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;
3) what is meant by a source of ignition and how combustion spreads from such a source. This section discusses these topics.
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.
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.
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.
- 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.)
- Oxygen (usually in the form of air) must be present and mixed in suitable proportion with the fuel vapor to form an ignitable mixture.
- A source of ignition (high temperature and sufficient energy to start the chemical reaction of combustion) must be present.
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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.
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.)
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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)
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 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.
Effect of Oxygen Concentration on Flammable Range
The oxygen concentration has a major impact on the combustion process. 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
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 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).
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.
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:
- Easily ignited —chemically active vapors and gases such as:
- 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.
- 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.
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.
Carbon Disulfide 0.15 mJ (millijoules)
Ethylene 0.07 mJ
Acetylene 0.02 mJ
Hydrogen 0.02 mJ
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
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.