Flammable vs Combustible: What’s the Difference?

Flammable and combustible are terms used to describe how easily a liquid ignites. While we usually use the term to describe liquids, it is not the liquid that burns – it’s the vapor given off by the liquid.

So, what is the difference between flammable and combustible? Flammable liquids burn at normal working temperatures while combustible liquids need heat before they will ignite. They also have different flashpoints.

Official Definition of Flammable and Combustible

The National Fire Protection Association defines the different classes of flammable and combustible liquids in its Flammable and Combustible Liquids Code, also known as NFPA 30.

The graphic above shows that flammable liquids have a flashpoint below 100 F (37.8 C). Combustible liquids have a flashpoint above 100 F.

The NFPA and OSHA differ in how they classify flammable liquids. In 2015, OSHA adopted the Globally Harmonized System (GHS) of classification and labeling of chemicals. This change removed the term combustible from OSHA standard 29 CFR 1910.106. Thus, all liquids with a flashpoint lower than 199.4 F (93 C) are flammable.

Important Physical Properties of Flammable and Combustible Liquids

Flammable and combustible liquids vary in complexity. But, there are some crucial similarities between the two types of liquid. Here are three defining physical characteristics to keep in mind when handling flammables and combustibles.

Flashpoint is the main physical property that defines flammable and combustible liquids. It is the minimum temperature at which the vapors given off by a liquid could ignite when mixed with air, near the surface of the liquid.

Boiling Point
The boiling point is the temperature that liquid boils. It occurs when the vapor pressure of the liquid equals the atmospheric pressure. Boiling liquids produce a lot of vapors and the harder they boil, the more vapors they release.

Flammable/Explosive Range
The flammable range of a liquid is a measure of the vapor concentration in the air where an explosion can occur, based on normal atmospheric and temperature variables. If the vapor concentration is "too lean," there is not enough fuel for ignition. If the vapor concentration is "too rich," there is not enough oxygen for ignition. The flammable range is the danger zone for fires.

Where to Find Info on Combustible or Flammable Liquids

Chemical suppliers provide a Safety Data Sheet (SDS) with each product they deliver. It contains all the chemical, physical and safety information for the product – including whether it is flammable or combustible. The physical properties described above appear in section 9 of any SDS. Section 7 describes the safe handling and storage requirements for each chemical. Follow the instructions outlined in your SDS to prevent injuries and fires.

Why Do I Need this Information?

Many different industries use flammable and combustible liquids – from hair salons to industrial manufacturing plants. With such a wide range of applications, your chances of working with or around flammable or combustibles is high. It’s important to understand the differences between these properties. Improperly using, transporting or storing flammable or combustible materials is easy to do, but the consequences can be dangerous.

Now that I Know the Differences, Where Do I Store Flammables and Combustibles?

Anytime you work with flammable or combustible liquids, the risk of fire exists. Proper storage and handling is the first step to reducing those risks. Always use approved safety storage cabinets that meet OSHA and NFPA construction and design requirements. Look for cabinets certified by independent third-party testing agencies such as FM Approvals, MPA Dresden or UL.

Here are some other things to keep in mind when storing flammable and combustible liquids:

Keep It Separate
Never store incompatible liquids near one another. This also means storing certain chemical groups away from other chemical groups – like oxidizers and flammables. Justrite supplies shelf dividers to help organize chemicals within a safety cabinet.

Ensure Proper Ventilation
Vapors are invisible and denser than air. They can ignite easily in areas with poor ventilation, even at ambient temperatures. All Justrite cabinets include dual vents with built-in flame arresters with bungs. While NFPA 30 does not require ventilating safety cabinets, it does state to leave the vent opening sealed with the supplied bungs if the cabinet is not vented.

Note: If venting the cabinet, consult the local fire marshal and/or the authority having jurisdiction to ensure local codes are met, and a professional engineer or EHS officer to ensure the fire resistance is not compromised.

Justrite Oily Waste Can, 14 Gallon, Foot-Operated Self-Closing Cover – Model No. 09500

Manage Spills
Proper cleanup and spill containment is vital when working with flammable and combustible chemicals. Justrite safety cabinets feature patented SpillSlope® shelves that slope backwards to safely direct spills towards the leakproof sump in the bottom of the cabinet. Use sorbents or rags to clean up spills, but never leave them within the cabinet as a preventative method. To prevent spontaneous combustion, dispose of cleaning rags in Justrite oily waste cans.

Eliminate Human Error
Despite our best efforts, nobody is perfect. Flammable safety cabinets are designed to protect workplaces in case of a fire, by allowing workers time to evacuate. But, these cabinets are only effective when doors are closed. What happens if the doors are accidentally left open?

Justrite safety cabinets with self-closing doors shut automatically in a fire situation. Fusible links hold doors open but will melt if the temperature reaches 165 F (74 C), slamming the doors shut. For added security, cabinets also feature three self-latching bullets located at the top, center and bottom of the door as a fail-safe.

Justrite Products Make Storing Flammable and Combustible Liquids Easy

Justrite supplies specialized products and services to help you handle and store flammable and combustible liquids safely and provide you with peace of mind. Learn more about flammable and combustible safety cans and cabinets.