Flammability Testing

Closed cup and open cup flash point tests differ primarily vapor containment during testing, which affects the temperature at which an ignition (or flash) is typically observed.

In closed cup methods, the sample is tested in a sealed (or nearly sealed) test cup which reduces the vapor loss to the atmosphere. During testing, vapors accumulate above the liquid pool into the vapor space of the sealed test cup, allowing ignition to occur at lower temperatures. As a result, closed cup tests typically produce lower (and more conservative) flash point values. These methods are generally preferred for safety assessments, regulatory compliance, and hazard classification.

In open-cup methods, the sample is tested in a lidless open cup where the liquid surface is exposed directly to the surrounding air. During testing, vapors dissipate into the environment, requiring a higher temperature to reach ignitable concentrations. Therefore, open cup tests typically yield higher flash point values and are often used to evaluate conditions more representative of open‑air industrial processes.

The difference in flash point temperature between closed and open cup tests can range from a few degrees to over 20°C, depending on the volatility of the sample tested.

Each ASTM closed cup flash point test method comes with its own strengths, weaknesses, and limitations. The differences primarily lie in their intended application range, apparatus design, heating approach, and suitability for different sample types.

ASTM D56 uses the Tag closed cup apparatus and is designed primarily for testing low-viscosity, volatile liquids with flash points below 93 °C (200 °F). It employs a relatively small brass cup and does not include stirring. The method is dynamic, meaning the sample is continuously heated at a controlled rate while an ignition source is periodically applied.

ASTM D93, the Pensky-Martens closed cup method, employs a relatively large brass cup and covers a much broader flash point range of 40 °C to 370 °C. A key distinguishing feature of D93 is the built-in stirring mechanism, which promotes uniform temperature distribution and vapor generation, making it more suitable for viscous liquids. Like D56, it is a dynamic method with continuous heating and periodic ignition source application. Due to its broader applicability and improved handling of complex matrices, D93 is more widely specified in regulatory contexts.

ASTM D3828 differs more fundamentally from the other two methods. It is a small-scale closed cup method that uses a very small sample volume of 2 to 4 mL of sample per test trial and covers an extremely wide flash point range of -30°C to 300°C. Unlike D56 and D93, D3828 uses a stainless-steel test cup and operates as an equilibrium method where the sample is brought to a specific temperature, allowed to stabilize, and then subjected to an ignition source. Additionally, it is the only closed cup flash point method that can be used to determine the flash point of solid samples. ASTM D3828’s small sample requirement and versatility make it particularly well suited for hazardous, reactive, and extremely flammable samples, as well as situations where sample quantity available for testing is limited.

While the flash point of a material can be used as a foundation for understanding the potential for flammability, it is critical to recognize that fires and/or explosions can still occur at process temperatures below this value. For example, there are certain amines and halogenated hydrocarbons that require large volumes or quenching distances to ignite. Therefore, standard flash point testers may potentially not have sufficiently large enough vapor spaces to adequately study the flammability potential of these materials. In these instances, we recommend performing Temperature Limits of Flammability (ASTM E1232) testing using a 12L glass flask, which is effectively a large-scale flash point test, to ensure there is sufficient volume for flame propagation. Otherwise, these materials may incorrectly be classified as non-flammable or combustible if standard flash point testers are used.

Processes where liquid materials are sparged, aggressively agitated, or aerosolized may also result in ignitions despite the process operating below the temperature of the experimentally determined flash point. There are currently no standardized test methods that can be performed to adequately study these turbulent scenarios, as the current standard flash point test methods are limited to testing quiescent environments. In these instances, custom large scale (5L or 12L) tests that utilize mechanical stirring or aerosolization of the liquid material should be considered.

There are also instances where liquid mixtures that contain both flammable and nonflammable components have the potential to emit flammable vapors but will not ignite during a flash point test, regardless of the test method used. This occurs when a highly volatile nonflammable component inerts the closed vapor space and prevents an ignition. These materials may still form flammable atmospheres, especially if they flash-vaporize in air. In these instances, ASTM recommends either Temperature Limit of Flammability (LTL per ASTM E1232), Lower Flammability Limit (LFL per ASTM E681 or E918), or flash point testing after allowing the samples to partially evaporate and performing tests at various degrees of evaporation.

NFPA 69 outlines several strategies for preventing a deflagration by eliminating one or more elements of the fire triangle (fuel, oxidizer, and ignition source) before an explosion can occur.

One of the most common strategies is oxidant concentration reduction, typically achieved through inerting. In this approach, an inert gas such as nitrogen or carbon dioxide is introduced into a vessel or process system to reduce the oxygen concentration below the limiting oxidant concentration (LOC) required to support combustion. By maintaining oxygen levels below this threshold, ignition and flame propagation are prevented even if a flammable fuel is present.

Another key prevention strategy is fuel concentration control, which involves maintaining the concentration of flammable gases, vapors, or combustible dusts outside the flammable range. This is often accomplished through dilution ventilation, process control, or continuous monitoring to ensure concentrations remain below the lower flammable limit (LFL) or above the upper flammable limit (UFL). NFPA 69 provides guidance on how these concentration limits must be maintained and monitored, including the use of instrumentation and safety interlocks that shut down processes if concentrations approach hazardous levels.

Ignition source control is also a fundamental component of deflagration prevention. NFPA 69 emphasizes eliminating or minimizing potential ignition sources such as static electricity, hot surfaces, mechanical sparks, electrical equipment, or open flames. This may involve equipment grounding and bonding, proper classification of hazardous locations, use of intrinsically safe electrical devices, temperature control of equipment surfaces, and preventive maintenance to reduce mechanical spark hazards.

Together, oxidant reduction, fuel concentration control, and ignition source control form the core framework for preventing deflagrations in industrial processes handling flammable gases, vapors, or dusts.

There are several ways to mitigate the severity of an explosion (or deflagration) in a vessel or enclosure, which include venting, suppression, and containment.

NFPA 68 outlines engineering methods designed to reduce the severity of a deflagration by relieving pressure and controlling the combustion event so that equipment or enclosures are not catastrophically damaged. The primary method described is deflagration venting, in which a vessel, enclosure, or building is equipped with explosion vents designed to open at a predetermined pressure. When a deflagration occurs, these vents rapidly release the burned and unburned gases, thereby limiting the maximum pressure inside the enclosure. Venting may be achieved using rupture panels, hinged panels, or rupture discs.

In addition to conventional venting, NFPA 68 also allows for flameless venting devices, which incorporate a flame-arresting element so that pressure is relieved while preventing the flame front and burning dust or gases from propagating into surrounding occupied areas. For situations where vent discharge would be hazardous indoors, the standard also describes ducted venting, where the vent is connected to a duct that directs the explosion products to a safe outdoor location, although the duct increases flow resistance and must be accounted for in vent sizing calculations. Finally, the standard recognizes venting through building panels or walls designed as explosion relief surfaces. Together, these approaches mitigate the severity of a deflagration by limiting internal pressure buildup and safely directing the combustion products away from personnel and critical equipment, thereby reducing structural damage and secondary hazards.

While NFPA 68 primarily addresses explosion venting, it recognizes that other explosion protection strategies may also be applied depending on the process and equipment involved. One such method is explosion suppression, which involves detecting the early pressure rise or flame associated with a developing deflagration and rapidly discharging a suppressant, typically an inert powder or gas, into the vessel or enclosure. The suppressant absorbs heat and interferes with the combustion process, effectively quenching the flame and limiting the pressure rise before damaging levels are reached.

Another approach is explosion-resistant design or containment, in which process vessels, ducts, or enclosures are engineered to withstand the maximum explosion pressure that could be generated during a deflagration. In this case, the equipment is constructed with sufficient mechanical strength so that it remains intact even if a deflagration occurs internally.

These methods may be used individually or in combination to effectively manage explosion hazards. Selection of the appropriate strategy depends on factors such as vessel size, process conditions, chemicals used, available venting locations, and whether the release of combustion products to the surrounding area can be safely tolerated. Together, techniques such as venting, suppression, and containment provide complementary means of limiting the consequences of a deflagration and protecting personnel, equipment, and facilities.

Samples should be shipped in leak-proof containers that are well-secured within the package. For hazardous samples, absorbent pouches or special packaging might be required. All of our quotes and the Sample Test Options Form outline how much sample to send for a particular test - please send at least that quantity to receive adequate test results. Sending unnecessarily large samples is also inadvisable, as it raises shipping costs, and an extra disposal fee may be charged to you for egregious examples.

There are many, many variables that can contribute to the behavior of a fire and/or when one takes place. While we try our best to account for these discrepancies in advance, unusual or unique samples may exhibit inconsistent behaviors when tested. As such, repeated trials may uncover a lower result and other observations that do not appear in a single trial, ensuring we report the most conservative data - and thus, help you to be the safest you can be.

All of the test methods we perform at Fauske & Associates adhere to national or international standards. Our operating procedures are reviewed annually as part of our ISO accreditation, and our scientists and technicians are trained rigorously on these methods. Furthermore, many of our tests undergo annual verifications on reference materials that are compared against available literature data.

While both are flammability limit tests performed in round vessels, the setups vary considerably. ASTM E918 utilizes a 5.3 L stainless-steel vessel, the ignition source used is a fuse wire, and the ignition criteria is dependent on pressure rise. ASTM E681 uses a 5 L glass flask instead, the ignition source is a set of two electrodes that create a spark between them, and the ignition criteria is purely visual. In general, we recommend that clients opt for testing via ASTM E918 for a few reasons: it is more cost-effective, it can achieve higher temperatures and pressures, and it can be performed to a higher standard of precision. However, there are times in which ASTM E681 is preferable. This is especially true for samples that have reactivity issues with the fuse wire or vessel. Unfortunately, it is limited to ambient pressure and ~150 C.

Many flammability properties are useful to know for internal safety and process design. Key properties to focus on early are the flash point (the temperature at which an external ignition source can cause a fire to occur in the vapor space of a chemical), the autoignition temperature (the temperature at which the material's vapors will ignite without an external ignition source), the flammability limits (the highest and lowest concentrations of the material in an atmosphere, usually air, that will allow the material to burn), and the explosion severity (the highest pressure rise observed in the ignition of a chemical at a specified temperature and pressure).

The main goal for all three combustibility methods are to evaluate if a material at a given temperature will be able to sustain burning once ignited. That being said the application of the data is what drives many of the differences.

• ASTM D4206
• It is a nonregulatory standard
• Often used for product safety, R&D, and formulation studies
• Performed at a single temperature of 49°C
• UN/DOT L.2
• UN/DOT Regulated
• Used worldwide for dangerous goods classification for global shipping compliance
• Performed at different temperatures with different hold times.
• 60.5°C and 75.0°C
• 30s and 60s
• 49 CFR 173 Appendix H
• US Code of Federal Regulations
• Required for US Hazmat classification and used in US domestic shipping
• Performed at different temperatures while keeping the hold time constant
• 60°C, 75°C, and slightly above the flashpoint (FP+20°C)

For deciding which method to use the use of the data needs to be evaluated. If looking for a value for internal use in product development or internal hazard development ASTM D4206 provides you with that information. If you need sustained combustion data for shipping purposes you will first need to identify if you are shipping within the US or internationally. If shipping within the US 49 CFR 173 Appendix H method is sufficient whereas if the material is being shipped internationally UN/DOT L.2 will be required along with flash point for shipping classification.

The most common use of sustained combustibility data is for shipping classification. Depending on the flash point and ability to sustain combustion at specific temperatures, a material can be deemed flammable for the purpose of shipping. The cut-off values for classification do vary slightly based on a few different standards (UN/DOT & CFR); however, for more in-depth information about a materials ability to sustain combustion fire point testing can be performed. Fire point testing can provide specific temperatures when a material transitions from its flash point to sustain burning for an extended period of time.

As always, the most applicable results rely on data generated from a test design that models the process conditions as much as possible. By performing testing at process condition or upset conditions data can best reflect what to expect in real life. Things like temperature and pressure can change results for flammability limits, limiting oxygen concentration, and even explosion severity. Due to the effects of pressure and temperature on flammable properties it is not always advisable to extrapolate data generated at ambient conditions to systems that deviate in pressure and temperature. Since flammable properties are heavily linked and driven by reaction kinetics and vapor pressure of individual components changing temperature and pressure can have meaningful impacts. A good example of this is the effect of temperature on flammability limits. As a system increases temperature flammability limits widen leading to lower LFL values and higher UFL values. If using data generated at standard temperature and pressure to develop safety limits on an elevated pressure and/or temperature system incidents can still occur.

Flammability Limits:

• Temperature - By increasing temperature, the flammable range of material widens which could shift a process which was originally deemed to be safe as it was below the LFL to now be in the flammable region. By increasing the temperature, you have changed the vapor pressure of the material along with altering the reaction kinetics driving wider than standard flammable region.

• Pressure – When pressure is increased often times flammability limits widen due to having more moles of reactants present along with heat loss becomes less of a problem. By increasing the amount of molecules in a given space the transfer of energy from one molecule to another is more likely and energy loss as a reaction propagates gets lower.

Limiting Oxygen Concentration:

• Temperature - Temperature has a similar effect as to the flammability limits where increasing the temperature does lower the LOC. This is because of some of the same reasons as the flammability limits where at higher temperatures the reaction kinetics have changed to be more favorable. As you increase temperature the amount of collisions and interactions of fuel and oxygen molecules is more frequent.

• Pressure – The effect of pressure can cause the LOC to decrease (Babrauskas pg 120). As you increase pressure the density of molecules in the given space increases, allowing for differences in energy transfer and the likelihood that a reaction will cascade into flame propagation.

Explosion Severity:

• Temperature – It effects outputs such as P_max, dP/dt, and flame speed. P_max increases due to increased flame temperatures which translate directly into higher pressures due to the relationship of pressure and temperature from the ideal gas law. dP/dt also increases as it is directly driven by kinetics so using the standard Arrhenius kinetic equation k=Ae-EaRT increasing temperature will increase the speed of the reaction. Finally, increased temperature will lead to a faster laminar burning velocity due to the density of the gas changing. As you increase temperature the density of the gas decreases therefore increasing the burning velocity.

• Pressure – Effects outputs such as dP/dt, temperature rise, and P_max. Going back to the Arrhenius kinetic equation, the pre-exponential factor is driven by the frequency of collisions of molecules within the system. As you increase the amount of total molecules in the system the likelihood of them colliding increases, which has a direct impact on the kinetics of the reaction. Increasing pressure also means potential energy exist within the system from the increased moles of reactants. Due to this higher max temperatures and pressures are seen as they are linked through the ideal gas law. Though max pressure does increase as starting pressure increases it often follows the same pressure ratio of starting pressure to max overpressure.

Though the core approach of the two methods are very similar where you change the concentration of fuel until reactions are no longer observed the experimental set up is different, putting limitations on each method.

• ASTM E681 – This test method uses a glass flask with a visual ignition criterion to determine flammability limits. The use of the glass flask has some pros and cons. Some of the pros being that the operator can see exactly what is happening inside the test vessel and note all observations. Some of the cons being that with it being a glass vessel inside of an oven elevated pressure and higher temperatures are not possible to test. The indication of an ignition also has some strengths and weaknesses. With it being purely visual the operator is able to see if flames are present and how far they propagate; however, with it being purely visual these observations are influenced by flame color, speed, viewer angle, and how dark the room is. It is possible to have multiple people watch the same trial and come to different conclusions.

• ASTM E918 – This method uses a closed stainless steel vessel with a strict pressure rise criterion to determine flammability limits. Again, this method has pros and cons. Some of the pros are that with it being a closed stainless steel vessel elevated pressure and temperature is done with ease and minimal changes to the test set up. Also, using a pressure rise criteria for what constitutes an ignition creates standardization and uniformity from lab to lab and user to user. Some of the cons being that with it being a stainless steel vessel observations on what is occurring inside of the vessel are reliant on pressure and temperature sensors and does not currently allow for visual observation of flame propagation. Another drawback is that using a stainless steel vessel the method is limited by material compatibility that does not interfere with the chemistry at work.

Vessel volume and geometry are important aspects to consider for test design. Most ASTM and NFPA methods recommend using equipment that allow for spherical propagation to best assess flammability, but little guidance is given on how to choose the correct test volume. As testing as been performed on a wider range of materials that are extending outside of the cookie cutter hydrocarbons it is starting to be observed that certain chemicals require extra room to fully propagate. This can very well lead to improper scale up of test data. The need for additional room for propagation can be driven by factors such as a material’s ability to self-quench meaning localized ignitions are present but unable to propagate. This is often due to heat loss to the surrounding walls of the vessel which stops or quenches the reaction. To minimize this effect the test vessel can be increased to reduce the surface area to volume ratio. Some chemicals that are particularly susceptible to quenching are ones that have low heats of combustion, materials which have reaction products with high heat capacities, materials with slow burning velocities, and materials which have high activation energies. These characteristics either pull energy out of the system or make it more difficult for propagation to occur. By increasing the vessel volume and reducing the surface area to volume ratio the test data is most easily and accurately scaled up.

GHS and NFPA are both systems for classification of material hazards and both are often found on SDS. Though both systems classify hazards of materials the thought process and approach is quite different.

• GHS – The Global Harmonized System is intended to be a to standardizes hazard communication with the goal of creating clear and easy to understand pictograms and signal words for SDS and placarding of materials. With the intent of the GHS being a universal system, it is managed by the United Nations to replace a wide variety of systems for standard hazard identification regardless of language.

• NFPA – The National Fire Protection Agency approaches hazard communication through the lens of hazard response. This can be seen in the way NFPA 704 identifies hazard levels specifying “under emergency conditions” with each level.

Though both systems use a 0-4 system for labeling hazard it is incredibly important that these systems are not mixed up as they are not interchangeable. It is paramount to understand that when looking at both systems it is understood that they operate in opposite directions. In GHS classification 0 holds the highest hazard level whereas with NFPA 4 is the highest hazard level. Mixing these two systems up can potentially be lethal.