Explosion Limits Determination for New Chemical Substances: LEL and UEL Testing

Explosion limits determination is a core physicochemical safety test that establishes the lower and upper explosion limits (LEL and UEL) of gases and vapors of new chemical substances. Testing performed by accredited third-party laboratories in accordance with standards such as ASTM E681, EN 1839, and GB/T 21844 provides the flammability data required for new chemical substance registration, GHS classification, safety data sheet preparation, and process safety design. This article explains the technical principles, sample requirements, test items, methods, applications, and frequently asked questions related to explosion limits testing for new chemicals.

Technical Overview

The explosion limits of a chemical define the concentration range of its gas or vapor in air, expressed as a percentage by volume, within which an ignited mixture will sustain flame propagation. The lower explosion limit (LEL, also known as the lower flammability limit or LFL) is the minimum concentration at which a flame can propagate away from the ignition source, while the upper explosion limit (UEL, or UFL) is the maximum concentration above which the mixture is too rich in fuel to burn. Mixtures below the LEL are too lean to ignite, and mixtures above the UEL are too rich to ignite, so the interval between the two values—known as the explosion range—represents the hazardous composition window that must be avoided or actively controlled.

For new chemical substances, published flammability data generally do not exist, so experimentally measured explosion limits are essential for hazard identification, GHS classification of flammable gases and vapors, safety data sheet (SDS) development, and registration under new chemical substance notification schemes required by environmental and chemical regulators. Explosion limits are not intrinsic constants of a substance: they depend on initial temperature, initial pressure, ignition source energy, test vessel geometry and volume, direction of flame propagation, turbulence, and humidity. For this reason, all results must be reported together with the exact test conditions, and accredited third-party laboratories apply internationally recognized standards to ensure the data are reproducible and acceptable for regulatory submission.

Testing Samples

Gases and Gas Mixtures

Compressed or liquefied gases are supplied in suitable cylinders or lecture bottles and are metered into the test apparatus by the partial pressure method. Gas mixtures are tested as delivered after verification of their composition. Testing is normally performed at ambient temperature (typically 20 °C to 25 °C) and atmospheric pressure unless process conditions require evaluation at elevated states.

Liquid Chemicals with Sufficient Vapor Pressure

Flammable and combustible liquids that can generate a measurable vapor-air mixture at the test temperature are injected into an evacuated apparatus using gas-tight syringes and allowed to vaporize completely before ignition. High-boiling liquids may be evaluated using a temperature-controlled apparatus, within the limits of the equipment. A sample purity of 99% or higher is recommended, and water content and major impurities should be reported, because halogenated or inhibiting impurities can shift the measured limits significantly.

Solids and Special Cases

Solids that melt, sublime, or release flammable vapor may be evaluated in the vapor phase at elevated temperature, provided they remain chemically stable under the test conditions. Thermally unstable or self-reactive substances require prior thermal stability screening (for example by calorimetry) and may be excluded from testing or handled under enhanced safety precautions. Combustible dusts are assessed using dedicated dust explosion tests, such as 20-liter sphere testing, which fall outside the scope of gas and vapor explosion limit methods.

Testing Items

• Lower explosion limit (LEL) in volume percent at the specified temperature and pressure
• Upper explosion limit (UEL) in volume percent at the specified conditions
• Explosion range, calculated as the difference between the UEL and the LEL
• Explosion limits at elevated temperature and/or pressure where process conditions require it
• Complementary limiting oxygen concentration (LOC) determination for inerting design
• Full test report covering apparatus, ignition source, flame propagation criteria, concentration bracketing data, repeatability demonstrations, and any deviations from the standard

Testing Methods

Spherical Glass Flask Method (ASTM E681, GB/T 21844)

The most widely used approach for new chemical hazard testing is the closed spherical glass flask method described in ASTM E681 and the corresponding Chinese standard GB/T 21844. A 12-liter (or, where permitted, 5-liter) spherical glass vessel is evacuated, the sample is introduced by syringe or partial pressure, and the mixture is homogenized with stirring. A central high-voltage electric arc or fuse wire ignites the mixture, and a positive result is recorded only when the flame detaches from the ignition source and propagates upward and outward beyond the defined observation criterion. The concentration is varied stepwise until the transition between propagation and non-propagation is bracketed; the reported limit is taken between the highest concentration showing no propagation and the lowest concentration showing propagation, with repeat firings performed at the bracketing concentrations to demonstrate repeatability. The flask is enclosed in a protective shielded chamber, and the entire procedure is operated remotely for personnel safety.

Tube and Closed Vessel Methods (EN 1839, GB/T 12474)

The European standard EN 1839 specifies both a tube method and a bomb (closed vessel) method. In the tube method, the mixture is ignited at the open base of a vertical tube and flame propagation over a minimum defined travel distance constitutes a positive result. The Chinese method GB/T 12474 follows a comparable approach for combustible gases in air. These methods allow testing at elevated initial temperatures and are frequently requested for submissions aligned with European regulatory practice.

Elevated Temperature and Pressure Method (ASTM E918)

When explosion limits are needed at process-relevant conditions rather than ambient conditions, ASTM E918 employs a heated pressure vessel to determine flammability limits at elevated temperatures and pressures within the apparatus rating. This supports the design of reactors, distillation systems, and vapor spaces operating above ambient conditions, and the screening of upset or runaway scenarios.

Limiting Oxygen Concentration (Related Test)

Often performed alongside explosion limit testing, the limiting oxygen concentration is determined by progressively replacing air with nitrogen (or another diluent) until flame propagation no longer occurs at fuel concentrations above the LEL. Standards such as EN 1839 and EN 14756 govern this determination, and the resulting LOC value, with an appropriate safety margin, defines the oxygen level required for nitrogen-inerted process and storage operations.

Calculation methods based on Le Chatelier's rule or group contribution estimations may be used for early screening, but they are not substitutes for measurement. Regulatory registration dossiers and process safety designs for new chemical substances require experimentally determined explosion limits generated under recognized standards.

Applications

• Regulatory registration of new chemical substances under environmental notification and inventory schemes, where explosion limits are part of the standard physicochemical hazard data set
• GHS classification of flammable gases, applying criteria consistent with ISO 10156, under which a gas is considered flammable if its LFL is 13% by volume or less, or if its flammability range is at least 12 percentage points
• Preparation of Section 9 (physical and chemical properties) of safety data sheets for new substances
• Process safety engineering, including ventilation design, nitrogen blanketing of vessels, and inerting levels derived from LOC data
• Setting alarm and shutdown thresholds for combustible gas detection systems, commonly at 25% of the LEL or lower
• Hazardous area classification of installations handling flammable gases and vapors, consistent with IEC 60079-10-1
• Transport classification of dangerous goods and fire protection planning
• Research, development, and scale-up of fine chemicals, pharmaceuticals, and specialty materials

FAQ

What is the difference between explosion limits and flammability limits?

The two terms are used interchangeably for gases and vapors: both describe the lower and upper concentration boundaries of flame propagation in air. Note that the phrase "explosion limits" is also sometimes applied to the thermal decomposition pressure limits of unstable substances, which is a completely different measurement and should not be confused with vapor-air flammability testing.

How much sample is required, and does purity matter?

Typical requirements are approximately 100 to 500 mL of liquid, or a gas cylinder or lecture bottle of sufficient volume, along with documentation of composition. Purity matters greatly: small amounts of inhibiting impurities or moisture can shift the measured limits, so samples should be well characterized before testing.

Can explosion limits be predicted instead of measured?

Estimation methods are useful for preliminary screening and for checking the plausibility of results, but regulatory submissions and safety-critical design decisions for new chemical substances require experimentally determined values obtained by accredited third-party laboratories under recognized standards.

How do temperature and pressure affect the results?

Raising the initial temperature generally lowers the LEL and raises the UEL, widening the explosion range, while pressure effects are substance-specific and can be significant for some compounds. Where processes operate above ambient conditions, testing under the relevant temperature and pressure, for example per ASTM E918, is recommended.

What is the typical turnaround time?

Routine ambient-condition determination of both limits, including bracketing, repeatability runs, and reporting, is generally completed within two to four weeks. Testing at elevated conditions, highly reactive samples, or complementary LOC determination may extend the schedule.

Are the results directly usable for classification and registration?

Yes, provided the report clearly states the test standard, apparatus, temperature, pressure, ignition source, observation criteria, and repeatability data. Results obtained at ambient conditions are directly applicable to GHS flammable gas classification, while data from elevated conditions support process-specific safety assessments.

In summary, explosion limits determination provides fundamental flammability data that underpin the safe handling, classification, and regulatory registration of new chemical substances. Engaging an accredited third-party laboratory with proven experience in flammability testing ensures that LEL and UEL values are technically defensible and accepted across regulatory and industrial applications.