Ignitable liquids: What they are and why the hazard matters

Ignitable liquids are widely used across industrial, commercial, and institutional occupancies. When improperly stored, handled, or protected, these liquids can create severe fire scenarios that exceed the capabilities of ordinary building construction and fire protection systems. Understanding ignitable liquids and the hazards they present is critical to preventing fires and limiting property damage when incidents occur.

What is an ignitable liquid?

Any liquid with a measurable flash point is considered an ignitable liquid, which includes both flammable and combustible liquids. Flammable liquids have flash points below 100°F, allowing them to create ignitable vapor-air mixtures at relatively low temperatures. Combustible liquids have flash points at or above 100°F and typically do not produce ignitable vapors at normal room temperatures.

It’s important to note that it’s not the liquid itself that burns, but the vapors they release. For an ignitable liquid to catch fire, it must first transform from a liquid into a vapor or gas. An ignitable vapor-air mixture is generated when the liquid reaches a minimum temperature known as the flash point.

However, both flammable and combustible liquids can pose significant fire hazards under the right circumstances. If a spill or release occurs, vapors can quickly form above the exposed or heated liquid surface. Once ignited, these liquids can generate high heat release rates, regardless of their flash point.

Common examples of ignitable liquids include:

• Alcohols
• Fuels
• Oils
• Resins
• Solvents
• Wide range of industrial chemicals

What are the hazards associated with ignitable liquids?

Liquid release and fire spread - Unlike solid combustibles, ignitable liquids can flow and spread following a release. A relatively small quantity can form a large pool, significantly increasing the burning surface area and fire severity. Fires may extend well beyond the point of origin, exposing adjacent areas and equipment.

Pool fires and fire growth - A pool fire involving ignitable liquid can grow rapidly, producing high radiant heat and flame heights. Testing has shown that even modest quantities of ignitable liquids can quickly overwhelm sprinkler systems designed for ordinary storage or manufacturing occupancies.

Container failure - Container construction influences fire development:

• Plastic containers are likely to fail early in a fire, releasing their contents and enlarging the fire.
• Metal containers may rupture violently or discharge burning liquid unless adequately cooled by fire protection systems.

Vapor and explosion hazard - If vapor from ignitable liquids accumulates in a confined space and reaches flammable concentrations, ignition can result in an explosion or deflagration. This hazard is most pronounced during leaks, spraying, heating, or transfer operations.

What controls can help prevent or limit ignitable liquid losses?

A fundamental principle for ignitable liquids is limiting the flow of liquid and controlling any liquid that is released. Fire protection systems alone cannot economically protect against all possible ignitable liquid fire scenarios. Effective loss prevention requires a combination of inherent safety features, passive protection, and active fire protection.

Storage and location - Ignitable liquid storage should be separated from occupancies not designed for the hazard. Proper location and separation reduce the likelihood that a liquid fire will impact critical buildings or processes.

• Where practical, use outdoor storage, detached buildings, or cutoff rooms.
• Store liquids on ground floors to reduce exposure.
• Limit storage quantities and heights based on container type and liquid characteristics.
• Consider FM approved cabinets, lockers, or prefabricated storage buildings for small or distributed quantities.

Limiting liquid release - Reducing fire severity begins with limiting the quantity and rate of liquid release. Stopping liquid flow is essential as sprinklers and special protection systems cannot control a sustained liquid release.

• Minimize the amount of ignitable liquid present at points of use.
• Avoid continuous or unrestricted flow where possible.
• Provide automatic safety shutoff valves to stop liquid flow during fire or leak conditions.
• Arrange transfer systems to isolate supplies quickly and completely.

Containment and drainage - Containment controls the spread of released liquids. Limiting pool size directly limits fire severity and sprinkler demand.

• Curbs, dikes, or containment basins limit pool size.
• Emergency drainage systems direct liquids away from critical exposures.
• Containment designs should account for the largest credible liquid release and sprinkler discharge.

Automatic fire protection - Automatic sprinklers should protect all areas where ignitable liquids are stored, staged, or used.

• Sprinkler designs must match the liquid type, container size, and storage or use arrangement.
• Areas with higher hazard may require foam water or compressed air foam (CAF) systems.
• Sprinklers are effective for cooling, exposure protection, and—under specific conditions—extinguishing certain pool fires, but they are not a substitute for controlling liquid flow.

Ventilation - Because ignitable liquids burn as vapor, ventilation is critical. Ventilation reduces both fire and explosion potential.

• Provide continuous low-level ventilation where vapor accumulation is possible.
• Exhaust vapors from floor level, where they tend to concentrate.
• Monitor ventilation systems to detect failures promptly.

Ignition source control - Controlling ignition sources significantly reduces fire probability.

• Use hazardous-location-rated electrical equipment where required.
• Prohibit smoking, open flames, and uncontrolled hot surfaces.
• Bond and ground containers and equipment to control static electricity.
• Strictly control hot work through permit systems.

Housekeeping, inspections, and training - Human factors play a major role in ignitable liquid losses.

• Clean spills promptly and remove waste regularly.
• Maintain clear access aisles and fire protection features.
• Inspect containers, piping, and safety devices routinely.
• Train employees on ignition hazards, spill response, and emergency shutdown procedures.

Ignitable liquids present unique and severe fire hazards due to their ability to spread, release vapor, and produce high-heat release rates. Experience and testing have demonstrated that effective protection depends on limiting the amount of liquid involved, stopping liquid flow early, containing releases, and providing fire protection systems specifically designed for the hazard.

Facilities that apply these loss prevention principles significantly reduce the likelihood of large-loss fires involving ignitable liquids and improve the resilience of buildings and operations when fires occur.

Additional resources:

• FM Data Sheet 7-32 ignitable liquid operations – Review engineering guidance for ignitable liquid handling and transfer operations focused on limiting liquid release and fire severity.
• Reach out to your reinsurance territory manager for a copy of FM’s “Understanding the Hazard: Ignitable liquid handling” to get a high-level overview of ignitable liquid hazards, common loss drivers, and risk considerations across storage, handling, and operations.
• FM Approved solutions – Storing ignitable liquids just got safer – Explore FM Approved storage solutions designed to address ignitable liquid hazards and support safer storage practices in a variety of facility environments.