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.
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
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.
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.
IGNITION PROPERTIES AND LIQUID CLASSIFICATION
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:
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)
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)
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
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.
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.
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
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.”
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
PHYSICAL, COMBUSTION & REACTIVITY PROPERTIES
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.
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
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.
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
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
FIRE CONTROL AND EXTINGUISHMENT
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.
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.
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|
|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%|
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.
- Zalosh, R. Industrial Fire Protection Engineering, John Wiley & Sons Ltd., West Sussex, England: 2003.
- "Globally Harmonized System of Classification and Labeling of Chemicals (GHS)”, United Nations, N Geneva, 2011.
- NFPA 30, Flammable and Combustible Liquids Code, National Fire Protection Association, Quincy, MA, 2012.
- Understanding Fire Protection for Flammable Liquids, National Fire Protection Association, Quincy, MA, 2003.
- NFPA 77, Recommended Practice on Static Electricity, National Fire Protection Association, Quincy, MA, 2007.
- Guidelines for Fire Protection in Chemical, Petrochemical, and Hydrocarbon Processing Facilities, American Institute of Chemical Engineers, New York, 2003.
- Reppermund, J. Generation and Control of Static Electricity in Coatings Operations, American Coatings Association Inc., Washington, DC , 2010, January 2010.
- ASTM D56 Standard Test Method for Flash Point by Tag Closed Cup Tester, ASTM International, West Conshohocken, PA, 2010.
- Mulligan, J. Handling Flammable Liquids, American Institute of Chemical Engineers, New York, July 2003.
- ASTM D92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester, ASTM International, West Conshohocken, PA, 2012.
- ASTM E659, Standard Test Method for Auto Ignition Temperature of Liquid Chemicals, ASTM International, West Conshohocken, PA, 2005.
- Hooker, P., Atkinson, G., Burrell, G. and Fletcher, J., "Fire Performance of Ordinary, Non-listed and Non-metallic IBCs with Aqueous Solutions of Flammable and Combustible Liquids”, Fire Protection Research Foundation, Quincy, MA, April 2012.
- Crowl, D. Minimize the Risks of Flammable Materials, American Institute of Chemical Engineers, New York, April 2012.
- Styrene Monomer: Environmental, Health, Safety, Transport and Storage Guidelines, Styrene Producers Association, Brussels, Belgium, 2007.
- UL Fire Protection Equipment Directory, Underwriters Laboratories Inc., Northbrook, IL, 2013.
- UL 162, Standard for Foam Equipment and Liquid Concentrates, Underwriters Laboratories Inc., Northbrook, IL, 1994.