CHAPTER.docx

CHAPTER 8

Fire Characteristics: Liquid Combustibles

OBJECTIVES

After studying this chapter, you should be able to:

•   Describe the flash point, fire point, and autoignition temperature of a flammable liquid.

•   List the three classes of flammable liquids, based on flash point and potential ambient temperatures.

•   Define the linear burning rate of a pool of liquid and explain why it varies with the diameter of the pool.

•   Describe the physical considerations that affect the rate of flame spread of flammable liquids.

•   Explain boilover.

•   Explain a boiling liquid/expanding vapor explosion (BLEVE).

Introduction

How many movies have you seen in which a lit cigarette ignites a gasoline spill? In reality, this is not a very likely occurrence. A cigarette is a small ignition source, and the spill is a large heat sink. Most likely, the gasoline will cool and quench the cigarette before the cigarette heats the gasoline—but that rather simple outcome would not serve the filmmaker’s intent to have the dramatic effect of flames destroying a car, a house, or a service station.

Liquids do ignite and burn, sometimes quite violently. The chemistry of a burning liquid is the chemistry of its vapor. If a compartment contains an ignitable volume fraction of n-hexane, the chemistry does not “know” whether the fuel entered the compartment as a gas or whether a small amount of spilled liquid vaporized. Thus, the combustion chemistry of burning gases discussed in the previous chapter provides a sufficient basis for understanding the chemistry of burning liquids. However, physical considerations also affect the ignitability and rate of flame spread of flammable liquids as well as the tactics used to limit these hazards. These are the topics covered in this chapter.

Ignition of Liquids: Flash Point, Fire Point, and Autoignition Temperature

It is the vapor of a liquid that burns; therefore, the principal property of a flammable liquid that affects its susceptibility to ignition is the ease with which the molecules vaporize to form a gaseous fuel–air mixture that is within the liquid’s flammability limits.

Placing a match just above a pool of a flammable liquid will not lead to ignition unless the vapor concentration exceeds the lower flammable limit of that vapor in air. In the Physical and Chemical Change chapter, we saw that at 32 °F (0 °C), the vapor pressure of methanol is 3.96 kPa (3.92 atm). The total pressure is 101 kPa (1 atm), so the volume percent, or mole percent, of methanol vapor in the air just above the liquid surface is 100 · 3.96/101 = 3.9 percent by volume. In the Fire Characteristics: Gaseous Combustibles chapter, we saw that the lower limit of flammability of methanol vapor in air is 6.7 percent by volume. Therefore, methanol should not be, and is not, flammable at 32 °F (0 °C).

The vapor pressures of liquids, however, increase sharply with increasing temperature. If a match flame were held next to a small enough quantity of methanol liquid for a long enough time (and if the match did not burn out), the liquid would be heated to 54 °F (12 °C). At this temperature, the vapor pressure of methanol is 7.17 kPa, and the percent by volume is 7.1 percent. This exceeds the lean flammability limit, so the liquid would ignite.

One of the standard tests for ignition of liquids, ASTM D92 [1], replicates this behavior. A small, open cup of cold methanol is heated gradually from below. A small flame from a tiny burner is passed across the liquid surface every 10 seconds. When the liquid reaches about 54 °F (12 °C), a flame moves rapidly across the surface, consuming the methanol vapor above the surface. After a fraction of a second, no further combustion would occur, because the combustible vapor has been consumed, and the heat transfer from the small flame is too little to overcome the evaporative cooling of the liquid surface and sustain the vaporization. By the time that additional vaporization can restore the original vapor concentration, the burner has been removed, and an ignition source is no longer present. Ten seconds later, the burner is passed over the liquid again, and the sequence repeats itself. The minimum temperature at which this behavior occurs (54 °F, 12 °C for methanol) is called the flash point of the liquid.

When the methanol is heated further (usually 10 °F to 30 °F, or 5 °C to 15 °C higher than the flash point), and the ignition flame is applied from time to time, combustion is sustained after removal of the ignition source. At this temperature, called the fire point of the liquid, the liquid temperature is high enough to maintain a supply of vapor as fast as it is consumed by the flame.

Multiple tests for flash point and fire point temperatures have been developed, which yield some variation in the measured values. These discrepancies arise because the thermal and flow environment above the liquid depends on four properties:

•   The intensity and size of the ignition source

•   The length of time for which the ignition source is held over the liquid

•   The rate of heating of the liquid

•   The degree of air movement over the liquid

Nevertheless, the measured values, especially the flash point, are widely used as guides to the safe handling of liquids. The flash point, being lower than the fire point, is a more conservative value to use.

Liquids can be divided into classes (which are divided further into subclasses) based on their flash points [2]:

1.  Class I: Liquids with flash points below 100 °F (38 °C) an indoor temperature that could be reached sometime during the year.

2.  Class II: Liquids with flash points at or above 100 °F (38 °C) but below 140 °F (60 °C), a temperature that could be reached with only a modest degree of heating.

3.  Class III: Liquids with flash points at or above 140 °F (60 °C), a temperature that would require considerable heating.

Table 8-1 Flash Points of Some Common Liquids

 

Flash Point *

 

 

°C

°F

Class I Liquids

gasoline

–45.5

–50

ethyl ether (anesthetic)

–28.9

–20

n-hexane

–3.9

25

JP-4 (jet aviation fuel)††

–18

0

acetone

–17.8

0

toluene

4.4

40

methanol

12.2

54

ethanol

12.8

55

turpentine†

35

95

Class II Liquids

No. 2 fuel oil (domestic)††

>38

>100

diesel fuel

40 to 55

104 to 131

Jet A (jet aviation fuel)††

47

117

kerosene

37.8

100

No. 5 fuel oil††

>54

>130

Class III Liquids

JP-5 (aviation jet fuel)††

66

151

SAE No. 10 lube oil††

171

340

tricresyl phosphate††

243

469

* Data from Reference [3], except as noted.

 Data from Reference [2]. This value was obtained using a closed-up method, which typically gives flash point values that are 5 °C to 10 °C lower than open-cup values.

†† Data from Reference [4]. These data were obtained using a closed-up method, which typically gives flash point values 5 °C to 10 °C lower than open-cup values.

Table 8-1  lists flash points for some common liquids. Notice the wide range, from –50 °F to 469 °F (–45.5 °C to +243 °C). Note also that these values are meaningful only for a bulk liquid. If a liquid with even a high flash point is formulated as a spray or a foam, is released with air present, and comes into contact with even a small ignition flame, the tiny amount of liquid in contact will immediately be heated to a temperature higher than its flash point and will start burning. The combustion enthalpy released will vaporize the surrounding spray or foam, and the fire will propagate (spread).

As an example of the fire potential of liquids in these three classes, consider a liquid spill on a summer day when the ground has been heated by the sun to 95 °F (35 °C). If the spill consisted of n-hexane, a Class I liquid, there would be a race between the wind dispersing this volatile chemical and the introduction of an ignition source. On a still day, the vapor would ignite; on a very windy day, it probably would not. If the spill consisted of kerosene, a Class II liquid, a fire hazard would exist only if the liquid was exposed to an additional heat source capable of raising the temperature of some part of the liquid by at least 25 °F (14 °C). JP-5, a Class III liquid, was designed to burn well in jet engines, but poses a significantly lower ignition risk during handling and storage than JP-4, a Class I liquid.

Fire points and flash points depend heavily on pressure. The flash point is the temperature at which the vapor pressure of the liquid equals the lower flammability limit. If the atmospheric pressure were 50 kPa instead of 100 kPa, the vapor pressure of the liquid need be only one-half as great to achieve the lower flammable limit. Therefore, flash points and fire points are lower than normal at pressures below atmospheric pressure and higher than normal at pressures above atmospheric pressure. This variation is very important in assessing the flammability in, for example, the vapor space of an aircraft fuel tank. It should also be considered in cities at higher elevations, such as Denver and Albuquerque, where the average atmospheric pressure is approximately 80 kPa (0.8 atm).

Autoignition, in which a vapor–air mixture is ignited strictly by heating, was discussed in the Fire Characteristics: Gaseous Combustibles chapter. Autoignition temperatures are typically hundreds of kelvins higher than flash points or fire points.

Burning Rates of Liquid Pools

Once a pool of a flammable liquid is ignited, the flames generally spread to cover the full surface area of the pool. The liquid will then burn at a more or less steady rate until the liquid is consumed. (Some very-low-volatility, high-fire-point fluids burn locally, perhaps with small, irregularly moving flames. There is some discussion of these later in this chapter.)

The rate of burning of a liquid pool is often expressed as a linear burning rate (in mm/s)—that is, the rate at which the surface of the pool recedes. The following discussion assumes that the liquid is sufficiently deep that steady burning can be established. Treatment of shallower pools is beyond the scope of this book.

The linear burning rate is readily converted to a linear mass burning rate (in kg/m²-s) by multiplying the linear burning rate by the density of the liquid (in kg/m³) and dividing the product by 1000 (to yield mm/m). Then, obtaining the heat of combustion of the liquid (J/g) from a handbook, the rate of heat release per unit area can be calculated, assuming complete combustion. The total heat release rate for the pool is the rate of heat release per unit area multiplied by the surface area of the pool.

To illustrate the magnitude of these rates, a pool of gasoline 3.3 ft (1 m) in diameter and 25 mm (1 in.) deep will be consumed in approximately 4 minutes. The average linear burning rate is 25 mm/240 s or about 0.1 mm/s. Gasoline (and many hydrocarbons of similar molecular weight) has a density of approximately 700 kg/m3 and a heat of combustion of approximately 45 MJ/kg. The surface area of the pool is πd2/4 =0.79 m2. Using these values, the mass burning rate of the pool can be calculated as about 0.06 kg/s. The average rate of heat release for this pool fire would be about 2.5 MJ/s or 2500 kW.

The linear burning rate of a pool of liquid depends not only on the nature of the liquid but also on the diameter of the pool.  Figure 8-1  shows the linear burning rate of n-hexane as a function of pool diameter from about 0.2 m to 2.5 m.

Figure 8-1 Effect of pool diameter on the linear burning rate of n-hexane [4].

Note

The burning rate of a pool smaller than 0.2 m in diameter is of less interest in fire protection, as its hazard would be limited to its role as an ignition source. For fires of such small diameter, the flames heat the lip of the container, and the hot container heats the adjacent liquid, increasing the local evaporation rate and, therefore, the overall burning rate. As the diameter increases, the central area of the pool becomes larger relative to the area of the ring near the lip, the edge effect becomes less important, and the linear burning rate decreases.

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For pools larger than 0.2 m in diameter, the burning rate increases with increasing pool diameter, reaching a limit at a diameter of approximately 3 m. The reason for this increase becomes apparent when we consider the three factors that control the burning rate of a liquid pool.

First, little of the flame radiation is needed to vaporize enough liquid fuel to sustain the fire. Each gram of n-hexane that burns releases 44,860 J. The rate of burning of the n-hexane is controlled by its rate of vaporization. To vaporize n-hexane, the latent heat of vaporization—that is, 371 J/g—must be supplied. Therefore, a little less than 1 percent of the combustion energy must return to the n-hexane surface, through the rising vapors, to maintain the vapor supply to the flame. (The bulk of the heat is convected upward with the combustion products and radiated sideways and upward.)

Second, the flame over a pool that is less than 0.2 m in diameter is a laminar diffusion flame. Its combustion is relatively complete, so little soot forms. The flame is optically thin, the emissivity of the flame is low, and the radiant intensity to the fuel surface is low. This small amount of radiation, combined with some conduction from the lip, suffices to vaporize enough liquid to keep the flame burning at a low level.

Third, the flame radiation to the surface increases with increasing pool diameter, up to a limit. At these larger diameters, the flames evolve from laminar to increasingly turbulent in nature. As this happens, more soot forms, and the optical thickness of the flame increases.  Figure 8-2  shows how the thermal radiation intensity increases with increasing sootiness.

The lowest curve in Figure 8-2 simulates an optically thin flame. As more soot is “added” to the flame, the radiant intensity increases. The “average” soot curve reaches a limiting value at about 2 m physical thickness. At this point, the flame is optically thick, and a further increase in the physical thickness will not increase the radiant output. The same is true for adding more soot (as in the “high soot” flame, which has eight times the soot of the “very low soot” flame)—this extra soot simply ensures that the “high soot” flame reaches the radiative limit at a smaller flame diameter.

Linear burning rates have been measured for other liquids [4] and have been extrapolated to limiting values for large pool sizes. For these pools, the flames are optically thick, so the energy radiated to the surface is constant. Thus one might expect that the linear burning rate would depend on how easy it is to vaporize the liquid.  Figure 8-3  demonstrates this relationship. For these five liquids, the linear burning rate correlates with the reciprocal of the latent heat of vaporization per unit volume (the product of the latent heat of vaporization per gram and the density). The slope of the correlation line is about 3 J/s•mm² (30 kW/m²). Thus the radiation from optically thick flames of any of the five combustibles imposes an average heat flux of approximately 30 kW/m² on the liquid surface, regardless of the chemical nature of the combustible.

Figure 8-2 Calculated radiative intensity coming from a hot, semi-transparent, sooty gas of various thicknesses, for three soot levels.

Figure 8-3 Burning rates of large liquid pools versus inverse of product of latent heat of vaporization (L) and density (p).

Flame Spread Rates over Liquid Surfaces

The previous section dealt with burning rates of burning liquid pools, where the flame covers the entire surface. In these scenarios, the spread over the surface is fast compared to the surface regression rate. This section addresses situations in which the rate of spread of the flame over the surface, after a local region of the surface has been ignited, is important.

Figure 8-4  shows data for the flame spread rate over the surface of a liquid, n-butanol (C4H9OH). (Note the logarithmic scale on the vertical axis.) This compound has a flash point of 110 °F (43 °C), as measured by the open-cup method described earlier. Above this temperature, the flame spread rate is 2 m/s (6.5 ft/s) and is independent of the liquid temperature. Below 110 °F (43 °C), the flame spread rate heavily depends on the liquid temperature. Indeed, at 68 °F (20 °C), the flame spread rate is only 1/100 of the value at 122 °F (50 °C).

Figure 8-4 Rate of flame spread over the surface of n-butanol [5].

This behavior is typical of flammable fluids. A flame will spread, albeit very slowly, over a liquid whose temperature is well below its flash point. To start the burning, the liquid must be heated locally to above its flash point. Then, if the radiative heat transfer from the flame is sufficient to heat the adjacent cold liquid (and induce convection currents in it), the flame will spread.

For a fluid whose temperature is well above its flash point, a combustible vapor concentration in excess of the lower flammability limit exists over the surface before the arrival of the flame, and the flame rapidly covers the entire surface. If the liquid is too warm, the vapor concentration just over the surface will exceed the upper flammability limit. In that case, air circulation over the liquid pool will decrease the combustible concentration with increasing height above the surface, and somewhere there will be a zone containing a near-stoichiometric mixture.

The direction and velocity of any air flow can have a large impact on flame spread over a liquid pool. A (co-flow) breeze in the same direction as the flame spread will increase the spread downwind. A (counterflow) breeze opposite to the flame spread direction will decrease and perhaps even halt the flame spread.

Hazards of Liquid Fuel Fires

With this understanding of the characteristics of fires of liquid fuels, it is helpful to identify five categories of liquid fires that constitute nearly all the encountered configurations.

1.  A pool of liquid, such as an open tank or the result of a spill. If the temperature of the liquid exceeds its fire point, it can be ignited and will sustain flaming. Given the proper equipment, fires on stagnant liquids are routinely extinguished using techniques such as those discussed in the Fire Fighting Chemicals chapter.

A serious outcome can result from boilover from a pool fire. The key condition for boilover is the presence of two immiscible layers:

•   An upper layer of a low density combustible liquid

•   A lower layer of a higher-density fluid whose boiling point can be exceeded due to heat transfer from the upper fluid layer

Although this phenomenon has been recognized for more than a century, boilover was not understood fully until about 25 years ago [6].

The classic case of boilover begins with a burning pool of hydrocarbon fuel. This liquid source could be as small as a deep-fat fryer or as large as a petroleum storage tank at an oil refinery. Hydrocarbon fuels consist of a mixture of highly volatile and slightly volatile components. A heated sample would start to burn at a fuel temperature below 212 °F (100 °C). As the fire burns in the open container, the more volatile components are driven out of the topmost few millimeters of liquid, and the temperature of this “slice” rises to approximately 572 °F (300 °C). Heat is conducted downward through the liquid to the next few millimeters, causing gasification of volatile components in that slice. The volatiles form bubbles below the surface. The motion of these bubbles greatly accelerates the mixing of the hotter upper fuel and the cooler lower fuel. Enhanced by this bubble-induced mixing, a hot zone spreads downward through the fuel.

Seeing the burning fuel, a nearby person applies water to quench the flames. The water, being of higher density than the hydrocarbon fuel, rapidly sinks to the bottom of the container. Now there are two layers that match the description given earlier: fuel on top and water on the bottom.

As the water sinks, it encounters hot oil at temperatures well above 212 °F (100 °C), and the subsurface water starts to boil vigorously. The volume of 1 kg of hot water vapor is more than 1000 times the volume of 1 kg of liquid water. Under the best of circumstances, the expanding water pushes up on the burning oil, which then overflows the container and spreads the fire. More seriously, the rapidly expanding water can propel blobs of the hot, burning oil out of the container. These airborne blobs have more surface area than the original pool surface, so the rate of oil combustion increases, heating the air surrounding the blobs and further accelerating their dispersion. The outcome is a rapid expansion of the fire outside the pan or tank. The consequences for nearby people or combustibles can be disastrous.

2.  A flowing liquid, such as from an overflowing or rapidly leaking tank. As in the first category, if the temperature of the liquid exceeds its fire point, the material can be ignited. Flowing liquid fires are very difficult, and sometimes impossible, to extinguish as long as the liquid continues to flow and the flames continue to move.

3.  A spray from a small orifice at high pressure (e.g., from a leaking hydraulic fluid line). For fires involving liquids in the form of sprays, the fire point is not a relevant measure of flammability. A pool of domestic (No. 2) fuel oil, a Class II fuel, at 68 °F (20 °C) cannot be ignited with a match (unless a wick is present). However, when the same oil in the form of a spray or foam comes into contact with even a very small ignition flame, the tiny amount of liquid in contact will immediately be heated to above its flash point and will start burning. The combustion energy released will vaporize the surrounding spray or foam, and the fire will propagate (spread). The concept of flammability limits still applies, and the values for a given chemical are similar to those for a vapor–air mixture of the same chemical. Because the flames are spatially linked to the orifice, these stationary fires can be attacked by relieving the fluid pressure and thereby turning a spray into a flow of potentially lower hazard, by inerting the environment, or by applying a gas-phase active fire suppressant.

4.  A thin liquid layer drawn up by capillarity (wicking action) over the surface of a porous medium, such as a fabric or paper. A wick can consist of any nonmelting porous material that the liquid is capable of wetting and that is in contact with the pool of liquid. The liquid is drawn up the wick by surface tension (capillarity), and the wick becomes covered with a thin film of the liquid. (As an example, immerse one corner of a handkerchief in a glass of water and observe what happens.) When an ignition source is applied to the wick (such as a match to a candlewick), the thin film of liquid is heated rapidly to above its fire point and it ignites. As it burns, additional liquid is drawn up the wick and feeds the fire. Such a fire is in itself quite small. However, if the flame from this small fire comes in contact with a large liquid pool or a combustible solid, its heat could eventually warm the fuel immediately adjacent to it so that the fire would spread from the wick to a portion of the adjacent fuel, ultimately growing into a much larger fire.

5.  A confined liquid in a pressure vessel, heated by an external fire. Liquefied gases, such as propane, are stored in tanks that are designed to withstand high pressures from within. The compression of a gas to form a liquid enables storage of approximately 1000 times the mass of the chemical in the tank volume. At atmospheric pressure, propane boils at –44 °F (–42 °C); its vapor pressure is 101 kPa (1 atm) at this temperature. At 77 °F (25 °C), its vapor pressure is 960 kPa (9.6 atm); and on a hot day at 100 °F (38 °C), its vapor pressure is 1320 kPa (13 atm). Thus the tank is designed to withstand at least this pressure. It is fitted with a pressure relief valve, which opens if the liquid overheats and generates an excessive pressure. A typical setting for the pressure relief valve on a propane tank is about 2000 kPa.

If a fire should be burning near the outside of the tank, the temperature of the tank will rise due to the radiative and convective heat transfer from the flames. The temperature, and thus the pressure, of the propane inside this container will rise due to conduction through the tank wall. The liquid propane, which was initially above its boiling point, now boils vigorously.

As the wall of the steel tank gets hot, its tensile strength diminishes. (At about 930 °F (500 °C), the yield point of steel is approximately half of its normal value.) The combination of increased internal pressure and weakened steel ruptures the tank. The liquid contents, which were boiling at perhaps 10 atm, are now suddenly at 1 atm. A sudden expansion of the vapor occurs, resulting in an enormous eruption of vapor and liquid aerosol called a boiling liquid/expanding vapor explosion (BLEVE; pronounced “blev-ee”).

Note

As can be seen from the name, a BLEVE is a change of state from a liquid to a gas, leading to a major pressure increase and (catastrophic) failure of the container. The use of the word “explosion” is actually misleading, because an explosion involves chain branching chemistry. The liquid whose expansion leads to a BLEVE need not be flammable [7]. For example, BLEVEs of water and liquid nitrogen are possible.

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The potential for loss of life and property can be increased significantly if the expelled fluid is ignited by contact with the external fire. This can lead to a burning cloud that can extend as much as hundreds of meters in diameter and propel pieces of the container as far as 1 kilometer. An example is shown in

Figure 8-5 The fireball formed from an ignited BLEVE. The small, dark object above the plume is a helicopter in the background.

© Ivan Cholakov/ShutterStock, Inc.

WRAP-UP

Chapter Summary

•   Three temperatures characterize the ability to ignite a liquid. The flash point is the temperature at which piloted ignition occurs, but is not sustained. The flash point is the temperature at which the vapor pressure of the liquid equals the lower flammability limit. At the slightly higher fire point, the flame is sustained. The autoignition temperature is hundreds of kelvins higher and applies to unpiloted ignition.

•   Class I liquids have flash points below 100 °F (38 °C), Class II liquids have flash points between 100 °F and 140 °F (38 °C and 60 °C), and Class III liquids have flash points at or above 140 °F (60 °C).

•   For a liquid pool of diameter less than about 0.2 m, the linear burning rate (surface regression rate) decreases as the pool diameter increases. In this pool-size range, fuel vaporization is affected by the container edges, which are heated by flame radiation. For pool diameters greater than 0.2 m, the burning rate rises with increasing diameter. The flames become turbulent and sootier, so flame radiation increases and vaporizes the fuel faster. A limit to the burning rate is reached at pool diameters near 2 m, where the flames are optically thick and the flame radiation to the fuel surface is no longer increasing.

•   If the liquid is at or above its flash point, the flame spread rate is fast, and the entire pool is engulfed within seconds. In such a case, the liquid is already evaporating sufficiently to reach the lower flammability limit in the vapor phase over the fuel surface. As the liquid temperature decreases, flame radiation must both heat the liquid to the flash point temperature and supply the heat of vaporization. As a result, the flame spread rate decreases sharply.

•   Liquid fuel fires generally fit into five categories: (1) a liquid pool, (2) a flowing liquid, (3) a spray from a small orifice at high pressure, (4) a thin liquid layer drawn up by wicking action over the surface of a porous medium, and (5) a liquid confined in a …