responsibility for fire prevention lies ultimately with the management of each
facility. Management may delegate specific parts of fire prevention to subordinate
managers, supervisors, or specialists, but such personnel must be accountable to
management for properly carrying out the duties assigned them.
Employee Responsibility
Employees are responsible for their personal safety and for the safety of fellow
employees. This responsibility includes: following procedures outlined by management, using and maintaining equipment as instructed, and being alert to unforeseen
fire hazards. Consistent with their training, employees should help prevent the
spread of fire and minimize any losses.
Employee Fire Fighting Responsibility
Employees must be trained for the level of fire response expected of them. Consistent with this training, response might include varied roles, such as isolation of fuel
sources and suppression of incipient-stage fires. Employees expected to fight fires
beyond the incipient stage require appropriate training and personal protective
equipment.
120 Principles of Combustion
Effective fire prevention requires a thorough understanding of: 1) conditions under
which flammable and combustible materials vaporize; 2) what is meant by an ignitable or flammable mixture; and 3) what is meant by a source of ignition and how
combustion spreads from such a source. This section discusses these topics.
121 Definitions
Combustion (burning) is the rapid oxidation reaction between a reducing agent
(fuel) and an oxidizer (usually oxygen in the air) accompanied by the evolution of
heat and usually the production of flame.
Fuel means a material capable of combusting and particularly designates the material (gas, liquid or solid) which feeds a fire.
Flammable refers to any material that is easily ignited and burns rapidly, usually
gases or liquids. A flammable liquid is one having a flash point below 100°F, and a
vapor pressure not exceeding 40 psia at 100°F.
Combustible refers to a material that can burn and, with respect to accidental ignition and flame spread, the word implies a lower degree of risk than “flammable.”
The word combustible is frequently applied to solid fuels and to liquids having a
flash point at or above 100°F.
Lower Flammable Limit (LFL) is the lowest volume % of vapor in air that can be
ignited.
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Upper Flammable Limit (UFL) is the highest volume % of vapor in air that can be
ignited.
Limiting Oxygen Concentration (LOC) is the minimum oxygen concentration to
support combustion.
Flash Point is the lowest temperature at which a flame will ignite a vapor of a
combustible liquid in air.
Auto-Ignition occurs when a substance is mixed with air at a temperature high
enough to initiate an oxidation reaction that will generate enough heat to cause
spontaneous ignition. This can occur when the material released is hot or when the
material is released into a hot environment. (See Section 240 for a detailed discussion.)
Inerting is adding inert gas to reduce the oxygen concentration below the LOC (at
least 2 vol % below LOC is target).
Enriching is adding combustible gas to increase the volume percent above the UFL
(200% of UFL is target).
Dilution is adding air to dilute the volume % vapor below the LFL (25% of LFL is
target).
Deflagration is when a flame front propagates by heat and mass transfer. Flame
speeds up to 400 m/sec; pressures 7-9 bar. Most vapor cloud explosions would fall
into this category.
Detonation is when a flame front propagates by shock wave compression at supersonic flame speeds of 1,800-2,000 m/sec; pressures up to 20 bar. Generally requires
containment.
122 Fire Triangle
For a fire to start, three things must be present at the same time and place: fuel
(vapor), oxygen (air), and a source of ignition.
Chevron Corporation
1.
Fuel must be present in a vaporized form. Liquid fuel mists that are readily
converted to vapor or finely divided solid fuels have much the same combustion characteristics as vapor. (Carbon and some metals are exceptions to the
general rule that fuel must be in the form of vapor before it can burn.)
2.
Oxygen (usually in the form of air) must be present and mixed in suitable
proportion with the fuel vapor to form an ignitable mixture.
3.
A source of ignition (high temperature and sufficient energy to start the chemical reaction of combustion) must be present.
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123 Fuel
How hazardous a fuel is depends on factors such as the fuel’s vapor pressure, partial
pressure, flammable limits, and flash point. These factors, as wells as tests to
measure flammability, are discussed next.
Vapor Pressure
To understand the process of vaporization, consider the boundary surface between a
liquid and a closed air-free space above it. Molecules of the liquid tend to escape the
liquid state and assume the properties of gas. Other molecules previously released
may strike the liquid surface and re-enter it. When the number of molecules leaving
and re-entering the surface becomes equal, a state of equilibrium is said to exist. At
equilibrium, the pressure exerted by the molecules in the vapor state is called the
vapor pressure of the liquid at that temperature. Vapor pressure is characteristic of
any liquid. The vapor pressure of a liquid increases as its temperature is raised.
To permit easy comparison, vapor pressures of petroleum liquids are usually
measured at a standard temperature—100°F—by the Reid method described in
ASTM Standard D-323. See Section 2.3 of API RP 2003 (in “Industry Specifications”) for a discussion of vapor pressure and flammability.
From the fire protection standpoint, it is the vapor pressure of petroleum liquid at
the temperature at which it is handled that is significant. This vapor pressure
controls the composition of the air-vapor mixture over the liquid surface.
Because vapor pressure cannot readily be measured in the low range where it is
significant as a criterion of fire hazard, testing a liquid to determine its flash point is
generally used to determine fire hazard. (Flash point is discussed below.)
Partial Pressure
If the space above the liquid should already contain a gas, for example, air vaporization will proceed exactly as above. When equilibrium is established, the space
above the liquid will contain just as many vapor molecules as though the air were
not present. Moreover, the vapor exerts the same pressure as it would exert if it
occupied the entire volume by itself. The air is still present, however, and continues
to exert its original pressure. Thus the total pressure exerted by the vapor-plus-air
mixture will equal the sum of these two pressures. The pressures exerted by each of
these components are called their “partial pressures.” The partial pressure of the
vapor, divided by the total pressure of air-plus-vapor, is the volume percent concentration of the vapor.
Vol %vapor = Ppvapor/(Pair + Pvapor)
124 Flammable Limits
A fuel vapor-air mixture cannot be ignited unless the ratio of vapor to air lies within
certain well-defined limits called the lower and upper limits of flammability. These
limits are usually expressed in terms of volume percent at atmospheric pressure and
temperature. The smallest concentration (percentage by volume) of fuel vapor in a
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vapor-air mixture that can be ignited is called the lower flammable limit (LFL).
Similarly, the highest percentage by volume of fuel vapor in air in which ignition
can be produced is called the upper flammable limit (UFL). The region between
these two percentages is called the flammable range.
For gasoline vapors, this range extends from a little over 1% to almost 8% by
volume of gasoline vapor in air. Mixtures containing less than about 1% are said to
be too lean to burn; they cannot be ignited by any source of ignition, however
intense. Mixtures containing more than about 8% are said to be too rich to burn. A
closed space filled with such a mixture cannot be ignited, but the mixture can be
further diluted so that it will ignite and burn if the mixture is allowed to escape into
the open air.
Figure 100-1 lists representative hydrocarbons and their respective flammable
limits.
Effect of Oxygen Concentration on Flammable Range
The oxygen concentration has a major impact on the combustion process.
Figure 100-2 is a flammability diagram of a Methane/Nitrogen/Oxygen Mixture.
The vertical axis represents Methane in Oxygen. As you can see, the LFL is 5%
methane and the UFL is 60% methane. If you look at the line representing air on the
chart, the flammable range has been lowered to 5%-15%. As you continue to
decrease the oxygen level by adding nitrogen or methane, the flammable range
continues to narrow to a point where combustion is no longer supported. This represents the limiting oxygen concentration (LOC). For methane, the LOC is 12% in
nitrogen.
Besides increasing the flammable range, increasing the oxygen concentration has
the following effects:
•
•
Decreases the energy needed for ignition so that low energy ignition sources
now become hazardous
Increases the energy of combustion resulting in much more damage from
explosions
Inerting, Enriching and Dilution
Figure 100-3 is the flammability diagram for a methane/inert/air mixture. Note that
this diagram is different from Figure 100-2 in that it uses air as the basis instead of
oxygen and it shows various types of inert gases. The line marked Cst is the stoichiometric mixture line.
Inerting is the addition of inert gas to reduce the oxygen concentration below the
LOC. Using Figure 100-3, the LOC is reached at about 34% nitrogen and 6%
methane. This equates to a 12% LOC [.21 (100-34-6)]. Per NFPA 69, Chapter 2,
systems should be inerted to an oxygen concentration of at least 2% below the LOC
if the gas is being continuously monitored (for methane this would be 12%2%=10% maximum oxygen concentration) or 60% of the LOC if it is not being
continuously monitored (for methane this would be 12%(.6) or 7.2% maximum
oxygen concentration).
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Properties of Flammable Liquids and Gases(1) Part of table derived from NFPA 325M,  1994, National
Fire Protection Association, Quincy, MA. Used with permission.
Fig. 100-1
Product
Lower and Upper
Flammable Limits(2), %
by Volume
Auto-ignition
Temperature, °F
Flash
Point,°F
Autoignition
Temp., °F
Vapor
Density
(Air=1)
Closed
Cup
Boiling
Point,°F
LFL
UFL
Specific
Gravity
(Water=1)
Acetone
2.5
13
0.8
869
2.0
-4
133
Acetylene
2.5
100
-
581
0.9
(gas)
-118
Ammonia
15
28
0.7
1204
0.6
(gas)
-28
Benzene
1.3
7.9
0.9
928
2.8
12
176
1.6
8.4
-
550
2.0
(gas)
31
<59-199
100-1500
n-Butane
(3)
Crude Oil
(3)
NDA
(4)
NDA
(4)
.75-1.04
NDA
(4)
NDA
(4)
0.6
7.5
<1
428-600
5-6
Per
Specification
175-340
Ethane
3.0
12.5
-
882
1.0
(gas)
-128
Ethanol
3.3
19
.8
685
1.6
55
173
Ethyl
Mercaptan
2.8
18
0.8
572
2.1
<0
95
Ethylene
2.7
36
-
842
1.0
(gas)
-155
Gasoline(3)
(Super
Unleaded)
1.4
7.6
0.7 - 0.8
430-797
3-4
-49
77- 419
n-Hexane
1.1
7.5
0.7
437
3.0
-7
156
Hydrogen
4.0
75
-
932
0.1
(gas)
-422
Hydrogen
Sulfide
4.0
44
-
500
1.2
(gas)
-76
iso-Butane
1.8
8.4
-
860
2.0
(gas)
11
JP-4
1.3
8
-
464
-
-10 to +30
-
2
34
<1
725 - 896
2
-
77-419
Methane
5.0
15.0
-
999
0.6
(gas)
-259
Methyl
Alcohol
6.0
36
0.8
867
1.1
52
147
MTBE(3)
2.5
15.1
.74
435
3.1
-22
131
n-Pentane
1.5
7.8
0.6
500
2.5
<-40
97
Propane
2.1
9.5
-
842
1.6
(gas)
-44
Propylene
2.0
11.1
-
851
1.5
(gas)
-53
Toluene
1.2
7.1
0.9
896
3.1
40
231
Diesel
M85
(4)(5)
(1)
(2)
(3)
(4)
(5)
From NFPA 325M, “Properties of Flammable Liquids, Gases and Volatile Solids.”
In air at normal atmospheric temperature and pressure.
Data from Chevron Material Data Sheet.
“No data available.” Varies by type of oil. Crude oil is considered a flammable liquid.
15% methanol, 85% unleaded gasoline.
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Fig. 100-2
100 Introduction to Fire Prevention
Flammability Diagram for the System Methane-Oxygen-Nitrogen at Atmospheric Pressure and 26 C.
(Courtesy of Bureau of Mines Bulletin, #627, Flammability Characteristics of Combustible Gases and
Vapors)
Figure 100-3 can also be used to determine the effect of using different types of
inert gas. CO2 and steam are more effective inerting agents in that less inert is
needed to reach the LOC. In other words, the LOC for nitrogen is lower than it is for
CO2 or steam.
Enriching is the addition of flammable gas, usually methane, so that the vapor is
above the flammable range or too rich. Using Figure 100-3, methane would be
added until 200% of the UFL is reached. For methane this would be 15%x2=30%
minimum methane in the mixture.
Dilution is the addition of air until the vapor is below the flammable range or too
lean. Per NPFA 69, for dilution, the concentration should be held at or below 25%
of the LFL. Using Figure 100-3, air would be added until the mixture was
0.25x5=1.25% methane in air.
NFPA 69 is the key resource for the design of inerting enrichment and dilution
systems and for determining purge rates.
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Fig. 100-3
Fire Protection Manual
Flammability Diagram for Methane
Inerting, enriching and dilution systems are active protections which must be
closely managed. Incidents have occurred because these systems have been shutdown inadvertently. Key concerns include:
•
•
•
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Is the purge gas supply reliable? How do we know when the supply is lost?
How do we know we have purge flow to the equipment being purged?
What is the impact if purge gas is lost?
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Effects of Hydrocarbon Mixtures on the Flammable Range
The effect of mixing two or more flammable gases on the flammable range can be
predicted by the following formulas from NFPA 69, Appendix C.
P1 + P2 + P3 + … + Pn
LowerFlammableLimit = -------------------------------------------------------------------P1
P2
Pn
------------- + ------------- + … + ------------LFL 1 LFL 2
LFL n
P1 + P2 + P3 + … + Pn
UpperFlammableLimit = ----------------------------------------------------------------------P1
P2
Pn
-------------- + -------------- + … + -------------UFL 1 UFL 2
UFL n
(Eq. 100-1)
where:
P1, P2, etc. = the volume fractions of components 1, 2, 3, etc., of the mixture
LFL1, LFL2, LFL3, etc. = the lower flammable limits of components 1, 2, 3, etc., respectively
and
UFL1, UFL2, UFL3, etc. = the upper flammable limits at components 1, 2, 3, etc., respectively
Effects of Temperature on the Flammable Range
Increasing temperature of a vapor/air mixture widens the flammable range (see
Figure 100-4). The change in flammable range can be approximated by the
following equations:
LFL t
------------------- = 1 – 0.000721 ( t – 25° )
LFL25°
UFL t
-------------------- = 1 + 0.000721 ( t – 25° )
UFL25°
(Eq. 100-2)
where:
LFLt and UFL2 = the lower and upper flammable limits at temperature t in °C
LFL25° and UFL25° = the lower and upper flammable limits at 25°C
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and
t = the vapor temperature in °C
Fig. 100-4
Flammable Limits of a Combustible Vapor as a Function of Temperature, in Air at a Constant
Initial Pressure
Legend:
AIT
Auto-ignition temperature. The minimum
temperature at which a material begins to
self-heat at a high enough rate to result in
combustion—reported in the Data Guides
as the temperature in air at one atmosphere.
TL
Equilibrium temperature at which the
lower flammable limit composition exists
over liquid in dry air at one atmosphere
(theoretical flash point).
Tu
Equilibrium temperature at which the
upper flammable limit composition exists
over liquid in dry air at one atmosphere.
Effect of Pressure on the Flammable Range
Figure 100-5 demonstrates the effect of pressure on flammable range for natural gas
(methane). Clearly increasing pressure widens the flammable range.
Flash Point
A fuel’s flash point is the lowest temperature at which the vapor pressure of the
liquid is just sufficient to produce a mixture at the LFL. A fuel’s flash point is a
good indicator of flammability.
Flash point test. The flash point test involves the concepts of both volatility and
flammable range. Consider a flammable liquid at a temperature so low that the
vapor pressure over the liquid surface is insufficient to produce an equilibrium
mixture within the flammable range. As the temperature is slowly increased, the
vapor pressure will rise and the amount of vapor in the mixture above the liquid
surface will increase. Until the LFL is reached, introduction of a source of ignition
such as a small flame into the vapor space will not cause combustion. When the
composition reaches the LFL, introduction of a flame will cause ignition, and flame
will spread momentarily across the oil surface. This is the method used in the
flash point test. See Section 1-2, NFPA 30 in “Industry Specifications” for acceptable flash point tests.
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Fig. 100-5
100 Introduction to Fire Prevention
Effect of Pressure on Limits of Flammability of Natural Gas-Nitrogen-Air Mixtures at 26° C (Courtesy of
Bureau of Mines Bulletin 627, Flammability Characteristics of Combustible Gases and Vapors)
There is no generally accepted corresponding test to determine the higher temperature at which the vapor composition over a liquid passes out of the flammable range
into the “too rich” region. Consequently, temperatures corresponding to the UFL are
not as generally available as those for the LFL.
Contrary to what the words might seem to imply, “flash point” does not mean the
temperature at which a “flash” appears spontaneously; introduction of a source of
ignition is essential. Increasing flash point temperature is usually associated
with decreasing hazard, high flash point means low hazard and vice versa.
Flash points of fuels. The flash point of gasoline, the most commonly used volatile
petroleum liquid, is well below 0°F at atmospheric pressure. At ordinary handling
temperatures, the vapor pressure of gasoline is high enough to produce an equilibrium mixture above the UFL in a closed air space.
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Kerosenes and domestic fuel oils usually have flash points within the range 100°F
to 150°F. For industrial fuel oils, the minimum flash point is usually above 150°F.
This means that these oils will not usually produce ignitable mixtures over their
surfaces unless heated.
Crude oils vary widely as to their characteristics and may range from thin, light
gravity liquids more volatile than ordinary gasoline to thick heavy substances such
as asphalt. Therefore, to determine the flammability of any particular crude oil, it is
necessary to know its characteristics. So called “refinable” crude usually contains
sufficient light components to place it in a flash point classification comparable with
gasoline.
See Figure 100-1 for flash points for selected chemicals.
125 Source of Ignition
In the preceding discussion of the “fire triangle,” the third requirement for fire was
designated simply as “source of ignition,” with the reservation that, for the particular chemical reaction concerned, considerations of both temperature and energy are
involved.
An ignition source serves as the “starter” for the process of combustion. Thereafter,
the heat of combustion itself provides the energy for continuation of the reaction, so
long as properly proportioned supplies of fuel and air are available.
Different fuel substances have different ignition characteristics. The possibility of
ignition is influenced by ambient conditions, by fuel temperature, and by size, duration, and energy of the potential source of ignition.
It is difficult to strictly define an ignition source and to assign to any fuel substance
a particular ignition characteristic (such as the so-called “ignition temperature”) that
will be an unvarying property under all conditions. However, general characteristics can be discussed.
Refer to Section 220 for additional information on sources of ignition and their
control. Refer to Section 240 for a discussion of auto ignition.
Ignition Characteristics of Fuel
Fuel substances vary widely in their susceptibility to ignition. Variability depends
on physical state and chemical composition of the fuel, nature of the ignition source,
and conditions under which the two are brought together. For gas-air mixtures, ignition is commonly thought to be an almost instantaneous process, although this is not
true under some conditions discussed later. Solid substances usually must be vaporized before ignition can occur, thus involving a distinct time element. Although
rigid classification is impossible, common fuel substances fall roughly into three
groups:
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Easily ignited —chemically active vapors and gases such as:
Ignition Energy
•
Carbon Disulfide
0.15 mJ (millijoules)
Ethylene
0.07 mJ
Acetylene
0.02 mJ
Hydrogen
0.02 mJ
Less easily ignited—hydrocarbon gases and vapors, including all of the products of petroleum together with most oxygen-containing organic chemicals
such as alcohols, ethers, acetones, etc., that have higher ignition energy requirements.
Ignition Energy
•
Methane
0.47 mJ (millijoules)
Ethane
0.285 mJ
Propane
0.305 mJ
Methanol
0.215 mJ
Dimethylether
0.29 mJ
Methyl Ethylketone
0.53 mJ
Slowly ignitable at low temperature—Cellulose-containing solids such as
paper, wood, and rags which, under long exposure to relatively low temperatures, may dry out, char, finally glow and possibly burst into flame.
Within each group the energy required for ignition will vary, depending on the
nature of the ignition source and on the oxygen concentration. If the atmosphere
contains more than the normal amount of oxygen, ignition will be facilitated.
As a point of reference, a spark from the human body as a result of static buildup
walking on a carpet can be in the 1 - 10 mJ range.
126 Spread of Flame
In a vapor-air mixture of flammable proportions, flame that started at one point will
spread in all directions until all of the mixture is consumed. The rate at which the
flame front moves—the rate of flame propagation—is subject to wide variation.
Rates. For mixtures near the upper or lower limit of their flammable range (near
their UFL or LFL), at which the reaction is barely self-sustaining, the flame propagation rate is low—1/2 foot per second or less—and no significant pressure rise
results. Mixtures closer to the center of the flammable range produce more vigorous
flames that not only spread faster initially but also tend to accelerate. The flame
propagation rate is also influenced by the nature of the combustible material.
Maximum rates of flame propagation, with accompanying maximum explosive
effects, occur in mixtures slightly on the rich side of the flammable range.
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In the open, a petroleum-vapor air mixture may burn at speeds up to about 8 to 10
feet per second, with no spectacular manifestation beyond the appearance of the
flame and an outward rush of gases characteristic of an unconfined explosion.
Unconfined vapor cloud explosions (UVCE) are discussed in Section 1200.
If burning occurs in a closed space, the heat of combustion produces a rise in pressure. This can be eight to ten times the initial pressure. If the pressure exceeds the
strength of the container (e.g., a tank), failure will result and an “explosion” is said
to have occurred. If the container is strong enough (e.g., the cylinder of a gasoline
engine), there may be no external manifestation at all.
Deflagration/detonation. A deflagration occurs when the flame front propagates
by transferring heat and mass to the unburned vapor-air mixture ahead of the front.
Flame speeds can range from 1-350 M/sec and peak overpressures can be as high as
20 times the initial pressure. If the flame front continues unchecked, it can continue
to accelerate due to the shock wave produced in the front of the flame. A detonation occurs when the flame velocity reaches supersonic speeds of 2000-2500 M/sec.
Peak overpressures can range from 20-100 times the initial pressure. Figure 100-6
illustrates the impact of distance from the ignition source on flame speed.
Fig. 100-6
General Effect of Pipe Length on Flame Speed
3,3(/(1*7+,1',$0(7(56)520,*1,7,216285&(
The distance required for a deflagration burning at subsonic speeds to accelerate to a
detonation burning at supersonic flame speed depends on the flammable mixture,
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temperature, pressure, the enclosure and its length and configuration, and the ignition source. For these reasons, it is imperative that burning gases be controlled near
the ignition point while the burning rate is manageable. Normally this is considered
to be about 30 pipe diameters, or 15 feet maximum.
The tendency to detonate is associated with a high initial rate of flame travel and
high energy release per unit volume, so it is favored by high initial pressure,
oxygen-enriched atmospheres, and such fuels as hydrogen and acetylene. See
Section 1200 for more information.
Flashback velocity. A type of flame propagation called “flashback” is best
explained by example. Consider the Bunsen burner. Air and gas in premixed proportions within the flammable range can be ignited above the burner tube, where the
velocity has become slow by expansion of the stream, but the flame does not pass
down into the tube (or “flash back”) because the flow velocity of the mixture in the
Bunsen burner tube is higher than the rate of flame propagation in the turbulent
burning mixture above. If the flow velocity is sufficiently reduced, flashback can
occur. The rate at which flashback occurs is the flashback velocity. Earliest attempts
to measure the rate of flame propagation used the Bunsen burner method.
Flashback velocity depends on composition of the mixture, and to a lesser extent on
tube diameter. Velocities determined by the Bunsen burner method are reproducible
and are free of the variations and uncertainties that accompany burning rates of
mixtures initially at rest in pipes or vessels. Hence, velocities measured by the
Bunsen burner method may be used with confidence in the design of burners and in
the appraisal of risk of flashback in pipes, certain types of flame arrestors, etc.
Flashback velocity for a typical hydrocarbon vapor and air mixture is approximately 10 feet per second.
127 Explosion Hazard
Determining the percentage composition of mixtures of flammable vapor with air is
of great importance. Mixtures that are too lean to burn are not a fire hazard,
although if a mixture should be close to the LFL, consider the possibility that
changing conditions, such as an increase in temperature, might make the composition flammable.
Mixtures that are too rich to burn are not a fire hazard, but because such mixtures
may become diluted with enough air to become flammable, the surrounding conditions must be carefully reviewed to see if a potential explosion hazard exists.
Mixtures within their flammable range are potentially very dangerous because it
requires only the presence of a source of ignition to produce a fire or explosion. The
degree of hazard depends upon the volume of the mixture, its location, the degree of
confinement, and the likelihood that a source of ignition might accidentally occur.
Flammable mixtures in large volume in partially or wholly confined spaces (tanks,
basements, etc.) are particularly dangerous because the force of the explosion may
destroy the enclosure and damage adjacent equipment. The likelihood of severe
destructive effects is increased when the mixture is initially above atmospheric pressure in a vessel capable of sustaining a proportionally high pressure before failure.
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The hazard is particularly serious if people are required to be in or adjacent to such
structures.
128 Gas Testing
Petroleum vapors in the range from zero up to the LFL can be detected by portable
combustible gas indicators. The scale of combustible gas detectors generally is
marked in percentages of the LFL. The “Lower Flammable Limit (LFL),” marked
near the upper end of the scale, indicates 100% of the LFL. The instrument must be
calibrated for the hydrocarbon or flammable vapor being sampled. Refer to the
manufacturers' instructions for limitations on accuracy of the instrument when calibrated on a vapor other than the one being sampled, or when the atmosphere is
contaminated with inert gases, H2S or high moisture levels.
Tests of atmospheres within any confined space are required before hot work is
performed or personnel are permitted to enter. Plant, facility or department rules
generally prescribe the methods for testing and set limits within which entrance is
prohibited. These limits usually are only a small percentage of the LFL of the
composition (typically 10% LFL). In some states, legal requirements have been
established with respect to hot work (as a percentage of the LFL), and the entry of
spaces in which flammable or toxic vapors may be present (e.g., Cal/OSHA State of
California, Paragraph 6777, 6793, 6810). These limits are given as threshold limit
values (TLVs). Section 300 of this manual and API Publication 2207 cover gas
testing as applied to the cleaning of tanks and process vessels.
Testing for Rich Mixtures
Determining the exact composition of vapor mixtures within or above the flammable range is a special problem. A rough idea of the composition may be obtained
with a combustible gas indicator by the use of a dilution valve or by other special
techniques, but precise knowledge of the flammability of rich mixtures or those
containing inert gas must be derived from laboratory analysis or direct explosion
test. We seldom need to know the exact composition of mixtures above the LFL.
All such mixtures must be considered potentially dangerous because dilution with
air can bring them within the flammable range, a possibility that depends more on
the source of vapor and the physical surroundings than it does on the original
composition.
130 References
General References
Zabetakis, M. G., “Flammability Characteristics of Combustible Gases and Vapors,”
U. S. Department of the Interior, Bureau of Mines, Bulletin 627, 1965.
Coward, H. F., G. W. Jones, “Limits of Flammability for Gases and Vapors,” U. S.
Department of the Interior, Bureau of Mines, Bulletin 503, 1952
Hilado, C. J., S. W. Clark, “Autoignition Temperatures of Organic Compounds,”
Chemical Engineering, Sept. 4, 1972, pp 75-80
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Chemical Safety Data Sheets, Manufacturing Chemists Association, Inc.
Calcote, H. F., C. A. Gregory, Jr., C. M. Barrett, and Ruth B. Gilmer, “Spark Ignition Effect of Molecular Structure,” Ind. Eng. Chem. 44, 2659, 1952
American Petroleum Institute (API)
API 750
Management of Process Hazards
API 2003
Protection Against Ignitions Arising Out of Static, Lightning and Stray Currents
API 2207
Preparing Tank Bottoms for Hot Work
API 2214
Spark Ignition Properties of Hand Tools
American Society for Testing Materials (ASTM)
ASTM D-323
Vapor Pressure of Petroleum Products (Reid Method)
National Fire Protection Association (NFPA)
NFPA 325M
Fire Hazard Properties of Flammable Liquids, Gases, and
Volatile Solids
NFPA 69
Explosion Prevention Systems
Occupational Safety and Health Administration (OSHA)
Title 29 CFR, Part 1910.38a, “Employee Emergency Plans and Fire Prevention
Plans”
Chevron
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