Flammable liquids definitions

An aerosol is a material dispensed as a mist, spray, or foam by a propellant under pressure, and for some rules OSHA treats certain aerosols as Category 1 flammable liquids. See the definition in 1910.106(a)(1) and the note that certain aerosols are considered Category 1 for the purposes of paragraph (d) in 1910.106(a)(14).

• Practical point: if you have an aerosol product that meets the Category 1 criteria, handle and store it under the more restrictive rules that apply to Category 1 flammable liquids.

Flashpoint is the minimum temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near its surface, and it must be measured using the specified test procedures in the standard. See the definition and test-method directions in 1910.106(a)(14).

• Use the test method that fits the liquid's physical properties (viscosity, solids, surface film tendency) as explained in the subparagraphs.

Use the Tag Closed Tester (ASTM D-56) or an equivalent method when the liquid has a viscosity less than 45 SUS at 100 °F, contains no suspended solids, and does not form a surface film during testing. See the specific instruction in 1910.106(a)(14)(i).

• If your liquid meets those three conditions (low viscosity, no solids, no surface film), the Tag Closed Tester is the appropriate test method.

Use the Pensky-Martens Closed Tester (ASTM D-93) or an equivalent method when the liquid has a viscosity of 45 SUS or more at 100 °F, or contains suspended solids, or tends to form a surface film while under test. See 1910.106(a)(14)(ii).

• Note: Certain exceptions referenced in ASTM D-93 are allowed for materials specified in that standard's Note 1.

Determine the flashpoint of a mixture by testing the material as shipped using the appropriate procedure in 1910.106(a)(14)(i) or (ii). See the instruction at 1910.106(a)(14)(iii).

• Practical tip: do not estimate by component values; test the actual shipped mixture.

No, organic peroxides that undergo autoaccelerating thermal decomposition are excluded from the flashpoint determination methods specified in this subsection. See the exclusion in 1910.106(a)(14)(iv).

• For such materials you must use other appropriate methods or standards to assess hazard and handling requirements.

When an accurate boiling point is unavailable, or for mixtures without a constant boiling point, use the 10 percent point of a distillation performed per ASTM D-86-62 as the boiling point for the purposes of this section. See the procedure in 1910.106(a)(5).

• That ASTM procedure is incorporated by reference as specified in 1910.6, so follow the standard test protocol.

A flammable liquid is any liquid with a flashpoint at or below 199.4 °F (93 °C), and OSHA divides flammable liquids into four categories (1–4) based on flashpoint and boiling point. See the definition and categories in 1910.106(a)(19).

• Category 1: flashpoint below 73.4 °F (23 °C) and boiling point ≤ 95 °F (35 °C) (1910.106(a)(19)(i)).

• Category 2: flashpoint below 73.4 °F (23 °C) and boiling point > 95 °F (35 °C) (1910.106(a)(19)(ii)).

• Category 3: flashpoint ≥ 73.4 °F (23 °C) and ≤ 140 °F (60 °C) (1910.106(a)(19)(iii)).

• Category 4: flashpoint > 140 °F (60 °C) and ≤ 199.4 °F (93 °C) (1910.106(a)(19)(iv)).

• Follow the category-specific handling and storage requirements elsewhere in 1910.106.

If a Category 3 liquid with a flashpoint at or above 100 °F is heated for use to within 30 °F of its flashpoint, it must be handled under the requirements that apply to a Category 3 liquid with a flashpoint below 100 °F. See the handling instruction in 1910.106(a)(19)(iii).

• In practice, that means applying the more restrictive controls (storage, ventilation, transfer precautions) that the standard requires for the lower-flashpoint condition.

When a Category 4 flammable liquid is heated for use to within 30 °F of its flashpoint, it must be handled in accordance with the requirements for a Category 3 liquid with a flashpoint at or above 100 °F. See 1910.106(a)(19)(iv).

• This effectively raises the controls required when heating Category 4 liquids close to their flashpoint, so review handling and storage provisions for Category 3 accordingly.

If a liquid with a flashpoint greater than 199.4 °F (93 °C) is heated for use to within 30 °F of its flashpoint, it must be handled in accordance with the requirements for a Category 4 flammable liquid. See 1910.106(a)(19)(v).

• In short: heating a normally non-flammable liquid close to its flashpoint can make it subject to flammable-liquid controls.

Flammable liquids must be stored in tanks or containers that meet the design and construction requirements referenced in the standard, specifically tanks or containers that comply with [1910.106(d)(2)]. See the storage requirement in 1910.106(a)(32) and the construction reference at 1910.106(d)(2).

• Practical step: inspect containers and tanks to verify they meet the specific construction, closure, and venting rules in paragraph (d)(2) before storing flammable liquids.

A safety can is an approved container of not more than 5 gallons capacity with a spring-closing lid and spout cover and designed to safely relieve internal pressure if exposed to fire. See the definition in 1910.106(a)(29).

• Use safety cans that are approved/listed and keep their closures and pressure-relief features in working order.

Vapor pressure is defined as the pressure exerted by a volatile liquid, measured in pounds per square inch (absolute), as determined by the Reid Method (ASTM D323-68) incorporated by reference. See the definition in 1910.106(a)(30).

• When evaluating volatility or storage conditions, use the referenced test method to determine vapor pressure for regulatory comparisons.

Ventilation required by the section must be sufficient to prevent accumulation of vapor-air mixtures in concentrations over one-fourth of the lower flammable limit (LFL). See 1910.106(a)(31).

• In practice: design ventilation so vapor concentrations stay below 25% of LFL to meet the standard's objective.

SUS means Saybolt Universal Seconds per ASTM D-88-56 (or conversion tables), and "viscous" is defined as having a viscosity of 45 SUS or more; these values affect which flashpoint test method to use. See the SUS definition in 1910.106(a)(37) and the viscous definition in 1910.106(a)(38).

• Use the susceptibility to surface film and the 45 SUS threshold to decide between Tag Closed and Pensky-Martens flashpoint tests.

Tanks shall be built of steel for general service, except for specified exceptions such as underground tanks or when the liquid's properties require other materials; aboveground or indoor tanks must be noncombustible. See the materials rules in 1910.106(b)(1)(i).

• If you plan non-steel tanks, ensure they are designed to accepted engineering principles for the material and service conditions described in the standard.

Metal tanks must be fabricated by welding, riveting, caulking, brazing, bolting, or a combination of those methods, and any filler metal used in brazing must be nonferrous or an alloy with a melting point above 1000 °F. See the fabrication requirement in 1910.106(b)(1)(ii)(b).

• Follow acceptable fabrication methods and materials to maintain tank integrity for flammable liquid storage.

An automotive service station is the portion of property where motor-fuel flammable liquids are stored and dispensed from fixed equipment into vehicle fuel tanks and includes facilities for sale/service of tires, batteries, accessories, and minor maintenance; major repairs and painting are excluded. See the definition in 1910.106(a)(3).

• Use this definition to decide which portions of a facility are covered by the service-station-specific requirements in 1910.106.

A basement is a story of a building with one-half or more of its height below ground level and where access for firefighting is unduly restricted. See the definition in 1910.106(a)(4).

• When storing flammable liquids, treat basements as having special access constraints and apply storage limits accordingly.

Boilover is the violent expulsion of crude oil (or similar liquids) from a burning tank when lighter fractions burn off, a heat wave reaches underlying water strata, and the tank contents froth and are expelled. See the definition in 1910.106(a)(6).

• Be aware of boilover risk when storing crude or similar hydrophobic mixtures that could trap water under hydrocarbon layers in tanks.

Crude petroleum means hydrocarbon mixtures with a flashpoint below 150 °F that have not been processed in a refinery. See the definition in 1910.106(a)(10).

• Storage and handling of crude petroleum should account for its volatility and fire hazard as defined here.

For this section, a liquid is any material with fluidity greater than that of 300-penetration asphalt when tested per ASTM D-5-65; the test is incorporated by reference. See 1910.106(a)(17).

• Use the ASTM test to determine whether a material qualifies as a liquid under the standard before applying storage and handling rules.

A portable tank is a closed container with liquid capacity over 60 U.S. gallons that is not intended for fixed installation; such tanks fall under the standard's definitions and applicable requirements. See 1910.106(a)(25).

• Inspect and manage portable tanks per the applicable construction, venting, and storage rules in 1910.106 when capacities exceed 60 gallons.

A low-pressure tank is designed to operate above 0.5 p.s.i.g. but not more than 15 p.s.i.g., while a pressure vessel is designed for pressures above 15 p.s.i.g. See the definitions in 1910.106(a)(21) and 1910.106(a)(26).

• Use these pressure-based distinctions to determine applicable tank design, testing, and safety requirements.

“Approved” or “listed” means equipment evaluated and accepted by a nationally recognized testing laboratory as defined in [1910.7]; the flammable-liquids standard refers to that acceptance standard. See 1910.106(a)(35), 1910.106(a)(36), and the lab definition in 1910.7.

• When the standard calls for "approved" containers or devices, choose products listed by a recognized testing lab per 1910.7.

An unstable (reactive) liquid is one that will vigorously polymerize, decompose, condense, or become self-reactive under shock, pressure, or temperature in its pure state or as commercially produced or transported. See 1910.106(a)(20).

• Such liquids require special handling and controls beyond normal flammable-liquid precautions because of their reactivity hazards.

Mercantile occupancy is the use of a building or portion of a building for displaying, selling, or buying goods, wares, or merchandise. See the definition in 1910.106(a)(23).

• Apply the appropriate storage and quantity limits in 1910.106 based on this occupancy classification when flammable liquids are present.

A marine service station is where fuel flammable liquids are stored and dispensed from fixed equipment on shore, piers, wharves, or floating docks into fuel tanks of self-propelled craft, and includes all facilities used in connection with that service. See 1910.106(a)(22).

• Treat these operations under the marine-specific provisions of 1910.106 when fueling or storing flammable liquids for boats and other craft.

An atmospheric tank is a storage tank intended to operate at or near atmospheric pressure and must be built to acceptable, good standards of design. Atmospheric tanks may be built according to the consensus standards listed in 1910.106(b)(1)(iii)(a), including the Underwriters' Laboratories standards referenced in 1910.106(b)(1)(iii)(a)(1) and the American Petroleum Institute standards in 1910.106(b)(1)(iii)(a)(2) and 1910.106(b)(1)(iii)(a)(3).

• If you use one of the referenced standards (for example, UL or API standards), that is an acceptable way to comply. See 1910.106(b)(1)(iii)(a).
• Tanks used as production tanks for crude petroleum must follow the specific API/UL specifications cited in 1910.106(b)(1)(iii)(a)(3).

Yes — tanks designed for underground service that do not exceed 2,500 gallons capacity may be used aboveground. This allowance is stated directly in 1910.106(b)(1)(iii)(b).

• Even when permitted aboveground, you should ensure the tank installation and materials are suitable for the intended aboveground service and meet any other applicable installation and venting requirements in 1910.106.

No — atmospheric tanks shall not be used for the storage of a flammable liquid at a temperature at or above its boiling point. That prohibition is stated in 1910.106(b)(1)(iii)(d).

• If the stored liquid must be handled at or above its boiling point, a tank designed for pressure service (a low-pressure tank or pressure vessel meeting the applicable design codes) must be used; see related provisions for low-pressure tanks in 1910.106(b)(1)(iv).

Low-pressure tanks must not operate above their design pressure, and they must be built according to acceptable design standards. Specifically, 1910.106(b)(1)(iv)(a) requires that the normal operating pressure of the tank not exceed the tank's design pressure. 1910.106(b)(1)(iv)(b) allows construction to follow consensus standards such as API 620 or the ASME Boiler and Pressure Vessel Code Section VIII as referenced in 1910.106(b)(1)(iv)(b)(2).

• Atmospheric tanks built to the UL-derived standards referenced earlier may be limited to emergency venting conditions up to 2.5 psig and may be used under certain lower operating pressures as described in 1910.106(b)(1)(iv)(c).
• Pressure vessels may also be used as low-pressure tanks per 1910.106(b)(1)(iv)(d).

You must provide additional metal thickness or suitable protective coatings or linings to compensate for expected corrosion during the tank's design life. 1910.106(b)(1)(vi) requires these measures when tanks are not designed per the listed standards or when anticipated corrosion exceeds design allowances.

• Document the expected corrosion rate and the protective measures you provide (thicker metal, coatings, or linings).
• Regular inspection and maintenance programs help ensure the protective measures perform as intended over the tank's service life.

The distance between any two flammable liquid storage tanks shall not be less than 3 feet, and except as otherwise provided, the distance between any two adjacent tanks shall not be less than one-sixth the sum of their diameters. See 1910.106(b)(2)(ii)(a) and (b).

• Special rules: when one tank's diameter is less than one-half the adjacent tank's diameter, the distance may be as small as one-half the diameter of the smaller tank ([1910.106(b)(2)(ii) text]).
• If unstable flammable liquids are stored, greater spacing applies: at least one-half the sum of their diameters (1910.106(b)(2)(ii)(d)).
• When tanks are compacted in three or more rows or in irregular patterns, additional spacing or other access measures must be provided for firefighting access (1910.106(b)(2)(ii)(e).

The minimum separation between a liquefied petroleum gas container and a flammable liquid storage tank is 20 feet, except where the flammable liquid tank operates above 2.5 p.s.i.g. or has emergency venting permitting pressures above 2.5 p.s.i.g., in which case the general shell-to-shell spacing rules apply. See 1910.106(b)(2)(ii)(f).

• You must also take measures to prevent accumulation of liquid under adjacent LPG containers (for example, diversion curbs or grading) as required in the same paragraph.

Normal vents for atmospheric tanks must be sized either in accordance with API Standard 2000, by another accepted standard, or be at least as large as the filling or withdrawal connection (whichever is larger) and in no case less than 1¼ inch nominal inside diameter. This requirement appears in 1910.106(b)(2)(iv)(b)(1)–(3).

• If a tank has multiple fill or withdrawal connections that can be used simultaneously, vent sizing must be based on the maximum anticipated simultaneous flow (1910.106(b)(2)(iv)(d)).
• Vents must also be arranged to avoid localized overheating if vented vapors could ignite and the vent does not limit pressure to 2.5 p.s.i. or less (1910.106(b)(2)(iv)(e).

Tanks and pressure vessels storing Category 1 flammable liquids must be equipped with venting devices that are normally closed except when venting pressure or vacuum; tanks storing Category 2 flammable liquids and Category 3 flammable liquids with flashpoint below 100 °F must have normally closed vents or approved flame arresters. See 1910.106(b)(2)(iv)(f)(1) and 1910.106(b)(2)(iv)(f)(2).

• There are limited exemptions: certain crude-producing area tanks and small outside atmospheric tanks under specified capacities may have open vents as described in 1910.106(b)(2)(iv)(f)(2).
• Flame arresters may be omitted where their obstruction would risk tank damage; see 1910.106(b)(2)(iv)(g).

Every aboveground storage tank must have construction or devices that relieve excessive internal pressure caused by exposure fires, and the total emergency venting capacity must prevent rupture of the tank shell or heads. This is required by 1910.106(b)(2)(v)(a)–(c).

• The total venting capacity for many tanks is taken from Table H-10 in 1910.106(b)(2)(v)(c).
• For tanks designed for pressure over 1 p.s.i.g. with wetted area greater than 2,800 ft2, use the formula CFH = 1,107 A^0.82 to compute required cubic feet per hour (see 1910.106(b)(2)(v)(d)).
• You may reduce required airflow using approved protection factors such as drainage, water spray, or insulation per 1910.106(b)(2)(v)(f).
• Each commercial tank venting device must have stamped information on opening pressure, full-open pressure, and flow capacity as required in 1910.106(b)(2)(v)(h).

The wetted area to be used with Table H-10 is calculated as 55 percent of the total exposed area for a sphere or spheroid, 75 percent of the total exposed area for a horizontal tank, and the first 30 feet above grade of the exposed shell area for a vertical tank. This calculation is described just before Table H-10 in 1910.106(b)(2)(v)(c).

• Use the appropriate percentage or surface area portion to find the square feet (A) that corresponds to a CFH value in Table H-10, or use the formula in 1910.106(b)(2)(v)(d) for very large wetted areas.

If vents or vent drains permit internal pressures exceeding 2.5 p.s.i.g., the outlet of vents and vent drains must be arranged to discharge in a way that prevents localized overheating of any part of the tank if emitted vapors are ignited. This is required by 1910.106(b)(2)(iv)(e) and reinforced in 1910.106(b)(2)(v)(g).

• In practice, this means routing vent discharge away from tank surfaces and providing hardware or deflectors so that accidental ignition of vented vapors will not focus heat on one area of the tank shell or roof.

Yes — there are limited exemptions that allow open vents. Tanks of up to 3,000 barrels capacity containing crude petroleum in crude-producing areas, and outside aboveground atmospheric tanks under 1,000 gallons capacity containing liquids other than Category 1 flammable liquids, may have open vents as described in 1910.106(b)(2)(iv)(f)(2).

• Note that these exemptions are narrow and conditional; where vents allow pressures above 2.5 p.s.i.g., other protective arrangements for venting discharges and heating risks still apply (see 1910.106(b)(2)(iv)(e) and (v)(g)).

Vent outlets for those tanks must discharge outside, at least 12 feet above the adjacent ground, and be placed so vapors disperse away from buildings and openings. Under 1910.106(b)(2)(vi)(b), vents for tanks storing Category 1 or 2 flammable liquids (and certain Category 3 liquids) adjacent to buildings or public ways must: discharge at a safe point outside buildings not less than 12 feet above ground; discharge upward or horizontally away from nearby walls to aid dispersion; avoid eaves or obstructions that could trap vapors; and be at least five feet from building openings. These siting rules reduce the chance that flammable vapors accumulate near people or ignition sources.

Each tank vent must be at least 1¼ inches nominal inside diameter, and you must choose larger diameters from Table H-11 based on maximum flow (GPM) and vent line length. 1910.106(b)(3)(iv)(b) states vent pipes shall be not less than 1¼ inch nominal inside diameter and Table H-11 provides recommended diameters (for example, up to 300 GPM you may need 1½ in. for longer runs). Use the table to match your expected maximum filling or vapor flow and total equivalent pipe length (note the table assumes 50, 100, or 200 ft plus fittings) so the vent prevents blow-back at the fill opening.

A diked area must hold at least the full volume of liquid from the largest tank (with special rules for crude oil boilover tanks), and its walls must be liquid-tight and able to withstand full hydrostatic head. Specifically, 1910.106(b)(2)(vii)(c)(1) requires the diked volume be not less than the greatest amount of liquid that could be released from the largest tank (assuming full). For tanks with boilover characteristics the diked capacity must meet the provisions in 1910.106(b)(2)(vii)(c)(2). Walls must be constructed of earth, steel, concrete, or solid masonry, made liquid-tight and designed to withstand a full hydrostatic head per 1910.106(b)(2)(vii)(c)(3). Earthen walls 3 feet or more high must have a flat 2-foot-wide top section.

Diked walls have an average height limit of 6 feet and you must not keep loose combustible materials or drums inside the diked area. 1910.106(b)(2)(vii)(c)(4) restricts the walls of diked areas to an average height of 6 feet above the interior grade. And 1910.106(b)(2)(vii)(c)(6) expressly prohibits loose combustible materials, empty or full drums, or barrels inside the diked area because such items can increase fire load, block capacity, or create ignition risks.

For certain flammable liquids the fill pipe must minimize static generation by terminating within 6 inches of the tank bottom, and any fill/empty connections that are made or broken must be outside, away from ignition sources and at least 5 feet from building openings. 1910.106(b)(2)(viii)(e) requires fill pipes for Category 2 and some Category 3 liquids to be designed to minimize static electricity and to terminate within 6 inches of the tank bottom when entering from the top. Similarly, 1910.106(b)(2)(viii)(f) and 1910.106(b)(3)(v)(e) require filling and emptying connections that are made and broken to be located outside the building, free from ignition sources and at least 5 feet from any building opening; they must be closed and liquidtight when not in use and be properly identified.

Vent piping must be laid so it drains toward the tank without sags or traps where liquid can collect, and it must be protected from physical damage with the tank end entering through the top. 1910.106(b)(3)(iv)(d) requires that vent piping be constructed per paragraph (c), installed to drain toward the tank (no sags or traps), be located to avoid physical damage, and have the tank end enter the tank through the top. Proper slope and support prevent liquid accumulation that can block vents or cause unsafe back pressures.

If you calculate flow capacity for vent openings larger than 12 inches nominal pipe size, you must actually measure opening pressure, state the rating pressure and corresponding free orifice area, put the word "calculated" on the nameplate, and base the computation on a flow coefficient of 0.5. The rule text explains that for vents larger than 12 inches the flow capacity "may be calculated provided that the opening pressure is actually measured, the rating pressure and corresponding free orifice area are stated, the word 'calculated' appears on the nameplate, and the computation is based on a flow coefficient of 0.5 applied to the rated orifice area" (see the general venting provisions in 1910.106). Also, 1910.106(b)(3)(iv)(a) requires vent discharge points for certain flammable liquids to be outside and at least 12 feet above adjacent ground. Keeping measured opening pressure and the calculation details on the nameplate ensures the vent will perform as intended and helps demonstrate compliance.

Under 1910.106(b)(4)(iv)(e) the fill pipe must terminate within 6 inches of the bottom of the tank to minimize the possibility of generating static electricity for Category 2 flammable liquids and Category 3 flammable liquids with a flashpoint below 100 °F (37.8 °C), except for crude oils, gasoline, and asphalts.

• Scope: This requirement specifically applies to the liquids described in 1910.106(b)(4)(iv)(e).
• Why it matters: Terminating the fill pipe close to the tank bottom helps reduce splashing and turbulent flow that can create static electricity and an ignition hazard.
• Related installation points to check: ensure the fill pipe is installed to avoid excessive vibration as required by 1910.106(b)(4)(iv)(f) and that the fill-pipe inlet is located outside buildings, away from ignition sources and at least 5 feet from building openings, and is closed and liquidtight when not in use as required by 1910.106(b)(4)(iv)(g).
• Practical tip: If you are designing or inspecting a tank fill system, document the fill-pipe termination and related features and coordinate with an engineer familiar with 1910.106 requirements to ensure full compliance.