7: Flammable Liquids

Properties of Flammable and Combustible Liquids

To control the potential hazards posed by handling flammable and combustible liquids, several properties of these materials should be understood. Information on the properties of a specific liquid can be found in that liquid’s Material Safety Data Sheet (MSDS), or other reference material.

It is the liquid’s vapor rather than the liquid itself that ignites when mixed in certain proportions with air in the presence of an ignition source. Flammable and combustible liquids vaporize and form flammable mixtures with air when in open containers, when leaks occur, or when heated. Volatility is the tendency or ability of a liquid to vaporize. Vapor pressure is a measure of a liquid’s volatility. A high vapor pressure usually is an indication of a volatile liquid, or one that readily vaporizes. The boiling point is the temperature at which the vapor pressure equals atmospheric pressure, such that the pressure of the atmosphere can no longer hold the liquid in a liquid state and bubbles begin to form. In general, a low boiling point indicates a high vapor pressure and, possibly, an increased fire hazard.

An important characteristic of any flammable or combustible liquid is its flashpoint. Flashpoint is the minimum temperature at which the vapor concentration near the surface of the liquid is high enough to form an ignitable mixture. Any liquid with a flashpoint less than 100⁰ F is considered to be a flammable liquid. Any liquid with a flashpoint between 100°F - 200°F is considered combustible. In general, the relative hazard of a flammable liquid increases as the flashpoint decreases.

The proportion of vapor to air mixture that is ignitable is referred to as the flammable range, and is expressed in terms of percentage of vapor in air by volume. The flammable range is bounded by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). The LFL is the minimum concentration of flammable liquid vapor in air that will support the propagation of flame, or spread of flame through the entire volume of vapor-air mixture, upon contact with an ignition source. The UFL is the maximum concentration of vapor in air that will support the propagation of flame. It is important to note that vapor-air mixtures below the LFL may burn at the ignition source without propagating away from the point of ignition.

The auto ignition temperature is the minimum temperature at which a vapor-air mixture will spontaneously ignite, without the necessity of a spark of flame.

Vapor density is a measure of a vapor’s weight when compared to air. Air is assigned a value of 1. Heavier, or denser, vapors tend to sink to floor level while lighter, less dense vapors tend to rise to ceiling level. This property must be taken into account when working with flammable or combustible liquids outside of fume hoods. Most flammable liquid vapors are heavier than air. These vapors can travel some distance and encounter ignition sources remote from the workstation.

Storage Containers

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

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.

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. Venting is not required by any code or the local municipalities and may actually prevent the cabinet from protecting its contents. Therefore, vents should remain plugged at all times. Storage cabinets must also be conspicuously labeled "FLAMMABLE – KEEP FIRE AWAY".                                                                                                                       

Use only those refrigerators that have been designed and manufactured for flammable liquid storage. Standard household refrigerators must not be used for flammable storage. 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.

Handling Precautions

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

Flammable Aerosols

Flammable aerosols are in pressurized containers that may rupture when exposed to fire. As with flammable liquids, these should be stored in a flammable storage cabinet.

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