SECTION 2C: Flammable Liquids
Properties of Flammable and Combustible
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
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 100oF is considered to be a flammable liquid.
A liquid with a flashpoint between 100oF and 200oF is combustible. In
general, the relative hazard of a flammable liquid increase as the flashpoint
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 or 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 (top)
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.
Different types of containers are required depending on the quantities
and classes of flammable or combustible liquids. 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
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 Considerations (top)
- Quantities should be limited to the amount necessary for the work
- 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 (top)
- Control all ignition sources in areas where flammable liquids are
used. Smoking, open flames and spark producing equipment should not
- 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 (top)
Flammable aerosols are in pressurized containers that may rupture when exposed
to fire. As with flammable liquids, these should be stored in a flammable