Managing hazards associated with flammable and combustible liquids requires a comprehensive strategy tailored to the conditions of their use. While preventive measures, such as spill prevention and ignition control, should receive the utmost attention, measures to mitigate fires and explosions should also be addressed. A strategy relying solely on prevention could be ineffective as unforeseen circumstances may arise.

The primary objective should minimize the life safety risk associated with the use of these materials. Other secondary consequences, such as environmental exposure, business interruption, and property damage, should also be factored into the strategy. This strategy should consider various scenarios, such as the potential for static pool fires, two dimensional flowing fires, three dimensional spill fires, as well as pressurized or spray fires. Also, explosions can result from combustion of vapors in either a confined or unconfined setting.1

Measures to prevent and mitigate incidents can include various engineering and administrative controls. As a general rule, engineering controls, both passive and active, are preferred over administrative controls as they reduce the human factor. However, both are necessary as part of an overall protection strategy. There are many codes, standards, and guidelines that offer recommendations or requirements for these controls.2,3,4,5,6,7


Flammable and combustible liquids possess a range of physical, ignition, combustion, and reactivity properties that define the hazards of these materials. Some of these properties can also affect the ability to control or extinguish fires. Additionally, these hazards can be magnified when these liquids are subjected to elevated temperature and/or pressure and are handled in large volumes.

When compared to ordinary materials, such as wood, paper, and plastic, the properties of flammable and combustible liquids require extraordinary measures to prevent and mitigate fires and explosions. A complete understanding of these properties is essential before effective loss prevention and mitigation strategies can be implemented.

It should be noted that a number of important risk-related properties are not typically included in Material Safety Data Sheets (MSDS). Other references are required to obtain this data.


The most common method to classify liquids is the "closed cup” flash point,8 and for some materials, also boiling point. Flash point is the minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture with air.3 When ignited, it will produce a flash fire, but not necessarily continuous flaming combustion over the surface of the fuel sample.

There are a number of hazard classification systems and associated definitions in use for flammable and combustible liquids. This includes the UN Globally Harmonized System of Classification and Labeling of Chemicals (GHS)2 and NFPA 30.3

The NFPA 30 classification system is as follows:

Flammable Liquid
Class IA – Flash Point < 73 °F (22.8 °C) & Boiling Point < 100 °F (37.8 °C)
Class IB – Flash Point < 73 °F (22.8 °C) & Boiling Point ≥ 100 °F (37.8°C
Class IC – Flash Point ≥ 73 ºF (22.8 ºC) & < 100 ºF (37.8 ºC)

Combustible Liquid
Class II – Flash Point ≥ 100 °F (37.8 °C) & < 140 °F (60 °C)
Class IIIA – Flash Point ≥ 140 °F (60 °C) & < 200 °F (93 °C)
Class IIIB – Flash Point ≥ 200 °F (93 °C)

Table 1 lists ignition properties for a sample of flammable liquids and is sorted from lowest flash point to highest. Properties associated with electrostatic ignition include electrical conductivity, minimum ignition energy (MIE), and charge relaxation times. The auto ignition temperatures (AITs) are also shown.

Material NFPA Class Flash Point °F (°C) Boiling Temperature °F (°C) Electrical Conductivity (pS/m) MIE (mJ) Charge Relaxation Time (s) Auto Ignition Temperature °F (°C)
Diethyl Ether IA -49 (- 45) 95 (35) 30 0.29 1.4 356
Acetone IB - 4 (-20) 133 (56) 6 x 106 0.19 3.2 x 10-5 869
Heptane IB 25 (-4) 209 (98) < 1 x 101 0.2 ~ 100 399
Isopropyl Alcohol IB 53 (12) 181 (83) 3.5 x 108 0.53 5 x 10-7 750
Ethyl Alcohol IB 55 (13) 173 (78) 1.35 x 105 0.23 1.6 X 10-3 685
Styrene Monomer IC 88 (31) 295 (146) 10 2.2 450

Table 1. NFPA Classification & Various Ignition Properties4,7

Flash point is not an indicator of the risk associated with an electrostatic ignition or auto ignition. For example, acetone has a lower flash point than heptane. However, heptane has a longer charge relaxation time and lower auto ignition temperature than acetone. Acetone is also more conductive than heptane1,4,7,9

NFPA 775 defines a liquid as conductive if its conductivity is greater than 10,000 picoSiemens (pS) per meter, semi-conductive if its conductivity is between 50 and 10,000 pS/m, and nonconductive if its conductivity is less than 50 pS/m. Thus, diethyl ether, heptane and styrene are considered non-conductive, as are other hydrocarbons.

Heptane will not readily dissipate an accumulated electrostatic charge unless treated with an anti-static additive or a sufficient amount of time has passed, as indicated with its charge relaxation time. It is important to understand all these properties as they relate to the conditions of use and not rely solely on flashpoint. Depending on circumstances, flash point alone might not be the most significant measure of ignition risk.

Electrostatic discharges have been implicated as the ignition source in many fires and explos ions . However, thi s potent ial source of ignition is not always apparent nor is it always understood. The minimum ignition energies for the liquids in Table 1 range from 0.19 mJ to 0.53 mJ. The energy level where a person may feel an electrostatic discharge, around their home for example, is approximately 1 mJ.7

Another important parameter is the fire point, which can be equal to or just slightly above the flashpoint of some liquids. As defined by NFPA 30,3 the fire point is "the lowest temperature at which a liquid will ignite and achieve sustained burning when exposed to a test flame in accordance with ASTM D 92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester.”10 Liquids that have a flash point and not a fire point are excluded from certain provisions in NFPA 30.

Material Specific Gravity (Water = 1) Water Solubility Flammability Range LFL – UFL (% by vol.) Heat of Combustion Btu/lb. (kJ/g) (Net) Self-reactive or Unstable
Diethyl Ether 0.7 No 1.9 36 13,450
Acetone 0.8 Yes 2.5 12.8 11,259
Ethyl Alcohol 0.8 Yes 3.3 19 11,532
Heptane 0.7 No 1.0 6.7 19,104
Isopropyl Alcohol 0.8 Yes 2.0 12.7 12,993
Styrene Monomer 0.9 No 0.9 6.8 17,427

Table 2. Physical, Combustion & Reactivity Properties 1,4

Liquids being processed at an elevated temperature can also be ignited when their temperature reaches the auto ignition temperature (AIT) and are released into the atmosphere. As defined by ASTM E659,11 auto ignition temperature is "the ignition of a material, commonly in air, as the result of heat liberation due to an exothermic reaction in the absence of an external ignition source, such as spark or flame.”

Process conditions where flammable or combustible liquids are used under pressure also should be understood. Atomization of an ignitable liquid through an inadvertent leak will lower the effective flash point. This atomization produces an aerosol cloud having an effect as if the vapor pressure of the material were increased. A liquid considered as combustible at atmospheric pressure could behave as flammable if a sufficient ignition source were present near a spraying or misting discharge.4


Table 2 lists other examples of liquid properties that require consideration. All of these listed liquids have a specific gravity less than 1.0. In the event of a fire, these liquids would form a layer above water and could spread the fire over a larger area.

The water solubility of these liquids is also indicated. Water dilution of a burning liquid from a sprinkler system may not be an effective means to mitigate a fire as too much water may be required.1 However, blends consisting of flammable or combustible liquids and water do present a lower risk than when pure materials. The flash point and fire point of a water-diluted flammable of combustible liquid will increase and the heat of combustion would be reduced.12

It is also worth noting the differences in heat of combustion between the listed materials. Acetone and the alcohols have approximately 65% of the energy potential of hydrocarbons.

For ignition of a flammable vapor to occur, the vapor concentration must be within the flammability range. For example, styrene monomer has a lower flammable limit (LFL) of 0.9 % and an upper flammable limit (UFL) of 6.8 %. These flammable limits are defined as the concentration of vapor in air that can support combustion of the vapor.4

If ignition of a flammable atmosphere were to occur, mixtures slightly above stoichiometric would prove to be most energetic and have the lowest minimum ignition energy (MIE).6 The MIE is the minimum electrical energy necessary to ignite a vapor within the flammable limits of a given vapor.3 It should be noted that these limits apply to mixtures with air. If replaced with oxygen, it would substantially raise the UFL.13

Another property unique to styrene is its ability to undergo exothermic self-reactivity.14 This reactive monomer can undergo a violent self-reaction or polymerization generating sufficient heat and pressure to burst process vessels or storage containers. Phenolic inhibitors are added to monomers to retard self-reactivity. These inhibitors must be maintained at certain concentrations and may need to be replenished as they are consumed over time.

If nitrogen is used to inert the vapor space above the liquid, the presence of dissolved oxygen above a minimal level is required for effective inhibition.


Water can be used to control flammable and combustible liquids fires under certain conditions. However, foam-water is more effective as it has the ability to extinguish pool fires.4 An aqueous film-forming foam (AFFF) can be used with hydrocarbons, and an alcohol-resistant aqueous film-forming foam (AR-AFFF) is required for polar or water soluble flammable and combustible liquids.

The ability to extinguish flammable and combustible liquid pool fires with foam-water is a function of certain liquid properties. An example of this behavior can be illustrated when reviewing minimum application rates of foam-water on various pool fires.

Spray sprinklers are commonly used to discharge foam-water on flammable and combustible liquid fires. For example, Table 3 lists the minimum application rate or density for foam-solution per the UL Fire Protection Equipment Directory15 using K 8 (115) spray sprinklers. This data is developed using the UL 16216 test protocol. It utilizes a 50 ft.2 (4.6 m2) pan fire, and spray sprinklers deliver foam-solution directly onto a burning liquid pool.

Material Organic Family Water Solubility Minimum Application Rate or Density gpm/ft.2 (mm/min) AR-AFFF Foam-Solution % Concentration
Acetone Ketone Yes 0.34 (14) 3%
Isopropyl Ether Ether Slightly 0.30 (12.2) 3%
Isopropyl Alcohol Alcohol Yes 0.29 (11.9) 3%
n-Butyl Acetate Ester Partially 0.26 (10.4) 3%
Ethyl Alcohol (Denatured) Alcohol Yes 0.24 (9.8) 3%
Heptane Hydrocarbon No 0.22 (9.0) 1%

Table 3. Foam-Water Extinguishing Data for Representative Flammable Liquids15

Acetone, a water soluble ketone, requires a higher minimum application rate or density than heptane, a hydrocarbon. Also, as with the other partially oxygenated hydrocarbons, acetone requires a 3% foamwater concent rat ion whereas 1% i s sufficient for heptane. The water soluble or polar materials are destructive to the foam layer blanketing the burning pool, thereby rendering them more difficult to extinguish.

David P. Nugent is with Valspar Corporation.


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