Flammable liquid
A flammable liquid is defined by the Occupational Safety and Health Administration (OSHA) as any liquid with a closed-cup flash point at or below 199.4°F (93°C), where the flash point represents the lowest temperature at which the liquid's vapors can ignite when exposed to an ignition source such as a spark or flame.[1] These liquids are ubiquitous in industries, laboratories, and everyday products, including fuels like gasoline, solvents such as acetone and toluene, alcohols like ethanol, paints, and adhesives, all of which can release ignitable vapors at ambient temperatures.[2][3] The classification of flammable liquids originated in the late 19th century amid the oil and gas industry revolution and was formalized in fire codes by the National Fire Protection Association (NFPA) in the early 20th century. OSHA adopted these standards in 1970 under 29 CFR 1910.106 and revised them in 2012 to align with the Globally Harmonized System (GHS).[1] OSHA classifies flammable liquids into four categories based on flash point and boiling point to guide storage, handling, and safety measures: Category 1 includes liquids with flash points below 73.4°F (23°C) and boiling points at or below 95°F (35°C), such as diethyl ether; Category 2 covers those with flash points below 73.4°F (23°C) but boiling points above 95°F (35°C), like acetone; Category 3 encompasses flash points from 73.4°F (23°C) to 140°F (60°C), such as xylene; and Category 4 involves flash points from 140°F (60°C) to 199.4°F (93°C), such as certain fuel oils.[1] This classification aligns with the Globally Harmonized System (GHS) and differs from the National Fire Protection Association (NFPA) standards in NFPA 30, which designate liquids with flash points below 100°F (37.8°C) as flammable and those from 100°F to 200°F (37.8°C to 93.3°C) as combustible.[4][5] Flammable liquids pose significant fire and explosion risks due to their ability to form ignitable vapor-air mixtures, with additional health concerns from inhalation or contact requiring proper management.[5][6]Introduction and Definition
Definition
A flammable liquid is defined as any liquid capable of igniting in air near ambient temperatures, specifically one having a closed-cup flash point at or below 93 °C (199 °F) under standards such as those from the Occupational Safety and Health Administration (OSHA) and the Globally Harmonized System of Classification and Labelling of Chemicals (GHS).[1] Other standards, such as pre-2012 OSHA and NFPA guidelines, used a flash point below 100 °F (37.8 °C) to define flammable liquids, distinguishing them from combustible liquids with flash points from 100 °F to 200 °F (37.8 °C to 93.3 °C); some international standards employed thresholds around 55–60 °C. This definition emphasizes the liquid's ability to produce sufficient vapor to form an ignitable mixture with air under typical room conditions, distinguishing it from solids that require melting or other processes to release flammable vapors. Flammable liquids are differentiated from combustible liquids primarily by their lower flash points, which allow easier ignition; for instance, under OSHA and GHS criteria, flammable liquids have flash points of 93 °C or below, whereas combustible liquids historically encompassed those with flash points from 93 °C to approximately 200 °F (93 °C) in pre-2012 classifications.[1] The flash point itself—the minimum temperature at which a liquid's vapors can ignite when exposed to an ignition source—serves as the primary indicator of this behavior. Environmental factors, such as altitude, can influence the effective flash point, with higher elevations leading to lower flash points due to decreased atmospheric pressure that enhances vaporization.[7] Representative examples of flammable liquids include gasoline, which has a flash point of -40 °C, and ethanol, with a flash point of 13 °C, illustrating their high volatility in everyday applications.[8]Historical Development
The recognition of flammable liquids as a distinct hazard category emerged in the late 19th century amid the rapid expansion of the petroleum industry following Edwin Drake's 1859 oil well in Pennsylvania, which sparked the U.S. oil boom and widespread kerosene production for lighting. Early industrial accidents, including numerous fires and explosions from low-flash-point kerosene, prompted state-level regulations to establish safety standards for illuminating oils; states set varying thresholds between 100°F and 150°F (38–66°C).[9] These measures represented initial efforts to classify liquids based on ignition potential, driven by the kerosene era's fire hazards rather than comprehensive national codes.[10] Key milestones in formal classification began with the National Fire Protection Association (NFPA), founded in 1896 to address electrical fire risks but expanding to broader hazards. In 1913, the NFPA issued its first code on flammable liquids, titled "Suggested Ordinance for the Storage, Handling, and Use of Flammable Liquids," which provided model municipal regulations emphasizing safe storage and handling to prevent fires in industrial settings.[11] This was followed by the United Nations' inaugural Recommendations on the Transport of Dangerous Goods in 1956, which categorized flammable liquids by flash point and boiling point for international shipping, influencing global harmonization efforts.[12] In the U.S., the Occupational Safety and Health Administration (OSHA), established in 1970, adopted a flash point threshold of below 37.8°C (100°F) to define flammable liquids in its standards, distinguishing them from combustible liquids with flash points between 37.8°C and 93.3°C (100–200°F), a terminology rooted in earlier NFPA guidelines.[13][14] Pre-1980s classifications maintained a clear distinction between flammable liquids (flash point <100°F or 37.8°C, posing immediate vapor ignition risks at ambient temperatures) and combustible liquids (100–200°F or 37.8–93.3°C, requiring heating to ignite), as codified in NFPA 30 editions and OSHA's 29 CFR 1910.106, to guide storage, handling, and firefighting protocols.[9][15] The 1984 Bhopal disaster, involving a methyl isocyanate leak from a Union Carbide plant in India that killed thousands and highlighted vulnerabilities in storing and handling hazardous chemicals including flammable intermediates, spurred global regulatory updates; it directly influenced the U.S. Emergency Planning and Community Right-to-Know Act (1986) and OSHA's Process Safety Management standard (1992), emphasizing risk assessment for flammable substances.[16][17] By 2012, OSHA aligned its Hazard Communication Standard with the Globally Harmonized System (GHS), expanding the flammable liquids category to include all with flash points ≤93°C (199.4°F) and introducing subcategories based on flash point and boiling point ranges, thereby eliminating the prior combustible label for regulatory labeling while retaining it for certain fire code applications.[18][19] This shift promoted international consistency in hazard communication without altering core storage requirements under NFPA 30.[11]Physical and Chemical Properties
Key Properties
The flash point represents the lowest temperature at which a flammable liquid produces sufficient vapor to form an ignitable mixture with air near the liquid's surface or within a test vessel when exposed to an ignition source.[5] This property is determined using either closed-cup methods, where vapors are contained in a covered vessel, or open-cup methods, where vapors can escape into the surrounding air; closed-cup measurements typically yield lower values due to higher vapor concentration.[5] Closely related is the fire point, defined as the lowest temperature at which the liquid's vapors achieve sustained burning after the ignition source is removed.[20] It generally occurs 5–10 °C above the flash point, as additional heat is required to maintain vapor production at a rate that supports ongoing burning.[21] The autoignition temperature is the minimum temperature at which the liquid ignites spontaneously in air without an external spark or flame; for instance, gasoline autoignites at approximately 280 °C.[22] Vapor pressure, the equilibrium pressure exerted by the liquid's vapors at a given temperature, plays a critical role in flammability by determining the rate of vaporization needed to create ignitable mixtures; higher vapor pressures increase volatility and lower the temperatures required for ignition.[23] Flammability limits describe the range of vapor concentrations in air that can support flame propagation, bounded by the lower explosive limit (LEL), the minimum concentration for ignition, and the upper explosive limit (UEL), the maximum concentration beyond which combustion cannot occur due to insufficient oxygen. For gasoline, the LEL is 1.4% by volume and the UEL is 7.6% by volume, highlighting the narrow window for explosive vapor-air mixtures.[24] These properties can vary based on factors such as the liquid's purity, where contamination by lower-flash-point substances reduces the flash point and alters ignition behavior; the container material, which may cause corrosion or chemical reactions that degrade the liquid over time; and oxygen concentration in the environment, though it has minimal impact on flash points for most flammable liquids.[25][26][27]Measurement Techniques
The measurement of flammability properties in liquids relies on standardized laboratory procedures to ensure reproducibility and accuracy, primarily focusing on flash point, boiling point, autoignition temperature, and flammability limits. These techniques use specialized apparatus to simulate conditions under which ignition or vaporization occurs, providing data essential for hazard assessment. Flash point, the lowest temperature at which a liquid produces sufficient vapor to form an ignitable mixture with air, is determined using closed-cup or open-cup testers. The Pensky-Martens closed-cup method, outlined in ASTM D93, employs a manual or automated apparatus for petroleum products with flash points between 40°C and 370°C, and biodiesel blends from 60°C to 190°C. In this procedure, a sample is placed in a closed cup, stirred, and heated at a controlled rate (typically 5–6°C per minute), with an ignition source applied periodically until a flash is observed. Procedure A applies to distillate fuels and lubricating oils, Procedure B to viscous residual fuels requiring a different stirrer, and Procedure C to biodiesel. This method minimizes vapor loss, providing a conservative estimate of flammability.[28][29] For lower-viscosity liquids (below 5.5 mm²/s at 40°C) with flash points under 93°C, the Tag closed-cup tester per ASTM D56 is used, suitable for solvents and light oils. The apparatus consists of a brass cup immersed in a bath, heated gradually (1–2°C per minute for low temperatures or faster for higher), and tested with a flame source. Automated versions enhance precision by controlling ignition timing. This method is ideal for aviation fuels and paints, offering results comparable to Pensky-Martens for low-viscosity samples.[30][31] Higher flash points, above 79°C and up to 400°C (excluding fuel oils), are measured with the Cleveland open-cup apparatus under ASTM D92, which determines both flash and fire points for viscous materials like bitumen and lubricants. The sample is heated in an open cup at 5–6°C per minute, with a test flame passed over the surface every 2°C until ignition occurs; the fire point follows similarly but requires sustained burning. This open configuration allows greater air circulation, simulating real-world exposure but yielding slightly higher values than closed-cup methods.[32] Boiling point, critical for vapor pressure assessment, is evaluated through distillation under ASTM D86 for petroleum products such as gasolines, diesels, and kerosene. The procedure involves batch distillation in a flask with a condenser, recording the initial boiling point, temperatures at 10%, 50%, and 90% recovered volumes, and final boiling point, typically at atmospheric pressure. This yields the boiling range distribution, with automated systems ensuring consistent heat input (4–5 ml/min vapor rate). The method applies to fuels with up to 30% biodiesel but not to products leaving significant residues. Precision from interlaboratory studies shows repeatability within 0.5–2°C for key points in ethanol blends.[33] Autoignition temperature, the minimum temperature for spontaneous ignition without an external spark, is measured via ASTM E659 for liquid chemicals in air. A sample (typically 10 µl) is injected into a preheated 200–500 ml glass flask at atmospheric pressure, observed for 10 minutes for hot- or cool-flame ignition; testing increments by 5–10°C until the lowest ignition temperature is found (up to 750°C). The procedure accounts for vessel size and material effects on results, excluding materials with condensed phases at test temperature.[34] Flammability limits, defining the concentration range for combustible vapor-air mixtures, are determined by ASTM E681, focusing on lower (LFL) and upper (UFL) limits through upward flame propagation in a 12-liter spherical vessel. The vapor is mixed with air at varying concentrations (0.5–1% steps), ignited centrally with an electrical spark, and propagation assessed visually or via pressure rise; limits are the extrema where flame travels 15–18 cm or more. This method applies to gases and vapors at ambient temperature and pressure, with downward propagation limits narrower.[35][36] Calibration of testing equipment involves traceable standards, such as certified reference materials for flash point (e.g., toluene at 4°C), ensuring temperature accuracy within ±0.5°C using thermocouples or platinum resistance thermometers. Safety protocols in these tests mandate fume hood operation, explosion-proof ventilation, and personal protective equipment to mitigate ignition risks; inert atmospheres like nitrogen may purge apparatus pre-test to avoid premature flashes, while temperature controls prevent overheating. Error margins typically range from ±1–2°C for flash point determinations across methods, influenced by sample volatility and apparatus condition.[37][29]Classification Systems
Criteria Based on Flash Point and Boiling Point
The classification of flammable liquids relies primarily on two key physical properties: the flash point, defined as the lowest temperature at which vapors from the liquid ignite when exposed to an ignition source in the presence of air, and the initial boiling point, the temperature at which the liquid's vapor pressure equals standard atmospheric pressure (101.3 kPa). These metrics determine the ease with which a liquid can form an ignitable vapor-air mixture, with lower flash points indicating higher volatility and fire risk. Under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS), adopted by OSHA in its Hazard Communication Standard, a flammable liquid is any liquid with a flash point of 93 °C (199.4 °F) or less; however, many traditional and regulatory definitions limit "flammable" to those with flash points of 60 °C (140 °F) or less, treating higher-flash-point liquids as combustible.[19] Flammable liquids are subdivided into four categories based on flash point and, for the most volatile, boiling point, to reflect escalating hazard levels from category 4 (least hazardous) to category 1 (most hazardous). The criteria are as follows:| Category | Flash Point | Initial Boiling Point | Hazard Level |
|---|---|---|---|
| 1 | < 23 °C (73.4 °F) | ≤ 35 °C (95 °F) | Extreme (e.g., diethyl ether) |
| 2 | < 23 °C (73.4 °F) | > 35 °C (95 °F) | High (e.g., acetone) |
| 3 | ≥ 23 °C (73.4 °F) and ≤ 60 °C (140 °F) | Any (typically > 35 °C) | Flammable (e.g., kerosene) |
| 4 | > 60 °C (140 °F) and ≤ 93 °C (199.4 °F) | Any | Combustible (included in GHS) |