6.1 Flammable Materials

6.0 Safety Recommendations 

6.1 Flammable Materials 

Chemicals in a liquid form do not burn, their vapors do. Knowing how much vapor a chemical releases is important to understand its flammability risk.

6.1.1 Properties of Flammable and Combustible Liquids

Flammable and combustible liquids vaporize and form flammable mixtures with air when in open containers, when leaks occur, or when heated. To control these potential hazards, several properties of these materials, such as flashpoint, vapor pressure, vapor density, compatibility, combustible (explosive) range, boiling point and auto ignition temperatures must be understood. An explanation of these terms and other properties of flammable liquids is available below. Information on the properties of a specific liquid can be found in that liquid’s safety data sheet (SDS), or other reference material. 

Flashpoint - The lowest temperature at which a chemical releases vapors capable of igniting.

The lower the chemical's flashpoint, the more readily it will start to burn.

Example: A chemical with a flashpoint of 60 degrees F (15.5 degrees C) spills outdoors. The ambient temperature is also 60 degrees F (15.5 degrees C), and the spill produces enough vapors to create an ignitable mixture.

Vapor pressure - Vapor pressure is a measure of the pressure generated by the vapor a liquid or solid produces in a closed container.

Liquids with a high Vapor pressure:

• Often evaporate more rapidly
• Are more likely to release vapors

A liquid's vapor pressure increases as temperature rise, just as a covered pot of water produces more steam as it's heated.

Example: A worker recovers spilled gasoline by shoveling saturated dirt into an unvented drum. If the worker places the drum in the sun, the buildup of vapor pressure may cause it to distort, resulting in the crowning at the top and bottom - a potentially very dangerous situation.

Vapor Density - Vapor Density is a measure of a vapor's weight compared to an equal volume of air. Air has a density value of one.

 Vapors with vapor density values:

• Less than 1 are lighter than air and tend to rise and dissipate
• Greater than one are heavier than air and tend to concentrate in low places (e.g., sumps, manholes, and trenches) and create a health or fire hazard

If you use a high vapor density in a confined space:

• Know the vapors released by the solvent are likely to pool near the floor
• Ensure that you have adequate ventilation to prevent fumes from being trapped and building up to hazardous levels
• Use a respirator, if necessary

Example: hydrogen sulfide gas (vapor density of 1.19), Found in sanitary sewers, it can be expected to accumulate in lower areas.

Compatibility - Compatibility is a measure of how stable two or more chemicals are when mixed together. If you  combine incompatible chemicals, spontaneous heating or combustion can occur. Use, ship, or store chemicals only with chemicals with which they are compatible.

DOT Segregation Chart

Indicates materials that may not be shipped next to each other or on the same truck to prevent further problems in the event of a crash.

When determining storage, it's important to know:

• How stable the material is
• Whether it could have a hazardous reaction with other materials
• That flammables should not be stored with oxidizers, corrosives, or materials prone to spontaneous heating

When disposing of chemical waste:

• Ensure it's compatible with any waste already in the container
• Physically separate contained wastes that are incompatible

Combustible (Explosive) range - When a liquid combusts, It's the mixture of the vapor and the ambient air that ignites. Combustion requires fuel, oxygen, and an ignition source. The concentration at which a chemical's vapor in air is capable of burning determines its combustible range.

 Combustible range is unique to each chemical and:

• Falls between a lower and upper explosive limit (LEL an UEL)
• Indicates the range between the lowest and highest concentrations of vapor in air that will burn or explode

Wider the combustion range = more likely to produce fire or explosion

Examples:

• Hydrogen gas - Wide combustible range, LEL 4% - UEL 75%
• Methane gas - Narrow combustible range, LEL 5% - UEL 17% 

Mixtures below the LEL are too lean to ignite, and those above the UEL are too rich to ignite.

Boiling Point - Boiling point is the temperature at which liquid changes to a gas. A chemical's boiling point is usually in degrees Fahrenheit or Celsius at normal atmospheric pressure - liquids boil at a lower temperature in high elevations. 

Boiling points of Commonly Used Chemicals and Substances

• Butane: 31. F (-0.5 C)
• Formaldehyde: - 60 F (-0.51 C)
• Ammonia: -28.1 F (-2.16 C)
• Propane: -43.67 F (6.48 C)
• Methane: -258.69 F (-161.49 C)

It's important to know a chemical's boiling point when choosing an appropriate storage location.

For Example, a chemical with a low boiling point should not be stored in warm environments, as this could cause vapor pressures to become dangerously high and could lead to a fire or explosion.

Auto-ignition Temperature - the minimum temperature required to ignite a gas or vapor in air without a spark or flame being present

are indicated for common fuels and chemicals below:

 Chart Source: <https://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html>

6.1.2 Storage of Flammable and Combustible Liquids

Flammable and combustible liquids should be stored only in approved containers. Approval for containers is based on specifications developed by organizations such as the US Department of Transportation (DOT), OSHA, the National Fire Protection Agency (NFPA) or American National Standards Institute (ANSI). Containers used by the manufacturers of flammable and combustible liquids generally meet these specifications.

Safety Cans and Closed Containers

Many types of containers are required depending on the quantities and classes of flammable or combustible liquids in use. A safety can is an approved container of not more than 5 gallons capacity that has a spring closing lid and spout cover. Safety cans are designed to safely relieve internal pressure when exposed to fire conditions. A closed container is one sealed by a lid or other device so that liquid and vapor cannot escape at ordinary temperatures.

Flammable Liquid Storage Cabinets

A flammable liquid storage cabinet is an approved cabinet that has been designed and constructed to protect the contents from external fires. Storage cabinets are usually equipped with vents, which are plugged by the cabinet manufacturer. Since venting is not required by any code or the by local municipalities and since venting may actually prevent the cabinet from protecting its contents, vents should remain plugged at all times. Storage cabinets must also be conspicuously labeled "FLAMMABLE – KEEP FIRE AWAY".

Refrigerators

Use only those refrigerators that have been designed and manufactured for flammable liquid storage. Standard household refrigerators must not be used for flammable storage because internal parts could spark and ignite. Refrigerators must be prominently labeled as to whether or not they are suitable for flammable liquid storage.

Storage Considerations:

Quantities should be limited to the amount necessary for the work in progress.No more than 10 gallons of flammable and combustible liquids, combined, should be stored outside of a flammable storage cabinet unless safety cans are used. When safety cans are used, up to 25 gallons may be stored without using a flammable storage cabinet.

Storage of flammable liquids must not obstruct any exit.

Flammable liquids should be stored separately from strong oxidizers, shielded from direct sunlight, and away from heat sources.

See Anecdotes for a description of an incident involving a flammable material stored near a hot plate. 

6.1.3 Handling Precautions

The main objective in working safely with flammable liquids is to avoid accumulation of vapors and to control sources of ignition. Besides the more obvious ignition sources, such as open flames from Bunsen burners, matches and cigarette smoking, less obvious sources, such as electrical equipment, static electricity and gas-fired heating devices should be considered. Accounts of a few of the fires that have occurred in our laboratories may be found in Anecdotes.

Some electrical equipment, including switches, stirrers, motors, and relays can produce sparks that can ignite vapors. Although some newer equipment have spark-free induction motors, the on-off switches and speed controls may be able to produce a spark when they are adjusted because they have exposed contacts. One solution is to remove any switches located on the device and insert a switch on the cord near the plug end.

Pouring flammable liquids can generate static electricity. The development of static electricity is related to the humidity levels in the area. Cold, dry atmospheres are more likely to facilitate static electricity. Bonding or using ground straps for metallic or non-metallic containers can prevent static generation. Control all ignition sources in areas where flammable liquids are used. Smoking, open flames and spark producing equipment should not be used.

Whenever possible use plastic or metal containers or safety cans. When working with open containers, use a laboratory fume hood to control the accumulation of flammable vapor. Use bottle carriers for transporting glass containers.

Use equipment with spark-free, intrinsically safe induction motors or air motors to avoid producing sparks. These motors must meet National Electric Safety Code (US DOC, 1993) Class 1, Division 2, Group C-D explosion resistance specifications.

Many stirrers, Variacs, outlet strips, ovens, heat tape, hot plates and heat guns do not conform to these code requirements. Avoid using equipment with series-wound motors, since they are likely to produce sparks.

Do not heat flammable liquids with an open flame. Steam baths, salt and sand baths, oil and wax baths, heating mantles and hot air or nitrogen baths are preferable.

Minimize the production of vapors and the associated risk of ignition by flashback. Vapors from flammable liquids are denser than air and tend to sink to the floor level where they can spread over a large area.

Electrically bond metal containers when transferring flammable liquids from one to another. Bonding can be direct, as a wire attached to both containers, or indirect, as through a common ground system. When grounding non-metallic containers, contact must be made directly to the liquid, rather than to the container. In the rare circumstance that static cannot be avoided, proceed slowly to give the charge time to disperse or conduct the procedure in an inert atmosphere. 

6.1.4 Flammable Aerosols

Flammable liquids in pressurized containers may rupture and aerosolize when exposed to heat, creating a highly flammable vapor cloud. As with flammable liquids, these should be stored in a flammable storage cabinet. 

6.1.5 Flammable and Combustible Solids

Flammable solids often encountered in the laboratory include alkali metals, magnesium metal, metallic hydrides, some organometallic compounds, and sulfur. Many flammable solids react with water and cannot be extinguished with conventional dry chemical or carbon dioxide extinguishers. See Anecdotes for descriptions of incidents involving such materials.

Ensure Class D extinguishers, e.g., Met-L-X, are available where flammable solids are used or stored. Sand can usually be used to smother a fire involving flammable solids. Keep a container of sand near the work area. If a flammable, water-reactive solid is spilled onto skin, brush off as much as possible, then flush with copious amounts of water.

NEVER use a carbon dioxide fire extinguisher for fires involving lithium aluminum hydride (LAH). LAH reacts explosively with carbon dioxide. 

6.1.6 Catalyst Ignition

Some hydrogenated catalysts, such as palladium, platinum oxide, and Raney nickel, when recovered from hydrogenation reactions, may become saturated with hydrogen and present a fire or explosion hazard.

Carefully filter the catalyst. Do not allow the filter cake to become dry.

Place the funnel containing moist catalyst into a water bath immediately. Purge gases, such as nitrogen or argon, may be used so that the catalyst can be filtered and handled in an inert atmosphere.