Post Tags

Modeling the Fundamental Burning Velocity

The Flammability Department at Fauske & Associates (FAI) has recently added determination of fundamental burning velocity to its ISO 17025 scope. The fundamental burning velocity (SU) is defined as the velocity of a laminar flame under set conditions of temperature, pressure, and composition of an unburned gas. The fundamental burning velocity can be calculated from deflagration test data with the following relationship (Senecal and Beaulieu, 1997):

Modeling the Fundamental Burning Velocity - Figure 1Afterwards, the burning velocity result is corrected using the following equation from NFPA 68. This equation adjusts the burning velocity of an unknown material based on the results of a propane calibration test performed in the experimental apparatus of interest along with the accepted propane burning velocity of 46 cm/s.

Modeling the Fundamental Burning Velocity - Figure 2

Estimation of Burning Velocity from Closed Vessel Testing

Burning velocity is determined in tandem with explosion severity testing, which FAI performs to EN 15967, another recent addition to FAI’s ISO 17025 scope. Data obtained from the explosion severity test include the maximum overpressure (Pmax) and the maximum rate of pressure rise (dP/dt), from which the deflagration index (KG) is calculated. An example time-pressure curve for a deflagration of 10% methane in air is seen in Figure 1. Data from this graph provides Po, P, Pe, and dP/dt.

Modeling the Fundamental Burning Velocity - Pressure vs. Time GraphCombining pressure vs time data with the remaining constants (R and k) of Eq (1) gives burning velocity at a particular pressure. This calculation is then extrapolated across the entire deflagration event. Burning Velocity vs Pressure (barg) is afterwards plotted between 0.1 and 3.0 barg (blue) as well as between 0.3 and 2.0 barg (orange), as seen in Figure 2. A least squares linear fit is applied between 0.3 and 2.0 barg (black). The resulting line is then extrapolated to P (barg) = 0 to obtain the Burning Velocity result.

Measuring the Fundamental Burning Velocity - Burning Velocity vs. PressureUsing data from approximately 40 ignitions across a range of fuel concentrations, FAI is able to create a burning velocity curve with respect to fuel concentration to determine the maximum burning velocity for a fuel in air. An example curve of methane can be seen in Figure 3.

Burning Velocity of MethaneFAI has performed verification testing to compare burning velocity results to literature values, which can be seen below in Table 1. The reported burning velocity results are corrected using the above NFPA 68 equation with FAI’s result for propane. Verification testing is performed on an annual basis to ensure validity of testing results and to satisfy requirements set by ISO 17025.

Modeling the Fundamental Burning Velocity - Table 1Applications in Process Safety

In terms of process safety, the burning velocity is vital when performing vent sizing calculations for mitigation of deflagration events. Hybrid gaseous mixtures containing more than one flammable component would further benefit from such testing, as flammability data on mixtures are typically not readily available. Making assumptions and averaging burning velocity data for gaseous mixtures may also not be appropriate in instances where components exhibit a vast difference in flame speed, such as hydrogen (~310 cm/s) and methane/propane (~40 cm/s). By implementing Senecal and Beaulieu’s estimation method, test data can be preferably relied upon, with less assumptions being made. This could also prevent system designs from being overly conservative and costly.

In addition to vent sizing per NFPA 68, determining burning velocity is also a new requirement for testing fire safety hazards linked with propagating thermal runaway within battery systems, specifically UL 9540A Edition 4. In this test, a thermal runaway is triggered in the battery, which then results in a rapid increase of the cell temperature. The temperature rise precedes the emission of flammable gases from the cell. These gases are then collected, analyzed, and a custom gas mixture is synthesized in larger quantities for deflagration parameter testing. The installation, operation, and maintenance of battery energy storage systems requires lower flammability limit, explosion severity, and burning velocity test data on these custom gas mixtures. The data are then used to determine the necessary fire and explosion protection safeguards for these battery systems.

More information on UL 9540A Edition 4 testing and requirements can be found in last quarter’s newsletter.

Further Development

With the growing demand for obtaining burning velocity values for various chemicals or chemical mixtures, Fauske & Associates is taking further measures in developing additional methods to visually determine the fundamental burning velocity. FAI is currently in the process of designing and building a new glass tube apparatus that will be used to measure the velocity of a flame propagating along the vertical tube. This tube method will satisfy the requirements of ISO 817 – Annex E. Currently, FAI is anticipating the apparatus to be completed by the end of 2020 and added to the ISO 17025 Scope in 2021.

In addition to the tube method, FAI will be studying the estimation of the fundamental burning velocity using Britton’s correlation provided in NFPA 68:

Britton’s correlation provided in NFPA 68The heat of combustion values for gaseous mixtures will be estimated using ASTM’s CHETAH software which implements Benson’s method of group additivity.

For more information, contact us at flammability@fauske.com.

Explore FAI's Flammability Testing Services & Resources

References