The Fundamentals of Gas & Vapor Flammability Testing

What is Gas & Vapor Flammability Testing?

Gas and vapor flammability testing evaluates how easily a gas or vapor ignites and sustains combustion under set conditions. It determines key properties like flammable limits, ignition temperature, and explosive concentration ranges—essential for classifying fire and explosion hazards.

Flammability testing protects workers, equipment, and facilities. It guides the safe design of systems, storage, and procedures for handling flammable gases and vapors. Industries such as chemical manufacturing, petroleum refining, pharmaceuticals, and hazardous material storage use this testing to reduce fire and explosion risks.

In this blog, we’ll cover the fundamentals of gas and vapor flammability testing—how it’s done, key concepts, and how the results shape safety practices across industries.

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Flash Point

The flash point is the lowest temperature where a flammable liquid produces enough vapor to ignite briefly under test conditions. This value accounts for atmospheric pressure variations from the standard 101.3 kPa. The fire point is the temperature at which the liquid burns for at least five seconds after removing the ignition source. Common test methods include the Pensky-Martens Closed Cup and Cleveland Open Cup. These tests gradually heat the liquid and attempt ignition to find these key temperatures.

Flash and fire points help assess a liquid’s volatility and flammability. These traits directly affect how safely industries store, handle, and transport flammable liquids. Low flash points mean higher fire risk; higher flash points mean lower hazard. Identifying these points helps safety teams take the right precautions, prevent fires, and meet regulations that protect people, property, and the environment.

Organizations use recognized standards to perform these tests, including ASTM D93 (Pensky-Martens Closed Cup), ISO 2719, ASTM D3828 (Small Scale Closed Cup), and ASTM D92 (Cleveland Open Cup).

Flammability Limits (LFL/UFL)

Flammability limit testing determines the lower and upper concentration limits—known as the Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL)—at which a chemical can ignite in air under specified conditions. This test is essential for volatile liquids and gases with sufficient vapor pressure to form combustible mixtures, such as refrigerants. The LFL represents the minimum concentration of a substance in air required for flame propagation, while the UFL is the maximum concentration at which ignition can still occur.

Knowing the flammability limits of a substance is vital for evaluating explosion risks and designing safe handling systems. These values guide decisions about ventilation requirements, safe storage, and operational conditions, especially in facilities managing volatile chemicals. LFL and UFL data are particularly useful during the development of new materials, in post-incident investigations, or when environmental or formulation changes could impact flammability. Testing ensures processes remain outside flammable concentration ranges, reducing the likelihood of ignition and enhancing workplace safety.

LFL and UFL are determined in a controlled lab setting by incrementally adjusting concentrations of the substance in air and observing flame propagation using an electrical ignition source. Once the LFL is established, UFL testing begins above a theoretical upper limit and is gradually adjusted until sustained ignition occurs. For refrigerants, tests follow ASHRAE 34 and ASTM E681, which define the methodology, ignition criteria, and required air moisture levels.

Autoignition Temperature (AIT)

Autoignition Temperature (AIT) is the lowest temperature at which a chemical vapor ignites spontaneously in air without external ignition. This occurs when air oxidation produces enough heat to start combustion. Several factors affect AIT, including vapor concentration, ignition delay, test volume, pressure, oxygen content, and catalytic surfaces. Although commonly used for gases, this test also applies to liquids and solids that fully vaporize.

Knowing a substance’s AIT is crucial for assessing fire and explosion risks in industrial settings. Engineers use AIT data to create accurate Safety Data Sheets (SDS) and select safe materials and equipment. AIT also guides safe storage and transport to prevent spontaneous ignition. Regulators often require AIT data to ensure equipment operates within safe temperature limits.

Technicians determine AIT through controlled lab testing following ASTM E659 – Standard Test Method for Autoignition Temperature of Chemicals.

Limiting Oxygen Concentration (LOC)

Limiting Oxygen Concentration (LOC) is the lowest oxygen level in air below which a combustible gas cannot ignite or sustain combustion, even with enough fuel and an ignition source. This threshold marks the point where gas-air mixtures are no longer explosive. LOC testing helps define safe operating conditions in environments where flammable gases are present.

LOC testing is essential for preventing gas explosions, especially when inert gases like nitrogen are used to reduce oxygen levels. It helps determine the maximum oxygen concentration allowed to keep processes safe. You should perform LOC testing when new materials are introduced, during process changes, or as part of safety reviews. The results guide the design of safe systems, support regulatory compliance, and improve workplace safety.

LOC testing follows the same principles as dust LOC testing but skips the dispersion step. The test follows European Standard EN 15967 and uses a pressure ratio of 2.0 as the ignition threshold.

Explosion Severity (Pmax and Kg)

The Gas Explosion Severity Test measures explosibility by determining two key parameters: maximum explosion pressure (Pmax) and explosion severity index (Kg). Specifically, Pmax is the highest pressure reached during a gas explosion under standard conditions, while Kg measures the rate of pressure rise. Together, these values provide a detailed understanding of gas explosive behavior and help assess the risks of industrial gas incidents.

This test is especially essential for industries handling flammable gases to ensure safe equipment and facility design. Moreover, knowing explosion severity helps engineers develop protective measures such as deflagration containment, explosion venting, and suppression systems. Additionally, the results support hazard analysis, process safety management, and regulatory compliance. Testing is required when a gas is identified as explosible, when processes change, or when safety regulations demand it, ensuring that risks are identified and mitigated effectively.

The Gas Explosion Severity Test follows the EN 15967 standard, which sets procedures for evaluating both Pmax and Kg.

Conclusion

Gas and vapor flammability testing is an essential part of managing fire and explosion risks in industrial environments. By identifying the flammable characteristics of gases and vapors—such as flash point, flammability limits, autoignition temperature, limiting oxygen concentration, and explosion severity—companies can design safer processes, implement effective controls, and comply with safety regulations. These tests not only help prevent accidents but also guide emergency preparedness, engineering design, and regulatory documentation.

At Prime Process Safety Center, we specialize in high-precision flammability testing using advanced equipment and industry-approved methods. Moreover, our experienced team delivers reliable, defensible data tailored to your needs—whether you’re designing new systems, updating safety protocols, or meeting compliance requirements. By partnering with us, you can ensure your operations are grounded in proven science and best-in-class process safety practices.